# A question on Special Relativity

1. Jan 25, 2012

### alexela

Hi,
I don't study Physics at school, so please excuse me if i sound crude or miseducated.

I was reading about Einsteins theory of Special Relativity, and was wondering if i understand it correctly. In the book it states that einstein proposed that the speed of motion of a body through space and time combined, adds up to the sum total of the speed of light?
So if something were to travel at half the speed of light through space, then it would age half as quickly than something stationary in space?

is this correct?

i would appreciate any help. thanks

2. Jan 25, 2012

### phinds

Yes, that is basically correct. You would probably find it interesting to Google "the twin paradox", which will explain it a bit more.

3. Jan 25, 2012

### clem

If "something were to travel at half the speed of light through space, then it would age half as quickly than something stationary in space" IS ALL WRONG!
Wait until you study relativity somewhere to try to understand it.

4. Jan 25, 2012

### alexela

Thanks,
This brings another question to my attention.
If a photon travels at the speed of light through space, then how can it travel through time?
Surely, in the instant that the photon reached the speed of light, time would stand still relative to it. However, for a stationary observer of the photon, time would continue as normal, and so in the next instant would the observer see the photon if it can no longer traverse time to the second instant, or is it trapped forever in the past?
I know this sounds stupid because our eyes work and we can see light, but somehow the question makes sense to me.

5. Jan 25, 2012

### phinds

Excellent point, and basically correct, although photons don't "reach the speed of light" since that would imply that they were NOT going at the speed of light just before they reached it and that doesn't happen. They travel at c as soon as them come into existance.

Your point above is the basis for what it is that there is no such thing as "the frame of reference of a photon". It just doesn't make sense.

I'm afraid I'm not following you on that one.

6. Jan 25, 2012

### ghwellsjr

That's not quite correct. If you represent the speed of motion of a body as a fraction of the speed of light (a number between 0 and 1) and you also represent the amount of time slowing down by a fraction (again a number between 0 and 1) then the relationship between these two numbers is not that their sum equals 1 but rather the squares of the two numbers adds up to 1.

So if you were going at .6 the speed of light, your time would be going at .8 of normal because .62 + .82 = .36 + .64 = 1.

If you tried this with both numbers at .5 you would get .52 + .52 = .25 + .25 = .5, which does not equal 1.

7. Jan 25, 2012

### Snip3r

thats a brilliant question (atleast for me) i had a similar doubt in this thread

however i dint understand ghwellsjr's sol given over there. bobc2's explanation looked ok

8. Jan 25, 2012

### alexela

Thank you very much, thats the kind of answer i was looking for, something i can learn a bit from.

9. Jan 25, 2012

### alexela

yes i have probably got confused, i guess what i mean in a rough and exagerated example is that, if you were to conduct an experiment in which you had a camera to take snap shots of a photons journey through space, and it was set to take continuous snapshots, the photon would appear in the snap shot which was taken at the same time the photon came into existence. But then, for the photon time would stand still, if the photon were to look over at the camera it would have ceased to take any more snap shots, but as an observer behind the camera it will continue clicking away. Although the photon can move freely around space at c without any time elapsing from its moment of creation, for the camera time would continue, and my question is would the snap shots taken after the photon was created be blank because they were taken at a time that the photon hasn't progressed to?

10. Jan 25, 2012

### ghwellsjr

How does a camera take a snapshot? Have you ever thought about it? Does a camera take pictures of light in transit? Or does it take pictures of other things in transit that are illuminated by light that reflects off of those things? If you had a high speed bullet, light would reflect off of it and come back into the camera and you take a series of snapshots to show its progress. But how would you capture the progress of a flash of light? (Let's make it brighter than an individual photon.)

11. Jan 25, 2012

### phinds

If I understand it correctly, this kind of thinking is a fundamental flaw in such thought experiments. Simply put, there is no frame of reference for a photon and as soon as you assume one you leave the real of the real world, after which point, you can come to any conclusion you like, because it follows from a fallacy.