# A thought experiment with theory of relativity.

1. Oct 17, 2011

### Supreethsj

Hi,
I had a question on theory of relativity. It is more of a thought experiment.

As per the theory of relativity, as the speed of an object increases time slows down. If any particle travels faster than the speed of light, theoretically it has to travel back in time. So from this we can deduce that when a particle is exactly at the speed of light t=0.

Thought experiment- let's say a fly sits on a photon that is emitted by the sun. We know that from out standpoint of time, the photon takes approximately 8min 20 sec to reach earth. But let's say that the fly has a stopwatch and it starts the watch when it leaves the sun. As the photon and the fly are traveling at the speed of light, t=0, hence the fly clocks 0 seconds confirms that the photon traveled instantly from sun to earth. Concurring from this, even if the photon travels across the universe, the result i.e t=0 stands same.

Also according to Einstin's theory, time and space are considered to be a single diminution called 'Time Space'. So as the speed of a particle tends towards the the speed of light, it is not just that time slows down but also space has to contract, eventually at exactly the speed of light space also become 0 (null). This brings us to Nothing.

I am a novice. I would love your comments on the same. Thank you.

2. Oct 17, 2011

### Pengwuino

The contradiction you've thought up is the reason why particles can't go the speed of light and why it makes no sense to talk about what "time" a photon sees.

Time is meaningless to objects that travel at the speed of light.

3. Oct 17, 2011

### Calimero

Well, never seen fly sitting on a photon, and there are good reasons for that. First, flies like rotten meat, not photons. Second, they have mass, which means that there exists no frame of reference in which they can move at the speed of light.
However, you can ask how does world looks like from the rest frame of the photon. And answer is, in relativity photons don't have inertial rest frame. It would violate very important postulate that speed of light is the same in any rest frame.

4. Oct 17, 2011

### ghwellsjr

Your question is asked frequently. That is why it is covered in the FAQ section. There you will find that instead of your word "Nothing", the word "meaningless" is applied (as Pengwuino) pointed out. You can find a lot more threads on this subject by searching this forum for "meaningless". Lots and lots. And now there is one more.