# Photons and Relativity

Consider a Photon from Sun and travels with a velocity $c$. Now think we are that photon. For us, it looks like Sun is moving away from us with a velocity $c$. So, why don't we get attracted back towards Sun, because the mass of Sun would be infinite for us since it moves away from us with a velocity $c$.

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Related to your second q: physics.stackexchange.com/questions/27794/… . I can't seem to find a post explaining why photons do not experience passage of time, though. – Manishearth Dec 29 '12 at 8:49
Hi Inquisitive. Could you please revise your second question. I don't force you. But, Manish has already given you a link. If you think its helpful, you could erase your second question. Or, you could clarify it to ask something specifically. But - to me, it looks the same. It is a good question though. But, There are many questions related to yours. I'll try to provide some useful links. Another piece of advice: A revision may sometimes enable the undownvoting for other users :-) – Waffle's Crazy Peanut Dec 29 '12 at 9:11
@CrazyBuddy Done – Inquisitive Dec 29 '12 at 9:14
@Inquisitive probably the biggest issue with this question is that it shows a significant lack of understanding of even the basic concepts of special relativity. You're asking what we'd see if we pretend to be that photon, but we can't do that; a photon does not have a reference frame. So it's a meaningless question. – David Z Dec 29 '12 at 9:59
@DavidZaslavsky: Now, it reminds me of this question: physics.stackexchange.com/q/16018 – Waffle's Crazy Peanut Dec 29 '12 at 10:00

You have completely mixed the modern and classical concepts of relativity. If you're talking about mass increment, you shouldn't calculate speed of Sun based on absolute time & space notion.

For you as a photon, space will be contracted to zero and time will be dilated to infinity. So, you can't calculate a speed (which is a time-like spacetime event) of Sun.

While its a nice satisfactory explanation, its not the real one.
Relativistic physics doesn't allow you to take position of a photon. In other words, relativistic physics doesn't allow photons to be an observer. Its because a photon can see itself stationary which breaks the framework of relativistic physics. Relativistic physics doesn't allow photons to be at rest in any reference frame.

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How it is possible for a photon to be in motion w.r.t to itself? – Inquisitive Dec 29 '12 at 9:57
There is no such thing as "with respect to a photon." – David Z Dec 29 '12 at 9:59
@Inquisitive Its not possible. That's why Relativistic Physics doesn't allow photons to be an observer. – Evil Angel Dec 29 '12 at 10:00

When anything moves at the speed of light, all of our physical models break down. If you were to watch a spaceship speed up to the speed of light, you would see a clock on the ship slow down and come to a complete stop when it hit the speed of light (assuming you could even see it at this point). The ship would also contract so much in the direction of motion that it would become infinitesimally small and, when it hit c, it's length would hit zero. As a result, matter cannot ever reach the speed of light (as far as we know).

It is a common misconception that mass changes when you travel at relativistic speeds, but this is not actually the case. The kinetic energy of an object increases relativistically, but the physical mass of the object (and, therefore it's gravitational effect on surrounding spacetime) does not actually change.

By our current notion of relativity, it is completely possible that there exists another universe within our own, in which photons are at rest and we are moving at 3e8 m/s and have energy but no other way of interacting with the type of "matter" that exists here. Our world is inaccessible to this one and vice-versa. There could be completely different laws of physics in this parallel universe, we have no idea because it exists (or doesn't) in a singularity.

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I'm not sure that the last paragraph is (a) correct or (b) at all relevant. – Kyle Kanos Jul 7 '14 at 19:25
It's not completely relevant, and it's very possibly untrue. The point was just to point out that no object that moves at the speed can interact with any matter (aside from transporting energy from one place to another), as the speed of light is a singularity. It is the equivalent of saying that there are wormholes at the center of black holes; we don't know if it's true and we have no way of testing it, but it's possible according to our current theories. – Hausdorf Jul 7 '14 at 22:10