# Light and its properties

1. Aug 31, 2006

### Put a quark in it

Greetings everyone! I am new to the Physics Forums, and I am really glad to have found this place. I just finished Brian Greene's book The Elegant Universe, and needless to say I am a little confused on some things (most things, actually). One of the most interesting topics discussed, I thought, was that of light and its properties.

According to the text, light only moves through the three extended spatial dimensions, while ignoring the fourth (time). So, in other words, light isn't affected by time. A photon emitted from a star a billion years ago is still the same "age" today as it was from its initial emission. Does that mean that if one were to see the world through the point of view of a photon, that everything would simply be completely frozen? I guess my question is, if light isn't affected by time, how is it that we are able to measure that in a year it has traveled a light year from us, instead of two, or ten, or an infinite number of light years? Since it is unaffected by time, why doesn't it reach infinity in what would seem to us as an instant?

2. Aug 31, 2006

### Danger

Welcome to PF, Quark. Let me start by saying that you have just about the coolest username I've seen. Wish I'd thought of it.
I think, if I'm interpreting your question correctly, that you're neglecting the difference between subjective and objective time. It's sort of the same as when someone wonders why a superfast particle doesn't turn into a black hole because of relativistic mass gain. At light speed, which of course is the speed of a photon, time passage is 0, length is 0, and mass is infinite. That, of course, doesn't translate to the rest of the universe because photons are obviously massless (rest mass) and moving forward in time. (The time thing eludes me a bit as well, but they couldn't move in space if they didn't also move in time. They just happen to move so fast that to them, no time passes from point 'A' to point 'B'.)
Okay, I'm over my head here. Ignore the last half of this until someone more knowledgeable can confirm or deny it.

3. Aug 31, 2006

### trinitron

4. Aug 31, 2006

### Put a quark in it

5. Aug 31, 2006

### chroot

Staff Emeritus
If you could "ride along on a photon," (and you cannot) you would see every point in space as if it existed in the same spot. You could travel any distance at all in an instant, so you are effectively in every place at once.

Note that the passage of time for an person riding a photon is very different from the passage of time of a person observing the photon fly past him.

- Warren

6. Aug 31, 2006

### Danger

And it's never pointless to ask a question. Science wouldn't exist if people didn't ask questions. It just so happens that some can't be readily answered.

7. Aug 31, 2006

### quasar987

Is it not only distances in the direction of motion that would shrink to a point? As a photon, you would be at rest in a 2 dimensional frozen universe. How boring! Perhaps this is the fate God reserves for the sinners! To spend the rest of eternity as a photon! :surprised

8. Aug 31, 2006

### Danger

It would be preferable to spending the rest of eternity dead, but I don't believe it any more than I do the heaven and hell thing.

9. Aug 31, 2006

### quasar987

Unfortunately, that we don't believe in the ultimate punishment does not make us immune to it! But it's pointless to worry about that.

I'm positive that I would prefer death over being trapped inside a photon for all eternity. But would I prefer this torture over the chance of being re-incarnated into a starving eternopian? That is what I fear about death. Not that it's over, but precisely the opposite: that I may be coming back to live an unpleasant existence!

10. Sep 1, 2006

### Gelsamel Epsilon

I'd rather be riding a photon that would be heaps awesome. Better then hell at least.

11. Sep 1, 2006

### LURCH

I think there is something of an answer in the link from Trinitron:
Time does not elapse for the photon, but it does for us. So in answer to your OP,
No, kinda the opposite. Photons experience nothing because (aside from being innanimate) they experience no time. So to them, we would appear to be infinitely fast, all evennts in the universe take zero amount of time to occur. It is from our frame of refference that they are completely frozen.
AND
Because light doesn't experience time, but we do. Since it's our measurement, it takes place according our clocks and rulers. What we measure as the distance to Poxima Centari would be about 24,000,000,000,000 miles, and a photon would take just over 4 years to get there. But to the photon, the distance was zero, and the trip took no time at all.

Last edited: Sep 1, 2006
12. Sep 1, 2006

### Mk

Speed is distance divided by time elapsed.