/ minkowski ) spacetime setting, we discuss them in the next section. ) rather than formulating a revised law of gravity / inertia using relative quantities, barbour and bertotti attacked the problem using the framework of lagrangian mechanics, replacing the elements of the action that involve absolute quantities of motion with new terms invoking only relative distances, velocities etc. their first ( 1977 ) theory uses a very simple and elegant action, and satisfies everything one could wish for from a mach - heavy theory : it is relationally pure ( even with respect to time : while simultaneity is absolute, the temporal metric is derived from the field equations ) ; it is nearly empirically equivalent to newton ' s theory in a world such as ours ( with a large - scale uniform, near - stationary matter distribution ) ; yet it does predict novel effects such as the ones mach posited with his thick bucket. among these is an ‘ anisotropy of inertia ’ effect — accelerating a body away from the galactic center requires more force than accelerating it perpendicular to the galactic plane — large enough to be ruled out empirically. barbour and bertotti ' s second attempt ( 1982 ) at a relational lagrangian mechanics was arguably less machian, but more empirically adequate. in it, solutions are sought beginning with two temporally - nearby, instantaneous relational configurations of the bodies in the universe. barbour and bertotti define an ‘ intrinsic difference ’ parameter that measures how different the two configurations are. in the solutions of the theory, this intrinsic difference quantity gets minimized, as well as the ordinary action, and in this way full solutions are derived despite not starting from a privileged inertial - frame description. the theory they end up with turns out to be, in effect, a fragment of newtonian theory : the set of models of newtonian mechanics and gravitation in which there is zero net angular momentum. this result makes perfect sense in terms of strict relationist aims. in a newtonian world in which there is a nonzero net angular momentum ( e. g., a lone rotating island galaxy ), this fact reveals itself in the classic “ tendency to recede from the center ”. since a strict relationist demands that bodies obey the same mechanical laws even in ‘ rotating ’ coordinate systems, there cannot be any such tendency to recede from the center ( other than in a local subsystem ), in any of the relational theory ' s models


a temptation to think of at least acceleration as ‘ true ’ or ‘ absolute ’. if such a realist believes motion to be by nature a relation rather than a property ( and as we saw in the introduction, not all philosophers accept this ) then she will feel obliged to accord some sort of existence or reality to the structure — e. g., the structure of galilean spacetime — in relation to which these motions are defined. for philosophers with such realist inclinations, the ideal relational account of motion would therefore be some version of mach - heavy. the special theory of relativity ( str ) is notionally based on a principle of relativity of motion ; but that principle is ‘ special ’ — meaning, restricted. the relativity principle built into str is in fact nothing other than the galilean principle of relativity, which is built into newtonian physics. in other words, while there is no privileged standard of velocity, there is nevertheless a determinate fact of the matter about whether a body has accelerated or non - accelerated ( i. e., inertial ) motion. in this regard, the spacetime of str is exactly like galilean spacetime ( defined in section 5 above ). in terms of the question of whether all motion can be considered purely relative, one could argue that there is nothing new brought to the table by the introduction of einstein ' s str — at least, as far as mechanics is concerned. as dorling ( 1978 ) first pointed out, however, there is a sense in which the standard absolutist arguments against ‘ strict ’ relationism using rotating objects ( buckets or globes ) fail in the context of str. maudlin ( 1993 ) used the same considerations to show that there is a way of recasting relationism in str that appears to be very successful. str incorporates certain novelties concerning the nature of time and space, and how they mesh together ; perhaps the best - known examples are the phenomena of ‘ length contraction ’, ‘ time dilation ’, and the ‘ relativity of simultaneity. ’ since in str both spatial distances and time intervals — when measured in the standard ways — are observer - relative ( observers in different states of motion ‘ disagreeing ’ about their sizes ), it is arguably most natural to restrict oneself to the invariant spacetime separation given by the interval between two points : [ dx2 + dy2 + dz2 — dt2 ] — the four - dimensional analog of the p


sizes ), it is arguably most natural to restrict oneself to the invariant spacetime separation given by the interval between two points : [ dx2 + dy2 + dz2 — dt2 ] — the four - dimensional analog of the pythagorean theorem, for spacetime distances. if one regards the spacetime interval relations between masses - at - times as one ' s basis on which space - time is built up as an ideal entity, then with only mild caveats relationism works : the ‘ relationally pure ’ facts suffice to uniquely fix how the material systems are embeddable ( up to isomorphism ) in the ‘ minkowski ’ spacetime of str. the modern variants of newton ' s bucket and globes arguments no longer stymie the relationist because ( for example ) the spacetime interval relations among bits of matter in newton ' s bucket at rest are quite different from the spacetime interval relations found among those same bits of matter after the bucket is rotating. for example, the spacetime interval relation between a bit of water near the side of the bucket, at one time, and itself ( say ) a second later is smaller than the interval relation between a center - bucket bit of water and itself one second later ( times referred to inertial - frame clocks ). the upshot is that, unlike the situation in classical physics, a body at rest cannot have all the same spatial relations among its parts as a similar body in rotation. we cannot put a body or system into a state of rotation ( or other acceleration ) without thereby changing the spacetime interval relations between the various bits of matter at different moments of time. rotation and acceleration supervene on spacetime interval relations. it is worth pausing to consider to what extent this victory for ( some form of ) relationism satisfies the classical ‘ strict ’ relationism traditionally ascribed to mach and leibniz. the spatiotemporal relations that save the day against the bucket and globes are, so to speak, mixed spatial and temporal distances. they are thus quite different from the spatial - distances - at - a - time presupposed by classical relationists ; moreover they do not correspond to relative velocities ( - at - a - time ) either. their oddity is forcefully captured by noticing that if we choose appropriate bits of matter at ‘ times ’ eight minutes apart, i - now am at zero distance from the surface of the sun ( of eight minutes ‘ past ’, since it took


either. their oddity is forcefully captured by noticing that if we choose appropriate bits of matter at ‘ times ’ eight minutes apart, i - now am at zero distance from the surface of the sun ( of eight minutes ‘ past ’, since it took 8 minutes for light from the sun to reach me - now ). so we are by no means dealing here with an innocuous, ‘ natural ’ translation of classical relationist quantities into the str setting. on the other hand, in light of the relativity of simultaneity ( see note ), it can be argued that the absolute simultaneity presupposed by classical relationists and absolutists alike was, in fact, something that relationists should always have regarded with misgivings. from this perspective, instantaneous relational configurations — precisely what one starts with in the theories of barbour and bertotti — would be the things that should be treated with suspicion. if we now return to our questions about motions — about the nature of velocities and accelerations — we find, as noted above, that matters in the interval - relational interpretation of str are much the same as in newtonian mechanics in galilean spacetime. there are no well - defined absolute velocities, but there are indeed well - defined absolute accelerations and rotations. in fact, the difference between an accelerating body ( e. g., a rocket ) and an inertially moving body is codified directly in the cross - temporal interval relations of the body with itself. so we are very far from being able to conclude that all motion is relative motion of a body with respect to other bodies. it is true that the absolute motions are in 1 - 1 correlation with patterns of spacetime interval relations, but it is not at all correct to say that they are, for that reason, eliminable in favor of merely relative motions. rather we should simply say that no absolute acceleration can fail to have an effect on the material body or bodies accelerated. but this was already true in classical physics if matter is modeled realistically : the cord connecting the globes does not merely tense, but also stretches ; and so does the bucket, even if imperceptibly, i. e., the spatial relations change. maudlin does not claim this version of relationism to be victorious over an absolutist or substantivalist conception of minkowski spacetime, when it comes time to make judgments about the theory ' s ontology


spatial relations change. maudlin does not claim this version of relationism to be victorious over an absolutist or substantivalist conception of minkowski spacetime, when it comes time to make judgments about the theory ' s ontology. there may be more to vindicating relationism than merely establishing a 1 - 1 correlation between absolute motions and patterns of spatiotemporal relations. the simple comparison made above between str and newtonian physics in galilean spacetime is somewhat deceptive. for one thing, galilean spacetime is a mathematical innovation posterior to einstein ' s 1905 theory ; before then, galilean spacetime had not been conceived, and full acceptance of newtonian mechanics implied accepting absolute velocities and, arguably, absolute positions, just as laid down in the scholium. so einstein ' s elimination of absolute velocity was a genuine conceptual advance. moreover, the scholium was not the only reason for supposing that there existed a privileged reference frame of ‘ rest ’ : the working assumption of almost all physicists in the latter half of the 19th century was that, in order to understand the wave theory of light, one had to postulate an aetherial medium filling all space, wave - like disturbances in which constituted electromagnetic radiation. it was assumed that the aether rest frame would be an inertial reference frame ; and physicists felt some temptation to equate its frame with the absolute rest frame, though this was not necessary. regardless of this equation of the aether with absolute space, it was assumed by all 19th century physicists that the equations of electrodynamic theory would have to look different in a reference frame moving with respect to the aether than they did in the aether ' s rest frame ( where they presumably take their canonical form, i. e., maxwell ' s equations and the lorentz force law. ) so while theoreticians labored to find plausible transformation rules for the electrodynamics of moving bodies, experimentalists tried to detect the earth ' s motion in the aether. experiment and theory played collaborative roles, with experimental results ruling out certain theoretical moves and suggesting new ones, while theoretical advances called for new experimental tests for their confirmation or — as it happened — disconfirmation. as is well known, attempts to detect the earth ' s velocity in the aether were unsuccessful. on the theory side, attempts to formulate the transformation laws for electrodynamics in moving frames — in such


