# How are non-contact forces possible

1. Jun 18, 2013

### Feather17

My question: how is matter influenced by forces that are non-matter? To elaborate more specifically: the four fundamental forces are non-contact forces, and I do not understand how in a classical reality that this is possible.
Thank you.

2. Jun 18, 2013

### WannabeNewton

As far as classical fields go, the fields are not just some kind of background upon which dynamics happens but rather the fields themselves have their own dynamics through their interactions with particles. They carry their own momentum, energy etc. A particle in a field interacts with the field locally (i.e. no action at a distance) and disturbances in the field propagate across the field (bounded above by $c$) and in effect the field "mediates" forces.

3. Jun 18, 2013

### Jolb

My answer may just cause more confusion for you, since the interactions between particles and fields leads to some of the most difficult physics (quantum field theory, for example), but hopefully it's a little comforting... Here goes:

Aside from gravity, physicists are fairly convinced that the fundamental forces are actually "mediated" by certain particles. In other words, in order for two charged particles to interact electromagnetically, a photon must be transferred between the two particles. Likewise, for quarks to interact via the strong force, a gluon traveling between the particles "mediates" the exchange of momentum and energy associated with the strong force. For the weak force, the mediating particles are the W and Z bosons. If gravity were like the other forces, then it would be mediated by a graviton (or a few different gravitons, like in higher spin gravity).

So in one way it is legitimate to think of interactions in quantum field theory as always being carried by some particle. But be careful when you think about quantum field theory: sometimes these particles are actually "virtual" particles which have weird properties such as being "off-shell." There are also nonlocal quantum mechanical effects (though they aren't exactly interactions [energy/momentum transfers], but rather correlations) between distant points in space which seem to occur just via the wavefunction, without any transfer of particles to coordinate the measurements at distant points.

4. Jun 18, 2013

### Crazymechanic

to the OP , by the way don't think of these forces like the EM force as "non-matter" they are matter or part of it I should say. Just because you don't see them with your eyes doesn't mean their not there.
You don't see microwaves , (high frequency em radiation) yet your food and everything that pretty much conducts heats up and feels them pretty fine.Everything that can influence matter is also matter or energy and energy.
Energy and all kinds of forces is a part of space time.So everything you see or don't see is actually physical.

5. Jun 18, 2013

### WannabeNewton

This is a stretch. Classically the 4-potential is not physical; it is simply a mathematical artifact with gauge freedom $A_{a}\rightarrow A_{a} + \nabla_{a}\varphi$ and it is only the electromagnetic field that is physical. In QM this changes because of the AB effect and the 4-potential gets its own primacy but classically what you say is a stretch. In fact, since general relativity is a classical field theory, we can appeal to the gauge freedom of the metric tensor to again argue the existence of unphysical characteristics of quantities which find their way into field equations.

6. Jun 18, 2013

### mrspeedybob

I like Feynman's answer to this question. Informative and entertaining to listen to.

Last edited by a moderator: Sep 25, 2014
7. Jun 18, 2013

### ModusPwnd

The way you say this is as though "physical" is a meaningful, technical word. But its not, is it? It not a technical word and thus isn't really meaningful to consider when it comes to science. I think that whether something is physical or not is a matter of taste.

8. Jun 18, 2013

### sophiecentaur

Isn't the whole point of Modern Physics that "Classical Reality" is just a subset of a wider reality? It can hardly be surprising that the Classical approach fails to explain everything. If it did, there would have been no need for QM or Relativity.

9. Jun 18, 2013

### Jolb

Thanks for this--I agree that crazymechanic's post
is a really naive thing to say. Without delving into the philosophical issues, what you're saying is that everything you see and everything you don't see is physical. Well, what else is there besides things you see and things you don't see? You're saying "Everything with (A) and everything with (not A) has property (B)" which is equivalent to "Everything has property (B)." In your context, (B) is being physical, so you're arguing that everything is physical. Hopefully you've been exposed to some philosophy where they explore the ideas of "existence" vs. "physical existence." (Does mathematics exist physically?)

Anyway, I vote to end this sub-conversation because there are much more relevant things to discuss here.

Last edited: Jun 18, 2013
10. Jun 18, 2013

### WannabeNewton

No. Open up any classical electromagnetism text and you will see $F_{ab}$ (i.e. the electromagnetic field) differentiated from $A_{a}$ with regards to the classically primitive existence of $F_{ab}$. This changes in QM because of the AB effect but this distinction between the two (the 4-potential vs the electromagnetic field) is something every basic electrodynamics text goes into (at least the ones that develop the 4-potential formalism). These distinctions show up when considering things like the initial value formulation of Maxwell's equations and Einstein's equations. This is not a matter of semantics.

11. Jun 18, 2013

### ModusPwnd

But why do you call that distinction "physical" vs "non-physical"? Do they use the word "physical" in the text? I doubt it, but I would be interested if they did. AFAIK, its a non-scientific word that does not provide any insight.

12. Jun 18, 2013

### WannabeNewton

13. Jun 18, 2013

### ModusPwnd

hmm, I dont buy it. It looks like they are conflating "observable" with "physical" in that wikipedia. Is the claim that physical and observable are the same? The other wikipedia says "physically observable", but I dont know what that means. What is something that is non-physically observable? I wish they defined "physical". It wreaks of sloppy language to me.

14. Jun 18, 2013

### AnTiFreeze3

I'm not sure why you think your disapproval is an argument, or is even worth saying.

WBN: According to Newton's 2nd Law, force equals mass times acceleration.

