# What does a light particle see?

1. Jan 17, 2007

### Coldie

Here's something I've always wondered about but never got around to asking anyone. We know that when an object moves very close to the speed of light, outside events from the perspective of the object begin to occur faster and faster, correct? So for every foot that such an object travels, a certain amount of time will pass in the world around it, that amount of time increasing to infinity the closer the object gets to the speed of light, right? So, with this being said, doesn't that mean that an infinity would have to pass in order for a light particle to move any distance? Do we know what a light particle sees while it's moving?

2. Jan 17, 2007

### artikk

When an object moves very close to the speed of light, or in the case of the photon => at the speed of light, for it time does not exist at all because all of its velocity is concentrated in the spatial dimension at 3x10^8 m/s.

3. Jan 17, 2007

### ZapperZ

Staff Emeritus
No, this is incorrect.

If A is moving with respect to B, B will see A's clock being slower. However, A will ALSO see B's clock being slower. There is nothing special about A or B's reference frame. If one sees one's clock slower, while the other sees a clock faster, then there's an asymmetry with this scenario and you might have a preferred frame.

So your assumption that ".... from the perspective of the object begin to occur faster and faster..." is incorrect. That object will STILL see time dilation in other inertial frames, the same way other frames will see time dilation in that object's frame.

Zz.

4. Jan 17, 2007

### Staff: Mentor

Last edited: Jan 17, 2007
5. Jan 17, 2007

### bernhard.rothenstein

what does a light particfle see

In the title of the therad I see the term "see". In the statement of the problem that term is not present. Taking into account what physicists mean by "see" the development of the iodea seems interesting.

6. Jan 17, 2007

### pervect

Staff Emeritus

The very short version is that it is an anthropomorphic fallacy to think of photons as experiencing "time".

While it's wrong to talk of photons experiencing "time", wrong because it's self contradictory, one can make a good case for photons having a coordinate system in which they are at rest. This sort of coordinate system typically has two null coordinates and two space-like coordinates.

The coordinate along which a photon travels will be, by definition, a null coordinate.

A null coordinate is not timelike - nor is it spacelike. So we have a mathematical description of what it is that photons "experience" - that is a null coordinate. And we have a word. But we can't anthropormophize this concept, because photons aren't like us, and we aren't like photons, and it is wrong to ignore the difference.

7. Jan 17, 2007

### Coldie

8. Jan 17, 2007

### robphy

Thanks... but that wasn't me.

9. Jan 17, 2007

### HallsofIvy

My first reactions was "photons don't have eyes!" My second was "hey, even if a photon had eyes, it couldn't see- it IS light so can't SEE light!"

But I take it the key phrase is "So, with this being said, doesn't that mean that an infinity would have to pass in order for a light particle to move any distance?"

Well, no, a light particle would not move in its own frame of reference. But then nothing moves in its own frame of reference! Any object is, by definition, always at the origin in its own frame of reference.

Perhaps what you are looking for is this: time does not change at all in a frame of reference moving at speed c with respect to any other. Of course, that is only true of a photon. Rather than talking about what a photon sees, the point is that a photon is literally in "the eternal now"!

10. Jan 18, 2007

### Hootenanny

Staff Emeritus
Further to HallsofIvy's comment, I would just like to emphasise that photons do not have a frame of reference. A particle's frame of reference is defined as a reference frame where the particle is at rest; however, since c is invariant from any inertial reference frame, it is not possible to define a frame of reference where the photon is at rest, hence photons do not have a reference frame.