Ball of light (not ball lightning)

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Ball of light (not ball lightning)

Postby idont_know12 » Thu Aug 16, 2007 5:18 am UTC

Suppose you had a perfectly spherical ball of glass, the internal surface of which was given mirror characteristics. At the 'top' of the ball is a small hole. Into this hole is shone a very bright light. Would, over time, the reflections of light within the ball 'build up'? Would the rate of photoinput exceed the escape of the previously-inputted light waves, and/or the rate of decay of the light waves? If not, what would happen? Could you create a self-contained ball of light that would shatter when thrown and act as a literal flashbomb?

This is a scenario I've been wondering over for quite a while now, and I was hoping you guys could help, given the collective superior knowledge on these types of things.

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Postby skeptical scientist » Thu Aug 16, 2007 6:03 am UTC

Well, any material will absorb some of the light moving through it, and reflectors are also imperfect, so you'll reach a steady state where the amount of light entering the ball is the same as the amount of light absorbed by the mirrors and glass, or which bounce back out of the hole. So I don't think it would be anywhere near as bright a light as you could get with a chemical reaction in a flashbang.
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Postby po2141 » Thu Aug 16, 2007 7:43 am UTC

Suppose you have a sphere made of some theoretical, non-absorbing, perfectly reflecting material, and you crammed it full of light. And assuming it didn't all instantly escape when you try to put the lid on.

So you throw this ball of light into a room of armed terrorists to try and blind them temporarily wit the flash. All the light in the ball will pretty much escape in the same instant and will all emanate from the same place. This will give an almost unimaginably short pulse of light, in the order of a fraction of a nanosecond. I doubt anyone would notice it :(

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Postby Graham's Number » Thu Aug 16, 2007 10:24 am UTC

Add to the fact that it is physically impossible to create a perfectly reflecting surface and you'll see it is impossible. Any light you shone into the sphere and managed to contain would be absorbed in a matter of nanoseconds and the sphere would just be slightly warmer.
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Postby Bondolon » Thu Aug 16, 2007 10:28 am UTC

Graham's Number wrote:Add to the fact that it is physically impossible to create a perfectly reflecting surface and you'll see it is impossible. Any light you shone into the sphere and managed to contain would be absorbed in a matter of nanoseconds and the sphere would just be slightly warmer.


Oh, come on. It's not like it's physically impossible to create one that's really, REALLY reflective. We can get the coefficient of reflectivity up really, realllllllllllly high without attaining perfection and still consider it extremely successful. Hell, we can get the absolute temperature to within fractions of a billionth of absolute zero, and people are still complaining about efficacy. Not to say that the OP's idea was great, but one could imagine a really neat sphere with a timer, and an LED to charge it being thrown into a room and releasing a fairly large amount of radiation. Not that it'd be terribly effective, but your objection is far too scrutinizing.

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Postby po2141 » Thu Aug 16, 2007 10:42 am UTC

I suppose if the photons were Infra-Red flavoured (Harder with the absorby-reflecty) then the sphere could release a "thermal pulse" of, dependant on the intensity, significant destructive potential.

Here's a thought, if we use a particular wavelength, an the sphere is perfectly reflecty-non-absorby (it would have to be physically unbreakable as well I think), will there be a theoretical maximum number of photons that can be contained within the sphere? I can't see why there should be, but on the other hand, physics tends not to like infinite concentrations of stuff.

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Postby evilbeanfiend » Thu Aug 16, 2007 12:23 pm UTC

po2141 wrote:I suppose if the photons were Infra-Red flavoured (Harder with the absorby-reflecty) then the sphere could release a "thermal pulse" of, dependant on the intensity, significant destructive potential.

Here's a thought, if we use a particular wavelength, an the sphere is perfectly reflecty-non-absorby (it would have to be physically unbreakable as well I think), will there be a theoretical maximum number of photons that can be contained within the sphere? I can't see why there should be, but on the other hand, physics tends not to like infinite concentrations of stuff.


photons can occupy the same quantum state, light ain't really a stream of particles.
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Postby zenten » Thu Aug 16, 2007 1:05 pm UTC

po2141 wrote:I suppose if the photons were Infra-Red flavoured (Harder with the absorby-reflecty) then the sphere could release a "thermal pulse" of, dependant on the intensity, significant destructive potential.

Here's a thought, if we use a particular wavelength, an the sphere is perfectly reflecty-non-absorby (it would have to be physically unbreakable as well I think), will there be a theoretical maximum number of photons that can be contained within the sphere? I can't see why there should be, but on the other hand, physics tends not to like infinite concentrations of stuff.