happened — disconfirmation. as is well known, attempts to detect the earth ' s velocity in the aether were unsuccessful. on the theory side, attempts to formulate the transformation laws for electrodynamics in moving frames — in such a way as to be compatible with experimental results — were complicated and inelegant. a simplified way of seeing how einstein swept away a host of problems at a stroke is this : he proposed that the galilean principle of relativity holds for maxwell ' s theory, not just for mechanics. the canonical ( ‘ rest - frame ’ ) form of maxwell ' s equations should be their form in any inertial reference frame. since the maxwell equations dictate the velocity c of electromagnetic radiation ( light ), this entails that any inertial observer, no matter how fast she is moving, will measure the velocity of a light ray as c — no matter what the relative velocity of its emitter. einstein worked out logically the consequences of this application of the special relativity principle, and discovered that space and time must be rather different from how newton described them. str undermined newton ' s absolute time just as decisively as it undermined his absolute space ( see note ). einstein ' s str was the first clear and empirically successful physical theory to overtly eliminate the concepts of absolute rest and absolute velocity while recovering most of the successes of classical mechanics and 19th century electrodynamics. it therefore deserves to be considered the first highly successful theory to explicitly relativize motion, albeit only partially. but str only recovered most of the successes of classical physics : crucially, it left out gravity. and there was certainly reason to be concerned that newtonian gravity and str would prove incompatible : classical gravity acted instantaneously at a distance, while str eliminated the privileged absolute simultaneity that this instantaneous action presupposes. several ways of modifying newtonian gravity to make it compatible with the spacetime structure of str suggested themselves to physicists in the years 1905 - 1912, and a number of interesting lorentz - covariant theories were proposed ( set in the minkowski spacetime of str ). einstein rejected these efforts one and all, for violating either empirical facts or theoretical desiderata. but einstein ' s chief reason for not pursuing the reconciliation of gravitation with str ' s spacetime appears to have been his desire, beginning in 1907, to replace str with a theory in which not only velocity could


desiderata. but einstein ' s chief reason for not pursuing the reconciliation of gravitation with str ' s spacetime appears to have been his desire, beginning in 1907, to replace str with a theory in which not only velocity could be considered merely relative, but also acceleration. that is to say, einstein wanted if possible to completely eliminate all absolute quantities of motion from physics, thus realizing a theory that satisfies at least one kind of ‘ strict ’ relationism. ( regarding einstein ' s rejection of lorentz - covariant gravity theories, see norton 1992 ; regarding einstein ' s quest to fully relativize motion, see hoefer 1994. ) einstein began to see this complete relativization as possible in 1907, thanks to his discovery of the equivalence principle. imagine we are far out in space, in a rocket ship accelerating at a constant rate g = 9. 98 m / s2. things will feel just like they do on the surface of the earth ; we will feel a clear up - down direction, bodies will fall to the floor when released, etc. indeed, due to the well - known empirical fact that gravity affects all bodies by imparting a force proportional to their matter ( and energy ) content, independent of their internal constitution, we know that any experiment performed on this rocket will give the same results that the same experiment would give if performed on the earth. now, newtonian theory teaches us to consider the apparent downward, gravity - like forces in the rocket ship as ‘ pseudo - forces ’ or ‘ inertial forces ’, and insists that they are to be explained by the fact that the ship is accelerating in absolute space. but einstein asked : “ is there any way for the person in the rocket to regard him / herself as being ‘ at rest ’ rather than in absolute ( accelerated ) motion? ” and the answer he gave is : yes. the rocket traveler may regard him / herself as being ‘ at rest ’ in a homogeneous and uniform gravitational field. this will explain all the observational facts just as well as the supposition that he / she is accelerating relative to absolute space ( or, absolutely accelerating in minkowski spacetime ). but is it not clear that the latter is the truth, while the former is a fiction? by no means ; if there were a uniform gravitational field filling all space, then it would affect all the other bodies in the world — the earth, the stars, etc, imparting to them a downward acceleration away from


gtr ). there is one key element left out of this success story, however, and it is crucial to understanding why most physicists reject einstein ' s claim to have eliminated absolute states of motion in gtr. going back to our accelerating rocket, we accepted einstein ' s claim that we could regard the ship as hovering at rest in a universe - filling gravitational field. but a gravitational field, we usually suppose, is generated by matter. how is this universe - filling field linked to generating matter? the answer may be supplied by mach - heavy. regarding the ‘ accelerating ’ rocket which we decide to regard as ‘ at rest ’ in a gravitational field, the machian says : all those stars and galaxies, etc., jointly accelerating downward ( relative to the rocket ), ‘ produce ’ that gravitational field. the mathematical specifics of how this field is generated will have to be different from newton ' s law of gravity, of course ; but it should give essentially the same results when applied to low - mass, slow - moving problems such as the orbits of the planets, so as to capture the empirical successes of newtonian gravity. einstein thought, in 1916 at least, that the field equations of gtr are precisely this mathematical replacement for newton ' s law of gravity, and that they fully satisfied the desiderata of mach - heavy relationism. but it was not so. ( see the entry on early philosophical interpretations of general relativity. ) in gtr, spacetime is locally very much like flat minkowski spacetime. there is no absolute velocity locally, but there are clear local standards of accelerated vs non - accelerated motion, i. e., local inertial frames. in these ‘ freely falling ’ frames bodies obey the usual rules for non - gravitational physics familiar from str, albeit only approximately. but overall spacetime is curved, and local inertial frames may tip, bend and twist as we move from one region to another. the structure of curved spacetime is encoded in the metric field tensor gab, with the curvature encoding gravity at the same time : gravitational forces are so to speak ‘ built into ’ the metric field, geometrized away. since the spacetime structure encodes gravity and inertia, and in a mach - heavy theory these phenomena should be completely determined by the relational distribution of matter ( and relative motions ), einstein wished to see the metric as entirely determined by the distribution of matter and energy. but what the gtr field equations entail is, in general, only a partial -


be completely determined by the relational distribution of matter ( and relative motions ), einstein wished to see the metric as entirely determined by the distribution of matter and energy. but what the gtr field equations entail is, in general, only a partial - determination relation. we cannot go into the mathematical details necessary for a full discussion of the successes and failures of mach - heavy in the gtr context. but one can see why the machian interpretation einstein hoped he could give to the curved spacetimes of his theory fails to be plausible, by considering a few simple ‘ worlds ’ permitted by gtr. in the first place, for our hovering rocket ship, if we are to attribute the gravity field it feels to matter, there has got to be all this other matter in the universe. but if we regard the rocket as a mere ‘ test body ’ ( not itself substantially affecting the gravity present or absent in the universe ), then we can note that according to gtr, if we remove all the stars, galaxies, planets etc. from the world, the gravitational field does not disappear. on the contrary, it stays basically the same locally, and globally it takes the form of empty minkowski spacetime, precisely the quasi - absolute structure einstein was hoping to eliminate. solutions of the gtr field equations for arbitrary realistic configurations of matter ( e. g., a rocket ship ejecting a stream of particles to push itself forward ) are hard to come by, and in fact a realistic two - body exact solution has yet to be discovered. but numerical methods can be applied for many purposes, and physicists do not doubt that something like our accelerating rocket — in otherwise empty space — is possible according to the theory. we see clearly, then, that gtr fails to satisfy einstein ' s own understanding of mach ' s principle, according to which, in the absence of matter, space itself should not be able to exist. a second example : gtr allows us to model a single rotating object in an otherwise empty universe ( e. g., a neutron star ). relationism of the machian variety says that such rotation is impossible, since it can only be understood as rotation relative to some sort of absolute space. in the case of gtr, this is basically right : the rotation is best understood as rotation relative to a ‘ background ’ spacetime that is identical to the minkowski spacetime of str, only ‘ curved ’ by the presence of matter in the region of the star. on the other hand, there is one


is best understood as rotation relative to a ‘ background ’ spacetime that is identical to the minkowski spacetime of str, only ‘ curved ’ by the presence of matter in the region of the star. on the other hand, there is one charge of failure - to - relativize - motion sometimes leveled at gtr that is unfair. it is sometimes asserted that the simple fact that the metric field ( or the connection it determines ) distinguishes, at every location, motions that are ‘ absolutely ’ accelerated and / or ‘ absolutely rotating ’ from those that are not, by itself entails that gtr fails to embody a folk - leibniz style general relativity of motion ( e. g. earman ( 1989 ), ch. 5 ). we think this is incorrect, and leads to unfairly harsh judgments about confusion on einstein ' s part. the local inertial structure encoded in the metric would not be ‘ absolute ’ in any meaningful sense, if that structure were in some clear sense fully determined by the relationally specified matter - energy distribution. einstein was not simply confused when he named his gravity theory. ( just what is to be understood by “ the relationally specified matter - energy distribution ” is a further, thorny issue, which we cannot enter into here. ) gtr does not fulfill all the goals of mach - heavy, at least as understood by einstein, and he recognized this fact by 1918 ( einstein 1918 ). and yet … gtr comes tantalizingly close to achieving those goals, in certain striking ways. for one thing, gtr does predict mach - heavy effects, known as ‘ frame - dragging ’ : if we could model mach ' s thick - walled bucket in gtr, it seems clear that it would pull the water slightly outward, and give it a slight tendency to begin rotating in the same sense as the bucket ( even if the big bucket ' s walls were not actually touching the water. while gtr does permit us to model a lone rotating object, if we model the object as a shell of mass ( instead of a solid sphere ) and let the size of the shell increase ( to model the ‘ sphere of the fixed stars ’ we see around us ), then as brill & cohen ( 1966 ) showed, the frame - dragging becomes complete inside the shell. in other words : our original minkowski background structure effectively disappears, and inertia becomes wholly determined by the shell of matter, just as mach posited was the case. this complete determination of in


frame - dragging becomes complete inside the shell. in other words : our original minkowski background structure effectively disappears, and inertia becomes wholly determined by the shell of matter, just as mach posited was the case. this complete determination of inertia by the global matter distribution appears to be a feature of other models, including the friedman - robertson - walker - lemaitre big bang models that best match observations of our universe. finally, it is important to recognize that gtr is generally covariant in a very special sense : unlike all other prior theories ( and unlike many subsequent quantum theories ), it postulates no fixed ‘ prior ’ or ‘ background ’ spacetime structure. as mathematicians and physicists realized early on, other theories, e. g., newtonian mechanics and str, can be put into a generally covariant form. but when this is done, there are inevitably mathematical objects postulated as part of the formalism, whose role is to represent absolute elements of spacetime structure. what is unique about gtr is that it was the first, and is still the only ‘ core ’ physical theory, to have no such absolute elements in its covariant equations. the spacetime structure in gtr, represented by the metric field ( which determines the connection ), is at least partly ‘ shaped ’ by the distribution of matter and energy. and in certain models of the theory, such as the big bang cosmological models, some authors have claimed that the local standards of inertial motion — the local ‘ gravitational field ’ of einstein ' s equivalence principle — are entirely fixed by the matter distribution throughout space and time, just as mach - heavy requires ( see, for example, wheeler and cuifollini 1995 ). absolutists and relationists are thus left in a frustrating and perplexing quandary by gtr. considering its anti - machian models, we are inclined to say that motions such as rotation and acceleration remain absolute, or nearly - totally - absolute, according to the theory. on the other hand, considering its most mach - friendly models, which include all the models taken to be good candidates for representing the actual universe, we may be inclined to say : motion in our world is entirely relative ; the inertial effects normally used to argue for absolute motion are all understandable as effects of rotations and accelerations relative to the cosmic matter, just as mach hoped. but even if we agree that motions in our world are in fact all


