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melech
2010-Jun-29, 03:28 PM
I understand that virtual photons have an unlimited range because, being massless, they don't violate the uncertainty principle. But is there a theoretical reason why real photons also have an unlimited range and lifetime?

trinitree88
2010-Jun-29, 08:17 PM
I understand that virtual photons have an unlimited range because, being massless, they don't violate the uncertainty principle. But is there a theoretical reason why real photons also have an unlimited range and lifetime?

melech No. Photons can be created nilly willy....just plug in a light bulb and turn it on. Now let them travel a short distance to a very black object where they convert to heat. Short range and lifetime. Check the hyperphysics link and do a few calculations from it's sequential link to get a feel for it. pete SEE:http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html


link is off, go to uncertainty principle....then uncertainty principle concepts.....then to range of forces....then to calculation. The url's are screen by screen, it won't load the final one, odd.

mugaliens
2010-Jun-30, 06:43 AM
I understand that virtual photons have an unlimited range because, being massless, they don't violate the uncertainty principle. But is there a theoretical reason why real photons also have an unlimited range and lifetime?

As anything approaches c, time dilation effects slow down subjective time for the moving item. At c, time stops (rather, it has no meaning). Therefore, theoretically, a photon does not "last" over time. Rather, the photon itsef is created and destroyed at the same moment in subjective time, regardless of the distance it has travelled between its creation and destruction.

forrest noble
2010-Jun-30, 04:47 PM
melech,


I understand that virtual photons have an unlimited range because, being massless, they don't violate the uncertainty principle. But is there a theoretical reason why real photons also have an unlimited range and lifetime?

A lot of these ideas concerning "unlimited" are theoretical and seemingly could not be proven -- also theory varies on this and other related matters. Real photons like atomic particles, according to theory, have no half life -- meaning an unlimited potential life if valid.

They have come up with many clever experiments to test the life of protons, for instance, whereby they were looking for their decay and some very long period as a half-life. To date they have not found such decay of protons. The same thing would seemingly apply for electrons, although such a conclusive test probably cannot be conducted for electrons.

Photons are fairly hardy in that they can bounce (reflect/ refract), travel through some liquids and solids but they can be absorbed by matter, if not reflected or refracted. It would be better for their health :) if they avoided matter. On the plane of our solar/ stellar system some photons would probably be absorbed between here and lets say Pluto. On the plane of our galaxy a lower percentage of photons would be absorbed because the density of matter in galactic space decreases. In intergalactic space photons seemingly could last a very long time because matter is very sparse out there, maybe only one hydrogen atom or proton per cubic meter.

We know that a good percentage of photons survive in intergalactic space because we can see distant galaxies and based upon their luminosities, in most cases, we see little or no apparent dimming of their EM radiation based upon their redshift and the inverse square law of light. Even if not forever, at least the possibility of billions of years old ain't bad.

As to virtual photons having an unlimited life is a contradiction of the term virtual which means living for only a very short-lived existence. Virtual photons are also still a part of theory concerning their true characteristics or even whether they really exist at all.

melech
2010-Jun-30, 05:19 PM
Thanks. But are you suggesting that virtual photons do have a time limit on their existence? I thought that the the electric and magnetic fields, mediated by virtual photons, have an unlimited range--therefor the virtual photon, unlike the virtual particles mediating the strong and weak forces, should have an unlimited lifetime, enabling it to cross vast distances.

forrest noble
2010-Jun-30, 06:55 PM
melech,


I thought that the the electric and magnetic fields, mediated by virtual photons, have an unlimited range--therefor the virtual photon, unlike the virtual particles mediating the strong and weak forces, should have an unlimited lifetime, enabling it to cross vast distances.

Prevailing theory at the present time considers that electric and magnetic fields have an unlimited range but the virtual photons within these fields suddenly appear and disappear, being very short lived. This is the meaning of "virtual" concerning photons and particles.

"Many researchers have used the virtual photon to explain the electromagnetic forces. Several have speculated that all forces might be explained through some type of photon" whether virtual or otherwise.

