Another physics question

[Hari_Seldon]

It is said that the only three measurements you can make of a black hole are its mass, angular momentum and charge. The mass is proportional to the size of the event horizon and the angular momentum is determined by its shape. But how can you measure the charge? The reason I find this mysterious is that electric and magnetic interaction are mediated by exchange of photons and no photon can escape the black hole (except by Hawking radiation, but I don’t think that is what is going on here).

[Ring]

Electromagnetic interactions are mediated by virtual photons not real photons, and virtual photons are not constrained by light speed.

[robby]

*Originally posted by Ring *
**Electromagnetic interactions are mediated by virtual photons not real photons, and virtual photons are not constrained by light speed. **

Are you sure this is true, Ring? I was under the impression that there was no difference between virtual photons and real photons. Also, I thought that because virtual photons do in fact travel at c, it results in the characteristic inverse-square nature of electromagnetism. The Heisenberg Uncertainty Principle puts a limit on the energy and the length of time that the virtual photon can exist. The lower the energy of a given virtual photon, the longer it can exist, and hence, the farther it can travel and mediate the electromagnetic force. This results in the electromagnetic force weakening with distance.

[Angua]

*Originally posted by Hari Seldon *
**It is said that the only three measurements you can make of a black hole are its mass, angular momentum and charge. The mass is proportional to the size of the event horizon and the angular momentum is determined by its shape. **

Slight hijack here. How do you know the shape of a black hole? Surely (from my understanding of General Relativity) the angular momentum determines the shape and location of the event horizon and the stationary limit surface, and that if the black hole has no angular momentum, the two co-incide (Schwartzschild black hole) but if there is angular momentum, the two do not, except at the poles, and the degree of seperation of the two, which gives the black hole its shape. I.e. that the angular momentum determines its shape, and not the other way round as you suggest.

Sorry if that’s actually what you meant, I’m not trying to be pedantic, or anything like that.

Angua

[Angua]

Firstly, ignore my sentence about knowing the shape of a black hole!

Secondly,

*Originally posted by robby *
**Are you sure this is true, Ring? I was under the impression that there was no difference between virtual photons and real photons. Also, I thought that because virtual photons do in fact travel at c, it results in the characteristic inverse-square nature of electromagnetism. The Heisenberg Uncertainty Principle puts a limit on the energy and the length of time that the virtual photon can exist. The lower the energy of a given virtual photon, the longer it can exist, and hence, the farther it can travel and mediate the electromagnetic force. This results in the electromagnetic force weakening with distance. **

That’s correct, AFAIK. There is a difference however between real and virtual photons, except I never paid enough attention in my classes on quantum theory to remember what it is. I think its something to do with lifetimes, and the fact that virtual photons are carrying energy that isn’t really there, so they have to decay. I’m too much of an astronomer, as opposed to a particle or theoretical physicist to go into great depth about this.

[Ring]

*Originally posted by robby *
Are you sure this is true, Ring? I was under the impression that there was no difference between virtual photons and real photons.

Yes I’m sure. Virtual particles are not on the mass shell which means they violate energy conservation, and no real particle can do that.

** Also, I thought that because virtual photons do in fact travel at c, it results in the characteristic inverse-square nature of electromagnetism. The Heisenberg Uncertainty Principle puts a limit on the energy and the length of time that the virtual photon can exist. The lower the energy of a given virtual photon, the longer it can exist, and hence, the farther it can travel and mediate the electromagnetic force. This results in the electromagnetic force weakening with distance. **

For more see the physics faq
http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

Yet here we have something that, at least at first glance, isn’t supposed to be possible in special relativity: the virtual photon can go from one interacting particle to the other faster than light! It turns out, if we sum up all possible momenta, that the amplitude for transmission drops as the virtual particle’s final position gets further and further outside the light cone, but that’s small consolation. This “superluminal” propagation had better
not transmit any information if we are to retain the principle of causality.

[robby]

I did a brief search on-line, and found this web page:

http://www.ph.surrey.ac.uk/partphys/chapter5/QED.html

It states:

In particle physics, quantum particles can be thought of as being surrounded by clouds of virtual particles which are emitted and usually reabsorbed by the parent particles. These virtual particles have lifetimes that depend on the uncertainty principle. The more massive and energetic a particle is, then the more brief its lifetime and the smaller its range.

While it does not refer to the speed of virtual photons, it relates the lifetime of a virtual photon to its range. This implies the speed of such photons is finite.

On the other hand, this website appears to support Ring’s contention:

http://www.faqs.org/faqs/astronomy/faq/part4/section-11.html

virtual particles can pretty much “do” whatever they like, including travelling faster than light, so long as they disappear before they violate the Heisenberg uncertainty principle.

This website also covers the topic, and supports Ring’s contention:

http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

[robby]

*Originally posted by Ring *
For more see the physics faq
http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

I didn’t mean to talk past you, Ring. Kind of funny that we quoted the same website!

[Ring]

robby anytime you do a search on a physics subject check the various physics faqs first. These faqs have been vetted by innumerable Ph.D. physicists, and you can be sure the information is correct. The web is full of stuff posted by cranks.

[robby]

*Originally posted by Ring *
**robby anytime you do a search on a physics subject check the various physics faqs first. These faqs have been vetted by innumerable Ph.D. physicists, and you can be sure the information is correct. The web is full of stuff posted by cranks. **

True enough. I generally try to be somewhat selective, and prefer to cite web pages from reputable-looking sources, such as universities.

[lucwarm]

*Originally posted by Hari Seldon *
** But how can you measure the charge? **

I wouldn’t know a virtual photon if it bit me on the nose, but I will propose the following way of measuring the charge:

Hook an electrically neutral up to a spring scale in the vicinity of a black hole (be careful of that event horizon, though!). Repeat with an object of equal mass that possesses a charge. For example, you might use yourself as an object and rub your feet on the floor (on a dry day) between weighings.

Any change in readout on the scale arguably means the black hole has charge. If you know the distance to the black hole; the change in charge of the test object; and the change in scale readout, you oughtta be able to figure out the charge of the black hole.