; the inertial effects normally used to argue for absolute motion are all understandable as effects of rotations and accelerations relative to the cosmic matter, just as mach hoped. but even if we agree that motions in our world are in fact all relative in this sense, this does not automatically settle the traditional relationist / absolutist debate, much less the relationist / substantivalist debate. many philosophers ( including, we suspect, nerlich 1994 and earman 1989 ) would be happy to acknowledge the mach - friendly status of our spacetime, and argue nevertheless that we should understand that spacetime as a real thing, more like a substance than a mere ideal construct of the mind as leibniz insisted. ( nerlich ( 1994 ) and earman ( 1989 ), we suspect, would take this stance. ) some, though not all, attempts to convert gtr into a quantum theory would accord spacetime this same sort of substantiality that other quantum fields possess. this article has been concerned with tracing the history and philosophy of ‘ absolute ’ and ‘ relative ’ theories of space and motion. along the way we have been at pains to introduce some clear terminology for various different concepts ( e. g., ‘ true ’ motion, ‘ substantivalism ’, ‘ absolute space ’ ), but what we have not really done is say what the difference between absolute and relative space and motion is : just what is at stake? recently rynasiewicz ( 2000 ) has argued that there simply are no constant issues running through the history that we have discussed here ; that there is no stable meaning for either ‘ absolute motion ’ or ‘ relative motion ’ ( or ‘ substantival space ’ vs ‘ relational space ’ ). while we agree to a certain extent, we think that nevertheless there are a series of issues that have motivated thinkers again and again ; indeed, those that we identified in the introduction. ( one quick remark : rynasiewicz is probably right that the issues cannot be expressed in formally precise terms, but that does not mean that there are no looser philosophical affinities that shed useful light on the history. ) our discussion has revealed several different issues, of which we will highlight three as components of the ‘ absolute - relative debate ’. ( i ) there is the question of whether all motions and all possible descriptions of motions are equal, or whether some are ‘ real ’ — what we have called, in seventeenth century parlance, ‘ true ’. there is a natural


debate ’. ( i ) there is the question of whether all motions and all possible descriptions of motions are equal, or whether some are ‘ real ’ — what we have called, in seventeenth century parlance, ‘ true ’. there is a natural temptation for those who hold that there is ‘ nothing but the relative positions and motions between bodies ' ( and more so for their readers ) to add ‘ and all such motions are equal ’, thus denying the existence of true motion. however, arguably — perhaps surprisingly — no one we have discussed has unreservedly held this view ( at least not consistently ) : descartes considered motion ‘ properly speaking ’ to be privileged, leibniz introduced ‘ active force ’ to ground motion ( arguably in his mechanics as well as metaphysically ), and mach ' s view seems to be that the distribution of matter in the universe determines a preferred standard of inertial motion. ( again, in general relativity, there is a distinction between inertial and accelerated motion. ) that is, relationists can allow true motions if they offer an analysis of them in terms of the relations between bodies. given this logical point, and given the historical ways thinkers have understood themselves, it seems unhelpful to characterize the issues in ( i ) as constituting an absolute - relative debate, hence our use of the term ‘ true ’ instead of ‘ absolute ’. so we are led to the second question : ( ii ) is true motion definable in terms of relations or not? ( of course the answer depends on what kind of definitions will count, and absent an explicit definition — descartes ' proper motion for example — the issue is often taken to be that of whether true motions supervene on relations, as newton ' s globes are often supposed to refute. ) it seems reasonable to call this issue that of whether motion is absolute or relative. descartes and mach are relationists about motion in this sense, while newton is an absolutist. leibniz is also an absolutist about motion in his metaphysics, and if our reading is correct, also about the interpretation of motion in the laws of collision. this classification of leibniz ' s views runs contrary to his customary identification as relationist - in - chief, but we will clarify his relationist credentials below. finally, we have discussed ( ii ) in the context of relativity, first examining maudlin ' s proposal that the embedding of a relationally - specified system in min


- in - chief, but we will clarify his relationist credentials below. finally, we have discussed ( ii ) in the context of relativity, first examining maudlin ' s proposal that the embedding of a relationally - specified system in minkowski spacetime is in general unique once all the spacetime interval - distance relations are given. this proposal may or may not be held to satisfy the relational - definability question of ( ii ), but in any case it cannot be carried over to the context of general relativity theory. in the case of gtr we linked relational motion to the satisfaction of mach ' s principle, just as einstein did in the early years of the theory. despite some promising features displayed by gtr, and certain of its models, we saw that mach ' s principle is not fully satisfied in gtr as a whole. we also noted that in the absence of absolute simultaneity, it becomes an open question what relations are to be permitted in the definition ( or supervience base ) — spacetime interval relations? instantaneous spatial distances and velocities on a 3 - d hypersurface? ( in recent works, barbour has argued that gtr is fully machian, using a 3 - d relational - configuration approach. see barbour, foster and murchadha 2002. ) the final issue is that of ( iii ) whether absolute motion is motion with respect to substantival space or not. of course this is how newton understood acceleration — as acceleration relative to absolute space. more recent newtonians share this view, although motion for them is with respect to substantival galilean spacetime ( or rather, since they know newtonian mechanics is false, they hold that this is the best interpretation of that theory ). leibniz denied that motion was relative to space itself, since he denied the reality of space ; for him true motion was the possession of active force. so despite his ‘ absolutism ’ ( our adjective not his ) about motion he was simultaneously a relationist about space : ‘ space is merely relative ’. following leibniz ' s lead we can call this debate the question of whether space is absolute or relative. the drawback of this name is that it suggests a separation between motion and space, which exists in leibniz ' s views, but which is otherwise problematic ; still, no better description presents itself. others who are absolutists about motion but relationists about space include sklar ( 1974 ) and van


motion and space, which exists in leibniz ' s views, but which is otherwise problematic ; still, no better description presents itself. others who are absolutists about motion but relationists about space include sklar ( 1974 ) and van fraassen ( 1985 ) ; sklar introduced a primitive quantity of acceleration, not supervenient on motions relative to anything at all, while van fraassen let the laws themselves pick out the inertial frames. it is of course arguable whether any of these three proposals are successful ; ( even ) stripped of leibniz ' s aristotelian packaging, can absolute quantities of motion ‘ stand on their own feet ’? and under what understanding of laws can they ground a standard of inertial motion? huggett ( 2006 ) defends a similar position of absolutism about motion, but relationism about space ; he argues — in the case of newtonian physics — that fundamentally there is nothing to space but relations between bodies, but that absolute motions supervene — not on the relations at any one time — but on the entire history of relations. works cited in text - aristotle, 1984, the complete works of aristotle : the revised oxford translation, j. barnes ( ed. ), princeton : princeton university press. - barbour, j. and bertotti, b., 1982, “ mach ' s principle and the structure of dynamical theories, ” proceedings of the royal society ( london ), 382 : 295 - 306. - – – –, 1977, “ gravity and inertia in a machian framework, ” nuovo cimento, 38b : 1 - 27. - brill, d. r. and cohen, j., 1966, “ rotating masses and their effects on inertial frames, ” physical review 143 : 1011 - 1015. - brown, h. r., 2005, physical relativity : space - time structure from a dynamical perspective, oxford : oxford university press. - descartes, r., 1983, principles of philosophy, r. p. miller and v. r. miller ( trans. ), dordrecht, london : reidel. - dorling, j., 1978, “ did einstein need general relativity to solve the problem of space? or had the problem already been solved by special relativity?, ” british journal for the philosophy of science, 29 : 311 - 323. - earman, j., 1989, world enough and spacetime :


c., 1994, “ einstein ' s struggle for a machian gravitation theory, ” studies in history and philosophy of science, 25 : 287 - 336. - huggett, n., 2006, “ the regularity account of relational spacetime, ” mind, 115 : 41 - 74. - – – –, 2000, “ space from zeno to einstein : classic readings with a contemporary commentary, ” international studies in the philosophy of science, 14 : 327 - 329. - lange, l., 1885, “ ueber das beharrungsgesetz, ” berichte der koniglichen sachsischen gesellschaft der wissenschaften zu leipzig, mathematisch - physische classe 37 ( 1885 ) : 333 - 51. - leibniz, g. w., 1989, philosophical essays, r. ariew and d. garber ( trans. ), indianapolis : hackett pub. co. - leibniz, g. w., and samuel clarke, 1715 – 1716, “ correspondence ”, in the leibniz - clarke correspondence, together with extracts from newton ' s “ principia ” and “ opticks ”, h. g. alexander ( ed. ), manchester : manchester university press, 1956. - lodge, p., 2003, “ leibniz on relativity and the motion of bodies, ” philosophical topics, 31 : 277 - 308. - mach, e., 1883, die mechanik in ihrer entwickelung, historisch - kritisch dargestellt. 2nd edition. leipzig : brockhaus. english translation ( 6th edition, 1960 ) : the science of mechanics, la salle, illinois : open court press. - malament, d., 1995, “ is newtonian cosmology really inconsistent?, ” philosophy of science 62, no. 4. - maudlin, t., 1993, “ buckets of water and waves of space : why space - time is probably a substance, ” philosophy of science, 60 : 183 - 203. - minkowski, h. ( 1908 ). “ space and time, ” in einstein, et al. ( 1952 ), pp. 75 - 91. - nerlich, graham, 1994, the shape of space ( 2nd edition ), cambridge : cambridge university press. - neumann, c., 1870, ueber die principien der galilei - newton ' schen theorie. leipzig : b.


