# Einstein's Thought Experiment

1. Sep 1, 2004

### Gfoxboy

I just recently saw a video in Physics class about Einstein's thought experiment that led to his discovery of Special Relativity. I was just curious how he knew at that time that light always moves at c even when the observer of the light is moving. I know that is the most important part of SR and I'm sure it's true, I was just curious if someone had discovered that previous to Einstein's thought experiment with the clock and the tram.

2. Sep 1, 2004

### Alkatran

I believe it was the Michelson-Morley experiment which was the first to notice that they couldn't measure light at any other speed than c.

The lorentz contractions were all discovered before Einstein, he only figured out WHY it was working like that. (We're skewed through time)

Of course, that's a very, very big "only"

3. Sep 1, 2004

this only proved that light does not propagate as a wave, and there is no magical 'ether' in the universe in which light travels through.

The lorents contractions were widely used by Pointclare (a french Mathematician) but they had no idea about the implications of physics. They never had the slightest thought.
What helped einstein prove his theory was simoutinaety, and the fact that two events that happen at the exact same time, can be seen as happening at different times by an observer in another fame of reference.

4. Sep 1, 2004

### Alkatran

But light can be seen as a wave in certain circumstances, especially when you're talking about interference.

The fact that light moves at c would be the disproving of the ether.

5. Sep 1, 2004

Light is onlt thought of as a wave in a probability sence, not in a mechanical sence. It is never thought of as an actual wave propagating though ppace.

6. Sep 1, 2004

### Staff: Mentor

Of course it is - interference is one example, but there are plenty of others where light does act like a wave: radio communications, microwave ovens, lasers, etc.

Whether light is a wave or a particle depends on whether you need it to be a wave or a particle for certain observations.

If you're talking specifically about the aether, the fact that there is no aether does not mean that light can't still be a wave.

7. Sep 1, 2004

### pervect

Staff Emeritus
AFAIK, Einstein had two main clues that when combined together led him to the conclusion that light must always travel at 'x'.

These were Maxwell's equations, and the Micehlson-Morley experiment.

Maxwell's equations gave a result of the speed of light of 1/sqrt(uo*E0). It would be possible, of course, for these constants to vary with velocity, to make the speed of light vary with velocity. But the Michelson-Morley experiment showed that the speed of light didn't vary with the changes in velocity of the earth orbiting the sun.

Einstein thought a lot about various alternatives, and decided that the simplest one which matched experimental results was that the speed of light was constant to all observers.

8. Sep 1, 2004

### Alkatran

Why are we arguing over what we agree on?

9. Sep 1, 2004

### Gfoxboy

Thanks guys.

10. Sep 1, 2004

### davidhart890

Hi, I just finished reading the special Einstein issue of Discover (of which Dr Michio Kaku article 'Einstein [in a nutshell]' led me (ultimately) to this forum....

I thought I understood and now I'm sure I don't.
If I sat on a particle of light leaving point x at time y and 'you' did exactly the same (in a parallel direction) would I not view 'you' as stationary? the reading suggests different - any other answer other than yes is going to be really hard to swallow.
If you can help me understand this, I would be most grateful

regards,
David Hart

11. Sep 1, 2004

### pervect

Staff Emeritus
Well, the very short answer is that you can't sit on a particle of light. The light, being massless, can move at 'c'. You, having mass, can't move that fast. Trying to say you can is like dividing by zero in mathematics - it yields nonsensical results. In fact, if you study the Lorentz transformations, you'll see that moving at 'c' does involve dividing by zero.

12. Sep 1, 2004

### JasonRox

He didn't say physically sit on it!

He just indirectly said visualize yourself as a particle of light. If you can visualize yourself sleeping with Britney Spears when that's impossible, why not visualize yourself as a photon hitting Britney Spears.

Note: I don't have a thing for Spears.

13. Sep 2, 2004

### pervect

Staff Emeritus
Well, if you visualize that 2+2=5, you can conclude, quite logically, that you are the King of England. (See the derivation by the famous mathemetician, Charles Dogson, aka Lewis Carroll).

The point here is that one has to be careful about making impossible assumptions - given one false statement, one can logically prove anything. Moving at 'c' as a material body is one of those impossible assumptions that one should avoid.

ps: It doesn't violate any laws of physics to sleep with Britney Spears. Why, if you won a celebrity date with her, then there was a power blackout while you were out on the date, and then you both got stuck in an elevator alone for 12 hours, you might both get tired and go to sleep. I wouldn't think you'd have much luck having sex with her, though.

