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Is A Photon Torpedo Possible?

photons radiation pressure light

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#1 jwferk

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Posted 20 October 2015 - 07:54 PM

New thought experiment (although this one can be done in a lab)

 

Photons exert radiation pressure.  In fact, the effects of radiation pressure need to be accounted for in some ultrasensitive measuring devices.  They also make "solar sail" propulsion possible.

 

Consider building a two boxes.  One of these has a reflective surface on the inner side with 99.9% reflective efficiency.  The other has an inner reflective surface of 100% efficiency.  In the latter instance we truly have a "black box" since any light entering the box is continuously reflected on the inside of the box.

 

Put a standard light bulb next to each and turn it on.

 

Question - 

 

1 - will the first box (99.9% efficiency) get brighter with time as more photons enter from the light bulb?

2 - will one or both boxes eventually explode from radiation pressure as the number of photons entering exceeds those exiting (photons build up inside)?

3 - do the boxes heat up with time?

4 - what happens if we continuously shine a laser beam into either box?

5 - will it take box one (99.9% efficiency) longer to dim (photon escape) if the light bulb is left on for 2 hr instead of 1 hr?  Presumably there are more photons trapped inside.

 

Cheers,

jwferk aka kref


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#2 CraigD

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Posted 21 October 2015 - 07:46 PM

Consider building a two boxes. One of these has a reflective surface on the inner side with 99.9% reflective efficiency. The other has an inner reflective surface of 100% efficiency. In the latter instance we truly have a "black box" since any light entering the box is continuously reflected on the inside of the box.


You’d need something more complicated than simply a reflective surface inside the boxes, because a simple reflector would reflect incoming light as well as outgoing. There are optical devices, called optical isolators, that allow more light to pass thought them in one direction than the other, so you could in principle make a box like you describe, jwferk.

The problem with the scheme is that the best reflectors have less than 100% reflectance. The best, which multiple layers of dielectric materials, have reflectance on the order of 99.999% over a broad range of angles.

A bit of arithmetic can show that this would give a “half life” for the light stored inside a 1 m diameter box (for simplicity, spherical) of about 0.0002312 seconds. This is pretty amazing, but it means that after 1 second, the box would have only about 1/24325 (1/101302) its original amount of light. There are only about 1089 photons in the universe (source), so even if the box started with all of them, it would be empty in less than 0.07 s.

It is possible, via total internal reflection, to have 100% reflectance, but only when the “reflector” has a lower refractive index than what the light’s traveling through, and the light strikes the reflector at an angle less (more acute) than a critical value.

So, though a box with light striking a reflector at many angles, some near 90deg can’t store light, maybe you could do it with something like a loop of optical fiber.
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#3 freeztar

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Posted 12 February 2017 - 05:15 PM

New thought experiment (although this one can be done in a lab)

 

Photons exert radiation pressure.  In fact, the effects of radiation pressure need to be accounted for in some ultrasensitive measuring devices.  They also make "solar sail" propulsion possible.

 

Consider building a two boxes.  One of these has a reflective surface on the inner side with 99.9% reflective efficiency.  The other has an inner reflective surface of 100% efficiency.  In the latter instance we truly have a "black box" since any light entering the box is continuously reflected on the inside of the box.

 

Put a standard light bulb next to each and turn it on.

 

Question - 

 

1 - will the first box (99.9% efficiency) get brighter with time as more photons enter from the light bulb?

2 - will one or both boxes eventually explode from radiation pressure as the number of photons entering exceeds those exiting (photons build up inside)?

3 - do the boxes heat up with time?

4 - what happens if we continuously shine a laser beam into either box?

5 - will it take box one (99.9% efficiency) longer to dim (photon escape) if the light bulb is left on for 2 hr instead of 1 hr?  Presumably there are more photons trapped inside.

 

Cheers,

jwferk aka kref

 

Cool thought experiment. I thought by the title that you were going into photon torpedo territory, but really you are just working on the first hypothetical part which is "How does one store light?".

 

Some answers from my point of view:

1. I don't understand the assumption. Do you mean that the inside of the box would get brighter because it is absorbing more photons? If so, I think that is valid.

2. The radiation pressure is not really significant with photons. Well, at least naturally occuring ones at distance. I don't think radiation pressure would be a factor in a photon torpedo.

3. The 99.9% one would heat up slightly, but it would take an incredibly sensitive thermometer to detect the change.

4. I think you've already discovered the answer to this question. CraigD did an excellent job of describing the half life.

5. I'm not sure what you are asking here. Why should the amount of hours matter if they are so quickly absorbed?

 

On the topic of the title, Photon Torpedos (from Star Trek presumably), I'd like to add a few things.

 

In every Star Trek episode, the photon torpedos surge in brightness on the ship and then fly out relatively slow as an organized "tube" of light. It reminds me of old camera flashes that emitted that high pitch crescendo to give juice to the flash.

 

If we take that to mean that the ship intializes and fires pretty rapidly, then we know that there is some kind of concentration going on. This is "the box" problem that has already been discussed.

 

What has not been discussed is how you get a discrete package of light to travel as a concentrated cluster. Photons are omnidirectional. That is the first hurdle. They might be contained within some kind of gravitational field, but there is never any hardware shown when torpedos fly. Hmm...



#4 CraigD

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Posted 13 February 2017 - 07:26 AM

This old thread appeals both to the modern physics and the Star Trek fan! :)

Some info about the fictional photon torpedo from the official and fan lore of Star Trek:
Photon torpedoes are just physical objects, round-edged capsules about the size of a funeral casket. Inside the weapon versions (some versions don’t explode, but rather contain scientific instruments, or, you might recall from the movie Wrath of Khan, even the bodies of dead crew members) is some antimatter, with a mechanism that keeps it separate from its surrounding ordinary matter until the torpedo reaches it target, where it releases the antimatter to annihilate with the ordinary matter, making a really big bang. The torpedo glowing in flight is just a side-effect due to a short-lived “warp field” that the ships launching system surrounds it in – you might recall an episode of Voyager where the starship Voyager, it’s torpedo launchers damaged, just dump its last photon torpedoes in its trail, no glowiness, where they still blow up her pursuing enemies nicely.

I’m pretty sure, the original Star Trek writers and special effects makers, and the early fan “technical” writers and game developers, didn’t have this explanation, but rather thought that photon torpedoes were simply, torpedoes made out of photons, physically nonsensical as that may sound. Trek writers and fans have never been known for their strict adherence to the known laws of physics. ;)

Enough STrek fan lore. The question at hand is, how could you actually make a torpedo out of photons?

I think feeztar’s on the right track here

What has not been discussed is how you get a discrete package of light to travel as a concentrated cluster. Photons are omnidirectional. That is the first hurdle. They might be contained within some kind of gravitational field, but there is never any hardware shown when torpedos fly. Hmm...

An object that contains photons with gravity is a black hole. A black hole that is made of nothing but photons is a kugelblitz. (John Wheeler wrote about them in 1955)

The problem with a kugelblitz photon torpedo is, since light can’t escape from it, it wouldn’t glow. You can get around that by imagining that the glow of a photon torpedo isn’t from the photons making its kugelblitz, but from hot matter in an accretion disk, which also explains its glowing tail, as accretion disk matter stripped away due to friction with the interstellar medium.

With a bit of Hawking radiation science, you can even come up with a plausible way that the kugelblitz torpedo can be made with just the right mass-energy to evaporate explosively at a preset range. :)



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