Reading through some of the threads in "Against the Mainstream" I see a fair amount of discussion about the nature of gravity (space-time warpage vs. quantum). The concept of gravity being "quantum" or particle driven really intrigued me so I spent some time googling around to learn more about it. I am now puzzling over this page:

http://hyperphysics.phy-astr.gsu.edu...es/funfor.html

At first the information on that page interested me because of the similarities between the electromagnetic force and gravity. However, now that I have thought about it for a bit, I am very much interested in the electromagnetic force for its own sake (perhaps I will get back around to thinking about gravity someday later).

Anyhow, I was hoping that someone could give me a little help in understanding a few things about the electromagnetic force. From the page linked above I understand that the electromagnetic force obeys an inverse square law (much like gravity), that its "range" is infinite (much like gravity), and that the force of attraction/repulsion is a consequence of the exchange of photons. My question is in relation to the following thought experiment:

Suppose we create a universe exactly like our own but there is nothing in it but space. Then we create (all of a sudden and out of nowhere/nothing) two particles. The particles are motionless relative to each other, but they are charged, one negatively and one positively. The particles are separated by some distance - lets say 10 light years.

My interpretation would leave me to believe that the two particles would attract each other regardless of the distance involved. (Whether they would actually start moving toward each other I do not know - would the force of the attraction have to overcome some amount of inertia?) My interpretation also holds that the electromagnetic field (the charge) will expand away from each particle at lightspeed (c), and that the charge/field would occupy a potentially unlimited volume of space (limited only by c and the amount of time that the particle continues to exist).

First question: is this interpretation correct?

Second question: at what point does each object begin to feel the attaction of the other? Does the charge have to reach all the way over to the other particle, or does the attraction start when the two charges/fields meet each other (in the middle)? (ie. if they were to start moving in response to each other's fields, would that begin 10 years after creation (remember 10 light years apart), or 5 years after creation?)

Third question: Why is it that the electromagnetic force is not described as a warpage of space-time (as gravity)? Is it just because it does not affect the path of a photon the way gravity does? (Also have there ever been any actual experiments to see if a VERY strong magnet can bend light?)

Thanks in advance to anyone who takes the time to read and respond.


Sheki

Yes, pretty much.

Until the "field" reaches the other particle, so a ten year delay in this case.

The reason we can think of gravity as a curve in space is because of the equivalence principle. That is, both the force on an object and it's resistance to that force are proportional to the mass, so in a given gravitational field, all objects will follow the same path regardless of their mass. For the electromagnetic force, that's not the case. Different objects will follow different paths, depending on their charge-to-mass ratio, so it can't be thought of as a curvature of space in the same simple sense.

I'm sure that it's been tested, but I don't know of any specific experiments offhand. Note, though, that if this were the case to any large extent, a magnet would distort your vision of things near it. Since people have worked with some pretty powerful magnets, I'd think this effect would be noticed even if nobody was looking for it specifically.

Thanks Grey.

So the charge from a 12 billion year old free electron would occupy/pervade almost the entire known universe? Seems odd that something so finite could have such an infinite effect...mind boggling really.

Is this a statement based on extrapolation from theory alone, or is this based on experimental evidence? (I would be very interested in the experimental design).

I see. But do not the objects "within a gravitational field" (odd way to put it given that you are essentially stating that there is no such thing as a "gravitational field" ) follow different paths depending on the mass of those objects? What I mean is that each object within the gravitational field of another body also has its own gravitational field proportional to its own mass. So two very massive objects should accelerate toward each other faster than one massive object and one less massive object. Or am I mistaken here? Presuming not, then how is this different than what you described for the electromagnetic force?

Stands to reason, but if I am not mistaken here, we needed to use the mass of the sun to originally test whether or not gravity affects the path of a photon. Would we not need a similarly powerful magnet under similar conditions (or reasonably scaled) to test magnetism's ability to do the same? Do such magnets even exist? And mightn't they need to act on a photon over a similar path (1/2 an AU?) as those detected as having been "curved" by the sun?

By the way, I know these types of questions can really try peoples patience (like the why,why,why questions of a toddler). I appreciate the effort.


Sheki

There are a couple of fundamental attributes of your hypothetical "space" that you need to take into account.

1. Permittivity. That is the ability of "space" to propagate and contain electric fields
2. Permeability. That is the ability of "space" to propagate and contain magnetic fields

Without these there is no propagation of static electric field lines, nor magnetic flux lines.

By "playing" with these values in your hypothetical space, you can effect a change in the speed of propagation of the information that is the time and location of the instantaneous creation of the particles.