Virtual photons of a particular nature are thought to be the "carriers" of forces in much of quantum physics. Their existence is well accepted but whether or not or how they contribute to any observed forces, depends upon what theory you are considering.

melech
2010-Jul-01, 04:54 PM
forrest noble, If the virtual photons suddenly appear and then disappear in a short lifetime, how is the electric force carried across all space without interruption? I always assumed that each photon represented a quantum of the force for its entire journey. Does the virtual photon generate its "replacement" photon before returning to the vacuum?

Also, please note that I'm not a scientist, only an interested layman, but I thought that the theory that ALL forces are carried by virtual particles was the fully accepted view in Quantum theory. You're suggesting that there are alternative explanations for how forces are mediated across distances. Could you please expand on this, or point me to some relatively non-technical books?

Nereid
2010-Jul-01, 05:12 PM
forrest noble, If the virtual photons suddenly appear and then disappear in a short lifetime, how is the electric force carried across all space without interruption? I always assumed that each photon represented a quantum of the force for its entire journey. Does the virtual photon generate its "replacement" photon before returning to the vacuum?

Also, please note that I'm not a scientist, only an interested layman, but I thought that the theory that ALL forces are carried by virtual particles was the fully accepted view in Quantum theory. You're suggesting that there are alternative explanations for how forces are mediated across distances. Could you please expand on this, or point me to some relatively non-technical books?(bold added)

AFAIK, no alternatives exist which are part of the generally accepted scientific mainstream, which is the standard for answers in this section of BAUT (except for gravity of course, where there is no particle mediation, in GR).

I myself do not know of any material which explains electromagnetism as mediated by exchanges of virtual photons, which is quite accessible, and which is also scientifically accurate. Not that there aren't any, of course (there almost certainly must be)! Just that I don't know of any, offhand.

AriAstronomer
2010-Jul-02, 11:16 AM
To forest noble in post #4 who said: Photons are fairly hardy in that they can bounce (reflect/ refract), travel through some liquids and solids but they can be absorbed by matter, if not reflected or refracted.

I was under the impression that photons do not reflect, and do nothing except come into existence through emittance and then become absorbed by something else. What really happens is they hit a material, the photon is absorbed by the electron, and a new photon is emitted. Although the end result is the same, I think it's unwise to confuse the concept. Please someone correct me if I'm wrong.

Nereid
2010-Jul-02, 02:46 PM
Indeed. forest noble's characterisation is not a good one, if you're concerned with accuracy (it might be appropriate as part of answer, for a particular audience).

It's been well-understood, for quite a while, that photons don't behave like that; think of the two-slit experiment, for example. The detailed physics of reflection (say), in QED (the contemporary theory which best describes electromagnetism) are nothing like what was written.

trinitree88
2010-Jul-02, 06:24 PM
(bold added)

AFAIK, no alternatives exist which are part of the generally accepted scientific mainstream, which is the standard for answers in this section of BAUT (except for gravity of course, where there is no particle mediation, in GR).

I myself do not know of any material which explains electromagnetism as mediated by exchanges of virtual photons, which is quite accessible, and which is also scientifically accurate. Not that there aren't any, of course (there almost certainly must be)! Just that I don't know of any, offhand.

Nereid. I recall Grey giving an excellent summary on this, but my advanced search through his posts yielded nothing. It was to the effect that the attraction or repulsion of electric charges depended upon the symmetries of the wave equations involved. It was a few years ago. pete

Grey
2010-Jul-02, 06:57 PM
Nereid. I recall Grey giving an excellent summary on this, but my advanced search through his posts yielded nothing. It was to the effect that the attraction or repulsion of electric charges depended upon the symmetries of the wave equations involved. It was a few years ago. peteHow about taking a look at this (http://www.math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html) link?

Nereid
2010-Jul-02, 07:34 PM
Nereid. I recall Grey giving an excellent summary on this, but my advanced search through his posts yielded nothing. It was to the effect that the attraction or repulsion of electric charges depended upon the symmetries of the wave equations involved. It was a few years ago. peteHow about taking a look at this (http://www.math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html) link?
Those (Baez) pages are, nearly always, great to read.

Does this work, for our OP melech? Let's see ...

Grey
2010-Jul-02, 08:45 PM
Does this work, for our OP melech? Let's see ...Well as a straight answer to the OP the answer is that any particle (or at least any stable particle) has an unlimited range and lifetime. There's no limit to the distance an electron can travel, for example. Not violating the uncertainty principle really has nothing to do with why virtual photons have an unlimited range.

