How do Virtual Photons Cause Attraction?
jeff...@howamazing.com
be attracted to a proton (+)?
I understand that all particles
(eg electrons, protons, gauge
bosons, and even our universe)
are fluctuations in a
"Sea of Zero-Point Energy".
If you could e-mail me an answer,
I'd appreciate it.
- Jeff Relf -
Saco
> What causes an electron (-) to
> be attracted to a proton (+)?
Electromagnetic interaction. As to how do virtual photons cause
attraction, the short answer is - because they can. For the long
answer look at the reference in my previous message.
Phil Gardner
The experts attribute the attraction to the continued emission and
transmision of virtusl photons and the interaction of these with the
real particles. But none of them can provide an account of how they
do it that is intelligible to the rest of us.
[Moderator's note: This "expert" recommends reading Feynman's "QED"
-- KS]
You can either settle for the fact that they are mathematical devices
that correctly predict the observed motions of the interacting
fermions but whose physical existence can never be demonstrated or
simply assume that each real particle can sense and respond to the
wave functions of other particles, or to some function of each of
them, eg a quantum potential.
Phil Gardner
John Baez
Phil Gardner <pej...@oznetcom.com.au> wrote:
>The experts attribute the attraction to the continued emission and
>transmision of virtual photons and the interaction of these with the
>real particles. But none of them can provide an account of how they
>do it that is intelligible to the rest of us.
Right - but the main reason is that as soon as anyone understands
this stuff, he or she becomes what you would call an "expert", and
is no longer one of "the rest of us".
So here's my advice to anyone who really wants to understand virtual
particles: become an "expert"!
>[Moderator's note: This "expert" recommends reading Feynman's "QED"
>-- KS]
Yes, that's certainly the best way to get started on understanding
this stuff. To dig deeper one should really take a course on quantum
field theory, or work ones way through a textbook on the subject.
It's a lot of work, but it's also incredibly fun! My current favorite
among elementary textbooks is:
Michael E. Peskin and Daniel V. Schroeder, An Introduction to Quantum
Field Theory, Addison-Wesley, 1995.
Phil Gardner
No way, life is too short. If I were to spend the time and energy
needed to use quantum field theory effectively what would I achieve?
The ability to duplicate the testable predictions of all the experts
who have gone before. Yes. New insights into physical processes at
the subatomic level? I doubt it. The conviction that virtual photons
and their continual emission and absorption by electrons or other
particles have any physical reality at all, are anything more than
mathematical devices conjured up by physicists who could not find any
simpler model that worked? I doubt it. An answer to the very basic
question: "Do the observed changes in momentum and kinetic energy of
an electron interacting with another fermion of opposite charge really
require any frame dependent transfer of momentum and energy between
the particles?" I doubt it.
Phil Gardner
Saco
news:<744cb7cd.02042...@posting.google.com>...
> No way, life is too short. If I were to spend the time and energy
> needed to use quantum field theory effectively what would I achieve?
Understanding physics in subatomic level.
> The ability to duplicate the testable predictions of all the experts
> who have gone before. Yes. New insights into physical processes at
> the subatomic level? I doubt it.
Well when the classical electrodynamics was developing nobody could
expect invention of radio. After development of quantum theory many
people were thinking that physics is finished and that the rest is
just calculation. The progress in quantum electronics made possible
the existence of internet and computers. Who knows what the progress
in quantum field theory might lead?
Daryl McCullough
between electrons is easily understood using the basketball
model: If I on the left (representing one
electron) throw a basketball (representing a virtual
photon) to you on the right (representing another electron),
then the resulting momentum exchange pushes you further
to the right and me further to the left.
Recently someone (Matt McIrvin) described how
attractive forces can be understood as an interference effect.
Understanding the attraction between an electron and a positron
requires an analysis of the interference between the effects
of leftward-pushing and rightward-pushing virtual photons.
This explanation is nice, but shows intuitiveness symmetry
breaking: the repulsive case is much simpler to understand.
However, it seems to me that hole theory (if you don't
like Dirac's hole theory for positrons, you can instead
think about hole theory for conductors in solid state
physics) makes it as easy to understand attraction as
repulsion. If one electron repels all other electrons,
then of course a hole will tend to drift towards an
electron as other electrons move farther away. It doesn't
seem like you need any detailed consideration of interference
effects.
Hole theory is sufficiently like QED that I think that the
explanations for attractive forces must be essentially the
same. So, either there is an explanation in the QED case
that doesn't involve detailed calculation of interference,
or the same interference effect must secretly be at work
in the hole case, as well. But I don't see it. Can anyone
show how these two explanations of attraction are related?
--
Daryl McCullough
Ithaca, NY
JG
news:<744cb7cd.02042...@posting.google.com>...
> ba...@galaxy.ucr.edu (John Baez) wrote:
> > To dig deeper one should really take a course on quantum
> > field theory, or work ones way through a textbook on the subject.
