# Photons in GR

1. Nov 1, 2006

### Bos

Hey,
I understand GR pretty well and I realize how and why light is constant and appears constant from any frame of reference. My question is that even though nothing can reach light speed except a photon (or massless particle), how would other photons appear to them? In other words, from a photon's percpective, the other photons couldn't appear to be "evading them at light speed" as they do for any other particle because wouldn't this mean that they're in fact moving faster than light? I'll restate the question once more just for clarity. Since two objects traveling side by side at a constant velocity perceive each other to be static, why isn't this the same for the actual photons themselves? And if it is, how can this be possible becuase then the photon is seeing light at a velocity of zero, not c. Hope this makes sense.

thanx

2. Nov 1, 2006

3. Nov 1, 2006

### pmb_phy

That is only true in SR. It is not a valid assumption in GR. In GR the cordinate speed of light varies with the gravitational field.

Best wishes

Pete

4. Nov 1, 2006

### MeJennifer

It seems to me that an accelerating observer or an inertial observer in a gravitational field will not experience the speed of light as constant except for in their immediate vicinity.

5. Nov 1, 2006

### Bos

Ok, but even in SR, would a photon perceive another photon to be static or evading it at speed c?

6. Nov 1, 2006

### quasar987

Even in special relativity and considering only inertial frames, there is no why to the constant of the speed of light. To be best of my knowledge, it is just an experimental fact that we agreed to take as a postulate.

(which reminds me of a thread I wanted to start; check ou the general physics forum)

7. Nov 1, 2006

### Bos

I'm not questioning the constancy of the speed of light. I take that for what it is. I'm questioning whether or not a photon would see another photon traveling beside it to be static or moving at speed c?

8. Nov 1, 2006

### robphy

Ok then.
Start by drawing two photon worldlines, say A and B.
Then try to draw signalling photons from events on A to B.

Now, toward defining a reference frame using radar methods (so that a time standard and a related distance standard can be defined), try to draw signalling photons from B to A.

9. Nov 1, 2006

### Bos

I see where you're going with this but don't quite get it literally. I get that you need photons in order to see and they have to travel to your eye but I feel like this is missing the point. If you just think simply about it, if you and I were photons, what would we see. Either it would appear that neither of us were moving, or that we'd each see each other moving away at c, since we're both photons. I know nothing with mass can travel at light speed but that's beside the point. Photons can't "see" like we can but they're certainly a frame of reference. So why can't we just put ourselves in that frame?

10. Nov 1, 2006

### robphy

By "see" all I mean is that a photon from a distant event meets the worldline of the observer.

What is your definition of a "frame of reference"?
Be precise, using concepts [spacetime-geometric, measurement-theoretic, etc...] from your relativity course. For an idea of what kind of definition I am looking for, see my post in the first link I posted.

Then let's see if it can be applied to a photon.

11. Nov 2, 2006

### jnorman

photons do not travel as packets of energy - they only exist as probability waves prior to absorption. they are always, however, detected as particles - but this does not mean they travel as particles: they do not. since at the speed of light, there is no distance between one place in the universe and any other place, the photon, once it is emitted, is essentially everywhere in the universe simultaneously. hence the concept of a photon traveling by all possible paths to get wherever it is finally absorbed. hence, your question has no meaning.

12. Nov 4, 2006

### Bos

13. Nov 6, 2006

### jnorman

bos - i have not made a mistake. my answer is quite correct. you are simply overlaying a sub-C perspective. i will use a simpler analogy for you - consider the apparent distance between the earth and a star which we measure from earth to be 10 LY's away. we then get aboard our spaceship which can travel at 0.999999C and go to the star. we find that, lo and behold, it only takes us about 2 years to get there - what is this?? have we actually travelled faster than the speed of light and travelled 10 LY in only 2 years? or is the star "really" only a bit less than 2LYs away from earth?

perhaps it is you who should do a bit more thinking before responding...

14. Nov 6, 2006

### Bos

OK. Sorry for being defensive earlier. I have had people be really rude in the past and I can't stand it. I shouldn't have taken it out on you. Anyway, as far as your scenario goes, now I'm just confused. First off, what is a sub-C perspective (if it's complicated then don't bother). Second, why is the star only a bit less than 2LYs away if it was measured to be 10? What am I missing? Again I apologize, please clear this up for me.

15. Nov 7, 2006

### jnorman

bos - in preface to my response, i must admit to some guile in my postings on this forum. i am one of those people who often profess ignorance by asking seemingly simple questions, just to illustrate (again) just how little we actually understand about the nature of reality. for example, i might innocently inquire "how does a magnet work?", or "what is a field?" and then sit back and watch as several members duke it out, well knowing that no one knows how a magnet works, or what a field is...

so, in that vein, i have posted two previous queries about the scenario above regarding the "actual distance" between earth and that star, which you may enjoy reading:

and