Well, we talked about that in the last video. The earth is spinning and then it's spinning around, it's orbiting around, the sun at a nice clip and then the whole solar system is orbiting around the center of the galaxy at a nice clip. The galaxy itself might be moving, so if you have some absolute frame of reference that's defined by the ether, well we are going to be moving relative to it.
And if we're moving relative to it well maybe you just measure the speed of light in different directions and see whether the speed of light is faster or slower in a certain direction and then that might help you identify-- well, one, validate that the ether exists-- but also think about what our velocity is relative to the ether, relative to that absolute frame of reference. But the problem in the 19th century is that we didn't have any precise way of actually measuring--or a precise enough way of measuring--the speed of light where we could detect the relative difference due to the light going for or against, or into or away from, the actual direction of the ether wind.
And so the experiment that is usually cited with first kind of breaking things open, starting to really make a dent in this whole idea of a luminiferous ether, is the Michelson-Morley Experiment. Michelson-Morley Experiment. They recognized, okay, we can't measure the speed of light with enough precision to detect has it gotten slowed down by the ether wind or sped up by the ether wind, but what we could do, and this is what Michelson and Morley did do, and I'm gonna do an oversimplification of the experiment, is that, okay, you have a light source, you have a light source right over here.
So, you have a light source. And so that's going to send light in this direction. It's going to send light just like that. And what you do is you have a half-silvered mirror that allows half the light to pass directly through it and half of it to be reflected.
So let's put a half-silvered mirror right over here. So, there's a half-silvered mirror. And so half of this light will bounce off like this, and this is just a simplification of it. Let me do it a little neater than that. So half will bounce off like that.
And then the other half will be able to go through it. Will be able to go through it. It's a half-silvered mirror. And then we make each of those light rays-- we've essentially taken our original light ray and split it into two-- well then we'll then bounce those off mirrors.
Bounce those off mirrors that are equidistant. And there are some adjustments when you actually have to factor in everything, but just as a simple notion, these things are just now going to bounce back. So, this one is now going to bounce back.
It's half-silvered, it can go through, or part of it can go through, that mirror. So that's that ray. And then this one is going to bounce back. It means that they found no difference between the speed of light while travelling through ether.
Michelson Morley interferometer sent white light for the actual observations and yellow light from a sodium flame through a half-transparent mirror. The mirror was used to split the coming light beam into two separate beams travelling perpendicular to each other.
After leaving this mirror, beams moved out to the long arms end where they faced back reflection into the middle. These two beams then recombine to produce a pattern of constructive and destructive interference. Michelson claimed that if the speed of light was constant concerning the ether medium through which the Earth moves, then that motion can be detected. The details of Michelson experiment set up are:.
The beam of light gets incident at a half-silvered glass plate. This plate acts as a beam splitter, which splits the light beam into two coherent beams. One beam transmits, and the other reflects. The beam transmitted strikes the mirror, say, M1, and gets reflected.
The beam reflected strikes the mirror, say, M2, which again gets reflected. The returned beams reach the telescope, which is used for interference patterns produced by these two rays. Now, if there is an aether wind blowing, someone looking through the telescope should see the halves of the two half-pulses to arrive at slightly different times, since one would have gone more upstream and back, one more across stream in general.
To maximize the effect, the whole apparatus, including the distant mirrors, was placed on a large turntable so it could be swung around. An animated applet of the experiment is available here —it makes the account above a lot clearer! Let us think about what kind of time delay we expect to find between the arrival of the two half-pulses of light.
You can check it with your calculator. Now, what about the cross-stream time? The actual cross-stream speed must be figured out as in the example above using a right-angled triangle, with the hypoteneuse equal to the speed c , the shortest side the aether flow speed v , and the other side the cross-stream speed we need to find the time to get across.
Therefore the across-stream roundtrip time, assuming the aether velocity is much less than that of light, is. This means the time delay between the pulses reflected from the different mirrors reaching the telescope is about one-hundred-millionth of a few millionths of a second.
It seems completely hopeless that such a short time delay could be detected. However, this turns out not to be the case, and Michelson was the first to figure out how to do it. The trick is to use the interference properties of the lightwaves. Instead of sending pulses of light, as we discussed above, Michelson sent in a steady beam of light of a single color.
This can be visualized as a sequence of ingoing waves, with a wavelength one fifty-thousandth of an inch or so. Now this sequence of waves is split into two, and reflected as previously described. One set of waves goes upstream and downstream, the other goes across stream and back. Finally, they come together into the telescope and the eye. If the one that took longer is half a wavelength behind, its troughs will be on top of the crests of the first wave, they will cancel, and nothing will be seen.
If the delay is less than that, there will still be some dimming. However, slight errors in the placement of the mirrors would have the same effect. This is one reason why the apparatus is built to be rotated. On turning it through 90 degrees, the upstream-downstream and the cross-stream waves change places. Now the other one should be behind. Thus, if there is an aether wind, if you watch through the telescope while you rotate the turntable, you should expect to see variations in the brightness of the incoming light.
To magnify the time difference between the two paths, in the actual experiment the light was reflected backwards and forwards several times, like a several lap race. In fact, nothing was observed. The light intensity did not vary at all. Again, nothing was seen. Finally, Michelson wondered if the aether was somehow getting stuck to the earth, like the air in a below-decks cabin on a ship, so he redid the experiment on top of a high mountain in California. Again, no aether wind was observed.
It was difficult to believe that the aether in the immediate vicinity of the earth was stuck to it and moving with it, because light rays from stars would deflect as they went from the moving faraway aether to the local stuck aether. The only possible conclusion from this series of very difficult experiments was that the whole concept of an all-pervading aether was wrong from the start. Michelson was very reluctant to think along these lines.
He had discovered that his equations predicted there could be waves made up of electric and magnetic fields, and the speed of these waves, deduced from experiments on how these fields link together, would be , miles per second. This is, of course, the speed of light, so it is natural to assume that light is made up of fast-varying electric and magnetic fields.
But what is the speed to be measured relative to? The whole point of bringing in the aether was to give a picture for light resembling the one we understand for sound, compressional waves in a medium. The speed of sound through air is measured relative to air. If the wind is blowing towards you from the source of sound, you will hear the sound sooner. So what does light travel at , miles per second relative to?
There is another obvious possibility, which is called the emitter theory: the light travels at , miles per second relative to the source of the light. The analogy here is between light emitted by a source and bullets emitted by a machine gun. The bullets come out at a definite speed called the muzzle velocity relative to the barrel of the gun. If the gun is mounted on the front of a tank, which is moving forward, and the gun is pointing forward, then relative to the ground the bullets are moving faster than they would if shot from a tank at rest.
The simplest way to test the emitter theory of light, then, is to measure the speed of light emitted in the forward direction by a flashlight moving in the forward direction, and see if it exceeds the known speed of light by an amount equal to the speed of the flashlight.
Actually, this kind of direct test of the emitter theory only became experimentally feasible in the nineteen-sixties. It is now possible to produce particles, called neutral pions, which decay each one in a little explosion, emitting a flash of light.
It is also possible to have these pions moving forward at , miles per second when they self destruct, and to catch the light emitted in the forward direction, and clock its speed.
It is found that, despite the expected boost from being emitted by a very fast source, the light from the little explosions is going forward at the usual speed of , miles per second.
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