graham, 1994, the shape of space ( 2nd edition ), cambridge : cambridge university press. - neumann, c., 1870, ueber die principien der galilei - newton ' schen theorie. leipzig : b. g. teubner, 1870. - newton, i., 2004, newton : philosophical writings, a. janiak ( ed. ), cambridge : cambridge university press. - newton, i. and i. b. cohen, 1999, the principia : mathematical principles of natural philosophy, i. b. cohen and a. m. whitman ( trans. ), berkeley ; london : university of california press. - norton, j., 1995, “ mach ' s principle before einstein, ” in j. barbour and h. pfister ( eds. ) mach ' s principle : from newton ' s bucket to quantum gravity : einstein studies, vol. 6. boston : birkhauser, pp. 9 - 57. - norton, j., 1993, “ a paradox in newtonian cosmology, ” in m. forbes, d. hull and k. okruhlik ( eds. ) psa 1992 : proceedings of the 1992 biennial meeting of the philosophy of science association. vol. 2. east lansing, mi : philosophy of science association, pp. 412 - 20. - – – –, 1992, “ einstein, nordstrom and the early demise of scalar, lorentz - covariant theories of gravitation, ” archive for history of exact sciences, 45 : 17 - 94. - pooley, o., 2002, the reality of spacetime, d. phil thesis, oxford university. - ray, c., 1991, time, space and philosophy, new york : routledge. - roberts, j. t., 2003, “ leibniz on force and absolute motion, ” philosophy of science, 70 : 553 - 573. - rynasiewicz, r., 1995, “ by their properties, causes, and effects : newton ' s scholium on time, space, place, and motion — i. the text, ” studies in history and philosophy of science, 26 : 133 - 153. - sklar, l., 1974, space, time and spacetime, berkeley : university of california press. - stein, h., 1977, “ some philosophical prehistory of general relativity, ” in minnesota studies in the philosophy of science 8 : foundations of


l., 1974, space, time and spacetime, berkeley : university of california press. - stein, h., 1977, “ some philosophical prehistory of general relativity, ” in minnesota studies in the philosophy of science 8 : foundations of space - time theories :, j. earman, c. glymour and j. stachel ( eds. ), minneapolis : university of minnesota press. - – – –, 1967, “ newtonian space - time, ” texas quarterly, 10 : 174 - 200. - wheeler, j. a. and ciufolini, i., 1995, gravitation and inertia, princeton, n. j. : princeton u. press. notable philosophical discussions of the absolute - relative debates - barbour, j. b., 1982, “ relational concepts of space and time, ” british journal for the philosophy of science, 33 : 251 - 274. - belot, g., 2000, “ geometry and motion, ” british journal for the philosophy of science, 51 : 561 - 595. - butterfield, j., 1984, “ relationism and possible worlds, ” british journal for the philosophy of science, 35 : 101 - 112. - callender, c., 2002, “ philosophy of space - time physics, ” in the blackwell guide to the philosophy of science, p. machamer ( ed. ), cambridge : blackwell. 173 - 198. - carrier, m., 1992, “ kant ' s relational theory of absolute space, ” kant studien, 83 : 399 - 416. - dieks, d., 2001, “ space - time relationism in newtonian and relativistic physics, ” international studies in the philosophy of science, 15 : 5 - 17. - disalle, r., 1995, “ spacetime theory as physical geometry, ” erkenntnis, 42 : 317 - 337. - earman, j., 1986, “ why space is not a substance ( at least not to first degree ), ” pacific philosophical quarterly, 67 : 225 - 244. - – – –, 1970, “ who ' s afraid of absolute space?, ” australasian journal of philosophy, 48 : 287 - 319. - earman, j. and j. norton, 1987, “ what price spacetime substantivalism : the hole story, ” british journal for the philosophy of science, 38 : 515 - 525. - ho


287 - 319. - earman, j. and j. norton, 1987, “ what price spacetime substantivalism : the hole story, ” british journal for the philosophy of science, 38 : 515 - 525. - hoefer, c., 2000, “ kant ' s hands and earman ' s pions : chirality arguments for substantival space, ” international studies in the philosophy of science, 14 : 237 - 256. - – – –, 1998, “ absolute versus relational spacetime : for better or worse, the debate goes on, ” british journal for the philosophy of science, 49 : 451 - 467. - – – –, 1996, “ the metaphysics of space - time substantialism, ” journal of philosophy, 93 : 5 - 27. - huggett, n., 2000, “ reflections on parity nonconservation, ” philosophy of science, 67 : 219 - 241. - le poidevin, r., 2004, “ space, supervenience and substantivalism, ” analysis, 64 : 191 - 198. - malament, d., 1985, “ discussion : a modest remark about reichenbach, rotation, and general relativity, ” philosophy of science, 52 : 615 - 620. - maudlin, t., 1993, “ buckets of water and waves of space : why space - time is probably a substance, ” philosophy of science, 60 : 183 - 203. - – – –, 1990, “ substances and space - time : what aristotle would have said to einstein, ” studies in history and philosophy of science, 531 - 561. - mundy, b., 1992, “ space - time and isomorphism, ” proceedings of the biennial meetings of the philosophy of science association, 1 : 515 - 527. - – – –, 1983, “ relational theories of euclidean space and minkowski space - time, ” philosophy of science, 50 : 205 - 226. - nerlich, g., 2003, “ space - time substantivalism, ” in the oxford handbook of metaphysics, m. j. loux ( ed. ), oxford : oxford univ pr. 281 - 314. - – – –, 1996, “ what spacetime explains, ” philosophical quarterly, 46 : 127 - 131. - – – –, 1994, what spacetime explains : metaphysical essays on space and time, new york : cambridge univ


314. - – – –, 1996, “ what spacetime explains, ” philosophical quarterly, 46 : 127 - 131. - – – –, 1994, what spacetime explains : metaphysical essays on space and time, new york : cambridge univ pr. - – – –, 1973, “ hands, knees, and absolute space, ” journal of philosophy, 70 : 337 - 351. - rynasiewicz, r., 2000, “ on the distinction between absolute and relative motion, ” philosophy of science, 67 : 70 - 93. - – – –, 1996, “ absolute versus relational space - time : an outmoded debate?, ” journal of philosophy, 93 : 279 - 306. - teller, p., 1991, “ substance, relations, and arguments about the nature of space - time, ” philosophical review, 363 - 397. - torretti, r., 2000, “ spacetime models for the world, ” studies in history and philosophy of modern physics, 31b : 171 - 186. - st. andrews school of mathematics and statistics index of biographies - the pittsburgh phil - sci archive of pre - publication articles in philosophy of science - ned wright ' s special relativity tutorial - andrew hamilton ' s special relativity pages descartes, rene : physics | general relativity : early philosophical interpretations of | newton, isaac : views on space, time, and motion | space and time : inertial frames | space and time : the hole argument | zeno of elea : zeno ' s paradoxes


by staff writers chicago il ( spx ) jan 11, 2013 technologically valuable ultrastable glasses can be produced in days or hours with properties corresponding to those that have been aged for thousands of years, computational and laboratory studies have confirmed. aging makes for higher quality glassy materials because they have slowly evolved toward a more stable molecular condition. this evolution can take thousands or millions of years, but manufacturers must work faster. armed with a better understanding of how glasses age and evolve, researchers at the universities of chicago and wisconsin - madison raise the possibility of designing a new class of materials at the molecular level via a vapor - deposition process. " in attempts to work with aged glasses, for example, people have examined amber, " said juan de pablo, uchicago ' s liew family professor in molecular theory and simulations. " amber is a glass that has been aged millions of years, but you cannot engineer that material. you get what you get. " de pablo and wisconsin co - authors sadanand singh and mark ediger report their findings in the latest issue of nature materials. ultrastable glasses could find potential applications in the production of stronger metals and in faster - acting pharmaceuticals. the latter may sound surprising, but drugs with the amorphous molecular structure of ultrastable glass could avoid crystallization during storage and be delivered more rapidly in the bloodstream than pharmaceuticals with a semi - crystalline structure. amorphous metals, likewise, are better for high - impact applications than crystalline metals because of their greater strength. the nature materials paper describes computer simulations that singh, a doctoral student in chemical engineering at uw - madison, carried out with de pablo to follow - up some intriguing results from ediger ' s laboratory. growing stable glasses several years ago, he discovered that glasses grown this way on a specially prepared surface that is kept within a certain temperature range exhibit far more stability than ordinary glasses. previous researchers must have grown this material under the same temperature conditions, but failed to recognize the significance of what they had done, ediger said. ediger speculated that growing glasses under these conditions, which he compares to the tetris video game, gives molecules extra room to arrange themselves into a more stable configuration. but he needed singh and de pablo ' s computer simulations to confirm his suspicions that he had actually produced a highly evolved, ordinary glass rather than an entirely new material. " there ' s interest in making these materials on the computer because you have direct access to the structure, and you can therefore determine the relationship between the arrangement of the molecules


