What is the mechanism that transmits force?

[danlightbulb]

Hello all,

New to this forum. Have had an interest in physics for a long time and studied it for a while at undergraduate level.

Anyway, my question is this:

What is the mechanism which actually transmits force? I'm thinking about electric or magnetic force. When two objects are physically separate, how is the force actually transmitted from one object to another?

I'm aware of electromagnetic fields, and how the force is proportional to the distance between the objects, but this doesn't explain how the force is actually transmitted through space between the objects.

Thanks

 

[Drakkith]

Well, in quantum theories forces are transmitted by virtual particles.
http://en.wikipedia.org/wiki/Force_carrier

However I think you may be disappointed in that answer. Unfortunately all I can really say is that we don't know. We can watch one particle interact with another, and do math about how the interaction works, but that's it.

 

[HomogenousCow]

The question itself is a bit flawed..I mean when you ask why, you are asking for an explanation in terms of axioms, it just so happens that the principle "the charge creates a field, the field pushes the other charges" is the axiom here.

 

[lightarrow]

I'm aware of electromagnetic fields, and how the force is proportional to the distance between the objects, but this doesn't explain how the force is actually transmitted through space between the objects.

Sometimes people (included me, many years ago) ask themselves this, the first time, because they are used to think in terms of "contact" between bodies, because of our sensorial experience. Of course, if you can't give up to such an explanation, you will never be satisfied (I'm not talking about you specifically, of course). Physics however is not interested in what it should be to satisfy an inner need of us, but in what it actually happens.
Of course this doesn't absolutely imply that we won't in the future find another description of what happens in the transmission of forces, that uses the concept of contact between some kinds of (really existing) bodies; at the moment however there isn't.

 

[danlightbulb]

Well, in quantum theories forces are transmitted by virtual particles.
http://en.wikipedia.org/wiki/Force_carrier

However I think you may be disappointed in that answer. Unfortunately all I can really say is that we don't know. We can watch one particle interact with another, and do math about how the interaction works, but that's it.

Yes you're right, I was hoping for a more revealing answer into the actual physical method as you probably guessed.

I have read up on the current theories (wikipedia is useful), but the whole gauge boson / virtual particle idea just doesn't cut it for me I'm afraid. It feels like its a mathematical construct which describes the behaviour but unfortunately doesn't reveal the actual mechanism.

Are there any theories out there as to what the actual mechanism might be in reality? May be more of a philosophical question than a physics one.

 

[Naty1]

In classical theory, fields transmit forces; as already noted, in quantum mechanics virtual particles. These are convenient mathematical constructs that so far are pretty good at describing observations. Here are excerpts from several different explanations of 'particles'...

it is quantum FIELDS, extended versions of particles, that expand in space with expanding space and can become 'particles', which we describe as quanta [localized versions] of fields...what we detect. There are a variety of ideas about how this might happen.

For an overview:
Gravitational fluctuations in accelerated expansion is what gives rise to the primordial perturbations. The idea is usually pitched in terms of particle/anti-particle pairs that pop out of the vacuum, only to get pulled apart by the rapidly inflating background. Eventually they are pulled outside the horizon, never to meet again. Of course, this description has the same conceptual shortcomings as the description of the Hawking effect {for Black Holes} in terms of particle/anti-particle pairs -- it isn't quite right.
and
Quantum fluctuations in the inflationary vacuum become quanta [particles]
at super horizon scales...it seems that expansion of geometry itself, especially inflation, can produce matter...The evolution of quantum fluctuations is from their birth [at Planck Scale] in the inflationary vacuum and their subsequent journey out to superhorizon scales where they become real life perturbations...[particles at detection]

So we have quantum field perturbations...what do they mean:

[These quotes may be from MTW..I did not keep the reference source]
... there is no preferred vacuum state, and the interpretation of the field states in term of particles appears to be difficult...Such arbitrariness and ambiguity of the particle concept have led some theoreticians like Davies to affirm that “particles do not exist” , a view shared by several relativists. ... other theoreticians ... hold that QFT is fundamentally a formalism for describing processes involving particles, such as scattering or decays...
This view similar to that I posted aready from Rovelli: there are some fundamental differences among theorists about what particles are...they are not easy to define.

