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RanDonimus
2010-Aug-07, 04:16 PM
Greetings.
I thought this might be something related to the topic at hand.
Let's say light always travels at its constant speed c. Hypothetically speaking, if i switch on my flashlight and point it to the night sky, assuming there are no abstacles or interferences to the path of the photons far far in the distances of the universe, like a hypothetical complete vaccum space (like a vaccum tube) through which the photons travel. Will the photons travel to infinity and beyond without stopping or losing energy on the way? Does the energy run out? Or is it infinite?

pzkpfw
2010-Aug-07, 10:51 PM
Welcome, RanDonimus. At BAUT we have a mostly one-question-one-thread style, so I've moved your question to it's own thread. Report this post (see the "/!\" at lower left) if you'd like the title of the thread changed.

mugaliens
2010-Aug-08, 05:26 AM
Will the photons travel to infinity and beyond without stopping or losing energy on the way? Does the energy run out? Or is it infinite?

To infinity? Nope.

And beyond? Doubly nope.

As for energy running out, theoretically, once a photon has been sent on its merry way, until/unless it interacts with anything, it'll continue on, unabated.

grant hutchison
2010-Aug-08, 05:24 PM
This is one of those questions that gets more complicated the more you think about it.
Key points:
1) A photon doesn't use energy to keep travelling. It has a fixed quantity of energy, and if nothing removes that energy from it, it will (potentially) keep going forever in exactly the same state.
2) If it loses energy, it changes wavelength, but keeps travelling at the same speed. So if it dips in and out of some distant gravity well, it will gain energy on the way in, and then lose the same amount of energy on the way out, so that it ends up in the same state as previously.

The expansion of the Universe complicates matters, since it means that photons which travel for a significant proportion of the life of the Universe are actually not received in the same state as they were emitted.

Grant Hutchison

JohnD
2010-Aug-08, 08:29 PM
I thought I had read that a photon can decay, but that it is unliklley to do so within the lifetime of the Universe.
Several discussions assert that this is so, but are usually contradicted as well.

Try Physics Forum: http://www.physicsforums.com/archive/index.php/t-165262.html

John

grant hutchison
2010-Aug-08, 09:47 PM
I thought I had read that a photon can decay, but that it is unliklley to do so within the lifetime of the Universe.I'd guess that, like one of the contributors on the thread you linked to, you actually read that protons might decay. That's a possibility that has been seriously investigated.

Grant Hutchison

grapes
2010-Aug-08, 09:53 PM
I thought I had read that a photon can decay, but that it is unliklley to do so within the lifetime of the Universe.
Several discussions assert that this is so, but are usually contradicted as well.

Try Physics Forum: http://www.physicsforums.com/archive/index.php/t-165262.html

JohnIf you read through that discussion closely, I think they are talking about protons, not photons. At least, as far as the length of the lifetime is concerned.

Nereid
2010-Aug-08, 09:54 PM
Greetings.
I thought this might be something related to the topic at hand.
Let's say light always travels at its constant speed c. Hypothetically speaking, if i switch on my flashlight and point it to the night sky, assuming there are no abstacles or interferences to the path of the photons far far in the distances of the universe, like a hypothetical complete vaccum space (like a vaccum tube) through which the photons travel. Will the photons travel to infinity and beyond without stopping or losing energy on the way? Does the energy run out? Or is it infinite?
Will the photons travel to infinity and beyond without stopping or losing energy on the way?

As has already been pointed out, in the rest frame of a distant, co-moving observer, photons will seem to have 'lost' energy - i.e. they will be redshifted.

Eventually, all photons will be absorbed by some cosmic plasma or other, once their (local) frequency gets close to the (local) plasma frequency. In large (~10 Mpc+) chunks of space, this will happen first in rich clusters of galaxies, then in the cosmic web, and finally in the voids.

caveman1917
2010-Aug-08, 09:58 PM
1) A photon doesn't use energy to keep travelling. It has a fixed quantity of energy, and if nothing removes that energy from it, it will (potentially) keep going forever in exactly the same state

Minor nitpick, a photon [ETA: by itself] has an undefined quantity of energy. Two observers will generally disagree about the energy of the same photon due to the doppler effect of their relative motion. It is the fact that a photon doesn't have a valid frame of reference that the energy can't be defined relative to the photon itself, only relative to arbitrary observers who will all disagree.

RanDonimus
2010-Aug-08, 10:11 PM
Eventually, all photons will be absorbed by some cosmic plasma or other, once their (local) frequency gets close to the (local) plasma frequency. In large (~10 Mpc+) chunks of space, this will happen first in rich clusters of galaxies, then in the cosmic web, and finally in the voids.

