# Can light be compressed?

Discussion in 'Physics & Math' started by Twist, Apr 22, 2004.

1. ### TwistRegistered Member

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12
Hi, I am not a physicist, please excuse obvious mistakes and tailor replies for the layman.

I have a balloon, the interior of which is a perfect mirror. I have a cork which fits perfectly in the balloon and is also a perfect mirror, and seals perfectly. So basically a sphere the interior of which is a perfect mirror.

I have a vacuum chamber in a vacuum chamber. The inner vacuum chamber is used to inflate the balloon by lowering the pressure outside of the balloon. The pressure inside the baloon is lowered by its exposure to the outer vacuum chamber (Hmm, I mean the ball is inflated, but contains a lower pressure than atmospheric).

The vacuum chambers are transparent, so light enters the balloon as it is inflated. The balloon is corked with the perfectly mirrored cork. So I now have a balloon full(?) of light that is not going anywhere. Is this possible?

If I release the vacuum chambers I will compress the balloon as the atmospheric pressure applies to the outside of the balloon. I now have stored compressed light. Is this possible?

I uncork the balloon in a dark room. The light rushes out providing a brief illumination. Is this right?

I guess I am asking if light compresses?

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3. ### JaredsterRegistered Senior Member

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50
well in theory if you were able to "capture" a bunch of photons in a balloon and compress it, the luminosity of light inside would just get greater, since you are increasing the light/cm^3 ratio.

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5. ### James RJust this guy, you know?Staff Member

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30,256
You'd have a problem getting light into your balloon in the first place, since if it had a perfectly reflective interior then no light could penetrate from the outside to the inside either.

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7. ### transioCreative GeniusRegistered Senior Member

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41
Of course, there would have to be nothing else inside the baloon either, except space.

Of course, being that there's no thing but space and photons in your baloon, it would have no perceivable volume.

I say we try it!

8. ### John ConnellanValued Senior Member

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3,636
Why would there be no perceivable volume? What if the space had a volume of 0.5 cubic meters?

9. ### TwistRegistered Member

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12
Thanks for the answers

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James R, the light will get in before the perfectly sealing cork is fitted.

10. ### TwistRegistered Member

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12
Transio, why would there have to be nothing else in the balloon? Would any matter in there absorb the light slowly?

So because there is nothing in it except photons it has no volume? ooh.

11. ### dsdsdsValued Senior Member

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1,677
My watch absorbs light so I can see what time it is in a movie theater. That's what you're talking about here, right? Why would we want to compress light? More intensity?

12. ### MarkRegistered Senior Member

Messages:
150
this is an interesting line of questioning and shows an intuition that the light inside a perfectmirror balloon could act like a gas

it could compress and decompress
maybe it could exert a pressure

maybe the light would become "hotter" as it is compressed, the way gas can heat up during compression

is there an analogy with the "gas law"?

the socalled gas law (PV = nkT) describes behavior of an ideal gas
and every Freshman Physics student must learn it very well and use it in homework. is there some analogy with how photons inside a perfectmirrorballoon would act?
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yes, there is plenty of analogy
-----------------

light exerts a pressure on surfaces and certainly would on a perfectmirror
inner wall

you can compress the light
the pressure would increase
the energy density would increase

even the frequency of the light would increase due to doppler with bouncing off the walls as the walls move in to compress
the wavelength would shorten and the light would
get "hotter" in a certain sense

there is very much analogy with a gas
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also when you let the balloon expand the light pressure inside
would get less

just like with a gas

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also there are formulas, not exactly the same as the Gas Law but
roughly analogous to it
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also this decompression of light with expansion has happened in the real world and is the reason that the Microwave background
which used to have temperature 3000 kelvin now
has temperature only around 3 kelvin
because space has expanded by a factor of approximately 1000
since the time the light of the CMB got loose and began its travels
-------------

so it is a very good intuition to think of a bunch of photons as a little bit like a bunch of gas molecules

13. ### betaRegistered Senior Member

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129
There should be no limit to how many photons you can get in the same volume.
Also, a mirror, no matter what qualities you bestow on it, should still cause a very small loss of energy (red shift) due to the Compton scattering of photons from some electrons in the mirror.
This will eventually degrade the energy contained in the balloon.