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a small constant. We can write the law like this: Az Ap > ñ/2, but we shall |
explain it in more detail later. This rule is the explanation of a very mysterious |
paradox: If the atoms are made out of plus and minus charges, why don't the |
minus charges simply sit on top oŸ the plus charges (they attract each other) and |
get so close as to completely cancel them out? Whg are atoms so bñg? Wlhy ïs the |
nueleus at the center with the electrons around it? It was first thought that this |
was because the nucleus was so big; but no, the nucleus is 0er small An atom |
has a diameter of about 10~ em. The nueleus has a diameter of about 10” !3 em. |
--- Trang 60 --- |
Tf we had an atom and wished to see the nucleus, we would have to magnify 1t |
until the whole atom was the size of a large room, and then the nucleus would |
be a bare speck which you could just about make out with the eye, but very |
nearly aÏÏ the uueighf of the atom is in that infnitesimal nucleus. What keeps the |
electrons from simply falling in? This principle: If they were in the nucleus, we |
would know their position precisely, and the uncertainty principle would then |
require that they have a very /arøe (but uncertain) momentum, i.e., a very large |
kimelic energu. With this energy they would break away from the nucleus. They |
make a compromise: they leave themselves a little room for this uncertainty and |
then jiggle with a certain amount of minimum motion in accordance with this |
rule. (Remember that when a crystal is cooled to absolute zero, we said that the |
atoms do not stop moving, they still Jiggle. Why? TIf they stopped moving, we |
would know where they were and that they had zero motion, and that is against |
the uncertainty principle. We cannot know where they are and how fast they are |
moving, so they must be continually wiggling ¡in therel) |
Another most interesting change in the ideas and philosophy of science brought |
about by quantum mechanies is this: it is not possible to predict ezacflu what will |
happen in any circumstance. For example, it is possible to arrange an atom which |
is ready to emit light, and we can measure when it has emitted light by picking up |
a photon particle, which we shall describe shortly. We cannot, however, predict |
tuhen 1% is goïng to emit the light or, with several atoms, œhúch ơne is goïng to. |
You may say that this is because there are some internal “wheels” which we have |
not looked at closely enough. No, there are no internal wheels; nature, as we |
understand it today, behaves in such a way that it is ƒundamentall impossible |
to make a precise prediction oŸ ezacfl that uiil happen in a given experiment. |
This is a horrible thing; in fact, philosophers have said before that one of the |
fundamental requisites of science is that whenever you set up the same conditions, |
the same thing must happen. This is simply nøÝ frue, it is no£‡ a fundamental |
condition of scienece. “Phe fact is that the same thing does not happen, that we |
can ñnd only an average, statistically, as to what happens. Nevertheless, science |
has not completely collapsed. Philosophers, incidentally, say a great deal about |
what 1s œbsolutel necessar for seience, and 1t is always, so far as one can see, |
rather naive, and probably wrong. Eor example, some philosopher or other said |
1t is fundamental to the scientifc efort that If an experiment is performed in, say, |
Stockholm, and then the same experiment is done in, say, Quito, the sœrne resulis |
must occur. Phat is quite false. It is not necessary that sc¿ence do that; it may be |
a fact oƒ czperience, but it is not necessary. For example, if one of the experiments |
--- Trang 61 --- |
1s to look out at the sky and see the aurora borealis in Stockholm, you do not see it |
in Quito; that is a diferent phenomenon. “But,” you say, “that is something that |
has to do with the outside; can you close yourself up in a box in Stockholm and |
pull down the shade and get any diference?” Surely. If we take a pendulum on |
a universal Joint, and pull it out and let go, then the pendulum will swing almost |
in a plane, but not quite. Slowly the plane keeps changing in Stockholm, but not |
in Quito. The blinds are down, too. The fact that this happened does not bring |
on the destruction of science. What ¡s the fundamental hypothesis of science, |
the fundamental philosophy? We stated it in the first chapter: he soÏe test oƒ |
the 0ualiditU oƒ am tdea is czpertment. TÝ it turns out that most experiments work |
out the same in Quito as they do in Stockholm, then those “most experiments” |
will be used to formulate some general law, and those experiments which do not |
come out the same we will say were a result of the environment near Stockholm. |
W©e will invent some way to summarize the results of the experiment, and we do |
not have to be told ahead of time what this way will look like. If we are told that |
the same experiment will always produce the same result, that is all very well, |
but ifƒ when we try it, i§ does no, then it does nmoøý. We just have to take what we |
see, and then formulate all the rest of our ideas in terms of our actual experience. |
Returning again to quantum mechanics and fundamenta] physics, we cannot øO |
into details of the quantum-mechanical principles at this time, of course, because |
these are rather dificult to understand. We shall assume that they are there, and |
go on to describe what some of the consequences are. Ône of the consequences 1s |
that things which we used to consider as waves also behawve like particles, and |
particles behave like waves; in fact everything behaves the same way. Thhere is |
no distinction between a wave and a particle. So quantum mechanics unifies the |
idea of the field and its waves, and the particles, all into one. Now it is true that |
when the frequeney is low, the fñeld aspect of the phenomenon is more evident, or |
more useful as an approximate description in terms of everyday experiences. But |
as the frequency increases, the particle aspects of the phenomenon become more |
evident with the equipment with which we usually make the measurements. In |
fact, although we mentioned many Írequencies, no phenomenon directly involving |
a frequeney has yet been detected above approximately 1012 eyeles per second. |
We© only deduce the higher Írequencies from the energy of the particles, by a rule |
which assumes that the particle-wave idea of quantum mechanics is valid. |
Thus we have a new view of electromagnetic interaction. We have a new kind |
of parficle to add to the electron, the proton, and the neutron. hat new particle |
1s called a pho£on. "The new view of the interaction of electrons and photons that |
--- Trang 62 --- |
1s electromagnetic theory, but with everything quantum-mechanically correct, is |
called qguantum clectrodunamics. Thĩs fundamental theory of the interaction of |
light and matter, or electric field and charges, is our greatest success so far In |
physics. In this one theory we have the basic rules for all ordinary phenomena |
except for gravitation and nuclear processes. For example, out of quantum |
electrodynamiecs come all known electrical, mechanical, and chemical laws: the |
laws for the collision of billiard balls, the motions of wires in magnetic fields, |
the specifc heat of carbon monoxide, the color of neon signs, the density of salt, |
and the reactions of hydrogen and oxygen to make water are all consequences |
of this one law. All these details can be worked out if the situation is simple |
enouph for us to make an approximation, which is almost never, but often we can |
understand more or less what is happening. At the present từme no exceptions |
are found to the quantum-electrodynamic laws outside the nucleus, and there we |
do not know whether there is an exception because we simply do not know what |
is goiïng on in the nucleus. |
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