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this will become clear when we discuss the following phenomena: If we were |
to charge a body, say a comb, electrically, and then place a charged piece of |
paper at a distance and move the comb back and forth, the paper will respond by |
always pointing to the comb. lf we shake it faster, it will be discovered that the |
paper is a little behind, £Öere ?s a đelœw in the action. (At the frst stage, when |
we move the comb rather slowly, we fnd a complication which is r =agnetism. |
Magnetic inÑuences have to do with charges ?m relaliue mmotion, so magnetic |
forces and electric forces can really be attributed to one field, as two diferent |
aspects of exactly the same thing. A changing electric field cannot exist without |
magnetism.) IÝ we move the charged paper farther out, the delay is greater. Then |
an interesting thing is observed. Although the forces between two charged objects |
should go inversely as the sguare of the distance, it is found, when we shake a |
charge, that the inÑuenece extends uer rmuch farther ou‡ than we would guess at |
first sipht. That is, the efect falls of more slowly than the inverse square. |
Here is an analogy: If we are in a pool of water and there is a Ñoating cork |
very close by, we can move it “directly” by pushing the water with another |
cork. If you looked only at the bwo cor&s, all you would see would be that one |
moved immediately in response to the motion of the other—there is some kind of |
“;nteraction” between them. OÝ course, what we really do is to disturb the t0afer; |
the ater then disturbs the other cork. We could make up a “law” that if you |
pushed the water a little bit, an object close by in the water would move. Tf it |
were farther away, of course, the second cork would scarcely move, for we move |
the water /ocaliu. On the other hand, if we jiggle the cork a new phenomenon |
1s involved, in which the motion of the water moves the water there, etc., and |
tuaues travel away, so that by jiggling, there is an inÑuence 0erg rmuch ƒarther out, |
an oscillatory infuence, that cannot be understood from the direct interaction. |
Therefore the idea of direct interaction must be replaced with the existence of |
the water, or in the electrical case, with what we call the electromagnetic field. |
The electromagnetic field can carry waves; some of these waves are ljghứ, |
others are used in radio broadcasis, but the general name is elecfromagnetic |
tuaues. 'hese oscillatory waves can have various ƒreguencies. The only thing that |
is really diferent from one wave to another is the ƒrequenec oj oscillalion. lỶ we |
shake a charge back and forth more and more rapidly, and look at the efects, we |
get a whole series of diferent kinds of efects, which are all unifed by specifying |
but one number, the number of oscillations per second. 'Phe usual “pickup” that |
we get from electric currents in the circuits in the walls of a building have a |
frequency of about one hundred cycles per second. If we increase the frequency to |
--- Trang 58 --- |
Table 2-1 |
The Electromagnetic Spectrum |
trequency in Rough |
oscillations/sec Name behavior |
102 Electrical disturbance Eield |
5 x 107 - 108 Radio broadcast |
10Ẻ EFM—TV |
1019 Radar Waves |
5x 1012-10!” Light ' |
1018 X-rays |
10?! ^-rays, nuclear |
10? ^-rays, “artificial” Particle |
10? ^-rays, in cosmiCc rays |
500 or 1000 kilocycles (1 kilocycle = 1000 cycles) per second, we are “on the air,” |
for this is the requenecy range which is used for radio broadcasts. (Of course it |
has nothing to do with the ør! W©e can have radio broadcasts without any air.) lÝ |
we again increase the requency, we come into the range that is used for EM and |
TV. Going still further, we use certain short waves, for example for rœdar. Still |
higher, and we do not need an instrument to “see” the stuf, we can see it with |
the human eye. In the range of frequeney from 5ð x 101! to 101 eyeles per second |
our eyes would see the oscillation of the charged comb, 1Ÿ we could shake it that |
fast, as red, blue, or violet light, depending on the frequency. Frequenecies below |
this range are called infrared, and above it, ultraviolet. The fact that we can see |
in a particular frequency range makes that part of the electromagnetic spectrum |
no more impressive than the other parts from a physicist's standpoint, but from |
a human standpoint, of course, i% ¡s more interesting. IÝ we go up even higher in |
Írequency, we get x-rays. X-rays are nothing but very high-fequency light. If we |
go still higher, we get gamma rays. These two terms, x-rays and gamma rays, |
are used almost synonymously. Ũsually electromagnetic rays coming from nuclei |
are called gamma rays, while those of high energy from atoms are called x-rays, |
but at the same frequency they are indistinguishable physically, no matter what |
their source. If we go to still higher frequencies, say to 102 eycles per second, we |
fnd that we can make those waves artificially, for example with the synchrotron |
--- Trang 59 --- |
here at Caltech. We can fñnd electromagnetic waves with stupendously high |
frequencies—with even a thousand times more rapid oscillation——in the waves |
found in cosznic ras. These waves cannot be controlled by us. |
2-3 Quantum physics |
Having described the idea of the electromagnetic field, and that this fñeld |
can carry waves, we soon learn that these waves actually behave in a strange |
way which seems very unwavelike. At higher frequencies they behave much |
more like particles! Tt 1s guantum rmechanics, discovered just after 1920, which |
explains this strange behavior. In the years before 1920, the picture of space |
as a three-dimensional space, and oŸ time as a separate thing, was changed by |
Binstein, fñrst into a combination which we call space-time, and then still further |
IntO a cur0ed space-time to represent gravitation. So the “stage” is changed |
into space-time, and gravitation 1s presumably a modification of space-time. |
'Then it was also found that the rules for the motions of particles were incorrect. |
The mechanical rules of “inertia” and “forces” are romg——Newton's laws are |
turong——in the world of atoms. Instead, it was discovered that things on a small |
scale behave 0othing like things on a large scale. 'Phat is what makes physics |
difcult—and very interesting. It is hard because the way things behave on a |
small scale is so “unnatural”; we have no direct experience with it. Here things |
behave like nothing we know of, so that it is impossible to describe this behavior |
in any other than analytic ways. It is difcult, and takes a lot of imagination. |
Quantum mechanics has many aspects. In the first place, the idea that a |
particle has a defnite location and a defñnite speed is no longer allowed; that is |
wrong. To give an example of how wrong classical physics is, there is a rule in |
quantum mechanics that says that one cannot know both where something is and |
how fast it is moving. The uncertainty of the momentum and the uncertainty |
of the position are complementary, and the product of the two is bounded by |
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