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View Full Version : How Real Are Virtual Particles?

batman1
2009-Jan-22, 08:04 AM
i read that they are real and that they travel faster than light,would that mean they are time traveling back in time to an earlier time? http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

alainprice
2009-Jan-22, 12:54 PM
They're very much virtual, since you can never measure one. However, the same particle but real can be measured easily.

The uncertainty principle does not apply to virtual particles since they don't last long enough to satisfy the HUP. They can travel faster than light, but again, we're talking about very small distances. Does this have any physical significance, I don't know.

trinitree88
2009-Jan-22, 05:04 PM
They're very much virtual, since you can never measure one. However, the same particle but real can be measured easily.

The uncertainty principle does not apply to virtual particles since they don't last long enough to satisfy the HUP. They can travel faster than light, but again, we're talking about very small distances. Does this have any physical significance, I don't know.

alainprice. I think I disagree here. Yukawa used the Heisenberg Uncertainy Principle to calculate the possible mass of an exchange particle between two nucleons separated by about a fermi...typical nucleon separation distances. Using Einstein's modified version of (Delta t)(Delta E) = to or > h/2 pi, he solved for the time interval to traverse the distance, and predicted the limiting mass/energy of the exchange particle . m=E/c2..from Eintein's Special Theory...substituting the E from the Heisenberg Uncertainty. He was pretty close. One need never see the exchange particle, so it was virtual until the nucleus was agitated sufficiently to promote it into existence. The agitation involved accelerator lab high speed collisions. That's how a virtual particle is made "real". The pion turned out to be real.
The prosed W bosons, of shorter range and hence larger mass/energy, were also later found, at much higher collisional energies. They too, fit HUP. pete

alainprice
2009-Jan-22, 06:53 PM
For a large number of interactions, they do behave well. If we're talking about repeated experiments, I agree with you. If we talk about a single event, or the true nature of the particle, I'm not so sure.

Just because, in theory, something can travel faster than light does not mean we can transmit information like this. It's a quantum process, we get random results and then compare. We must compare results at less than the speed of light.

gzhpcu
2009-Jan-22, 08:09 PM
They're very much virtual, since you can never measure one. However, the same particle but real can be measured easily.

The uncertainty principle does not apply to virtual particles since they don't last long enough to satisfy the HUP. They can travel faster than light, but again, we're talking about very small distances. Does this have any physical significance, I don't know.

Virtual particles come in pairs. They appear at small distances (Planck) as a virtual particle - virtual antiparticle pair, and quickly annihilate each other. They can do so because of the Heisenberg Uncertainty Principle.
(http://science.jrank.org/pages/7195/Virtual-Particles.html)

Virtual particles do not travel faster than the speed of light. (unless they are virtual neutrinos... which we do not know if neutrinos exist)

alainprice
2009-Jan-22, 09:40 PM
Virtual particle pairs are only one type of virtual particle.

Magnetic and electric fields are made up of virtual photons. Then there are particles involved in high energy collisions which are virtual and then degrage into real particles. Then...

trinitree88
2009-Jan-22, 10:54 PM
Virtual particle pairs are only one type of virtual particle.

Magnetic and electric fields are made up of virtual photons. Then there are particles involved in high energy collisions which are virtual and then degrage into real particles. Then...

alainprice. My understanding is that the electron and the proton exchange the electromagnetic force via virtual photons, like a constant fusillade of cannonballs, with the photons being the balls, here. Just as "shaking" the nucleus promotes pions into the "real" world, shaking the atom promotes various virtual photons into the real world ,too...as light. Shaking the quarks in a baryon promotes colored gluons. This is the model of exchange forces and their carriers....(the search continues for a conclusive experimental determination of a graviton).
Changing the signs of the wave functions of the particles involved determines if the interaction is attractive or repulsive...the like charges repel,thing. pete

P.S. If you don't add the "shaking" energy, the promotion never happens, and the virtual particles remain that forever. That's Conservation of mass/energy, amongst others.

Hornblower
2009-Jan-22, 11:42 PM
How "real" is anything? What is the definition of "real"?

If the predicted effects of the little beasties are in good agreement with observations, I would consider them to be real unless the theory turns out to be bad.

gzhpcu
2009-Jan-23, 04:26 AM
Virtual particle pairs are only one type of virtual particle.

Magnetic and electric fields are made up of virtual photons. Then there are particles involved in high energy collisions which are virtual and then degrage into real particles. Then...

Yes, but virtual photons as force carriers do not exceed the speed of light.

Tensor
2009-Jan-23, 10:37 PM
Yes, but virtual photons as force carriers do not exceed the speed of light.

Not true. In Feynmann's Path Intergral approach (Sum over Histories) you have to include all possible paths, even those that include faster than light movement. Since Feynmann's approach is mathematically equivalent to Schwinger's Variation approach (Path differention), the faster than light movement is included in all virtual particle force exchanges.

gzhpcu
2009-Jan-24, 05:20 AM
The energy of the virtual particle is defined via http://www.scienceforums.net/forum/latex/img/6865ae0b62abdcde3b9cc35573b5d365-1.gif (http://javascript%3Cb%3E%3C/b%3E:;).

In physics, particularly in quantum field theory, configurations of a physical system that satisfy classical equations of motion, such as the above, are called on shell, and those that do not are called off shell. The equation draws a 4d circle (shell) in Minkowski space, a particle obeying it is said to be on shell, while one disobeying it is said to be off shell

Feynmann virtual particles are allowed to be off shell.

In this view, the virtual particles can travel faster than light. This is not a violation of causality though since you can't measure them going faster than light.

Energy conservation is probably one of the most fundamental laws we have, so there is another interpretation, namely that energy is conserved, but http://www.scienceforums.net/forum/latex/img/6865ae0b62abdcde3b9cc35573b5d365-1.gif (http://javascript%3Cb%3E%3C/b%3E:;) is violated.

In this interpretation, the virtual particle never goes faster than light.

Seems like both are equally valid viewpoints.