# Thought Experiment - Measuring speed of light

1. Sep 19, 2007

### Xori

Here's my thinking: The faster you go relative to the universe, the more the universe appears to contract. However, c remains constant as measured by distance/time.

Now, what about to a photon? To a photon, the entire universe would appear as a single point, correct? To this same photon, c would remain constant as observed by the photon. But how can the photon know this? It's entire distance is infinitely small, and therefore it can't perform the distance/time measurement.

Is my logic wrong?

2. Sep 19, 2007

### JesseM

A photon does not have its own reference frame in relativity--the whole concept of each observer having their own frame is based on the idea of their having a network of measuring-rods and sychronized clocks moving along with them, but this doesn't make sense in the case of a photon. See this thread for an older discussion about this.

3. Sep 19, 2007

### Math Jeans

Perhaps it would see a point, but even if a photon had a point of view, it would be impossible to know what it is like. If the theory of relativity is correct, then a single moment from the reference frame of a photon is equal to an infinite amount of time in the normal reference frame.

4. Sep 19, 2007

### JesseM

What do you mean "if the theory of relativity is correct"? The Lorentz transformation gives nonsensical answers if you try to plug in v=c, so the phrase "the reference frame of a photon" simply has no well-defined meaning according to relativity.

5. Sep 19, 2007

### genneth

Replace v=c with v->c, and generally replace instances of "to be" with "tends to". It's sloppy, but occasionally it's worth the shorthand. However, in this case, I agree with you -- it's important to realise that there is no frame in SR that corresponds to the rest frame of a photon.

6. Sep 19, 2007

### JesseM

Even talking about limits, "then a single moment from the reference frame of a photon is equal to an infinite amount of time in the normal reference frame" isn't really correct. In the limit as a ship's velocity approaches c, we will see the ticks of its clocks stretched out arbitrarily long in our frame, but it will also see the ticks of our clocks stretched out arbitrarily long in its own frame. However, it will also see clocks that are synchronized in our frame and which lie along the direction of motion as being very out-of-sync and close together--if two clocks at opposite ends of the galaxy are synchronized and 20,000 light years apart in our frame, then in the frame of someone moving at very close to the speed of light along the line between them, both clocks would be ticking very slowly, but the clock they're moving towards will show a date close to 20,000 years ahead of the clock they're moving away from, and the distance between the clocks would be very small so it would take a very small time to pass from the first clock to the second (and in the limit as v approaches c, 'very small' approaches zero, while these clocks would approach being completely frozen at their respective dates).

7. Sep 21, 2007

### huyen_vyvy

in the photon's ref frame, it takes zero time to travel across any distance. so things still make sense?

8. Sep 21, 2007

### Staff: Mentor

In a photon's reference frame (to the extent that we can speak of such a thing), all distances along the direction of motion are zero. If you're going to try to apply time dilation to this situation, you'd better also apply length contraction, too.

9. Sep 21, 2007

### Mentz114

What ?? This does not make sense unless you have a universal frame of reference.
I'm not even going to read the rest. Go study SR.

10. Sep 21, 2007

### Xori

I wans't implying a universal frame of reference. I meant as you go faster relative to everything else, it contracts. If you go the speed of light (as a photon does), everything else in the universe appears to have a length of 0.

So how can a photon possibly measure the speed of light if everything is infinitely close?

11. Sep 21, 2007

### Mentz114

Hi Xori,
I see what you are trying to say. Your question has been well answered above.
Anyhow, I don't think photons exist as free entities. They exist when they are created and when they are absorbed. In between they are quanta of a field that obeys Maxwells equations, and beyond that we can't say much about them.

12. Sep 21, 2007

### tabchouri

As already said, the only dimention that is contracted is the direction of the mouvement.
If an object closes to the speed of light (or if we assume that the light has a frame of reference), the hole universe would seem to be just plane perpendicular to the mouvement, not a point.
That's my 2 cents :)

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