Another thing to think about. If these particles have charge, they probably also have mass, so there is a double attraction going on. Both Coulomb's law and Newton's law of gravitation would apply simultaneously.

__________________
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Yes, I had intended to address this in the first lines of the thought experiment "a universe exactly like our own". Heck, otherwise if we were creating a new universe we could change all of the constants if we wanted!

Good point, but perhaps we should leave that aside in the interests of not muddying the waters?

Sheki

It is pretty amazing.

Well, we can't have charges appear out of nowhere, of course. But if you take a charge and start wiggling it, the effect of that wiggling propagates out at the speed of light, so I'd call that experimental evidence. That's just electromagnetic radiation, of course, so this experiment happens all the time!

Usually we'd think of testing the force at any given point by introducing an imaginary test particle that doesn't actually affect the system. So for such a test particle, perhaps you can see that there's a pretty noticable difference. You're correct that if you allow the test particle to affect the system that there will be some slight differences, but it's not quite the same thing. In the gravitational case, both objects contribute to the curvature, and you could check that by introducing a third test object a seeing the interaction. But for the electromagnetic case, the electromagnetic effect is entirely determined by the charge alone, and it's only the acceleration of the object that is affected by the charge-to-mass ratio.

Sorry if that's not clear. How about a concrete example? If I have a large negative charge and I bring a proton or a positron close by, they'll both be attracted, but their paths will be very different. However, if I were to bring in another test charge, both the proton and the positron would affect that third particle's path in exactly the same way, so I can't just attribute the differing paths to a difference in the curvature caused by the particles themselves. Does that make sense?

Actually, you can see the relativistic effects on photons from the gravitational difference between the top and bottom of a building. You can search on "Pound and Rebka" and "Mossbauer effect". The key here, though, is that the strength of the electromagnetic force is about 10^36 more powerful than the gravitational field. So for example, just this week in an introductory physics lab I was teaching the students were looking at the deflection of an electron beam by a magnetic field. It was only a moderately powerful field, but the acceleration the electrons were experiencing was on the order of a trillion times as great as they would experience if they were skimming the Sun. If magnets had any effect on light, it's very small. And of course, we don't expect it to, since it strictly affects charged objects, and photons show no sign of carrying charge.

No trouble. It's fun answering these questions, or I wouldn't be here doing it.

Well, that's not really what I was getting at, but I understand what you are saying. To clarify the question that I am really interested in I think I need to modify the thought experiment:

Same though experiment as above but instead of creating both particles simultaneously, we create one, and then create the other (10 ly away) 9 years later.

In this version the field from particle 1 will reach particle 2 one year after particle 2's creation. However, the field from particle 2 will not reach particle 1 for another 9 years after that.

So will particle 2 "feel" the field generated by particle 1 (ie. become attracted toward it) after that first year?

Will particle 1 "feel" particle 2's field (become attracted toward it) 9 years later?

Or is there a 10-year wait no matter what (ie. 10 years after the creation of the second particle, both particles start to feel the effect of the other)?

I suspect the answers are "yes", "yes", and "no". But I am nowhere near certain.

It is clear, and yes that does make sense. But what I take from this is that the electromagnetic force has a variable effect on other particles of various charge, whereas gravity has a uniform effect on other particles regardless of their charge. What I do not understand is why the electromagnetic force is described in terms of a particle exchange while gravity is assumed not to. To the uninitiated (like me) it seems easier to accept that there might be a "graviton" particle and associated field that all particles possess (even photons), than it is to accept "warpage of space-time". The particle theory only requires us to accept that there is a particle responsible for an attractive field (that lacks a charge dichotomy), but which we have not yet detected. Whereas the warped space theory requires us to accept the existence of a "space-time" medium that cannot be directly observed, and that mass, for reasons unclear, can by its nature "warp" that medium. (And I guess this is where I came in...wondering about "against the mainstream" ideas about gravity.)

Sheki

You're correct, with the caveat that it's not actually possible to magically create particles. This may sound pedantic, but with an interaction model of the electromagnetic force, there is a sense it which it is nonsensical to talk about such a thing, and I think that it can actually have an effect on things.

We have two impressive theories. One is general relativity, which uses a model involving the curvature of space, and which seems to do an excellent job of describing the nature of gravitational interactions and the universe on the largest scales. The other is quantum mechanics, which talks about forces being carried by virtual particles, and does an excellent job of describing the universe on the smallest scales, and is able to fully explain the electromagnetic and strong and weak nuclear forces. At this point, we do not have a full theory that is able to unify the two and give us a quantum picture of gravity.