> > It's a lot of work, but it's also incredibly fun! My current favorite
> > among elementary textbooks is:
> >
> > Michael E. Peskin and Daniel V. Schroeder, An Introduction to Quantum
> > Field Theory, Addison-Wesley, 1995.
> No way, life is too short. If I were to spend the time and energy
> needed to use quantum field theory effectively what would I achieve?
> The ability to duplicate the testable predictions of all the experts
> who have gone before. Yes. New insights into physical processes at
> the subatomic level? I doubt it. The conviction that virtual photons
> and their continual emission and absorption by electrons or other
> particles have any physical reality at all, are anything more than
> mathematical devices conjured up by physicists who could not find any
> simpler model that worked? I doubt it. An answer to the very basic
> question: "Do the observed changes in momentum and kinetic energy of
> an electron interacting with another fermion of opposite charge really
> require any frame dependent transfer of momentum and energy between
> the particles?" I doubt it.
The exchange of virtual photons is not required to explain
electromagnetic interactions but it's an elegant way to describe
what's going on at the microscopic level. You can use Coulomb's
equation to describe the electromagnetic force between the two
particles, F = Kq'q/r^2, but by introducing a propagator (virtual
photon), you can describe all fundamental interactions, i.e., gravity,
electromagnetic, electro-weak and strong. Each force has its own
propagator and all interactions are described through the exchange of
virtual particles. "The quantum concept of continual emission and
absorption of virtual photons by the charge source is no more or less
fictitious than the classical concept of a field surrounding the
source."[1]
This representation helped me understand how protons in nuclei
overcome the repulsive force from Coulomb's Law through the exchange
of virtual pions. The discovery of the neutral pion in 1947 seemed to
support this concept of virtual particle exchange (Yukawa's
prediction), although describing strong interactions with nuclei via
virtual pion exchange turned out to be an overly simplistic view of
the nuclear-force quantum. HTH
Jerry
1. Donald H. Perkins, Introduction to High Energy Physics,
Addison-Wesley, 1986.
Charles Francis
>ba...@galaxy.ucr.edu (John Baez) wrote in message
>news:<aaard5$pqr$1...@glue.ucr.edu>...
>> To dig deeper one should really take a course on quantum
>> field theory, or work ones way through a textbook on the subject.
>> It's a lot of work, but it's also incredibly fun! My current favorite
>> among elementary textbooks is:
>>
>> Michael E. Peskin and Daniel V. Schroeder, An Introduction to Quantum
>> Field Theory, Addison-Wesley, 1995.
>No way, life is too short. If I were to spend the time and energy
>needed to use quantum field theory effectively what would I achieve?
>The ability to duplicate the testable predictions of all the experts
>who have gone before. Yes.
One would hope so.
>New insights into physical processes at
>the subatomic level? I doubt it.
That depends. Actually I think you probably get more insight from
Feynman's QED.
>The conviction that virtual photons
>and their continual emission and absorption by electrons or other
>particles have any physical reality at all, are anything more than
>mathematical devices conjured up by physicists who could not find any
>simpler model that worked? I doubt it.
Since this is the view of many physicists anyway, I think you would have
to go a lot further than read Peskin & Schroeder.
>An answer to the very basic
>question: "Do the observed changes in momentum and kinetic energy of
>an electron interacting with another fermion of opposite charge really
>require any frame dependent transfer of momentum and energy between
>the particles?" I doubt it.
Well I hope you would get a clear answer to that. I do not know why you
say frame dependent however, the theory is covariant and, Yes,
covariance does imply that it is really necessary for momentum and
kinetic energy to be transmitted from one fermion to another by means of
an intermediary.
As for whether you chose to think of the intermediary as a particle, as
Feynman does, or whether you chose to think that "virtual" particles are
real, as Feynman also does, or whether you wish to think of it instead
as some sort of quantised field with properties that can be described in
mathematics but not thought of with physical intuition as most
physicists appear to, all of that is an open question. I follow Feynman,
but it is a question that requires far more than a reading of Peskin &
Schroeder to answer.
Regards
--
Charles Francis
Surendar Jeyadev
Phil Gardner <pej...@oznetcom.com.au> wrote:
>
>No way, life is too short. If I were to spend the time and energy
>needed to use quantum field theory effectively what would I achieve?
Pleasure.
--
Surendar Jeyadev jey...@wrc.xerox.bounceback.com
Remove 'bounceback' for email address
Phil Gardner
> In article <744cb7cd.02042...@posting.google.com>, Phil
> Gardner <pej...@oznetcom.com.au> writes
>
> > (Snip) .............. what would I achieve?
>
> > (Snip) ........................ An answer to the very basic
> >question: "Do the observed changes in momentum and kinetic energy of
> >an electron interacting with another fermion of opposite charge really
> >require any frame dependent transfer of momentum and energy between
> >the particles?" I doubt it.
>
> Well I hope you would get a clear answer to that. I do not know why you
> say frame dependent however, the theory is covariant and, Yes,
> covariance does imply that it is really necessary for momentum and
> kinetic energy to be transmitted from one fermion to another by means of
> an intermediary.