AnTiFreeze3: Nah, I just choose not to accept that. No, I don't have a particular reason for feeling this way, aside from maybe a primitive feeling in my gut that tells me not to trust this, but I do know that I want to voice my dissonance without looking for myself to see if this is true. Additionally, evidence put forth in support of your claim will also not feel right.

15. Jun 18, 2013

### ModusPwnd

Why not? I'm not sure why you think your statement is even worth saying, but you did. Im just trying to explore an idea and maybe learn something. Your comment here is rude and unhelpful. Have anything useful to add besides a naked critique? What is the difference between "observable" and "physical"? Is there a difference? If I replace the word "physical" with "observable" in his link it makes a lot more sense.

Also, I just scanned my griffths text on the 4 potential and EM field like he suggested, I see nothing mentioning it as being physical or not. But if I missed it I would like to know.

16. Jun 18, 2013

### WannabeNewton

Looks like Griffiths: http://postimg.org/image/gd36ejp37/ [Broken]
and Wald: http://postimg.org/image/66ukvk4oz/ [Broken] like the word too. See here as well: https://www.physicsforums.com/showthread.php?t=433308
and here: http://physics.stackexchange.com/qu...ial-and-gauge-invariance-in-quantum-mechanics

Last edited by a moderator: May 6, 2017
17. Jun 18, 2013

### ModusPwnd

K^2 says this: "In QM, vector potential is physical, but not observable." I like this one because it explicitly differentiates physical from observable. But I dont understand it. What is the difference? I understand what being "not observable" means, but what does being physical, in this context, mean? Otherwise, it looks like physical and observable are completely interchangeable in the other links, doesn't it?

And why does Griffiths call E and B physical? My first thought is that he does so because they are observable (or measurable), but K^2's quote throws a wrench in that logic.

The stack exchange quote says "physical observable". What is a non-physical observable?

They are all confusing to me... What does "this difficulty is not of a physical character." mean? It also says "physically, Maxwell's equations do admit a well posed initial value formulation". Does he mean its observable? Is that why its "physically"?

I think I need a precise technical definition of "physical" otherwise this all just looks like sloppy, historically minded language to me. (from the old days when there was a physical realm and a non-physical realm to consider)

I should probably make my own thread on these questions. (Hopefully AnTiFreeze3 approves...)

Last edited: Jun 18, 2013
18. Jun 18, 2013

### WannabeNewton

It is here that the two words seem to have some discrepancy amongst all those different posts i.e. it is here that the lack of a standard definition of "physical" becomes a pain. You can see yourself that some people interchanged the two in those posts quite often whereas K^2 differentiated the two. As far as I can tell, the most obvious difference can be seen by reading the first answer here: http://physics.stackexchange.com/qu...ial-and-gauge-invariance-in-quantum-mechanics and comparing it to the first answer here: http://physics.stackexchange.com/questions/65757/observables-what-are-they

So it has physically observable effects but it is unlike what is described in the second link. However at this point I certainly agree that semantics starts creeping up.

19. Jun 18, 2013

### Staff: Mentor

As WannabeNewton points out, many textbooks use the word "physical", but I dont know of any that actually define it. I think the common definition would be something along the lines of "of or pertaining to physics". So I would tend to call just about everything that you can assign a variable to "physical". But that is just me.

20. Jun 18, 2013

### Staff: Mentor

The two classical fields are gravity and EM. They are both clearly non-contact forces, and clearly they exert forces on matter. So you are making some sort of counterfactual assumption.

Perhaps you could explain why you think non-contact forces are impossible. That may help illuminate the wrong assumption.

21. Jun 18, 2013

### AnTiFreeze3

It's sweet that my approval means so much to you. You can go ahead and make that thread , because I don't mind when someone asks a question, but it does get on my nerves when someone disagrees with evidence without supplying a reason, aside from "nah, it doesn't feel right."

Have a good day.

22. Jun 18, 2013

### Integral

Staff Emeritus
At this time the OP has yet to reply and has not been active for nearly 12hrs. He has enough to read. I am locking this, lets give him a chance to catch up.

Feather17 when you return please Report this post and the thread will be reopened.

Last edited: Jun 18, 2013
23. Jun 18, 2013

### PhanthomJay

Say Feather17, if you are still with us, even so-called "contact'' forces on objects, like friction and normal forces, are actually electromagnetic forces due to electron interaction at the surfaces. There is no such thing as a fifth fundamental 'contact' force. The objects actually never touch.

24. Jun 19, 2013

### Feather17

Thanks for replies
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I think reality is far from classical - If I think of it as information, it makes sense to me what energy and forces 'are'. but, I can't understand it in a traditional physics sense, which could very likely be due to a lack of knowledge, and thus the reason for the question.
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It looks like I need to look more into photons role in fields, as I don't understand this well.
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Also, mass influencing space is another example but a little different than what was being discussed. I get how a boweling ball warps a net metaphorically acting as the 'fabric' of space, but I do not understand how matter influences space in actuality.
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There are lots of equations for "how much" something will be influenced, but I keep growing concerened about how it does it.

25. Jun 19, 2013

### WannabeNewton

Ah but those are the type of "how" questions that are beyond physics and enter into the realm of metaphysics, at least until someone can actually develop a formalism that can codify those "how" questions mathematically. For example we know "how" test particles free fall in curved space-time and "how" mass-energy affects the curvature of space-time in the sense that these "how"s are codified by the geodesic equation and Einstein's equations respectively, but we don't know "how" they do it in the sense that you are asking.