You'd get quantum leakage though (don't think that's the right term).

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Postby evilbeanfiend » Thu Aug 16, 2007 1:58 pm UTC

do you mean quantum tunnelling?
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Postby po2141 » Thu Aug 16, 2007 2:39 pm UTC

whether particles or waves or whatever, would you be able to squeeze an infinite amount of EM energy into this storage medium of ours? What is this leakage? assuming perfect reflectivity and non-absorbance, where does the leaking energy go?

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Postby evilbeanfiend » Thu Aug 16, 2007 2:45 pm UTC

well there should be a non 0 chance that a photon can quantum tunnel i.e. it can jump outside the sphere

but again, if you make the sphere thick enough this probability should get very small letting you store light for long enough for some purpose
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Postby MFHodge » Thu Aug 16, 2007 5:42 pm UTC

I've thought about this before also, and would like to toss out this idea:

Does it even need to be a perfect sphere? If we're assuming perfect reflection, wouldn't any closed shell work equally well?
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Postby Thunderbird4! » Thu Aug 16, 2007 5:43 pm UTC

Let us assume that we can make a perfect sphere of some material that reflects 100% of the light on the inside. Now we take a portion of this and change it so EM radiation can get in but not out. We then let this build up inside the sphere. Could we then break/open the sphere releasing a directed "explosion" of radiation that might kill someone?

(This was one of my dreams not so long ago...)
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Postby skeptical scientist » Thu Aug 16, 2007 9:26 pm UTC

Thunderbird4! wrote:Let us assume that we can make a perfect sphere of some material that reflects 100% of the light on the inside. Now we take a portion of this and change it so EM radiation can get in but not out. We then let this build up inside the sphere. Could we then break/open the sphere releasing a directed "explosion" of radiation that might kill someone?

(This was one of my dreams not so long ago...)

I dreamed that the glass doorknobs in my house were focusing lazers and if I had to get up to use the bathroom I would die horribly as they sliced me in half. I think it's about equally likely.
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Postby sheltrk » Fri Aug 17, 2007 3:53 pm UTC

Being a "laser jock", I had to register when I saw this discussion. Yes, you can store light energy in a resonant optical cavity. Imagine a laser cavity of any type, and remove the gain medium. If you pump light into it, it will bounce around for a while before it is scattered/absorbed/otherwise attenuated by various loss mechanisms. How long is a "while"? Well, that depends on how high the losses are. Assuming you have a simple 1 foot long linear cavity and modest, 99.9% reflective end mirrors, you can expect a photon lifetime of a microsecond or so. Not bad. With "super" mirrors of 99.9999% reflectivity (and *no* other losses considered), you could get a photon lifetime of 1 millisecond. That's pretty good! Add another three "9"s to the mirror reflectivity (not physically realizable, as far as I know), and you'd have a 1 second photon lifetime in the cavity, assuming no other losses.

Further, yes you can release all the stored optical energy "at once" (where "at once" is defined as one cavity round-trip time, or less in some cases) in such an arrangement. Add the gain medium back in, and there are several types of lasers that use this concept to produce high energy, short duration optical pulses. See:
http://tinyurl.com/2yjtx3

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Postby Toeofdoom » Fri Aug 17, 2007 4:12 pm UTC

Better (well, probably not really better) idea:

Get a (small) black hole, and shine a torch in so that the photons will orbit, then wait until the black hole fully decays (thats still the current theory, that they eventually die right....?), or chuck in a blob of antimatter of equal mass.

Only problem is that this isnt so useful for small scale things. Still, you could try baking the side of a planet with it, but overall you wont get out more power than you put in... unless by some odd gravity effect or something.
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Postby alexthesoso » Fri Aug 17, 2007 4:53 pm UTC

po2141 wrote:Suppose you have a sphere made of some theoretical, non-absorbing, perfectly reflecting material, and you crammed it full of light. And assuming it didn't all instantly escape when you try to put the lid on.

So you throw this ball of light into a room of armed terrorists to try and blind them temporarily wit the flash. All the light in the ball will pretty much escape in the same instant and will all emanate from the same place. This will give an almost unimaginably short pulse of light, in the order of a fraction of a nanosecond. I doubt anyone would notice it :(


congrats. youve just described the basic theory behind a laser. if it were a vacuum inside, and a perfectly reflective surface, and the entry way were a two way mirror of sorts, light in, none out, yeah, it would work.

