1. Lets say I have an area that is a total vacuum except for two parallel optical mirrors. I fire a beam of light perpendicular to the mirrors. Will that beam of light ever lose its energy? If I come back in 1000 years, will it still be going? If the energy does run out, will it leave photons behind? Can photons be isolated from the beam of light it is a part of?

2.

3. Originally Posted by Raggedjoe
Lets say I have an area that is a total vacuum except for two parallel optical mirrors. I fire a beam of light perpendicular to the mirrors. Will that beam of light ever lose its energy? If I come back in 1000 years, will it still be going? If the energy does run out, will it leave photons behind? Can photons be isolated from the beam of light it is a part of?
I assume that the mirrors are perfectly reflective for the purpose of this discussion.

A beam of light is nothing but a bunch of photons. The energy of each photon is given by the equation whre is energy is Planck's constant and is frequency. The mirrors do not change and therefore do not change the energy of the photons.

The energy of the beam is the sum of the energies of the photons in the beam, so if the mirror is perfectly reflective, all the photons are reflected and the energy of the beam is unchanged by the act of reflection. In this idealized case the beam can continue to bounce back and forth between the mirrors forever.

A real mirror is not 100% reflective and some of the photons go right through. So if you had a real mirror eventually all of the photons would escape.

The energy only "runs out" if all of the photons "run out".

The beam of light is just the ensemble of photons, so I don't quite understand what you mean by isolating the photons from the beam. You can use a beam splitter or partially reflective mirror to separate some photons from the main beam. There are detectors that can respond to a single photon.

4. I think that all light dissipates. No matter how focused of a beam you initially fired it would eventual weaken in strength and grow in radius.

5. Doesn't matter that the photons are pouring through each other? They dodge?

6. Originally Posted by Pong
Doesn't matter that the photons are pouring through each other? They dodge?
Photons are really really small and move really really fast. Imagine trying to hit a bullet in mid air with a second bullet. Now make the bullets move at the speed of light and make them (I think) smaller then an atom.

7. Originally Posted by Raggedjoe
Originally Posted by Pong
Doesn't matter that the photons are pouring through each other? They dodge?
Photons are really really small and move really really fast. Imagine trying to hit a bullet in mid air with a second bullet. Now make the bullets move at the speed of light and make them (I think) smaller then an atom.
...Now allow them, as you imply, infinite time to collide. See, forever is infinitely greater than "really really".

I guess the question (to DrRocket) is does a photon ever interact with another photon.

8. Originally Posted by Pong
Originally Posted by Raggedjoe
Originally Posted by Pong
Doesn't matter that the photons are pouring through each other? They dodge?
Photons are really really small and move really really fast. Imagine trying to hit a bullet in mid air with a second bullet. Now make the bullets move at the speed of light and make them (I think) smaller then an atom.
...Now allow them, as you imply, infinite time to collide. See, forever is infinitely greater than "really really".

I guess the question (to DrRocket) is does a photon ever interact with another photon.
Intuition says no, simply because light beams don't explode when stuck between two mirrors. Ironically, in an ideal set up it would be guaranteed to happen. If the two mirrors are represented by parallel lines, and the beam of light is a line perpendicular to both of them, then the photons would go through the exact same space every time. Lucky for us, we can't make mirrors and laser beams that precise.

9. Originally Posted by Raggedjoe
Originally Posted by Pong
Originally Posted by Raggedjoe
Originally Posted by Pong
Doesn't matter that the photons are pouring through each other? They dodge?
Photons are really really small and move really really fast. Imagine trying to hit a bullet in mid air with a second bullet. Now make the bullets move at the speed of light and make them (I think) smaller then an atom.
...Now allow them, as you imply, infinite time to collide. See, forever is infinitely greater than "really really".

I guess the question (to DrRocket) is does a photon ever interact with another photon.
Intuition says no, simply because light beams don't explode when stuck between two mirrors. Ironically, in an ideal set up it would be guaranteed to happen. If the two mirrors are represented by parallel lines, and the beam of light is a line perpendicular to both of them, then the photons would go through the exact same space every time. Lucky for us, we can't make mirrors and laser beams that precise.
Supposing the light is monochrome and in phase (i.e. a laser), could you get a standing wave between the mirrors, if the distance equals ?

