Light..

[foundaplanet]

I know this probably sounds a bit daft..but...

If light speed is immediate to the destination for the photon and to say that "for the photon there is no time"..... Does this mean that for the photon, the universe ends as soon as soon as it starts?...

Would this also imply that, the universe is born and dies at exactly the same time and that its entire existence is effectively immediate?

[acey]

If you were a photon and had consciousness then you would witness the beginning and end of the universe simultaneously (along with all other events in between). In effect, everything else would appear infinitely speeded up to you.

But photons don't have consciousness, or even any kind of clock that would indicate the passage of time for them (they don't wear out, for example). So the premise is strictly meaningless, and the suggested consequence does not follow.

[ollypenrice]

...but doesn't quantum theory cause us to reflect on our unerstanding of time?

Olly

[miguel87]

ummm, is it just me or... photons travel at the speed of light, not immediately, thats why you look back in time when viewing the moon, sun, stars etc.

[Zakalwe]

ummm, is it just me or... photons travel at the speed of light, not immediately, thats why you look back in time when viewing the moon, sun, stars etc.

As you approach the speed of light, then time appears to compress. At c, time is compressed to zero, so the photon arrives as soon as it leaves assuming the photon had an understanding of time (which it doesnt).

For observers not travelling at relativistic speeds, time travels at the "normal" rate.

[Glen]

ok first of all < thats me, second do photons have spin? Bearing in mind polarity/polarisation surly that's something to measure time over distance?

[grey]

my tidbit on this matter... a photon is simply a particle of energy having no mass and moving at the speed of light, time at c is 0 within that frame of reference. They are bundles of energy, that move at c, they move through space-time, and depending on the distance to the endpoint. Say a photon left the surface of the sun, it takes approx 8 minutes to reach the earth... how then do you relate? for the photon, time is 0 for the observer time is 8 or so minutes.. relativity my friends! You can't be the photon

cheers

[acey]

The point is that photons do not have a rest frame.

There are many ways you can use a photon to measure time; the question is whether you could measure time "as experienced" by a photon.

For example you have an apparatus in which a material object (cannonball, let's say) passes some point p1 at time t1 (measured by the laboratory clock). At this point a tiny stopwatch is shot onto the cannonball, sticks to it, and begins counting. The cannonball subsequently passes another point p2 at time t2, where a camera photographs it, thus enabling experimenters to read the time on the stopwatch. They find that if the cannonball is fast enough, and if the stopwatch is accurate enough, then the elapsed time on the stopwatch is a tiny bit less than the elapsed laboratory time.

They repeat the experiment, replacing the stopwatch with a radioactive sample of known halflife. The sample ought to decay before reaching p2 but instead lasts longer. Thinking about things from the cannonball's point of view (i.e. in its "rest frame"), they deduce that if the cannonball had consciousness, it would not feel that time was made any shorter when it flew through the air; instead it would think that the distance between p1 and p2 was less than the laboratory measurement. (In the laboratory rest frame, incidentally, the cannonball appears contracted in the direction of motion, but the cannonball itself doesn't feel squashed).

Someone extrapolates all this to say that if the cannonball were flying at the speed of light then the elapsed stopwatch time would be zero, and the cannonball would feel as if it had travelled no distance in no time: everything was already at the same place.

But you can't make a cannonball (or any kind of matter) travel at the speed of light. Nor can you stick a stopwatch on a photon (not even in some metaphorical sense). Or putting it another way, the photon has no rest frame. Which is why I said at the outset that the premise was meaningless. Certainly not a daft question, but a question that Einstein answered for us.

Quantum theory introduces spin, as has been mentioned (a photon has spin 1). But this is not to be thought of as the physical spinning of some material object: it corresponds to the classical notion of polarisation. You might have an apparatus where what happens at point p1 is that the photon passes through a polarising filter, and is therefore put in a known polarisation (i.e. spin) state. But when you measure the photon at point p2 you will have no way of knowing, from the photon itself, when that polarisation state was imposed. Quantum theory incorporates special relativity, so it does not alter the basic fact that for anything travelling at light speed (e.g. photons, gravitons) there is no rest frame.

All of this refers to physics as currently understood. Of course some new theory might come along to replace it all. But before giving us a new way of thinking about how it feels to be a photon, the new theory will have to account for a heck of a lot of experimental observations that are already sufficiently well explained by existing theory.

[Glen]

Thanks acey for the help, that clears a whole lot up. I get that time for a photon stops (from observer) due to c

and slows down for anything that could reach near it, or should I say, the faster something goes the slower time appears to pass..if observed.? I think. lol

Just trying to understand if It could be tied.

All a bit much for my brains, but I remember a while ago that they found a particle that enters the atmosphere and can be detected near ground level,

and it proved the cannon ball scenario you mention..for the life of me I can't remember much about it though.

As in it should of decayed further up.

Right then, rest frames it is, time to get on the books and have read up

Cheers

[acey]

I remember a while ago that they found a particle that enters the atmosphere and can be detected near ground level

It's the mu meson - a classic test of special relativity. It's the sort of thing I had in mind when I mentioned about half-life.

[Glen]

Ahh brilliant , thanks again! At least I'm on the right track

[Mr Spock]

Here's another one of my whacky theories. Light travels not through space, but through hyperspace. So what we see as the speed of light is the exit velocity from hyperspace when an object is encountered. This would explain why light particles have zero mass but yet are affected by gravity; appear both as a particle and a wave; always has a constant velocity despite its origins.