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Postby Xanthir » Fri Aug 17, 2007 5:20 pm UTC

Toeofdoom wrote:Better (well, probably not really better) idea:

Get a (small) black hole, and shine a torch in so that the photons will orbit, then wait until the black hole fully decays (thats still the current theory, that they eventually die right....?), or chuck in a blob of antimatter of equal mass.

Only problem is that this isnt so useful for small scale things. Still, you could try baking the side of a planet with it, but overall you wont get out more power than you put in... unless by some odd gravity effect or something.

You don't quite understand black hole decay. Decay doesn't mean the black hole goes away, leaving you with the original mass. It means the mass itself radiates out as pure energy due to Hawking radiation.

Black holes decay faster as they get smaller. According to theory, a micro-black hole will evaporate almost instantly and release all of its mass at once. Considering that a cheeseburger converting itself into energy would be about as powerful as the largest nuclear bombs that ever existed (about 20,000 TJ of energy, compared to roughly 60,000 TJ in the largest bomb ever exploded by the US), I think that a microgram of matter should work just fine as a bomb. ^_^


Edit: Reread the quoted post, and realized this wasn't quite was he was saying. Regardless, the decay of the black hole will release such an unimaginably large amount of energy that your dinky little flashlight wont' make a bit of difference. Note, though, that chunking antimatter at a black hole doesn't make it explode - it'll just eat it like anything else. Black holes are black because *nothing* can escape, people.

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Postby Toeofdoom » Fri Aug 17, 2007 6:13 pm UTC

Yeah, I knew how the decay thing worked. but the point is that when it finishes it would release all the orbiting energy because it doesnt have super strong gravity any more...

I was thinking the antimatter would work cause it like, cancels out matter, making it into energy, but really they cancel out their opposite particle I guess? Or it would turn into energy and be sucked back in anyway... although in that case it would suggest that detonating the whole thing at once would fix it. I never really investigated beyond high school physics, so I havent been informed how all this stuff works...

Still according to wikipedia there is a difference between a matter black hole and an antimatter black hole... not sure how relavent that is.

anyway, you wouldnt be using a flashlight for this, you'd be using something more like a giant lens refracting light from a star or something, then focusing it into a beam. That could help right?
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Postby Xanthir » Fri Aug 17, 2007 7:00 pm UTC

Toeofdoom wrote:Yeah, I knew how the decay thing worked. but the point is that when it finishes it would release all the orbiting energy because it doesnt have super strong gravity any more...

Nod, but the only reason it doesn't have gravity anymore is because it already released all of its energy into your face.

I was thinking the antimatter would work cause it like, cancels out matter, making it into energy, but really they cancel out their opposite particle I guess? Or it would turn into energy and be sucked back in anyway... although in that case it would suggest that detonating the whole thing at once would fix it. I never really investigated beyond high school physics, so I havent been informed how all this stuff works...

Best way to think about it is that matter and antimatter are both the same energy, they just have opposite 'patterns' (where the pattern is the mysterious thing that makes energy into solid matter). When you combine them, you cancel out patterns, which releases all that pent up energy.

If you threw antimatter into a black hole, it will do what it's supposed to - annihilate the matter and release energy. But the energy is just photons, travelling at the speed of light. Black holes are black because light can't escape. So the energy released from the combination of matter and antimatter can't escape either.

Still according to wikipedia there is a difference between a matter black hole and an antimatter black hole... not sure how relavent that is.

They likely are different things - antimatter does act differently from normal matter. If you throw antimatter into a matter black hole, though, it won't be antimatter for long; it'll just turn into energy (along with a corresponding mass of matter).

anyway, you wouldnt be using a flashlight for this, you'd be using something more like a giant lens refracting light from a star or something, then focusing it into a beam. That could help right?

Wouldn't solve the basic problem, that the energy release by the black hole evaporating will be enormously greater than whatever energy you have rotating around it.

Remember, black holes don't evaporate all at once. They bleed off mass one particle at a time via Hawking radiation. A big black hole takes billions of years or more to evaporate. Like I said, though, they evaporate faster as they get smaller, so that a tiny black hole explodes in a fraction of a second and blows up a city.

And really, once you have the capability to create black holes at will and refocus the light from a star via a giant space-based lens, do you really need a little light-bomb? It's like opening a can of soda with a backhoe.

