# If light appears frozen due to time dilation, how do its fields fluctuate?

1. Nov 25, 2012

### d3mm

I guess I have no problem with time dilation for stuff moving at less than c, but the step from less than to to c, is confusing me.

2. Nov 25, 2012

### ghwellsjr

Then don't take the step. It's impossible.

3. Nov 25, 2012

### Gingermolloy

Is it true that for a particle (like a photon) travelling at c, that Lorentz contraction makes its path length =Zero (i.e. it doesnt move)

And also that time dialation means that it does not age.

Therefore we end up with something that doesnt move and doesnt age??

Does it even exist???

?????!!!!! Help!!! Brain melting

4. Nov 25, 2012

### bcrowell

Staff Emeritus
FAQ: What does the world look like in a frame of reference moving at the speed of light?

This question has a long and honorable history. As a young student, Einstein tried to imagine what an electromagnetic wave would look like from the point of view of a motorcyclist riding alongside it. But we now know, thanks to Einstein himself, that it really doesn't make sense to talk about such observers.

The most straightforward argument is based on the positivist idea that concepts only mean something if you can define how to measure them operationally. If we accept this philosophical stance (which is by no means compatible with every concept we ever discuss in physics), then we need to be able to physically realize this frame in terms of an observer and measuring devices. But we can't. It would take an infinite amount of energy to accelerate Einstein and his motorcycle to the speed of light.

Since arguments from positivism can often kill off perfectly interesting and reasonable concepts, we might ask whether there are other reasons not to allow such frames. There are. One of the most basic geometrical ideas is intersection. In relativity, we expect that even if different observers disagree about many things, they agree about intersections of world-lines. Either the particles collided or they didn't. The arrow either hit the bull's-eye or it didn't. So although general relativity is far more permissive than Newtonian mechanics about changes of coordinates, there is a restriction that they should be smooth, one-to-one functions. If there was something like a Lorentz transformation for v=c, it wouldn't be one-to-one, so it wouldn't be mathematically compatible with the structure of relativity. (An easy way to see that it can't be one-to-one is that the length contraction would reduce a finite distance to a point.)

What if a system of interacting, massless particles was conscious, and could make observations? The argument given in the preceding paragraph proves that this isn't possible, but let's be more explicit. There are two possibilities. The velocity V of the system's center of mass either moves at c, or it doesn't. If V=c, then all the particles are moving along parallel lines, and therefore they aren't interacting, can't perform computations, and can't be conscious. (This is also consistent with the fact that the proper time s of a particle moving at c is constant, ds=0.) If V is less than c, then the observer's frame of reference isn't moving at c. Either way, we don't get an observer moving at c.

5. Nov 25, 2012

### Bill_K

Along the path of a light ray, the path length (proper time) is zero. But the elapsed coordinate time and distance in the reference frame of any observer is not zero. So to say the light ray doesn't move and doesn't age is not the case.

6. Nov 25, 2012

### d3mm

Thinking about this and the replies, especially the excellent one from bcrowell, leads me to the conclusion that I am thinking of time in intuitive but unrealistic way, hence am suffering from an apparent paradox. Or in other words, I should just accept that I don't have the sensory equipment to perceive the result properly.

Would that agree with the consensus?

7. Nov 25, 2012

### phinds

Yes. You will find that in cosmology, and even more so in quantum mechanics, the things being studied are simply not ones that are part of our everyday lives and thus our built up "intuition" just isn't useful. It would SEEM that time IS in our everyday experience, but when relativistic speeds come in, it is not.

8. Nov 25, 2012

### DrGreg

There is no time or distance relative to a photon. That doesn't mean time and distance are zero, it means they are undefined or meaningless. We have a related FAQ: Rest frame of a photon