Am more interested in the Photon's potential to continuously travel without stop. This is why I imagined a vaccum tube which would go on far far into the ends of the universe. A scenario where the photons would meet no interferences from any forms of matter. Would the photon carry on moving forever through this tube?

Nereid
2010-Aug-08, 10:29 PM
Am more interested in the Photon's potential to continuously travel without stop. This is why I imagined a vaccum tube which would go on far far into the ends of the universe. A scenario where the photons would meet no interferences from any forms of matter. Would the photon carry on moving forever through this tube?
No.

Why? Because a vacuum tube is full of matter! Albeit matter at a relatively low density, but it does contain matter.

In fact, most of interstellar space - and, indeed, much of interplanetary space - is a harder vacuum than that found in most vacuum tubes. Further, much (most?) of the interstellar medium (ISM) and all of the intergalactic medium (IGM) is a harder vacuum than any achieved, to date, in any lab here on Earth.

caveman1917
2010-Aug-08, 10:34 PM
Am more interested in the Photon's potential to continuously travel without stop. This is why I imagined a vaccum tube which would go on far far into the ends of the universe. A scenario where the photons would meet no interferences from any forms of matter. Would the photon carry on moving forever through this tube?

A photon doesn't stop by itself. So if it encounters no energy whatsoever - yes it will keep on going.
But as Nereid pointed out, that is completely theoretical, no hard vacuum exists - or can exist even.

RanDonimus
2010-Aug-08, 10:56 PM
No.

Why? Because a vacuum tube is full of matter! Albeit matter at a relatively low density, but it does contain matter.

In fact, most of interstellar space - and, indeed, much of interplanetary space - is a harder vacuum than that found in most vacuum tubes. Further, much (most?) of the interstellar medium (ISM) and all of the intergalactic medium (IGM) is a harder vacuum than any achieved, to date, in any lab here on Earth.

Yes good fellow, you are completely right. But as I've said before. My interest lies in the Photon's potential to carry on moving, possibly forever. Provided ofcourse, however theoretical, that these photons travel through a hypothetical matterless path. The key word here is Potential.

grant hutchison
2010-Aug-08, 11:04 PM
Minor nitpick, a photon [ETA: by itself] has an undefined quantity of energy.Fair enough. I should have stipulated my one observer (the guy with the flashlight in the OP).

Grant Hutchison

Jens
2010-Aug-09, 05:17 AM
Yes good fellow, you are completely right. But as I've said before. My interest lies in the Photon's potential to carry on moving, possibly forever. Provided ofcourse, however theoretical, that these photons travel through a hypothetical matterless path. The key word here is Potential.

Yes, I think that in a hypothetical total vacuum, a photon (and indeed anything else in the universe) would continue to travel forever, since there would be no way for it to lose energy.

clop
2010-Aug-09, 07:17 AM
I suppose that from the photon's point of view it gets to infinity in zero time anyway.

clop

mugaliens
2010-Aug-09, 07:58 AM
I suppose that from the photon's point of view it gets to infinity in zero time anyway.

clop

Exactly. And considering the fact we can see to the edges of the observable universe quite well (HDSF), I'd say the idea of all photons petering out along the way is non-sequitous with observation.

Ken G
2010-Aug-09, 03:47 PM
My interest lies in the Photon's potential to carry on moving, possibly forever. Provided ofcourse, however theoretical, that these photons travel through a hypothetical matterless path. The key word here is Potential.Questions like this cannot be answered experimentally, which means we can never actually know the answer (scientific answers are experimental). However, we can say what any given theory would say is the answer. That tells you something about the theory, not something about reality. But our best theory of photons posits that the photon does indeed have the potential to go on forever. No doubt, the theory posits that because we see no reason to design a theory that posits anything else, as our experiments only show photon losses for one of the reasons that you are asking past.

Nereid
2010-Aug-09, 04:05 PM
Yes good fellow, you are completely right. But as I've said before. My interest lies in the Photon's potential to carry on moving, possibly forever. Provided ofcourse, however theoretical, that these photons travel through a hypothetical matterless path. The key word here is Potential.
No one knows.

There may be interactions, of very low cross-section, with whatever dark energy is, that stops photons dead, after 10^10 Mpc.

There may be photon-virtual photon (or particle) interactions predicted by QED that give a mean free path of photons of only 10^100^100 Mpc in a hypothetical vacuum (I say "only" because of "possibly forever").

Maybe there is a spacetime-photon interaction that becomes important, in an as-yet-undiscovered extension of GR, that limits photons to co-moving distances of 999^999^999^999 Gpc.

Maybe ...