Here is a closely related view:
... there are two distinct notions of particles in QFT. Local particle states correspond to the real objects observed by finite size detectors. ... On the other hand, global particle states...can be defined only under certain conditions. Global particle states are simpler to define and they approximate well the local particle states detected by local measurements. Therefore the global particle states, when they are available, give a good approximate description of the physics of the “real” particles recorded by local detectors...In the paper we illustrate the difference between these two classes of states, and discuss their relation. The precise sense in which global states approximate local particle states is subtle...
Here is a quote I liked from Wikipedia. Keep in mind the quantum fluctuations that are grown with accelerated cosmological expansion can be thought of as a mix of 'real and virtual particles' ...which are also field amplitudes...[analogous to the real and imaginary numbers I mentioned in my first post.]
There is not a definite line differentiating virtual particles from real particles — the equations of physics just describe particles (which includes both equally). The amplitude that a virtual particle exists interferes with the amplitude for its non-existence; whereas for a real particle the cases of existence and non-existence cease to be coherent with each other and do not interfere any more. In the quantum field theory view, "real particles" are viewed as being detectable excitations of underlying quantum fields. ... In this sense, virtual particles are an artifact of perturbation theory, and do not appear in a non-perturbative treatment.
That last sentence AGAIN alludes to the difficulty of defining and modeling a 'particle'. What you 'see' depends on your model.

From a CArlo Rovelli's Introduction:

"..uniquely-defined particle states do not exist in general in QFT on a curved spacetime. ... in general, particle states are difficult to define in a background-independent quantum theory of gravity.."As you can probably tell from the above descriptions, we have a lot more to learn!

 

[Ibix]

Yes you're right, I was hoping for a more revealing answer into the actual physical method as you probably guessed.

I have read up on the current theories (wikipedia is useful), but the whole gauge boson / virtual particle idea just doesn't cut it for me I'm afraid. It feels like its a mathematical construct which describes the behaviour but unfortunately doesn't reveal the actual mechanism.

Are there any theories out there as to what the actual mechanism might be in reality? May be more of a philosophical question than a physics one.

Disclaimer - I don't know much about quantum field theories. However, I don't think you are going to get a better answer.

Try a simpler question: Imagine a pool table (a purely classical one). Viewed from the white ball, a red ball is just to the left of the pocket. Where do you aim the white ball to sink the red one? Left of the center, of course. It's easy enough to calculate your aim point. But what is the actual physical mechanism at work?

Conservation of energy and conservation of momentum would be the stock answer. But what is energy and what is momentum? You can't hold a chunk of energy in your hand, or a slice of momentum. You can calculate the kinetic energy of something, but you cannot separate the energy and the momentum from the ball. They are, ultimately, just book-keeping tools that are handy for figuring out what happens on a pool table. They turn out to be much more powerful than that, of course, which is the beauty of science.

What I'm getting at is that there never is an "actual physical mechanism" in the sense that you mean. When I talk about conservation of energy and momentum, I am just talking mathematical constructs. The same is true of quantum field theory - it's just that the concepts are weird enough that you are questioning more.

Does that make sense?

 

[lightarrow]

Furthermore, even if it could be all reduced to the contact between balls, what is a ball made of? Atoms separated in space but held together by forces which acts at a distance. And what are atoms made of? Particles separated in space held by forces at distance...
Fields nested in other fields, there never is a real "contact".

 

[danlightbulb]

Try a simpler question: Imagine a pool table (a purely classical one). Viewed from the white ball, a red ball is just to the left of the pocket. Where do you aim the white ball to sink the red one? Left of the center, of course. It's easy enough to calculate your aim point. But what is the actual physical mechanism at work?

Conservation of energy and conservation of momentum would be the stock answer. But what is energy and what is momentum? You can't hold a chunk of energy in your hand, or a slice of momentum. You can calculate the kinetic energy of something, but you cannot separate the energy and the momentum from the ball. They are, ultimately, just book-keeping tools that are handy for figuring out what happens on a pool table. They turn out to be much more powerful than that, of course, which is the beauty of science.

What I'm getting at is that there never is an "actual physical mechanism" in the sense that you mean. When I talk about conservation of energy and momentum, I am just talking mathematical constructs. The same is true of quantum field theory - it's just that the concepts are weird enough that you are questioning more.

Does that make sense?

Absolutely. I'm well aware that there are fundamental questions underpinning everything we see and do. I chose the force example for the reason below:

Furthermore, even if it could be all reduced to the contact between balls, what is a ball made of? Atoms separated in space but held together by forces which acts at a distance. And what are atoms made of? Particles separated in space held by forces at distance...
Fields nested in other fields, there never is a real "contact".

I was leaning up against a wall the other day and I thought to myself, my atoms are not actually in physical contact with the atoms of the wall. That's what started the question off.

These four fundamental forces are key to unraveling the whole universe.

 

[jtbell]

I have read up on the current theories (wikipedia is useful), but the whole gauge boson / virtual particle idea just doesn't cut it for me I'm afraid. It feels like its a mathematical construct which describes the behaviour but unfortunately doesn't reveal the actual mechanism.

It's the best we've got so far.

Are there any theories out there as to what the actual mechanism might be in reality? May be more of a philosophical question than a physics one.

Yes, there is a serious philosophical question lurking here: How will we know it's the "actual mechanism" rather than just another "mathematical construct?"