ps3

the author of the article said "virtual particles can travel slower or faster than light,forwards and bavkwards in time,however they are virtual.

i saw a video on youtube where a guy is trying to build a time machine. http://en.wikipedia.org/wiki/Ronald_Mallett swansont have you read the two links i provided?

anything going back in time would be paradoxical right?

so not something actully physical or really happening in the universe?

how come he says they are not paradoxical?

have you heard of his model?

Nothing goes back in time. It's just a way of looking at the math, a model. It is not some experimental result.

what is that stenger is talking about then?

You can model them that way — they exhibit that behavior. I refer you to the link I gave in post #6. It'll tell you the same thing, because you're asking the same question.

 

How about asking a different (hopefully better) question?

did you read the links i provided? what is it that the writer is talking about? because if these particles are able to do this,they would violate laws and causality and see weird paradoxes.

have these two links where the writer says that quantum particle's do in fact travel back in time,is this true? or i misread them? http://www.colorado.edu/philosophy/vstenger/Briefs/QuantumTimeTravel.html http://www.colorado.edu/philosophy/vstenger/void.html

here is where i got my info. http://www.colorado.edu/philosophy/vstenger/Briefs/QuantumTimeTravel.html http://www.colorado.edu/philosophy/vstenger/void.html

Told you this would happen

what are talking about?? this is different.

ok,i read it,so his "time machine" won't work? also do quantum particle's travel back in time?

In the early 90's a Professor proposed a theory for a "quantum Time machine" http://www.npl.washington.edu/AV/altvw45.html

well,my project is more on time travel.have you heard of yakir aharonov?

im doing a school project on them which has to be done soon.

Just to let everyone know, ps3 is known as Wolv1 on SciForums and Deuce2 on PhysOrg. Myself and the posts known as Rpenner and Prometheuse (on PhysOrg) and BenTheMan (a mod on Sciforums and a poster on PhysOrg) have all answered the thread starters questions multiple times. He just asks the same questions again and again and again. He was banned from SciForums for repeatedly ignoring the answers we provided and spamming several people's PM boxs with the same questions. The aforementioned people are either physics graduates or current postgraduates, so it's not like he's asking questions to people who don't know.

 

He will continue to ask these questions and I'm willing to be he's already PM'd a few of you. Don't bother replying, he'll just ask the same question rephrased again and again.

 

We can't work out why he's got such an obsession with it.

ya becuase you guys don't give simple answer's like these guys on here.

so just becuase relativity allows something doesnt mean it can work in the universe? so relativty could allow it but not the universe? so the solutions would have to be wrong for there to be no ctc's? or can the solution be right without any ctc's?

would something be wrong if ctc's did not exist?

causality would be violated if they existed?

what would be proof that ctc's exist?

ive read or been told,im not sure,that QG will probly prevent ctc from forming.

are they physicaly real? what is thought to prevent them?

so virtual particle's don't do any of that sci-fi stuff like a time loop or going back in time sort of thing?

im having trouble because i was told that they travel faster than light,but im all ears though.

why are virtual particle's allowed to travel faster than the speed of light?

need a bit of help till i turn in my paper.

OK - let me try and answer this once and for all. The question "do virtual particles travel faster than light" is a somewhat ill defined question unless you define what you mean by a virtual particle and what you mean by its velocity (or momentum).

 

Some people hold to the interpretation that:

 

"In perturbation theory, systems can go through intermediate "virtual states" that normally have energies different from that of the initial and final states. This is because of another uncertainty principle, which relates time and energy."

 

This is a quote from the page you link to. In that interpretation they define the energy of the virtual particle via [math]E^2 = p^2 c^2+m^2c^4[/math] and then say that since [math]p^2 c^2+m^2c^4[/math] is not equal to the square of the sum of the energies before (or after) the virtual particle was formed (or decayed), they say that energy was not conserved. This view is embodied in the Heisenberg uncertainty principle.

 

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.

 

However, this is not the modern way of interpreting this. Energy conservation is probably one of the most fundamental laws we have, so modern physicists would rather hang on to it. So they say that energy is conserved, but [math]E^2 = p^2 c^2+m^2c^4[/math] is violated. Since this equation draws a 4d circle in Minkowski space, a particle obeying it is said to be on-mass-shell, while one disobeying it is said to be off-mass-shell, or just off-shell. The more off-shell a particle is, the shorter its lifetime, which is the modern embodiment of the HUP.

 

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

 

So which interpretation is correct? Both are, and neither are at the same time! Since you cannot experimentally tell them apart - there is no measurement you can make that distinguishes them - then they are equally valid viewpoints. This is why they are called interpretations.

so causality is violated unobservable becuase we cant measure virtual particles? my friend said "While virtual particles can break causality, we can never observe virtual particles, and so we never can observe causality violations. What is a virtual particle? Virtual particles are how we understand that quantum interactions occur. For example, suppose two electrons come along and bump off of each other. One electron experiences the electric field of the other electron. The force between the two electrons is due to exchange of photons. These photons are virtual.

Given this, the influence of one electron on another is the result of a sum over all of the virtual photons exchanged---this is the Feynman path integral. While some virtual photons travel faster than the speed of light, some (in fact, many more) photons also travel slower than the speed of light. This means that the net effect is mediated at a speed that respects causality.

Summary: While one photon in the interaction may travel faster than the speed of light, the net effect of the interaction requires that one sum over all photons exchanged. Because the probability of a virtual photon traveling slower than the speed of light is MUCH greater than the probability of a virtual photon traveling faster than the speed of light, the net effect means that the information between the two electrons is exchanged in a manner which respects causality''