# Concentration of photons: How many can you slow down in a given unit of space?

Being able to slow down light to the speed of a bicicle as shown in this vid:

Led to a question: Given a unit of space e.g. 1cm^3, and then you slowed down light as it enters that unit of space, how many photons can you fit in that space? Is there a maximum photon concentration limit?

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There should be a maximum energy density for a given volume before the energy from the light collapses into a black hole. Other than that, I'm not aware of any limit for the amount of light you can "stack" in one location at a time. –  Brandon Enright Nov 26 '13 at 5:56
Always amuses me when Blackholes get involved in the discussion! Sorry to sound naive, is that prediction drawn from "general relativity"? Einstein field equations? –  hello_there_andy Nov 26 '13 at 17:36
You asked for a maximum. Black holes tend to be at the extreme end of lots of things. You can form a black hole from anything as long as you pack enough energy into a small enough space. If you have a specific question about forming a black hole from light, you should ask a question on the site. Comments aren't a good place for lots of technical rigor. –  Brandon Enright Nov 26 '13 at 17:38

In the specific case of slowing light with a Bose-Einstein condensate there will be a limit because the slowing of the light is due to an interaction of the light with the BEC to form a polariton. If you put too much energy in you'll destroy the BEC and it will stop slowing the light. Offhand I don't know what the limit is, but it will be a very small amount given that BECs need to be maintained at millikelvin temperatures.

More generally, there is no need to slow light to fit more light into a given space. Photons are bosons so there is no limit to the number you can put in the same quantum state. As Brandon says in his comment, ultimately the limit would be that enough energy in a small space will form a black hole. However this limit is many orders of magnitude above anything we're likely to achieve experimentally in the foreseeable future.

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