Number of particles in the universe

[MathematicalPhysicist]

I have a question that I didn't see covered in any book that I read in QFT (I read so far Srednicki which I finished and Peskin and Schroeder which I haven't finished), can the vacuum generate an infinite number of real particles? How do we generate real particles from virtual particles? Is there such a process in QFT?

I know that usually virtual particles are a mathematical tool, but then again also point particle is a mathematical tool.

Thanks.

 

[rootone]

I am by no means an expert in this area, but I've frequently come across the notion that this is the principle behind Hawkins radiation.
According to Hawkins and I gather this is now generally agreed, a black hole which is no longer accumulating matter will gradually lose mass due to this (predicted by QM mathematics) form of radiation.
The radiation arises due virtual particle pairs appearing at or just outside the event horizon.
One of the pair is absorbed by the black hole, and the other therefore has no virtual partner so it become 'real', and constitutes the outgoing radiation.
Something like that anyway, I am sure somebody will either be able to elaborate, or else slap me down for taking pop-sci presentations too seriously.

 

[phinds]

The radiation arises due virtual particle pairs appearing at or just outside the event horizon.
One of the pair is absorbed by the black hole, and the other therefore has no virtual partner so it become 'real', and constitutes the outgoing radiation.

Not really. Hawking himself said that the "virtual particle pair" description of what is known as Hawking Radiation is simply the best English language description he could come up with to describe something that really can only be described by the math, but pop-science takes it as gospel and repeats exactly what you just said.

 

[MathematicalPhysicist]

Any technical books or papers to read this technicality together with all the math you recommend?

 

[MathematicalPhysicist]

Is this Hawking radiation covered in his book called the "the large scale structure of spacetime"?

 

[mfb]

can the vacuum generate an infinite number of real particles?

The vacuum without a black hole or other spacetime distortions cannot create real particles, this would violate energy conservation.

How do we generate real particles from virtual particles?

Some Feynman diagrams can be drawn in a way that it looks like virtual particles become real, but I don't think this is a useful model. Also, Feynman diagrams are just a visualization of perturbation theory, which is just a handy approximation of the actual quantum field theory.

 

[dextercioby]

Is this Hawking radiation covered in his book called the "the large scale structure of spacetime"?

No, that is a book purely on General Relativity.

 

[MathematicalPhysicist]

@mfb and @dextercioby in what papers or books is Hawking radiation explained rigorously with all the math? obviously it appears in one of Stephen Hawking's papers, which one?

 

[mfb]

Google/Wikipedia -> Hawking radiation -> Chapter "Further reading" first entry -> Hawking, S. W. (1974). "Black hole explosions?". Nature 248 (5443): 30
Which step was hard to find?

 

[MathematicalPhysicist]

I didn't look at Wiki, should have done that.

 

[MathematicalPhysicist]

The vacuum without a black hole or other spacetime distortions cannot create real particles, this would violate energy conservation.
Some Feynman diagrams can be drawn in a way that it looks like virtual particles become real, but I don't think this is a useful model. Also, Feynman diagrams are just a visualization of perturbation theory, which is just a handy approximation of the actual quantum field theory.

@mfb is the same true also for sub-atomic black holes?

 

[ohwilleke]

A more sophisticated educated layman oriented description of what Hawking radiation really involves can be found in a recent post at the Backreaction blog
http://backreaction.blogspot.com/2015/12/hawking-radiation-is-not-produced-at.html

 

[mfb]

@mfb is the same true also for sub-atomic black holes?

Hawking radiation of very small black holes would include real massive particles.

 

[MathematicalPhysicist]

It's a pitty that the original paper in Nature of Hawking's is under a pay-wall (you would have thought that my university's library subscription will include it).

 

[BiGyElLoWhAt]

It's a pitty that the original paper in Nature of Hawking's is under a pay-wall (you would have thought that my university's library subscription will include it).

This, in my opinion, is one of the bigger problems with current science. You might be able to find an arxiv preprint, but I'm not so sure, with the author being Hawking. Takes science, which uses a universal language (math) to make it accessible to everyone, and makes it exclusive.

 

[MathematicalPhysicist]

The university library has a hard copy of this article, so I can photocopy it but then again you would argue that such material should be available for free use if we want more people to read scientists' work.

 

[BiGyElLoWhAt]

Exactly.

 

[Nugatory]

It's a pity that the original paper in Nature of Hawking's is under a pay-wall (you would have thought that my university's library subscription will include it).

It's widely available outside the paywall as well. for example: http://www.itp.uni-hannover.de/~giulini/papers/BlackHoleSeminar/Hawking_CMP_1975.pdf

And while you're at it, you'll want to read http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/hawking.html

 

[MathematicalPhysicist]

It's widely available outside the paywall as well. for example: http://www.itp.uni-hannover.de/~giulini/papers/BlackHoleSeminar/Hawking_CMP_1975.pdf

And while you're at it, you'll want to read http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/hawking.html

I was referring to the first paper by Hawking in the Wiki page here:https://en.wikipedia.org/wiki/Hawking_radiation#Further_reading i.e "Black hole explosions?".
I downloaded already the paper above that you cited, but it seems I first need to read the first article in this sequence of articles.
Do you have free access to this paper by Hawking?

 

[Jeff Rosenbury]

My understanding is that only particles that we can measure are "real". I think most scientists would also include those which have some easily quantifiable assumptions as well. For example we have not measured each proton in the sun, but we measure the sun's mass and assume it's made of normalish matter because every particle we have measured is made up of normal matter (for some definition of normal anyway). So we feel we have a fair estimate on the number of protons in the sun without counting each one.

Counting virtual pairs is a little tricky. We could guestimate the number in a volume of space. Or we could ignore them as unmeasurable. (Of course most of the protons in the sun are unmeasurable unless/until we go there...). So the answer to how many depends on what you decide to count.

Technically the only particles in the universe that exist are the ones with which you come into contact. Even that leaves the the question of where you end and the rest of the universe begins. (Do old toenail clippings count as you, or the universe?)

 

[Nugatory]

I downloaded already the paper above that you cited, but it seems I first need to read the first article in this sequence of articles.
Do you have free access to this paper by Hawking?

I don't know of any, but I'm not sure what you expect there that you won't find in the paper I linked.

 

[MathematicalPhysicist]

@Nugatory I don't expect that he iterates in the paper you linked what he has done in the first paper, obviously he just cites what he's done in his first article.
If that's not the case then I don't need to read the first article.

 

[alw34]

can the vacuum generate an infinite number of real particles?

Isn't this what happened during the inflationary period?? assuming an infinite universe...