At any interval of space, is the net density of that interval never equal to zero?
Can you picture a region of space without photons? I can't.
If you put walls around, then the walls themselves must have some positive temperature and so they must be radiating thermal glow. I think some EM field must be there.
Also there is the gravitational field, if you consider that as contributing. Or perhaps that doesn't count?
Frank Wilczek has an interesting new popularization book called The Lightness of Being, that is all about the modern physics vacuum. What is going on in empty space. A Nobel laureate who can write well. Ask the local librarian to order it. Great book.
It's just that I've never imagined the universe as a whole mass before aside from the beginning stages of the big bang. Basically, mass is space and space is mass. This greatly changes my outlook on the geometry of the universe.
Does this also mean that mass relates to light in that as time goes on, the net frequency of mass in the universe continues to drop? In other words, the term "expanding universe," most actually means "as time persists, the frequency of mass in the universe continually drops while the amount of mass stays constant."
Dark energy confuses the issue.
If you mean mass density, then I have to pass up this question. I don't feel comfortable trying to answer about the mass of the vacuum.
I thought you meant energy density. So I replied in post #2 with simple classical reasoning that says of course there are always photons. Empty space always has some thermal EM energy. That doesn't involve anything deep or sophisticated. Nothing about virtual particles and quantum field theory. That is a separate issue. Wikipedia would have an article about QFT or vacuum energy, I expect.
What I was saying is there are real thermal photons, not virtual but actual, everywhere in space. That has been known for around 100 years I guess.
People commonly treat energy as a purely relative thing. So you can peg the vacuum energy to be whatever you want and measure differences relative to it. then you can peg it at zero. that is physically correct. there is no absolute zero that anyone forces you to use.
But my point is whatever sliding scale you might adopt, whatever your conventions, there are going to be some real honest-to-gawd photons in any chunk of vacuum you take. Some EM energy. There might be a lot more stuff. Like Wilczek describes in his book. But at least there is that. How much will depend on the temperature.
But going over from that to treating space as a material with inertia, i.e. mass? I don't want to go there.
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