had actually produced a highly evolved, ordinary glass rather than an entirely new material. " there ' s interest in making these materials on the computer because you have direct access to the structure, and you can therefore determine the relationship between the arrangement of the molecules and the physical properties that you measure, " said de pablo, a former uw - madison faculty member who joined uchicago ' s new institute for molecular engineering earlier this year. there are challenges, though, to simulating the evolution of glasses on a computer. scientists can cool a glassy material at the rate of one degree per second in the laboratory, but the slowest computational studies can only simulate cooling at a rate of 100 million degrees per second. " we cannot cool it any slower because the calculations would take forever, " de pablo said. " it had been believed until now that there is no correlation between the mechanical properties of a glass and the molecular structure ; that somehow the properties of a glass are " hidden " somewhere and that there are no obvious structural signatures, " de pablo said. creating better materials ultrastable glasses achieve their stability in a manner analogous to the most efficiently packed, multishaped objects in tetris, each consisting of four squares in various configurations that rain from the top of the screen. " this is a little bit like the molecules in my deposition apparatus raining down onto this surface, and the goal is to perfectly pack a film, not to have any voids left, " ediger said. the object of tetris is to manipulate the objects so that they pack into a perfectly tight pattern at the bottom of the screen. " the difference is, when you play the game, you have to actively manipulate the pieces in order to build a well - packed solid, " ediger said. " in the vapor deposition, nature does it for us. " but in tetris and experiments alike, when the objects or molecules descend too quickly, the result is a poorly packed, void - riddled pattern. " in the experiment, if you either rain the molecules too fast or choose a low temperature at which there ' s no mobility at the surface, then this trick doesn ' t work, " ediger said. then it would be like taking a bucket of odd - shaped pieces and just dumping them on the floor. there are all sorts of voids and gaps because the molecules didn ' t have any opportunity to find a good way of packing. " " ultrastable glasses from in silico vapor deposition, " by sadamand


in the star wars : where science meets imagination exhibit, luke skywalker ' s landspeeder is on display for the first time. click on image for full size courtesy of landspeeder image © 2006 lucasfilm ltd. & tm photo : dom miguel photography star wars exhibition brings reality to fantasy news story originally written on april 16, 2008 a new museum exhibit shows that some of the robots, vehicles and devices from the star wars films are close to the types of things scientists have developed to use in space. the exhibition - - at the science museum of minnesota in st. paul, minn., from june 13 until august 24 - - showcases landspeeders, r2d2 and other items from the star wars films. visitors will learn how researchers today are pursuing similar technologies. the exhibit developers were surprised and excited to learn that many of today ' s scientists were inspired by the fantasy technologies they saw in the star wars movies. one of the goals of the exhibit is to be an inspiration for the kids will be the next set of future scientists. the exhibit contains film clips, props, models and costumes. visitors are encouraged to participate in hands - on exhibits and activities. shop windows to the universe science store! our online store includes fun classroom activities for you and your students. issues of nesta ' s quarterly journal, the earth scientist are also full of classroom activities on different topics in earth and space science! you might also be interested in : scientists have learned that mount hood, oregon ' s tallest mountain, has erupted in the past due to the mixing of two different types of magma. " the data will help give us a better road map to what a future... more the earth ' s mantle is a rocky, solid shell that is between the earth ' s crust and the outer core, and makes up about 84 percent of the earth ' s volume. the mantle is made up of many distinct portions or... more some geologic faults that appear strong and stable, slip and slide like weak faults, causing earthquakes. scientists have been looking at one of these faults in a new way to figure out why. in theory,... more the sun goes through cycles that last approximately 11 years. these solar cycle include phases with more magnetic activity, sunspots, and solar flares. they also include phases with less activity. the... more studying tree rings doesn ' t only tell us the age of that tree. tree rings also show what climate was like for each year of a tree ' s life,


learn something new every day more info... by email a block diagram is a graphical method used to explain the concept of a system without the need to understand the individual components within that system. this type of diagram might be used in a variety of industries to illustrate and educate individuals about how a system operates, either in part or in its entirety. block diagrams usually will have a logical, methodical flow from beginning to end. engineers and software programmers are examples of individuals who might be familiar with block diagrams. block diagrams essentially are synonymous with flow charts, but a block diagram is generalized in nature. sometimes block diagrams are used to conceal specific information or processes that might prove to be advantageous or detrimental, whichever the case might be. people who are being presented with a block diagram should be able to develop an understanding of what that block represents. to assist in understanding the block itself, lines should be drawn to the block representing various inputs, outputs or alternative choices. depending on the type of process being illustrated, blocks might serve in any capacity that is needed to adequately describe the process or parts of the process. for instance, a manufacturing cell of machine tools might include a drill press, a milling machine and a sanding machine. to illustrate a process within that cell, each machine tool might be represented by its own block. when the manufacturing process is illustrated in its entirety, a single block might be used to represent all of the components within that cell. a block diagram also can be used to illustrate how a computer program works or how parts of a program work. if, for instance, a program is needed to calculate four different methods of interest rates, a block might represent each of these lines of code for one of these methods. in this way, a supervisor does not need to understand the code itself, as it is written, as long as the purpose of that block is communicated effectively. some block diagrams can be used as a way to map out a process as a top - down diagram. for instance, a person who has an inspired project might use a block diagram as a way to convey the idea as a series of individual blocks, each of which helps support the main topic. later, these individual blocks might then be analyzed and further developed into additional block diagrams as needed. this method can be repeated until the process is mapped out to the satisfaction of all those involved with the project. if compiled and mapped out completely, the block diagram might resemble a pine tree type of structure of the entire project, which is typical for a top - down diagram.


p ] ( x ) to be the result of running p with input x. now we define p ( x ) to be the program p modified so that it no longer takes an argument or input but instead substitutes the ' hard - coded ' value of x instead. in other words [ p ( x ) ] = [ p ] ( x ). p ( x ) is, of course, another program. there are also many ways of implementing p ( x ). we could simply evaluate [ p ] ( x ) and write a program to simply print this out or return it. on the other hand, we could do the absolute minimum and write a new piece of code that simply calls p and supplies it with a hard - coded argument. whatever we choose is irrelevant to the following discussion. so here ' s the demand that we make of our programming language : that it ' s powerful enough for us to write a program that can compute p ( x ) from inputs p and x. this might not be a trivial program to write, but it ' s not conceptually hard either. it doesn ' t have gotchas like the quotation mark issue above. typically we can compute p ( x ) by some kind of textual substitution on p. with that assumption in mind, here ' s a theorem : any program p that takes one argument or input has a fixed point, x, in the sense that running p with input x gives the same result as just running x. given an input x, p acts just like an interpreter for the programming language as it outputs the same thing as an interpreter would given input x. so here ' s a proof : define the function f ( q ) = [ p ] ( q ( q ) ). we ' ve assumed that we can write a program that computes p ( x ) from p and x so we know we can write a program to compute q ( q ) for any q. we can then feed this as an input to [ p ]. so f is obviously computable by some program which we call q0. so [ q0 ] ( q ) = [ p ] ( q ( q ) ). now the fun starts : [ p ] ( q0 ( q0 ) ) = [ q0 ] ( q0 ) ( by definition of q0 ) = [ q0 ( q0 ) ] ( by definition of p ( x ) ) in other words q0 ( q0 ) is our fixed point. so now take p to compute the identity function.


) ( by definition of q0 ) = [ q0 ( q0 ) ] ( by definition of p ( x ) ) in other words q0 ( q0 ) is our fixed point. so now take p to compute the identity function. then [ q0 ( q0 ) ] = [ p ] ( q0 ( q0 ) ) = q0 ( q0 ). so q0 ( q0 ) outputs itself when run! what ' s more, this also tells us how to do other fun stuff like write a program to print itself out backwards. and it tells us how to do this in any reasonably powerful programming language. we don ' t need to worry about having to work around problems like ' escaping ' quotation marks - we can always find a way to replicate the escape mechanism too. so does it work in practice? well it does for haskell - i derived the haskell fragment above by applying this theorem directly, and then simplifying a bit. for c + +, however, it might give you a piece of code that is longer than you want. in fact, you can go one step further and write a program that automatically generates a self - replicator. check out samuel moelius ' s kpp. it is a preprocessor that converts an ordinary c + + program into one that can access its own source code by including the code to generate its own source within it. another example of an application of these methods is futamura ' s theorem which states that there exists a program that can take as input an interpreter for a language and output a compiler. i personally think this is a little bogus.


principles of design balance in design is similar to balance in physics. a large shape close to the center can be balanced by a small shape close to the edge. a large light toned shape will be balanced by a small dark toned shape ( the darker the shape the heavier it appears to be ) graduation of size and direction produce linear perspective. graduation of colour from warm to cool and tone from dark to light produce aerial perspective. graduation can add interest and movement to a shape. a graduation from dark to light will cause the eye to move along a shape. repetition with variation is interesting, without variation repetition can become monotonous. if you wish to create interest, any repeating element should include a degree of variation. contrast is the juxtaposition of opposing elements eg. opposite colours on the colour wheel – red / green, blue / orange etc. contrast in tone or value – light / dark. contrast in direction – horizontal / vertical. the major contrast in a painting should be located at the center of interest. too much contrast scattered throughout a painting can destroy unity and make a work difficult to look at. unless a feeling of chaos and confusion are what you are seeking, it is a good idea to carefully consider where to place your areas of maximum contrast. harmony in painting is the visually satisfying effect of combining similar, related elements. eg. adjacent colurs on the colour wheel, similar shapes etc. dominance gives a painting interest, counteracting confusion and monotony. dominance can be applied to one or more of the elements to give emphasis. relating the design elements to the idea being expressed in a painting reinforces the principal of unity. eg. a painting with an active aggressive subject would work better with a dominant oblique direction, course, rough texture, angular lines etc. whereas a quiet passive subject would benefit from horizontal lines, soft texture and less tonal contrast. unity in a painting also refers to the visual linking of various elements of the work.