To answer the original question: If the beam is sufficiently focussed (within an angle of (width mirror)/(1000 light years)), and the mirrors are a couple of light years apart, the light would still be between the mirrors after 1000 years.
If they are closer together (e.g. a few km), and the mirrors aren't perfect (e.g. 99.999 % reflection; 99.97% is supposedly the best commercially available mirror), the light would be absorbed/diffused/pass through the mirror pretty quickly (in the e.g. 5% would remain after 1 s, 1e-76% after a minute)

Now lets suppose the mirrors are indeed perfect. The continuous bombardment of photons would push them backwards (since the mirrors are in total vacuum, they can't be attached to anything), in case of a not perfectly focussed beam, the mirrors would also start tilting, and only a slight tilt would have the same effect as an imperfect mirror. Lets also suppose the focus is perfect and perfectly in the absolute centre of the mirror. The mirror would be pushed straight back. I think this would cause a Doppler effect, increasing the wavelength of the light, and thus decreasing it's energy. The light would never actually disappear, though, and it would still be there after 1000 years.

10. Originally Posted by Pong
Originally Posted by Raggedjoe
Originally Posted by Pong
Doesn't matter that the photons are pouring through each other? They dodge?
Photons are really really small and move really really fast. Imagine trying to hit a bullet in mid air with a second bullet. Now make the bullets move at the speed of light and make them (I think) smaller then an atom.
...Now allow them, as you imply, infinite time to collide. See, forever is infinitely greater than "really really".

I guess the question (to DrRocket) is does a photon ever interact with another photon.
It's called interference, no? And you see interference patterns if you perform a double-slit experiement!

Never forget that photons too are wavicles!

11. Originally Posted by Bender
Now lets suppose the mirrors are indeed perfect. The continuous bombardment of photons would push them backwards (since the mirrors are in total vacuum, they can't be attached to anything), in case of a not perfectly focussed beam, the mirrors would also start tilting, and only a slight tilt would have the same effect as an imperfect mirror. Lets also suppose the focus is perfect and perfectly in the absolute centre of the mirror. The mirror would be pushed straight back. I think this would cause a Doppler effect, increasing the wavelength of the light, and thus decreasing it's energy. The light would never actually disappear, though, and it would still be there after 1000 years.
You are right about the mirrors moving apart (I didn't think of that) but that would not cause the Doppler effect. The Doppler effect is caused by an increase in the space between photons. In this case we have increase of the distance the photons travel, but not an increase in the distance between photons.

12. Originally Posted by Raggedjoe
Originally Posted by Bender
Now lets suppose the mirrors are indeed perfect. The continuous bombardment of photons would push them backwards (since the mirrors are in total vacuum, they can't be attached to anything), in case of a not perfectly focussed beam, the mirrors would also start tilting, and only a slight tilt would have the same effect as an imperfect mirror. Lets also suppose the focus is perfect and perfectly in the absolute centre of the mirror. The mirror would be pushed straight back. I think this would cause a Doppler effect, increasing the wavelength of the light, and thus decreasing it's energy. The light would never actually disappear, though, and it would still be there after 1000 years.
You are right about the mirrors moving apart (I didn't think of that) but that would not cause the Doppler effect. The Doppler effect is caused by an increase in the space between photons. In this case we have increase of the distance the photons travel, but not an increase in the distance between photons.
Then were does the energy come from to accelerate the mirrors?
My reasoning was that the momentum has to remain constant, which means that the mirror has to get the momentum of the colliding photon + the momentum of the reflected photon. The energy also has to remain constant, so part of the energy of the photon would go to the kinetic energy of the mirror, and since , less energy for the photon means a lower frequency.
Am I missing something?

13. [quote="Raggedjoe"]
Originally Posted by Bender

You are right about the mirrors moving apart (I didn't think of that) but that would not cause the Doppler effect. The Doppler effect is caused by an increase in the space between photons. In this case we have increase of the distance the photons travel, but not an increase in the distance between photons.
Incorrect. First, the Doppler effect is caused by a decrease in the frequency of the photon itself, not an increase between the distance of the photons. Secondly, there would be an increase between successive photons.

Example. Photon 1 strikes mirror and reflects back the other way. In the process it imparts momentum to the mirror causing to move away fro the other mirror. Photon 2 following photon 1 must now travel a longer distance to catch up the mirror than photon 1 did. Thus the distance between photon 1 and the mirror when photon 2 strkes the mirror and refects back is greater than it would have been if the mirror had not moved after being struck by photon 1. After leaving the mirror, the gap between the two photns will be greater than it was before either struck the mirror.