14. Sep 8, 2004

### robert Ihnot

Last edited: Sep 8, 2004
15. Sep 8, 2004

### humanino

could not help

Because I am a very proud and arrogant french guy, I would like to point that Poincare is among the most important mathematicians of the beggining of the century, and was not so far behind Einstein, probably closer than Hilbert for instance :tongue2:

Poincare especially was the first to backup Einstein in France, and maybe also in Europe. Since physicists could not yet undestand Einstein's theory, and it looked to much like philosophy to them, horrible statements have been made. When Einstein was invited to the french Academie, a well-known scientist declared "Who needs the theory of this jewish guy who, thinking he is prosecuted, prides himself with an ununderstanble theory, invented by others anyway" :surprised

16. Sep 9, 2004

### ШЇSЭЯ

does light always travel at the same speed no matter where it comes from??
just curious, im new to this i need to read up, a lot
and where can i read that article by michio "Einstien [in a nutshell]"

17. Sep 9, 2004

### HallsofIvy

Yes, that's the whole point of special relativity. In "classical mechanics", if you were standing by the side of a road, I was in a truck traveling at 40 mph and throw you a ball at (relative to me) 30 mph, the ball would be traveling at 70 mph relative to you.

Experimental evidence shows that that is not true for light. If a I am moving toward you at 1/2 the speed of light (1/2 c) and shine a light at you (speed, relative to me c), you would see the light coming toward you at c, not 3/2 c.

18. Sep 29, 2006

### airlinemusic

Thats what I was looking for, eo and uo came from electrostatic, E, and
electromagnetic, H, Force equations.

Maxwell's equations dealing with dE/dt and dH/dt ended up with those
factors that each or both factored together can be equated to c.

In grad school Optics class, I recall that light comes from electron
energy level changes but when press on how it bounces off walls and
objects he gave something like its just re emitted or bounce without
little decay and did not go into too much detail.

After all it was the first chapter but I recall that more than what was
in the rest of the book.

19. Sep 29, 2006

### airlinemusic

After all that I wonder if Einstein ever settled for a final understanding
of his own ideas, he was always involved in Unified Field Theory that
brings us String Theory as a solution. Given the varoius ways to
solve equations it almost sounds like a Fourier series solution which
he could have applied long ago.

Ha, another French solution. Does the solution mean radiation is present
as strings or rings.

20. Sep 30, 2006

### bernhard.rothenstein

Einstein's thought experiment?

Please have a criical look at
arXiv.org > physics > physics/0510178

21. Sep 30, 2006

### rbj

please take a look at the post i just did at the Relativity and light thread in this forum. i think that it spells out, as a thought experiment, what Einstein was thinking about. it wasn't merely the negative result of the Michaelson-Morley experiment. it's because Einstein had the insight to insist that for two different observers, both inertial, unaccelerated - moving at a constant velocity but possibly moving relative to each other, both observers have equal claim to being "stationary" (and it's the other guy that's "moving") and that the laws of physics (namely Maxwell's Equations) both qualitatively and quantitatively apply exactly the same to both observers. that means both observers have the same permittivity of free space ($\epsilon_0$) and permeability of free space ($\mu_0$) and then, when they both solve Maxwell's equations, must get the same speed of light. if the speed of light was different for one compared to the other, then that person has, at least quantitatively, a different set of physical law than the other.

22. Sep 30, 2006

### bernhard.rothenstein

how could I see your relatiity and light?. i like solutions that result from the first postulate. I have seen an answer to the question based on the relative motion of a coil and a permanent magnet

23. Sep 30, 2006

### turbo

To the contrary, the fact that light seems to propagate at c through a vacuum can be interpreted as though the vacuum has a refractive index and a maximum speed at which light can propagate through it. Every material through which light can propagate has a "speed limit" of this type. If you will Google on "Klaus Scharnhorst", you will find numerous references to the Scharnhorst effect, in which the speed of light through a vacuum is expected to be higher if you can physically exclude some wavelengths of vacuum fluctuations, as in the gap between the conducting plates of a Casimir device. If he is right, the quantum vacuum field is the ether.

http://en.wikipedia.org/wiki/Scharnhorst_effect

24. Sep 30, 2006

### rbj

i'm not sure what it is that you're asking Bernhard?

25. Sep 30, 2006

### bernhard.rothenstein

light and relativity

My question is how could I see your "Light and relativity" post.