We do, however, know some features that such a theory would have to possess. Under a quantum theory of gravity, gravity would indeed be mediated by virtual particles (gravitons, which would have to be massless and have a spin of 2), just as the other forces. I think most physicists think that such a theory will eventually be developed. That might obviate the need for a curved space description of gravity. However, it is interesting that such a description is not only possible, but works extremely well. I expect that even with a quantum gravity theory, there might be a way to look at it that makes that curved space picture sensible, or there will be some other natural reason why gravity affects objects of any mass in the same way.

Layman-to-layman, I might suggest something...

It seems that physicists often consider several descriptions as being accurate while we lay-folk want to know "what the Real Deal" is.

What I mean is, I know that physicists sometimes think of gravitation as spacetime warpage and sometimes as a field in flat spacetime, a field that changes the lengths of rulers and the timescale of clocks.

Both descriptions are accurate, and mathematically they both lead to the same predictions about the way mass and gravitation behave.

I might suggest that a particle-exchange theory of gravitation will be another description that is compatible mathematically with spacetime warpage and field-in-flat-spacetime descriptions.

Apparently we have not arrived at a theory of quantum gravitation yet, due to troublesome mathematical difficulties. As I understand the problem, a gravitational field actually gravitates...but electromagnetism does not electromagnitate .

A gravitational field produced by mass X actually produces a field of mass X plus a self-gravitation factor of X1, which produces a field of X + X1 plus a tiny additional factor from the self-gravitation of the original field. Well, plus the self-self-gravitational addition...ad infinitum. "Troubling infinities" someone called them.

Electromagnetism doesn't have that complication, I think.

Put another way, the equations describing gravity are of a higher order mathematically than the equations describing electromagnetism.

But my personal guess is that if a quantum particle-exchange description of gravitation is found, it will be compatible with the spacetime-curvature description (and with the flat-field-and-distorted-clocks-and-rulers description).

__________________
Jim he allowed [the stars] was made, but I allowed they happened; I judged it would have took too long to make so many. Jim said the moon could 'a laid them... --Huckleberry Finn

The reference to permittivity and permeability wreaks of ether.

This leads us into somewhat philosophical issues about when one theory is different from another, if they have the same observational consequences. For example, a skeptical position might be that since the only way to chose one competing theories over another would be by observations that support one and contradict the other, but since these theories have identical observational consequences, there's no way we can ever choose between them and decide which is really true.

A slight variant on this is that, since we can't distinguish between them, we should just pick one that we'll decide is true by convention. Since that's the best we can do, we might as well be happy with it.

Others might suggest that there might be distinctions that are not observational, but perhaps related to the simplicity, elegance, or systematic power of the theory, and that these would allow us to decide between them. Of course, then you might get into disagreement with others about which theory is more elegant.

A reductionist perspective, on the other hand, suggests that what a theory really is is just the sum of its observational consequences. In that case, since the apparently incompatible theories have the exact same observational consequences, they should really be just considered different ways of expressing the very same theory.

If you're interested in exploring these issues further, especially as they relate to relativity, I'd recommend the book Space, Time, and Spacetime by Lawrence Sklar.

For myself, I tend to think that in many cases, there are slight observational consequences. So, for example, it really is true that postulating curved space, or postulating flat space that changes rulers and clocks can be shown to be completely equivalent. But I can also see how one could imagine that those really are just two ways of looking at the same thing.

However, having the electromagnetic force carried by virtual photons has some slightly different predictions from the electromagnetic field view, and we've actually confirmed those predictions. Although a quantum theory of gravity would have to agree with general relativity to the extent that we've tested it so far, I expect that there probably will be some differences that could probably be measured at some point.

Good one, but it's "reeks"

(This is a hint for the latest Invisible Idiot Game)

Bathcat wrote:

Well, while I am a layman when it comes to physics, I am no less a scientist (biologist), and I can attest that all of science (including physics) is concerned with "what the real deal is"!

Grey wrote:

Thanks for your help Grey. In reference to the above quote, I understand that the two theories do have different predictions (the existence of a graviton vs. the non-existence of a graviton). I think that I will hold out for a few more decades of research (I think I have that much time anyway) before making up my mind about it one way or the other.

Deep down, I hope that gravity turns out to be particle driven. I can't get it out of my head that particle driven gravity could be subject to manipulation (suggests the possibility of artificially generating gravitons and or developing technologies that can shield against gravitons). That would be pretty exciting.

Sheki

The operative definition of field is limit of the force for test-"charge" going to zero.

Experimentally, you measure the force for different test-"charges" and then you extrapolate to zero "charge".
The result does not depend on the specific test-"charges" used, hence depends only on the source "charge" (and position).

__________________
papageno


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