Virtual Particle's Violate Causality?

[ps3]

do they violate causality since they travel faster than light and violate conservation?

[Severian]

They don't travel faster than light, so, no, they don't violate causality.

[ps3]

what do virtual particle's violate?

[timo]

They do not obey the energy-momentum relation that a free particle of the respective field would have.

[ps3]

why can they violate causality?

[Severian]

No.

 

Interestingly in Quantum Field Theory you can show that particles with positive energy always must move forwards in time in order to preserve causality. This seems fair enough, but perhaps more oddly, you can also show that particles of negative energy must flow backwards in time to preserve causality (weird huh?).

 

This is actually where antiparticles come from since a negative energy particle moving backwards in time is actually identical to a positive energy antiparticle moving forwards in time. This is called the Feynman-Stueckleberg interpretation.

[ps3]

so what is really going back in time? this is what confuses me with anti particle and particle time travel,back in time sort of thing.

Edited October 16, 2008 by ps3

[Cap'n Refsmmat]

Nothing's going back in time. Antiparticles don't violate causality.

[ps3]

i read that virtual particle's CAN travel faster than light,because of the uncertainty principle?

[Gilded]

All of it falls within the bounds of the uncertainty principle. Virtual particles can violate rules all they want as long as they don't get caught, so to speak. Regarding causality, information isn't transmitted at superluminal speeds even by virtual particles with negative energies as noted by Severian.

[ps3]

so this mean's cause and effect is safe from being violated?

[ps3]

can virtual particle go faster than the speed of light?

[Kyrisch]

No. Virtual particles have normal mass. Theoretically, particles with imaginary (in the mathematical sense) mass can (called "tachyons").

[ps3]

i have some link's where it's said they DO violate causality. http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

 

http://www.ucolick.org/~mountain/AAA/030718.html

[swansont]

i have some link's where it's said they DO violate causality. http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

 

http://www.ucolick.org/~mountain/AAA/030718.html

 

First link: you misread it. "The virtual particle didn't "transmit" any information that I didn't have already; it is useless as a means of faster-than-light communication." That translates as: doesn't violate causality.

 

Second Link:

"First off a disclaimer: I'm not a particle physicist, and although I have a basic understanding of Quantum Field Theory, I can by no means claim to be an expert on virtual particles"

 

The "violation" he then quotes is the HUP.

[ps3]

what is the longest time a virtual particle can last?

[swansont]

what is the longest time a virtual particle can last?

 

It depends on its energy. The longest time approaches infinity.

[ps3]

so it can last infinitly?

[Gilded]

so it can last infinitly?

 

I think that would mean the particle has 0 energy. So no.

[Kyrisch]

so it can last infinitly?

 

It can last as near to infinity as the particle's energy is near zero.

 

[math]\Delta E \Delta t \leq \hbar[/math] Where [math]E[/math] is energy, [math]t[/math] is duration, and [math]\hbar[/math] is Dirac's constant.

[timo]

Wouldn't that depend on the value of [math]\Delta[/math], too?

[Severian]

The probability of decay is proportional to the Breit-Wigner distribution:

 

[math]\frac{m \Gamma}{\left(E^2-|\vec{p}|^2-m^2 \right)^2 + m^2 \Gamma^2}[/math]

 

where [math]\Gamma[/math] is the particle width and I have neglected the [math]c[/math]'s.

[ps3]

The probability of decay is proportional to the Breit-Wigner distribution:

 

[math]\frac{m \Gamma}{\left(E^2-|\vec{p}|^2-m^2 \right)^2 + m^2 \Gamma^2}[/math]

 

where [math]\Gamma[/math] is the particle width and I have neglected the [math]c[/math]'s.

 

what does this all mean?

 

i was told by a friend that causality is violated but since we can't see it happening it cannot affect us.is this true?

[ps3]

ive been told that virtual particle's are allowed to travel faster than light due to uncertainity principle. here is a link where i got my info. http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

[Riogho]

They can go as fast as they frikkin want to, they don't exist.