Virtual bosons and conservation of energy

[a dull boy]

I read this on a website called Physics for Idiots

"If an electron gets near another electron it emits a virtual photon which is absorbed by the second electron and let's it know it need to move away."


If a virtual photon is absorbed, doesn't than make it real, and so break conservation of energy?

Thanks, Mark

 

[TumblingDice]

If a virtual photon is absorbed, doesn't than make it real, and so break conservation of energy?

The energy is transferred as momentum, according to wiki:

we can imagine one particle emitting a virtual particle which is absorbed by the other. The virtual particle transfers momentum from one particle to the other.

More here: en.wikipedia.org/wiki/Force_carrier#Forces_from_the_particle_viewpoint

 

[a dull boy]

If the energy is transferred as momentum, wouldn't that make the virtual particle real? Meaning, if a virtual particle can exist only within the uncertainty principle, and before it disappears transfers momentum to a real particle, where does that momentum come from?

I would think this could be a source of unlimited energy, if you could freely transfer momentum from virtual particles to real particles.

I must be missing something...

Thanks, Mark

 

[ChrisVer]

the momentum the one particle gets is equal to the momentum the other particle is losing...

 

[mfb]

If the energy is transferred as momentum, wouldn't that make the virtual particle real?

No. Why should it?

Meaning, if a virtual particle can exist only within the uncertainty principle, and before it disappears transfers momentum to a real particle, where does that momentum come from?

Momentum is conserved at every interaction point.

I would think this could be a source of unlimited energy, if you could freely transfer momentum from virtual particles to real particles.

No. Energy and momentum are exactly conserved everywhere.

 

[kurros]

Actually I have related question; why do cosmologists worry about things like Boltzmann brains and so on? Isn't the idea there that "observers" could fluctuate out of the vacuum (given almost infinite time) and so with an infinity of universes or some such we would have to worry about the probability that we could be such observers?

But how does that make sense? Even granted that the vacuum could fluctuate into such a configuration, as stated above this would only be "allowed" for a completely negligible time interval, no matter how long we are allowed to wait for fluctuations. Am I missing something here?

 

[mfb]

Actually I have related question; why do cosmologists worry about things like Boltzmann brains and so on?

It would be weird to be one. And there is no reason why a Boltzmann brain should have a memory that looks like the world would follow any specific rules - this would be a very rare coincidence.

Even granted that the vacuum could fluctuate into such a configuration, as stated above this would only be "allowed" for a completely negligible time interval, no matter how long we are allowed to wait for fluctuations.

Still long enough to wonder about the world. This is very rare, but it could exist.