[science_guy (OP)]

I was wondering at this trivial peice of information, and I thought it might be a interesting question.  What is the largest cubed unit that can possibly not contain any trace of matter or energy (including light) at one time in the universe?

[JimBob]

The box Schrodinger's cat came in.

     ______________________________________________________

"I would engage you in a battle of wits, but it is against my moral code to attack the unarmed."

"You are about half right - JJ"

Half-wit, get it? (Har, Har Har, Har choke)

[lyner]

The average density of the universe is about 10^-27 kg per metre cubed  and the mass of proton is about 1.6 X10^-27 kg. So, as Hydrogen atoms are the most common atoms in the universe, there will be about one atom per metre cube. You could easily pick a  random cubic metre of space with no matter in it.
The photon question is much harder to answer. The 'size' of a photon will relate to its wavelength and the spectrum of the residual microwave radiation  which would be the very least you could expect to find is centred about the cm wavelength of microwave radiation. You could say that the lowest frequency photons (corresponding to sub - audio electromagnetic perturbations) have wavelengths of millions of km so they would seem to 'fill up' everywhere with 'something'. Remember, as the frequency of an em wave decreases, the energy of a photon is less. So there will be loads and loads of these low frequency photons everywhere - compared with the relatively few high energy X ray and gamma ray photons.

I just had a thought; Feynman discusses photons in terms of interactions between charged particles (ref. the well known Feynman diagrams).You could say, then, that for every pair of charged particles in the universe, there would be a photon because they are all affecting each other to some extent. That means the place is just chock full of the things! Far, far more photons than matter particles.

[Heliotrope]

It doesn't matter how small a box you have it will always have energy inside it.
The only limit I can see to the size of a box would be the Planck length.
So if you have a box which was a cubic Planck length then basically that's as small as it gets. The maths is unable to describe anything below that limit so you are left with one of a few conclusions.
Either you just ssy that because we can't describe scales that small there's nothing to describe and as such the fabric of reality breaks down.
Or you can say that there are things inside the box that we can't describe to we'd better get some better physics sorted out pronto !
Either way there's going to be something inside the box.

[JimBob]

The box Schrodinger's cat came in.

SERIOUSLY !!

If the size of the box is less than Planck's constant how are you going to tell anything is in it? Even if it were greater the size h, you still could not tell.

ALSO, mass or energy is not distributed evenly throughout the universe. There are areas where density of mass and/or energy is very low.

[Heliotrope]

Low is still some.
The question asked for no energy or matter at all.

The box can't be less than a Planck length along each edge. There are no lengths smaller than a Planck length.

Schrodinger's cat analogy is no longer accepted as a description of what is happening at the quantum level.
It was from a time when the Copenhagen Interpretation was the most popular. This has also been superceded.