what is fluorescence? fluorescence is the ability of certain chemicals to give off visible light after absorbing radiation which is not normally visible, such as ultraviolet light. this property has led to a variety of uses. let ’ s shed some further light on this topic ; consider the omnipresent “ fluorescent ” lights. just how do they work? fluorescent tubes contain a small amount of mercury vapor. the application of an electric current causes a stream of electrons to traverse the tube. these collide with the mercury atoms which become energized and consequently emit ultraviolet light. the inside of the tube is coated with a fluorescent material, such as calcium chlorophosphate, which converts the invisible ultraviolet light into visible light. the same idea is used to produce color television pictures. the screen is coated with tiny dots of substances which fluoresce in different colours when they are excited by a beam of electrons which is used to scan the picture. but fluorescent materials had practical uses even before we dreamed of color television. one of the most amazing of all fluorescent materials is a synthetic compound, appropriately called fluorescein. under ultraviolet light it produces an intense yellow - green fluorescence which during world war ii was responsible for saving the lives of many downed flyers. over a million pounds of the stuff were manufactured in 1943 and distributed to airmen in little packets to use as a sea marker. since the fluorescence is so potent that it can be seen when the concentration of fluorescein is as little as 25 parts per billion, rescue planes easily spotted the men in the ocean. aircraft carriers also made extensive use of fluorescein. the signal men on deck wore clothes and waved flags treated with the compound which was then made to glow by illumination with ultraviolet light. the incoming pilots could clearly see the deck and the need to use runway lights which would have drawn the attention of enemy aircraft was eliminated. certain natural substances also fluoresce under ultraviolet light. urine and moose fur are interesting examples. prisoners have actually made use of this property of urine and have used it as a secret ink. what about the moose fur? well, in canada and sweden there are hundreds of accidents each year involving the collision of automobiles with moose. some of these result in fatalities. some car manufacturers are now considering fitting their vehicles with uv emitting headlights to reduce moose collisions! how ’ s that for putting the right chemistry to work.


one of my favorite science fiction ideas is in a short story called light of other days about something called “ slow glass. ” light took decades to pass through. in this story, the idea was that people could buy glass windows that took so long for the light to pass through, that they could nostalgically watch long gone scenes, such as their children playing outside as toddlers long after they had gone off to college, or green fields with horses where now ugly cities grew. it was about the speed of light. here is a concept that is similar. instead of slow windows, it is slow walls. ravenbrick has made a nanotech wall that can slow down the day ’ s heat coming into a building. using phase - changing material at the molecular level, you get to transfer the warmth of the sun ’ s heat from the afternoon well into the night. ravenbrick makes several clean tech materials for building that greatly reduce energy needs, most notably windows that turn off the sun, like sage electrochromics windows do. the one that is new to me is this “ slow wall ”. they claim that their glass - clad smart wall : ravenskin could literally reduce your heating bill to zero! ( coupled with good building design, of course, you can ’ t expect a zero bill if you put leaky windows in their wall! ) their wall can delay solar heat gain from hot afternoons, to later that night, when you need it more. this helps regulate the internal temperatures of buildings. it has excellent r - values to begin with ( r - 11 or more ) so it insulates like a normal wall limiting the conduction and convection of heat. the magic – or science fiction – part is achieved by converting incoming sunlight to infrared, and then directing the flow of energy inward only when you want it to come through the walls. the problem with super well - insulated buildings is that sometimes you do want the suns heat getting in, and regular insulated walls are dumb walls that don ’ t know when to send the heat in and when to shut it out. the smart wall knows because you can tell it. susan kraemer writes at cleantechnica, csp - today, pv - insider, smartgridupdate and greenprophet and has been published at ecoseed, nrdc onearth, matternetwork, celsius, energynow and scientific american. as a former serial entrepreneur in product design she brings an innovator ' s perspective on inventing a carbon - constrained civilization :


textbooks. discussions of textbooks have been included on creation and evolution in the public education debate. the smith v. board of school commissioners of mobile county case brought forward a debate about scientific fact being presented in textbooks. in his book, surely you ' re joking, mr. feynman!, the late physics nobel prize laureate richard p. feynman described his experiences as a member of a committee that evaluated science textbooks. at some instances, there were nonsensical examples to illustrate physical phenomena ; then a company sent — for reasons of timing — a textbook that contained blank pages, which even got good critiques. feynman himself experienced attempts at bribery. largely in the us, but increasingly in other nations, k - 12 mathematics textbooks have reflected the controversies of new math and reform mathematics which have sought to replace traditional mathematics in what have been called the math wars. traditional texts, still favored in asia and other areas, merely taught the same time - tested mathematics that most adults have learned. by contrast " progressive " approaches seek to address problems in social inequity with approaches that often incorporate principles of constructivism and discovery. texts such as terc and cmp discourage or omit standard mathematics methods and concepts such as long division and lowest common denominators. for example an index entry to multiply fractions would lead to " devise your own method to multiply fractions which work on these examples ", and the formula for the area of a circle would be an exercise for a student to derive rather than including it in the student text. by the 2000s, while some districts were still adopting the more novel methods, others had abandoned them as unworkable. higher education in the u. s., college and university textbooks are chosen by the professor teaching the course, or by the department as a whole. students are typically responsible for obtaining their own copies of the books used in their courses, although alternatives to owning textbooks, such as textbook rental services and library reserve copies of texts, are available in some instances. in some european countries, such as sweden or spain, students attending institutions of higher education pay for textbooks themselves, although higher education is free of charge otherwise. with higher education costs on the rise, many students are becoming sensitive to every aspect of college pricing, including textbooks, and in many cases amount to one tenth of tuition costs. the 2005 government accountability office report on college textbooks said that since the 1980s, textbook and supply prices have risen twice the rate of inflation in the past two decades. a 2005 pi


phosphorescence is a specific type of photoluminescence related to fluorescence. unlike fluorescence, a phosphorescent material does not immediately re - emit the radiation it absorbs. the slower time scales of the re - emission are associated with " forbidden " energy state transitions in quantum mechanics. as these transitions occur very slowly in certain materials, absorbed radiation may be re - emitted at a lower intensity for up to several hours after the original excitation. commonly seen examples of phosphorescent materials are the glow - in - the - dark toys, paint, and clock dials that glow for some time after being charged with a bright light such as in any normal reading or room light. typically the glowing then slowly fades out within minutes ( or up to a few hours ) in a dark room. the study of phosphorescent materials led to the discovery of radioactivity in 1896. in simple terms, phosphorescence is a process in which energy absorbed by a substance is released relatively slowly in the form of light. this is in some cases the mechanism used for " glow - in - the - dark " materials which are " charged " by exposure to light. unlike the relatively swift reactions in a common fluorescent tube, phosphorescent materials used for these materials absorb the energy and " store " it for a longer time as the processes required to re - emit the light occur less often. quantum mechanical most photoluminescent events, in which a chemical substrate absorbs and then re - emits a photon of light, are fast, on the order of 10 nanoseconds. light is absorbed and emitted at these fast time scales in cases where the energy of the photons involved matches the available energy states and allowed transitions of the substrate. in the special case of phosphorescence, the absorbed photon energy undergoes an unusual intersystem crossing into an energy state of higher spin multiplicity ( see term symbol ), usually a triplet state. as a result, the energy can become trapped in the triplet state with only classically " forbidden " transitions available to return to the lower energy state. these transitions, although " forbidden ", will still occur in quantum mechanics but are kinetically unfavored and thus progress at significantly slower time scales. most phosphorescent compounds are still relatively fast emitters, with triplet lifetimes on the order of milliseconds. however, some compounds have triplet lifetimes up to minutes or even


thus progress at significantly slower time scales. most phosphorescent compounds are still relatively fast emitters, with triplet lifetimes on the order of milliseconds. however, some compounds have triplet lifetimes up to minutes or even hours, allowing these substances to effectively store light energy in the form of very slowly degrading excited electron states. if the phosphorescent quantum yield is high, these substances will release significant amounts of light over long time scales, creating so - called " glow - in - the - dark " materials. where s is a singlet and t a triplet whose subscripts denote states ( 0 is the ground state, and 1 the excited state ). transitions can also occur to higher energy levels, but the first excited state is denoted for simplicity. some examples of " glow - in - the - dark " materials do not glow by phosphorescence. for example, " glow sticks " glow due to a chemiluminescent process which is commonly mistaken for phosphorescence. in chemiluminescence, an excited state is created via a chemical reaction. the light emission tracks the kinetic progress of the underlying chemical reaction. the excited state will then transfer to a " dye " molecule, also known as a sensitizer or fluorophor, and subsequently fluoresce back to the ground state common pigments used in phosphorescent materials include zinc sulfide and strontium aluminate. use of zinc sulfide for safety related products dates back to the 1930s. however, the development of strontium aluminate, with a luminance approximately 10 times greater than zinc sulfide, has relegated most zinc sulfide based products to the novelty category. strontium aluminate based pigments are now used in exit signs, pathway marking, and other safety related signage. | this section requires expansion. ( october 2008 ) | see also - karl a. franz, wolfgang g. kehr, alfred siggel, jurgen wieczoreck, and waldemar adam " luminescent materials " in ullmann ' s encyclopedia of industrial chemistry 2002, wiley - vch, weinheim. doi : 10. 1002 / 14356007. a15 _ 519 - zitoun, d. ; bernaud, l. ; manteghetti, a. microwave synthesis of a long - lasting phosphor. j. chem. ed. 2009, 86, 72 -


| quantum field theory | | | it has been suggested that this article be merged with zero - point energy. ( discuss ) proposed since june 2012. | in quantum field theory, the vacuum state ( also called the vacuum ) is the quantum state with the lowest possible energy. generally, it contains no physical particles. zero - point field is sometimes used [ by whom? ] as a synonym for the vacuum state of an individual quantized field. according to present - day understanding of what is called the vacuum state or the quantum vacuum, it is " by no means a simple empty space ", and again : " it is a mistake to think of any physical vacuum as some absolutely empty void. " according to quantum mechanics, the vacuum state is not truly empty but instead contains fleeting electromagnetic waves and particles that pop into and out of existence. the qed vacuum of quantum electrodynamics ( or qed ) was the first vacuum of quantum field theory to be developed. qed originated in the 1930s, and in the late 1940s and early 1950s it was reformulated by feynman, tomonaga and schwinger, who jointly received the nobel prize for this work in 1965. today the electromagnetic interactions and the weak interactions are unified in the theory of the electroweak interaction. the standard model is a generalization of the qed work to include all the known elementary particles and their interactions ( except gravity ). quantum chromodynamics is the portion of the standard model that deals with strong interactions, and qcd vacuum is the vacuum of quantum chromodynamics. it is the object of study in the large hadron collider and the relativistic heavy ion collider, and is related to the so - called vacuum structure of strong interactions. non - zero expectation value if the quantum field theory can be accurately described through perturbation theory, then the properties of the vacuum are analogous to the properties of the ground state of a quantum mechanical harmonic oscillator ( or more accurately, the ground state of a qm problem ). in this case the vacuum expectation value ( vev ) of any field operator vanishes. for quantum field theories in which perturbation theory breaks down at low energies ( for example, quantum chromodynamics or the bcs theory of superconductivity ) field operators may have non - vanishing vacuum expectation values called condensates. in the standard model, the non - zero vacuum expectation value of the higgs field, arising from spontaneous symmetry


