Virtual Particle Confusion


by shifty123
Tags: confusion, particle, virtual
shifty123
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#1
Mar26-12, 10:45 AM
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Layman here. Two questions on Virtual Particles.

I am not trying to make some claim to spontaneous generation, just curious about how to expand the holes in my knowledge... which is about the level of a post grad engineer who likes to buy the popular physics books in stores to read.

1) Where is all the spontaneously created matter?
So, I understand that according to one physics interpretation that there are virtual particles (pairs of particles that are spontaneously created and destroyed alomst instaneously).

The most often written example is a matter and antimatter pair. So, I know that Charge Parity violation exists. So if there is not perfect parity in a matter anti matter collision, how come the universe has been crushed into a singularity yet, due to the spontaneously created matter that is left over from virtual particle annihilations?

2) Has there been any work on understanding the impact of the Big Rip (expansion of space ever accelerates) on virtual particles? The idea is this. The fastest they can go is light speed, or slower if they have mass, will call it speed c. Plankc Time has a constraint on being the smallest unit of time, t. This implies that the farthest apart a virtual particle interaction can occur c*t (simplified probably). My question is, if the Big Rip (Dark Energy) ever nets large enough acceleration, then space will have expanded before the virtual particles had a chance to annihilate. Is that about right?
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Vanadium 50
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#2
Mar26-12, 11:18 AM
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Quote Quote by shifty123 View Post
Where is all the spontaneously created matter?
There is none.
shifty123
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#3
Mar26-12, 12:21 PM
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Well. Thanks. I guess.

So does that mean virtual particles dont work the way I assumed? Or are virtual particles something of an old theory that has been disproven some time ago?

Bill_K
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#4
Mar26-12, 01:48 PM
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Virtual Particle Confusion


shifty123, Virtual particles are fine. But unfortunately the account of them you hear from popsci sources is rather far off.
So, I understand that according to one physics interpretation that there are virtual particles (pairs of particles that are spontaneously created and destroyed alomst instaneously).
The vacuum is not, as you might have heard, a foam, a "violent storm." Things are not being constantly created and destroyed at an incredible rate. Virtual particles are not constantly "popping in and out of existence." In fact, the vacuum state is time-independent. But - if you try to describe it in terms of states in which the particle number is definite, in the vacuum the number of particles is not definite. The vacuum state is partly two particles, partly four particles, etc. But again, this does not mean the vacuum "spends part of the time" in each of these states, it's a steady situation.

Here's an analogy: take a system with spin and prepare it in the spin up state, Sz = +. Now ask what is the spin along the x-axis, Sx. Answer: it's a quantum superposition, 50 percent Sx = +, 50 percent Sx = -. But one should not say the system is constantly jumping back and forth between the two!
shifty123
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#5
Mar26-12, 03:49 PM
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Thanks Bill_K. That helps a bit.

Its almost as if you knew about the violent storm, quantum foam etc, that I read about! Those exact words even.

If the vacuum state is time independent then that definitely takes care of the issue I was wondering about.

Ok. Is this the correct interpretation?

So I guess the answer is if there is a CP violation that occurs for fermion virtual particles, then regardless of the number of superposition states that they could possibly live in, there is should be a positive bias towards matter virtual particles surviving. However since the VPs are time independent there is no "time" for this imbalance to "manifest"?
nonequilibrium
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#6
Mar26-12, 06:39 PM
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Also, I don't think virtual particles are bound by the speed of light(?)
Demystifier
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Mar27-12, 10:38 AM
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Quote Quote by Bill_K View Post
But - if you try to describe it in terms of states in which the particle number is definite, in the vacuum the number of particles is not definite. The vacuum state is partly two particles, partly four particles, etc.
That's simply wrong. The vacuum is an eigenstate of the particle number operator with the zero eigenvalue. In other words, the number of particles is definite in the vacuum. What is not definite in the vacuum is the value of field.
Bill_K
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Mar27-12, 11:34 AM
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Congratulations, Demystifier, I figured somebody would walk into that. Did I say "number operator"?

The number operator for a Dirac field is N = N+ - N-, the number of electrons minus the number of positrons, also proportional to the total charge. Yes, N commutes with the Hamiltonian and any stationary state such as the vacuum state is an eigenvalue of N. But consider for example a photon self-energy diagram in which a photon temporarily becomes a positron-electron pair. I count two particles in the intermediate state, don't you? "Two" is obtained from N+ + N-.
Bob S
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#9
Mar27-12, 06:00 PM
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Quote Quote by shifty123 View Post
So does that mean virtual particles dont work the way I assumed? Or are virtual particles something of an old theory that has been disproven some time ago?
Virtual electron positron pairs are created in vacuum (as virtual pairs) in a strong Coulomb field around charged particles, and they "shield" the true (unrenormalized) charge of protons and nuclei. This polarization of the vacuum was first calculated by Uehling* in Phys Rev (1935), and used to correct the atomic energy level measurements in pionic atoms. See Apppendix 2 in Robert E. Shafer Measurement of Pion Mass Phys. Rev. 163, 1451 (1967). See attachment.

* Serber also in same issue of Phys Rev
Attached Thumbnails
Vac_Pol_Shafer2.jpg  
matonski
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#10
Mar27-12, 10:49 PM
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Virtual Particles: What are they? by Matt Strassler.
Demystifier
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Mar28-12, 03:34 AM
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Quote Quote by Bill_K View Post
But consider for example a photon self-energy diagram in which a photon temporarily becomes a positron-electron pair. I count two particles in the intermediate state, don't you? "Two" is obtained from N+ + N-.
Ah, you are talking about VIRTUAL particles. But virtual particles are not real, as explained, e.g., at the link provided by matonski in the post above. In particular, the probability of observing a virtual particle is exactly zero. Or more precisely, that probability is not even defined by the theory, which talks only about the probabilities for observation of final particles (out states in the S-matrix).

Saying that virtual particles are real is like saying that when you have one apple, you actually have
2 apples and -1 apple because you can write
1 apple = 2 apples + (-1 apple)
nonequilibrium
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#12
Apr1-12, 02:24 PM
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But Demystifier, don't certain versions of de Broglie Bohm interpret virtual particles to be as real as any normal particle? (same ontological status) Yet that seems kind of incompatible with your 1 = 2 + (-1) argument.
Bob S
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#13
Apr1-12, 05:08 PM
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I think that Uehling's model for the disturbance of virtual electron positron pairs in strong Coulomb fields is a real effect
Phys. Rev. 48, 5563 (1935) "Polarization Effects in the Positron Theory"
and has been rigorously tested in recent years (muonic and pionic atoms) and found to be still correct. So are we just arguing semantics?

See http://philoscience.unibe.ch/documen.../uehling35.pdf
Demystifier
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#14
Apr16-12, 02:58 AM
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Quote Quote by mr. vodka View Post
But Demystifier, don't certain versions of de Broglie Bohm interpret virtual particles to be as real as any normal particle?
No, I don't think there is such a version of dBB.
hbinzhang
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#15
Apr16-12, 07:51 AM
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virtual particles just participate in the interactions,the momentum can be indefinitely large!!


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