14. what if you had a sealed box and all inward facing sides were perfect mirrors, 100% reflecting. Then imagine you had some light inside of the box somehow. Would the light just sit inside bouncing around forever?

15. what if you had a sealed box and all inward facing sides were perfect mirrors, 100% reflecting. Then imagine you had some light inside of the box somehow. Would the light just sit inside bouncing around forever?
There is always light. That is, light is Electromagnetic Radiation (EMR). EMR is a form of energy. Just like heat, or a falling stone. It is capable to cause an action occur. What this means is that energy affects an object natural inertial state.

EMR is always being absorbed, emitted, and reflected by atoms all the time. It is only considered light when this energy is at a specific frequency range. At this frequency range it is visible to the human eye, but it is the same event as a radio wave, or a microwave.

So the box or the mirrors are can really be reduced to an atom and an atom shooting light back and forth.

The photon is massless,[1] has no electric charge[11] and does not decay spontaneously in empty space. A photon has two possible polarization states and is described by exactly three continuous parameters: the components of its wave vector, which determine its wavelength λ and its direction of propagation. The photon is the gauge boson for electromagnetism, and therefore all other quantum numbers—such as lepton number, baryon number, and strangeness—are exactly zero.[12]
It also isnt really a true particle, it only has behavior that is similar to a particle. It also has behavior similar to a wave but it is not a wave either because a wave travels through a medium like water, but space between matter is not filled with more matter.

As for photons hitting each other, no. Light is not actually capable to physically hitting itself because a photon itself is not actually physical like we know and see physical things today. When light waves occupy the same space they create an area that has a high energetic state, which I believe is called a standing wave?

Anyway, there will always be energy moving between the mirrors as long as the mirrors have atoms. The fact is if you were able to somehow contain all the box in some kind of a special bubble that prevents anything from getting out, then yes the energy would be at work continuously. The mirrors would remain the same temperature and the energy in the form of photons would remain more or less the same in 'strength'.

16. Originally Posted by Pong
Originally Posted by Raggedjoe
Originally Posted by Pong
Doesn't matter that the photons are pouring through each other? They dodge?
Photons are really really small and move really really fast. Imagine trying to hit a bullet in mid air with a second bullet. Now make the bullets move at the speed of light and make them (I think) smaller then an atom.
...Now allow them, as you imply, infinite time to collide. See, forever is infinitely greater than "really really".

I guess the question (to DrRocket) is does a photon ever interact with another photon.
Photons can collide, but that is fairly exotic stuff. But to the point of this thread, photons don't bump into one another and "wear out". A photon generally maintains its identity quite well unless it interacts with electronsl, is absorbed, and some other photon is emitted or unless you look at effects of very strong gravitational fields that can change the wavelength of the photon.

The effect being discussed here is not one that affects individual photons. The reduction in the intensity of light with distance is simply explained in terms of elementary geometry. The surface area of a sphere is , and in particular the surface area grows like the square of the distance. So if there is light beam that is anything other than perfectly columnated, the area over which the beam is spread grows like the square of the distance. This means that number of photons per unit area in a light beam decreases as the square of the distance. Each photon maintains its identity and energy, but there are fewer photons per unit area the farther one is from the source.

17. And then of course photons are purely 'theoretical' energy clearly arrives from the sun we say it arrives as trillions of little energy packets called photons we have decided (for now) that photons exist (remember the 300 year old argument wave v particle?) photons as we describe them are able to pass through each other without interaction (forget QM for now please..) We have brains that were developed for the physical middle world ie to understand what we can feel and see, not for such grand stuff as 'passing through each other' or 'before time' etc etc.

As for the question your mirror would have to mimic 'free space' - even perfect mirrors would not work even in a vacuum, free space is a theoretical space with no external influence no gravitaional, electric magnetic fields and certainly NO little quantum thingies popping in and out of existence. Now since your mirrors have mass they also exhibit a minute gravitational field which, according to albert will affect the photons (though I suspect there will be a plank limit to negate this argument.

We have all tried it and all failed about 50 or so years ago I put a lamp and a photo cell inside a box with reflective surfaces - it failed miserably (yes I know why, so no answers please). Have fun and try it your self with boxes of different internal reflectivity and see if you can 'pulse' the light and get different persistence times suing a singe photocell lamp arrangement moved from box to box.

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