a field φ, which should be written as, is usually condensed to. virtual particles the presence of virtual particles can be rigorously based upon the non - commutation of the quantized electromagnetic fields. non - commutation means that although the average values of the fields vanish in a quantum vacuum, their variances do not. the term " vacuum fluctuations " refers to the variance of the field strength in the minimal energy state, and is described picturesquely as evidence of " virtual particles ". it is sometimes attempted to provide an intuitive picture of virtual particles based upon the heisenberg energy - time uncertainty principle : ( with δe and δt being the energy and time variations respectively ; δe is the accuracy in the measurement of energy and δt is the time taken in the measurement, and ħ is the planck constant divided by 2π ) arguing along the lines that the short lifetime of virtual particles allows the " borrowing " of large energies from the vacuum and thus permits particle generation for short times. although the phenomenon of virtual particles is accepted, this interpretation of the energy - time uncertainty relation is not universal. one issue is the use of an uncertainty relation limiting measurement accuracy as though a time uncertainty δt determines a " budget " for borrowing energy δe. another issue is the meaning of " time " in this relation, because energy and time ( unlike position q and momentum p, for example ) do not satisfy a canonical commutation relation ( such as [ q, p ] = i ħ ). various schemes have been advanced to construct an observable that has some kind of time interpretation, and yet does satisfy a canonical commutation relation with energy. the very many approaches to the energy - time uncertainty principle are a long and continuing subject. physical nature of the quantum vacuum according to astrid lambrecht ( 2002 ) : " when one empties out a space of all matter and lowers the temperature to absolute zero, one produces in a gedankenexperiment the quantum vacuum state. " photon - photon interaction can occur only through interaction with the vacuum state of some other field, for example through the dirac electron - positron vacuum field ; this is associated with the concept of vacuum polarization. according to milonni ( 1994 ) : "... all quantum fields have zero - point energies and vacuum fluctuations. " this means that there is a component of the quantum vacuum respectively for each component field ( considered in the conceptual absence of the other fields ), such as the electromagnetic field, the dirac electron


quantum fields have zero - point energies and vacuum fluctuations. " this means that there is a component of the quantum vacuum respectively for each component field ( considered in the conceptual absence of the other fields ), such as the electromagnetic field, the dirac electron - positron field, and so on. according to milonni ( 1994 ), some of the effects attributed to the vacuum electromagnetic field can have several physical interpretations, some more conventional than others. the casimir attraction between uncharged conductive plates is often proposed as an example of an effect of the vacuum electromagnetic field. schwinger, deraad, and milton ( 1978 ) are cited by milonni ( 1994 ) as validly, though unconventionally, explaining the casimir effect with a model in which " the vacuum is regarded as truly a state with all physical properties equal to zero. " in this model, the observed phenomena are explained as the effects of the electron motions on the electromagnetic field, called the source field effect. milonni writes : " the basic idea here will be that the casimir force may be derived from the source fields alone even in completely conventional qed,... " milonni provides detailed argument that the measurable physical effects usually attributed to the vacuum electromagnetic field cannot be explained by that field alone, but require in addition a contribution from the self - energy of the electrons, or their radiation reaction. he writes : " the radiation reaction and the vacuum fields are two aspects of the same thing when it comes to physical interpretations of various qed processes including the lamb shift, van der waals forces, and casimir effects. " this point of view is also stated by jaffe ( 2005 ) : " the casimir force can be calculated without reference to vacuum fluctuations, and like all other observable effects in qed, it vanishes as the fine structure constant, α, goes to zero. " see also references and notes - astrid lambrecht ( hartmut figger, dieter meschede, claus zimmermann eds. ) ( 2002 ). observing mechanical dissipation in the quantum vacuum : an experimental challenge ; in laser physics at the limits. berlin / new york : springer. p. 197. isbn 3 - 540 - 42418 - 0. - christopher ray ( 1991 ). time, space and philosophy. london / new york : routledge. chapter 10, p. 205. isbn 0 - 415 - 03221 - 0. - aip physics news update, 1996 - physical review focus dec. 1998 - walter dittrich & gi


and philosophy. london / new york : routledge. chapter 10, p. 205. isbn 0 - 415 - 03221 - 0. - aip physics news update, 1996 - physical review focus dec. 1998 - walter dittrich & gies h ( 2000 ). probing the quantum vacuum : perturbative effective action approach. berlin : springer. isbn 3 - 540 - 67428 - 4. - for an historical discussion, see for example ari ben - menahem, ed. ( 2009 ). " quantum electrodynamics ( qed ) ". historical encyclopedia of natural and mathematical sciences, volume 1 ( 5th ed. ). springer. pp. 4892 ff. isbn 3 - 540 - 68831 - 5. for the nobel prize details and the nobel lectures by these authors see " the nobel prize in physics 1965 ". nobelprize. org. retrieved 2012 - 02 - 06. - jean letessier, johann rafelski ( 2002 ). hadrons and quark - gluon plasma. cambridge university press. p. 37 ff. isbn 0 - 521 - 38536 - 9. - sean carroll, sr research associate - physics, california institute of technology, june 22, 2006 c - span broadcast of cosmology at yearly kos science panel, part 1 - david delphenich ( 2006 ). " nonlinear electrodynamics and qed ". arxiv : hep - th / 0610088 [ hep - th ]. - klein, james j. and b. p. nigam, birefringence of the vacuum, physical review vol. 135, p. b1279 - b1280 ( 1964 ). - mourou, g. a., t. tajima, and s. v. bulanov, optics in the relativistic regime ; § xi nonlinear qed, reviews of modern physics vol. 78 ( no. 2 ), 309 - 371 ( 2006 ) pdf file. - holger gies ; joerg jaeckel ; andreas ringwald ( 2006 ). " polarized light propagating in a magnetic field as a probe of millicharged fermions ". physical review letters 97 ( 14 ). arxiv : hep - ph / 0607118. bibcode : 2006phrvl.. 97n0402g. doi : 10. 1103 / physrevlett.


( 14 ). arxiv : hep - ph / 0607118. bibcode : 2006phrvl.. 97n0402g. doi : 10. 1103 / physrevlett. 97. 140402. - davis ; joseph harris ; gammon ; smolyaninov ; kyuman cho ( 2007 ). " experimental challenges involved in searches for axion - like particles and nonlinear quantum electrodynamic effects by sensitive optical techniques ". arxiv : 0704. 0748 [ hep - th ]. - myron wyn evans, stanisław kielich ( 1994 ). modern nonlinear optics, volume 85, part 3. john wiley & sons. p. 462. isbn 0 - 471 - 57548 - 8. " for all field states that have classical analog the field quadrature variances are also greater than or equal to this commutator. " - david nikolaevich klyshko ( 1988 ). photons and nonlinear optics. taylor & francis. p. 126. isbn 2 - 88124 - 669 - 9. - milton k. munitz ( 1990 ). cosmic understanding : philosophy and science of the universe. princeton university press. p. 132. isbn 0 - 691 - 02059 - 0. " the spontaneous, temporary emergence of particles from vacuum is called a " vacuum fluctuation ". " - for an example, see p. c. w. davies ( 1982 ). the accidental universe. cambridge university press. p. 106. isbn 0 - 521 - 28692 - 1. - a vaguer description is provided by jonathan allday ( 2002 ). quarks, leptons and the big bang ( 2nd ed ed. ). crc press. pp. 224 ff. isbn 0 - 7503 - 0806 - 0. " the interaction will last for a certain duration δt. this implies that the amplitude for the total energy involved in the interaction is spread over a range of energies δe. " - this " borrowing " idea has led to proposals for using the zero - point energy of vacuum as an infinite reservoir and a variety of " camps " about this interpretation. see, for example, moray b. king ( 2001 ). quest for zero point energy : engineering principles for ' free energy ' inventions. adventures unlimited press. pp. 124 ff. isbn 0 - 932813 - 94 - 1. - quantities satisfying a


example, moray b. king ( 2001 ). quest for zero point energy : engineering principles for ' free energy ' inventions. adventures unlimited press. pp. 124 ff. isbn 0 - 932813 - 94 - 1. - quantities satisfying a canonical commutation rule are said to be noncompatible observables, by which is meant that they can both be measured simultaneously only with limited precision. see kiyosi ito ( 1993 ). " § 351 ( xx. 23 ) c : canonical commutation relations ". encyclopedic dictionary of mathematics ( 2nd ed ed. ). mit press. p. 1303. isbn 0 - 262 - 59020 - 4. - paul busch, marian grabowski, pekka j. lahti ( 1995 ). " § iii. 4 : energy and time ". operational quantum physics. springer. pp. 77 ff. isbn 3 - 540 - 59358 - 6. - for a review, see paul busch ( 2008 ). " chapter 3 : the time – energy uncertainty relation ". in j. g. muga, r. sala mayato and i. l. egusquiza, editors. time in quantum mechanics ( 2nd ed ed. ). springer. pp. 73 ff. isbn 3 - 540 - 73472 - 4. - fowler, r., guggenheim, e. a. ( 1965 ). statistical thermodynamics. a version of statistical mechanics for students of physics and chemistry, reprinted with corrections, cambridge university press, london, page 224. - partington, j. r. ( 1949 ). an advanced treatise on physical chemistry, volume 1, fundamental principles. the properties of gases, longmans, green and co., london, page 220. - wilks, j. ( 1971 ). the third law of thermodynamics, chapter 6 in thermodynamics, volume 1, ed. w. jost, of h. eyring, d. henderson, w. jost, physical chemistry. an advanced treatise, academic press, new york, page 477. - bailyn, m. ( 1994 ). a survey of thermodynamics, american institute of physics, new york, isbn 0 – 88318 – 797 – 3, page 342. - jauch, j. m., rohrlich, f. ( 1955 / 1980 ). the theory of photons and electrons. the relat