>
Consider an isolated two particle system which goes from one uniform
motion state to another by way of an elastic interaction. A change of
reference frame changes the ratio, T2/T1(final/initial kinetic
energies) for each particle. Current theories (covariance and
whatever) insist that this requires the physical transfer of energy
from one particle to another, that the magnitude and direction of
transfer can be varied at will by an arbitrary change of reference
frame. That is why I say "frame dependent".
> As for whether you chose to think of the intermediary as a particle, as
> Feynman does, or whether you chose to think that "virtual" particles are
> real, as Feynman also does, or whether you wish to think of it instead
> as some sort of quantised field with properties that can be described in
> mathematics but not thought of with physical intuition as most
> physicists appear to, all of that is an open question. I follow Feynman,
> but it is a question that requires far more than a reading of Peskin &
> Schroeder to answer.
>
> Regards Charles Francis
What are your criteria for classing a particle as "real", as existing
physically? Mine are that it has:
A finite (non-zero) relativistic mass, M = p/v. A lifetime of not
less than 10^-28 s ( of more than 10 s if it mediates long range
interactions). A position within a laboratory reference frame that
can be defined approximately by some function of the time registered
by a standard clock in the laboratory.
I believe that anyone who believes that virtual photons are real
particles, as you seem to, should have some order of magnitude answers
to the following questions:
Given an isolated pair of charged leptons some 10^9 m apart is the
interaction between them still finite (an acceleration of order 2.5 x
10^-16 m s^-2) ? If so:
At what rate are the postulated virtual photons emitted? What is the
mean energy of one of these virtual photons? What is the mean
lifetime of one of them? How far out in space do they travel? Where
do they cease to exist?
Have you?
Phil Gardner
Charles Francis
<da...@cogentex.com> writes
>However, it seems to me that hole theory (if you don't
>like Dirac's hole theory for positrons, you can instead
>think about hole theory for conductors in solid state
>physics) makes it as easy to understand attraction as
>repulsion. If one electron repels all other electrons,
>then of course a hole will tend to drift towards an
>electron as other electrons move farther away. It doesn't
>seem like you need any detailed consideration of interference
>effects.
Another simple explanation based on the Stuckelberg-Wheeler
interpretation of positrons as electrons propagating backwards through
time is that the reversal of sign in the positron's momentum causes the
photon momentum to be subtracted rather than added, so the force is
reversed.
Regards
--
Charles Francis
Charles Francis
Gardner <pej...@oznetcom.com.au> writes:
>Charles Francis <cha...@clef.demon.co.uk> wrote in message news:<aasrf
>r$5mq$1...@news.state.mn.us>...
>> As for whether you chose to think of the intermediary as a particle, as
>> Feynman does, or whether you chose to think that "virtual" particles are
>> real, as Feynman also does, or whether you wish to think of it instead
>> as some sort of quantised field with properties that can be described in
>> mathematics but not thought of with physical intuition as most
>> physicists appear to, all of that is an open question. I follow Feynman,
>> but it is a question that requires far more than a reading of Peskin &
>> Schroeder to answer.
>What are your criteria for classing a particle as "real", as existing
>physically? Mine are that it has:
>A finite (non-zero) relativistic mass, M = p/v.
That isn't a good definition. You would do better to say non-zero
energy. I agree that I would have trouble discussing the reality of
particles with zero energy.
>A lifetime of not
>less than 10^-28 s ( of more than 10 s if it mediates long range
>interactions).
I don't have a specific time limit.
>A position within a laboratory reference frame that
>can be defined approximately by some function of the time registered
>by a standard clock in the laboratory.
I take it approximate can be quite vague, and include quite broad
statements to allow (near) momentum eigenstates, so we can say
"somewhere within a bubble chamber", or "a wave packet in a storage
ring"
>I believe that anyone who believes that virtual photons are real
>particles, as you seem to, should have some order of magnitude answers
>to the following questions:
>
>Given an isolated pair of charged leptons some 10^9 m apart is the
>interaction between them still finite (an acceleration of order 2.5 x
>10^-16 m s^-2) ?
Yes, in principle. Though detection is clearly impracticable.
>If so:
>At what rate are the postulated virtual photons emitted?
>What is the
>mean energy of one of these virtual photons?
All I know how to calculate is the net momentum transfer of all the
virtual photons, not how many there are, nor how much energy each one
has. These questions are intrinsically unanswerable because it is not
possible to write down a "photon number" observable operator as a
combination of field operators, and so photon number cannot be
physically determined or known. But to me not being able to say how many
virtual photons there are does not imply that virtual photons do not
exist.
>What is the mean
>lifetime of one of them?
All photons have a proper lifetime of 0.
>How far out in space do they travel?
There is no bound to where they can be absorbed. However the concept of
travelling is probably not valid. They are emitted and they are
absorbed. Quantum mechanics does not really allow us to discuss what
happens in between.
>Where do they cease to exist?
They cease to exist when they are absorbed by the receiving particle.
Regards
--
Charles Francis