Nereid
2010-Jul-02, 08:54 PM
Well as a straight answer to the OP the answer is that any particle (or at least any stable particle) has an unlimited range and lifetime. There's no limit to the distance an electron can travel, for example. Not violating the uncertainty principle really has nothing to do with why virtual photons have an unlimited range.
I was thinking more of melech's post #11:


forrest noble, If the virtual photons suddenly appear and then disappear in a short lifetime, how is the electric force carried across all space without interruption? I always assumed that each photon represented a quantum of the force for its entire journey. Does the virtual photon generate its "replacement" photon before returning to the vacuum?

Also, please note that I'm not a scientist, only an interested layman, but I thought that the theory that ALL forces are carried by virtual particles was the fully accepted view in Quantum theory. You're suggesting that there are alternative explanations for how forces are mediated across distances. Could you please expand on this, or point me to some relatively non-technical books?

neilzero
2010-Jul-02, 09:42 PM
I suppose the definition of electromotive force is somewhat different than it was 2 centuries ago in this context. Apparently EMF is not voltage and is not magnetic lines of force nor microwave photons? Definition please? Perhaps the title of this thread is inappropriate? Neil

Nereid
2010-Jul-02, 10:02 PM
I suppose the definition of electromotive force is somewhat different than it was 2 centuries ago in this context. Apparently EMF is not voltage and is not magnetic lines of force nor microwave photons? Definition please? Perhaps the title of this thread is inappropriate? Neil
What does EMF have to do with the questions asked by the OP (and subsequently elaborated upon)?

melech
2010-Jul-02, 10:34 PM
I don't see any link as referred to in Grey's posting. Perhaps my security system suppressed it? Would someone please re-post it?

Grey, you said that not violating the uncertainty principle has no bearing on the life of a virtual particle. However, I have read this in many semi-popular science sources. I'll quote from one at hand: Isaac Asimov in "Understanding physics. Vol 3, Signet Science library, pp.243-245, speaking of the time-energy uncertainty relationship states: "During the period of existence of the virtual particle it can move away from the parent particle, but it can only move a limited distance because it must be back when the time-uncertainty period is over. The more massive the particle, the greater the uncertainty represented by this energy and the shorter the time interval permitted its existence".
While Asimov is not specifically addressing the virtual photon, I've read other sources which say that its masslesness allows it to have an infinite lifetime, assuming it's not absorbed in its journey. Convereely, Yukawa's assumption about the massiveness of the virtual boson mediating the strong force is based precisely on the short effective distance of the force.
I hope I haven't been wrong about this, all these years.

Nereid
2010-Jul-03, 06:18 AM
I don't see any link as referred to in Grey's posting. Perhaps my security system suppressed it? Would someone please re-post it?

Grey, you said that not violating the uncertainty principle has no bearing on the life of a virtual particle. However, I have read this in many semi-popular science sources. I'll quote from one at hand: Isaac Asimov in "Understanding physics. Vol 3, Signet Science library, pp.243-245, speaking of the time-energy uncertainty relationship states: "During the period of existence of the virtual particle it can move away from the parent particle, but it can only move a limited distance because it must be back when the time-uncertainty period is over. The more massive the particle, the greater the uncertainty represented by this energy and the shorter the time interval permitted its existence".
While Asimov is not specifically addressing the virtual photon, I've read other sources which say that its masslesness allows it to have an infinite lifetime, assuming it's not absorbed in its journey. Convereely, Yukawa's assumption about the massiveness of the virtual boson mediating the strong force is based precisely on the short effective distance of the force.
I hope I haven't been wrong about this, all these years.
Grey's link is to a webpage entitled Some Frequently Asked Questions About Virtual Particles

The link is: http://www.math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

Please let us know if this is helpful, and which parts are too hard to follow.

melech
2010-Jul-05, 03:01 PM
Thanks for the posting. It was very helpful, although heavy sledding in places. It even dealt with questions I had not thought to ask, e.g., how the same force can be either attractive or repulsive.

trinitree88
2010-Jul-05, 03:38 PM
Nereid, Grey...thanks, that's the one. pete