york, isbn 0 – 88318 – 797 – 3, page 342. - jauch, j. m., rohrlich, f. ( 1955 / 1980 ). the theory of photons and electrons. the relativistic quantum field theory of charged particles with spin one - half, second expanded edition, springer - verlag, new york, isbn 0 – 387 – 07295 – 0, pages 287 – 288. - milonni, p. w. ( 1994 ). the quantum vacuum. an introduction to quantum electrodynamics, academic press, inc., boston, isbn 0 – 12 – 498080 – 5, page xv. - milonni, p. w. ( 1994 ). the quantum vacuum. an introduction to quantum electrodynamics, academic press, inc., boston, isbn 0 – 12 – 498080 – 5, page 239. - schwinger, j., deraad, l. l., milton, k. a. ( 1978 ). casimir effect in dielectrics, annals of physics, 115 : 1 – 23. - milonni, p. w. ( 1994 ). the quantum vacuum. an introduction to quantum electrodynamics, academic press, inc., boston, isbn 0 – 12 – 498080 – 5, page 418. - jaffe, r. l. ( 2005 ). casimir effect and the quantum vacuum, phys. rev. d 72 : 021301 ( r ), http : / / 1 – 5. cua. mit. edu / 8. 422 _ s07 / jaffe2005 _ casimir. pdf further reading - free pdf copy of the structured vacuum - thinking about nothing by johann rafelski and berndt muller ( 1985 ) isbn 3 - 87144 - 889 - 3. - m. e. peskin and d. v. schroeder, an introduction to quantum field theory. - h. genz, nothingness : the science of empty space - maybe this should discuss star trek and / or star gate : engineering the zero - point field and polarizable vacuum for interstellar flight - e. w. davis, v. l. teofilo, b. haisch, h. e. puthoff, l. j. nickisch, a. rueda and d. c. cole ( 2006 ) " review of experimental concepts for studying the quantum vacuum field "


by the temperature factors. high values, indicating lots of motion, are in red and yellow, and low values are in blue. notice that the interior of the protein has low b - values and the amino acids on the surface have higher values. you can click on the picture for an interactive jmol view. tip : temperature factors are a measure of our confidence in the location of each atom. if you find an atom on the surface of a protein with a high temperature factor, keep in mind that this atom is probably moving a lot, and that the coordinates specified in the pdb file are only one possible snapshot of its location. occupancy and multiple conformations macromolecular crystals are composed of many individual molecules packed into a symmetrical arrangement. in some crystals, there are slight differences between each of these molecules. for instance, a sidechain on the surface may wag back and forth between several conformations, or a substrate may bind in two orientations in an active site, or a metal ion may be bound to only a few of the molecules. when researchers build the atomic model of these portions, they can use the occupancy to estimate the amount of each conformation that is observed in the crystal. for most atoms, the occupancy is given a value of 1, indicating that the atom is found in all of the molecules in the same place in the crystal. however, if a metal ion binds to only half of the molecules in the crystal, the researcher will see a weak image of the ion in the electron density map, and can assign an occupancy of 0. 5 in the pdb structure file for this atom. occupancies are also commonly used to identify sidechains or ligands that are observed in multiple conformations. the occupancy value is used to indicate the fraction of molecules that have each of the conformations. two ( or more ) atom records are included for each atom, with occupancies like 0. 5 and 0. 5, or 0. 4 and 0. 6, or other fractional occupancies that sum to a total of 1. the two images shown are taken from the high - resolution structure of myoglobin in entry 1a6m : glutamine 8 is on the left, and tyrosine 151 on the right. in both cases, the depositors interpreted the experimental data as showing two conformations of the amino acid, with occupancies of


it ' s entertaining, the same way you or i would enjoy a video game in a completely sandboxed environment. they, however, are doing things in the real world. same layer of digital separation between you and the consequences of your actions, with the added thrill of knowing it ' s real. - does computer insecurity exist because of the nature of computers? to a point, yes. computers are powerful, but they are extremely dumb. they require humans to do their thinking for them, to design them in a way that is difficult to subvert, to program them in a way that is difficult to subvert, and to use them in a way that is difficult to subvert. the inherent difficulty of this is similar to the inherent difficulty ( maybe the impossibility ) of designing a " completely foolproof system " : a common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools. - douglas adams in both cases, you ' re very simply trying to pre - emptively outsmart someone willing to spend a lot of time and effort finding a way to misuse what you ' re designing once the finished product has left your hands. you effectively have to come up with the same ideas that the other person would have, and incorporate mechanisms to defeat that line of thinking. the more complex the system is internally, the more of those ideas become possible, and the less likely you are to have thought of everything. the more you put in place to prevent misuse, the more complexity you add. it ' s a vicious cycle.


mit professor ’ s book digs into the eclectic, textually linked reading choices of people in medieval london. cambridge, mass. - - following the 1997 creation of the first laser to emit pulsed beams of atoms, mit researchers report in the may 16 online version of science that they have now made a continuous source of coherent atoms. this work paves the way for a laser that emits a continuous stream of atoms. mit physicists led by physics professor wolfgang ketterle ( who shared the 2001 nobel prize in physics ) created the first atom laser. a long - sought goal in physics, the atom laser emitted atoms, similar in concept to the way an optical laser emits light. " i am amazed at the rapid progress in the field, " ketterle said. " a continuous source of bose - einstein condensate is just one of many recent advances. " because the atom laser operates in an ultra - high vacuum, it may never be as ubiquitous as optical lasers. but, like its predecessor, the pulsed atom laser, a continuous - stream atom laser may someday be used for a variety of applications in fundamental physics. it could be used to directly deposit atoms onto computer chips, and improve the precision and accuracy of atomic clocks and gyroscopes. it could aid in precision measurements of fundamental constants, atom optics and interferometry. a continuous stream laser could do all of these things better than a pulsed atomic laser, said co - author ananth p. chikkatur, a physics graduate student at mit. " similar to the optical laser revolution, a continuous stream atom laser might be useful for more things than a pulsed laser, " he said. in addition to ketterle and chikkatur, authors include mit graduate students yong - il shin and aaron e. leanhardt ; david f. kielpinski, postdoctoral fellow in the mit research laboratory of electronics ( rle ) ; physics senior edem tsikata ; mit affiliate todd l. gustavson ; and david e. pritchard, cecil and ida green professor of physics and a member of the mit - harvard center for ultracold atoms and the rle. a new form of matter an important step toward the first atom laser was the creation of a new form of matter - the bose - einstein condensate ( bec ). bec forms at temperatures around one millionth of a degree kelvin, a million times colder than interstellar space. ketterle ' s group had developed novel cooling techniques that were key to


the bose - einstein condensate ( bec ). bec forms at temperatures around one millionth of a degree kelvin, a million times colder than interstellar space. ketterle ' s group had developed novel cooling techniques that were key to the observation of bec in 1995, first by a group at the university of colorado at boulder, then a few months later by ketterle at mit. it was for this achievement that researchers from both institutions were honored with the nobel prize last year. ketterle and his research team managed to merge a bunch of atoms into what he calls a single matter - wave, and then used fluctuating magnetic fields to shape the matter - wave into a beam much like a laser. to test the coherence of a bec, the researchers generated two separate matter - waves, made them overlap and photographed a so - called " interference pattern " that only can be created by coherent waves. the researchers then had proof that they had created the first atom laser. since 1995, all atom lasers and bec have been produced in a pulsed manner, emitting individual pulses of atoms several times per minute. until now, little progress has been made toward a continuous bec source. while it took about six months to create a continuous optical laser after the first pulsed optical laser was produced in 1960, the much more technically challenging continuous source of coherent atoms has taken seven years since ketterle and colleagues first observed bec in 1995. a new challenge creating a continuous bec source involved three steps : building a chamber where the condensate could be stored in an optical trap, moving the fresh condensate and merging the new condensate with the existing condensate stored in the optical trap. ( the same researchers first developed an optical trap for becs in 1998. ) the researchers built an apparatus containing two vacuum chambers : a production chamber where the condensate is produced and a " science chamber " around 30 centimeters away, where the condensate is stored. the condensate in the science chamber had to be protected from laser light, which was necessary to produce a fresh condensate, and also from hot atoms. this required great precision, because a single laser - cooled atom has enough energy to knock thousands of atoms out of the condensate. in addition, they used an optical trap as the reservoir trap, which is insensitive to the magnetic fields used for cooling atoms into a bec. the researchers also needed to figure out how to move the fresh condensate -


##nsate. in addition, they used an optical trap as the reservoir trap, which is insensitive to the magnetic fields used for cooling atoms into a bec. the researchers also needed to figure out how to move the fresh condensate - chilled to astronomically low temperatures - from the production chamber to the science chamber without heating them up. this was accomplished using optical tweezers - a focused laser light beam that traps the condensate. finally, to merge the new condensate with the existing condensate in the science chamber, they moved the new condensate in the tweezers into the science chamber by merging the condensates together. a bucket of atoms if the pulsed atom laser is like a faucet that drips, chikkatur says the new innovations create a sort of bucket that collects the drips without wasting or changing the condensate too dramatically by heating it. this way, a reservoir of condensate is always on hand to replenish an atom laser. the condensate pulses are like a dripping faucet, where the drops are analogous to the pulsed bec production. " we have now implemented a bucket ( our reservoir trap ), where we collect these drips to have continuous source of water ( bec ), " chikkatur said. " although we did not demonstrate this, if we poke a hole in this bucket, we will have a steady stream of water. this hole would be an outcoupling technique from which we can produce a continuous atom laser output. " the big achievement here is that we have invented the bucket, which can store atoms continuously and also makes sure that the drips of water do not cause a lot of splashing ( heating of becs ), " he said. the next step would be to improve the number of atoms in the source, perhaps by implementing a large - volume optical trap. another important step would be to demonstrate a phase - coherent condensate merger using a matter wave amplification technique pioneered by the mit group and a group in japan, he said. this work is funded by the national science foundation, the office of naval research, the army research office, the packard foundation and nasa.