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Postby Toeofdoom » Sat Aug 18, 2007 4:01 am UTC

The point isnt to be practical, the point is to make a sphere that will trap light until it is destructively released. I cant see a black hole losing the light beforehand, so it should work fine, if you get the maths *exactly* right.

And maybe if you really want to charge it up so its more powerful than a black hole evaporation, you should charge it up with other black holes evaporating. There should be some way to charge it up significantly, because there definitely is enough energy in the universe.

Seems more plausible than getting a perfectly reflective, non absorbing vacuum sphere anyway.

Unless the problem is that once the energy gets in it makes the black hole explode later anyway, other than that I'd think it would work fine...

Anyway, I understand the matter/antimatter stuff now, and already knew the hawking radiation things.
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Postby gmalivuk » Sat Aug 18, 2007 9:00 pm UTC

Toeofdoom wrote:Unless the problem is that once the energy gets in it makes the black hole explode later anyway


Which it does.

Anyway, I understand the matter/antimatter stuff now, and already knew the hawking radiation things.


The above which suggests that you didn't already kow the hawking radiation things. The more massive a black hole, the longer it takes to evaporate. The more energy your pour into a black hole, the more massive it becomes.

Also, the light shell around a black hole isn't going to shrink with the event horizon as the hole itself radiates away. Rather, those photons which were in circular orbits will just spiral outward now that the radius of the natural light shell has shrunk a bit.* (It's an unstable orbit, anyway, so even with a black hole that is magically perfectly stable in size, it's going to take a long time to actually store up energy in the photon shell, seeing as it is basically infinitely thin.)

Personally, I'd say perfect reflectivity is more plausible...

* Imagine an object in a stable circular orbit some distance away from Earth's center. Then imagine that we somehow remove a bunch of Earth's mass in a way that doesn't purturb the object itself. Now, the object's speed is higher than the orbital velocity at its distance. It will start moving in an elliptical orbit with its perigee at the distance of the original orbit. The photon shell around a black hole is unstable, on the other hand, so no such elliptical orbit is possible. Instead, light will simply escape once the hole shrinks.
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Postby Toeofdoom » Sun Aug 19, 2007 3:52 am UTC

Just how far in does the energy have to go exactly to become a part of the black hole, increasing its mass? Beyond the event horizon, or hitting the singularity? I was thinking the light would be just outside or exactly on the event horizon...

good point about the infinitely thin thing though.

okay, so probably both ideas are ridiculously implausible at the moment...

And I did understand the idea of hawking radiation itself perfectly fine. I just wasnt informed about matter and antimatter being made up of the same things at a lower level. Related concepts, but still different...

EDIT: Rig up the flashlight battery with a few strobe lights, that'll fix em.
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Postby gmalivuk » Sun Aug 19, 2007 5:10 pm UTC

Once light (or anything else) is beyond the event horizon, it will not come back out, and will thus contribute to the size (and mass) of the event horizon. The photon shell is 1.5r, where r is the radius of the horizon. This radius allows for (unstable) circular orbits of photons. (The null geodesics are closed.) Nothing can just hang out inside this radius. It either falls in or escapes, depending on its angular speed. If you had a rocket, you could theoretically cancel out the gravity and hover at any point right down to the horizon itself. But light doesn't ride on a rocket.
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Postby po2141 » Mon Aug 20, 2007 9:36 am UTC

alexthesoso wrote:
po2141 wrote:Suppose you have a sphere made of some theoretical, non-absorbing, perfectly reflecting material, and you crammed it full of light. And assuming it didn't all instantly escape when you try to put the lid on.

So you throw this ball of light into a room of armed terrorists to try and blind them temporarily wit the flash. All the light in the ball will pretty much escape in the same instant and will all emanate from the same place. This will give an almost unimaginably short pulse of light, in the order of a fraction of a nanosecond. I doubt anyone would notice it :(


congrats. youve just described the basic theory behind a laser. if it were a vacuum inside, and a perfectly reflective surface, and the entry way were a two way mirror of sorts, light in, none out, yeah, it would work.


A laser (the kind with a light and two mirrors anyway) will only emit while the pumping light source is on (as near as makes any difference anyhoo), the question here is if you could trap the light from a source (also there was no mention of stimulated emission of radiation) for a significant period of time, and release all at once for some purpose or another. I think we have all decided by now that this would only be practical with materials with certain theoretical properties and probably won't be possible unless you are an evil supervillain with enough cash for a base on the moon and lots of free time and captured soviet scientists.


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