Do all virtual particle travel at light speed in a vacuum? else wouldn't that imply they should have rest mass however tiny? When they pop back out of existence do their mass disappear instantly? BTW what is the heaviest virtual particle ever found?
Mass – Can a Virtual Particle Have Mass?
massvirtual-particles
Related Solutions
There is absolutely nothing conventional about the mass of different particle species. For any particle moving in the vacuum, you may measure the total energy $E$ (including the latent energy) and the momentum $p$. It turns out experimentally - and Einstein's special theory of relativity guarantees - that the combination $$E^2 - p^2 c^2$$ doesn't depend on the velocity but only on the type of the particle. It is a quantity describing the particle type and we call it $$E^2 - p^2 c^2 = m_0^2 c^4$$ This determines the rest mass $m_0$ of the particle. The formula above works for any particle in the vacuum, any speed, and is always non-singular. Photons have $E=pc$ which implies that $m_0=0$. The rest mass of a photon is equal to zero.
Indeed, that's also the reason why one can't really have a photon at rest, $v=0$. If a speed of something is $c$ in one reference frame, it will stay $c$ in any (non-singular) reference frame - that's another postulate of the special theory of relativity. So one can't ever make the speed of the photon zero by switching to another (non-singular) reference frame.
But if you want to see some values for all quantities, you may imagine that a photon at rest could exist and its total mass would be zero. At speed $v$, the mass is increased to $$m_{total} = \frac{m_{0}}{\sqrt{1-v^2/c^2}}$$ For $m_0=0$ and $v=c$, the expression above is clearly a $0/0$ indeterminate form and its proper result may be anything. In particular, the correct value is any finite number. At the right speed, $v=c$, the massless photons can have any finite energy.
First of all, virtual particles are indeed a consequence of the uncertainty principle – without any quotation marks. Virtual particles are those that don't satisfy the correct dispersion relation $$ E = \sqrt{m^2 c^4 +p^2 c^2}$$ because they have a different value of energy by $\Delta E$. For such a "wrong" value of energy, they have to borrow (or lend) $\Delta E$ from the rest of the Universe. This is possible for a limited amount of time $\Delta t$ as long as the "negated" time-energy uncertainty relationship $$\Delta t \cdot \Delta E \leq \hbar / 2$$ is obeyed. One simply can't measure energy $E$ during too short an interval $\Delta t$ more accurately than with the error $\Delta E$ given by the formula above which makes it possible to borrow/lend this much energy for such a short time.
Pretty much by definition, virtual particles are effects that look like a temporary existence of a real particle which is bounded in time by the inequality above. The more virtual the particle is – the greater the deviation of the energy $\Delta E$ is – the shorter is the timescale over which the virtual particles may operate. In the limit $\Delta E\to 0$, the virtual particles become "real" which means that they may also be observed. For a nonzero value, they can't be observed and they're just "intermediate effects in between the measurements" that modify the behavior of other particles. Most explicitly, virtual particles appear as propagators (internal lines) of a Feynman diagram.
The electron is not necessarily "simulating" anyone, whatever "simulating" was supposed to mean. Instead, the electron may "emit" a virtual particle such as a photon. The emission of a real photon is impossible by the energy/momentum conservation: in the initial electron's rest frame, the energy is just $m_e c^2$ but it would get increased both by the extra kinetic energy of the final moving electron and by the positive photon's energy, thus violating the energy conservation law. But the electron may emit a virtual photon for which the energy conservation law is effectively violated (or the photon has a different energy, perhaps negative one, than it should have) which is OK for the time $\Delta t$ described above. As long as the photon disappears before this $\Delta t$ deadline arrives – it is absorbed by another charged particle, everything is fine and this intermediate history contributes to the probability amplitudes. That's why charged particles influence each other due to electromagnetism; this is how the virtual photons operate.
Concerning the last question, yes, virtual particles may interfere with the real ones. For example, if we study processes in an external electric field create by many coherent long-wavelength photons, there will still be Feynman diagrams with virtual photons in them. The amplitudes from these diagrams have to be added to the amplitudes with the real classical electric field, and only the result (sum) is squared in absolute value. That's what we mean by interference.
And yes, the effects of virtual particles on a isolated electron are equally likely in all directions and in this sense they "average out". An electron state with a sharply defined 3-momentum still remains an energy eigenstate and moves along a straight line. However, due to the constant emission and reabsorption of some virtual particles, the real electron-like energy eigenstate has a "cloud" of virtual photons around it. The symmetries of the theory such as the gauge symmetry and the Lorentz symmetry aren't broken by the virtual photons. After all, the virtual photons result from the theory whose Lagrangian does respect these symmetries and no anomaly breaks them.
Best Answer
Light speed is the limit for any transfer of energy/momentum and information.
As virtual particles are described by a four vector, it will have a length value which by definition is the invariant mass of a particle. Real particles have positive and fixed invariant mass. Virtual particles can have any value of invariant mass allowed within the limits of integration, where they are defined.
Here is the definition of a virtual particle, in this pictorial representation of the integration that must be carried out to get the crossection of e-e- scattering.
Virtual particles live only within integration limits, they have the quantum numbers of the named particle but their mass is off shell, within the limits of the implied integration.
They do not exist outside integration limits, which supply the energy for the interaction. If you are thinking of vacuum loops of pair produced particle antiparticle, they can only exist in corrections to real particle interactions. If no real particles supply four vectors for the interaction, there are no observable virtual particles.
Virtual particles cannot be observed. They can be stated as a mathematical hypothesis, but their mass has to be within the limits of the integration.
In e+e- annihilation , the closer to the mass of the Z the incoming energy is, the closer the virtual Z is to the on shell mass of 90+ GeV of the Z.