# Riding on a beam of light?

1. May 17, 2004

### chasingwind

It is often stated in the biographies that Einstein thought about the question what he would see if he rode on one beam of light right after another beam of light. After reading special relativity, however, I still have vague understanding of this question. Would anyone give me some hints? Many thanks.

2. May 17, 2004

### chroot

Staff Emeritus
It isn't physically possible, but if it were, the universe would appear to be a geometric point. In other words, it would take zero time (measured by your watch) to be anywhere in the universe. Time would stop, so to speak, and distance would no longer have meaning.

- Warren

3. May 17, 2004

### quartodeciman

As far as I can tell, young Einstein did not have an answer to his puzzle at the time. It was only later, after Einstein had absorbed the publications of Lorentz with transformation equations for the Maxwell theory, that Einstein began to realize that only observers with relative motions less than c could be validly considered. Between the Einstein puzzle (1895) and Einstein's Special Relativity (1905) came Einstein's learning electrodynamics (FĂ¶ppl's textbook on Maxwell theory and available research papers and summaries by Lorentz, PoincarĂ©, etc.).

4. May 17, 2004

### jcsd

All I remmebr about that thought experiment is that he concluded from it that light doesn't have a valid inertial reference frame.

5. May 17, 2004

### chasingwind

Chroot, with what tool did you derive this conclusion? Could you please show how?

6. May 18, 2004

### chroot

Staff Emeritus
chasingwind,

Lorentz length contraction is all you need.

- Warren

7. May 19, 2004

### hellfire

As you said, it is physically incorrect to have a reference frame with speed c. Anyway, assuming a speed very close to c, I think your description is incorrect. Distance would have a meaning since there is Lorentz contraction only in the direction of motion. The universe would be rather a bidimensional plane than a point.

Regards.

8. May 19, 2004

Staff Emeritus
Close to c is a completely different question. Now you are an inertial observer, have a rest frame in which you measure the speed of light as c, and have a relative speed with respect to all other inertial observers. The universe, however, is not an inertial observer.

Because of the effect of Lorentz transformation on angles, you will see the external stars bent around so that even the ones "behind you" from another observer's perspective will appear squeezed into a disk in front of you.

9. May 21, 2004

### Chronos

agree with chroot. from a photons point of view it is emitted and absorbed instantaneously. time and distance do not exist for the hapless photon. when you look at a distant galaxy, the poor little guys have no idea they are millions of years old and a long way from mom when your retina ends their journey.

traveling at the speed of light, were it possible, would be over before it started as you slammed into a star, asteroid or whatever was the first [and last] thing in your path. traveling very near the speed of light would be even more unpleasant. any luminous body in your path would be so severely blue shifted you would soon be, though not instantly incinerated.

Last edited: May 21, 2004
10. Jun 5, 2004

### jdavel

chasingwind,

Einstein was trying to imagine what an electromagnetic wave would look like if he could travel along with it. What he concluded was that he'd see static E and B fields that would violate the laws (Maxwell's equations) that govern these fields. Specifically, they would have non-zero curl, which is impossible for static fields in empty space. And since E and B fields exert forces on charged particles, this would mean that Newton's laws of motion would be different depending on whether you were stationary with respect to the beam or moving along with it. So either:

a) Newton's laws have to depend on your speed,

or

b) the speed of light can't depend on your speed.

Einstein chose b, which turned out to be right.