Is gravity unique among the "Fundamental forces"?

[mothythewso]

I admit first-off I have absolutely no expertise in this area, this question may be utterly absurd, but:

As I understand things, the 3 fundamental forces that conform to quantum theory have an absolute lower limit, based somehow on the Planck constants; nothing can have a lower energy than the Planck limit, no particle can have a dimension smaller than the Planck length, etc. But why does gravity have to have a lowest possible limit, a quantizeable value. Again, as I understand things, gravity acts by bending space-time, it's thought of as being as being a "geometrical" phenomenon. Does a geometrical function, a geometrical progression that has a limit of zero, ever actually reach zero. Of course not. So why does gravity have to be quantizeable lowest value? And another thought: If in a truly empty space (I know such a thing doesn't exist, virtual particles and all) 2 masses appear simultaneously 1 light year apart, how long will it take for gravity to interact with them? If the 2 masses are sufficiently small, can one reasonably treat the force between as zero? I would really appreciate a non-condescending, thoughtful reply, even if it proves that I'm totally out to lunch!

mothythewso

[Strange]

As I understand things, the 3 fundamental forces that conform to quantum theory have an absolute lower limit

 

I don't think that is quite true. However, the forces are quantized, which means they are mediated or represented by (virtual) particles. But the electromagnetic force (as the simplest example) falls of with the square of distance in the same way that gravity does. They will both get smaller and smaller with distance but they will never reach a minimum value and never stop (although they may become insignificant compared to other forces after some time (e.g. we can ignore the gravity of Jupiter when calculating the orbit of the moon).

 

The difference with gravity is, as you say, that it is an effect of the geometry of space-time so it isn't really considered a force. (Although it can still be treated as a force, because using Newton's laws is much simpler than GR and usually good enough).

 

The reason that people want to quantize gravity is to unify it with quantum mechanics. There are conditions (such as black holes or the early universe) where we can't really explain / describe what happens because we don't have a theory of quantum gravity.

 

 

And another thought: If in a truly empty space (I know such a thing doesn't exist, virtual particles and all) 2 masses appear simultaneously 1 light year apart, how long will it take for gravity to interact with them?

 

Changes in gravity propagate at the speed of light.

 

 

If the 2 masses are sufficiently small, can one reasonably treat the force between as zero?

 

Often you can, yes. It depends what you are trying to calculate and how accurately.

Edited January 11, 2015 by Strange

[elfmotat]

Gravity is different from the other three forces in a number of important ways:

• It is described by a rank-2 tensor field. The other three forces are described by rank-1 tensor fields, AKA vector fields. This translates into photons, gluons, and the weak bosons having spin-1, while the theoretical graviton is predicted to have spin-2.
• It couples to all other fields/matter. Anything with energy-momentum will generate a gravitational field. Electromagnetism only couples to fields/matter with charge. The weak force only couples to fields/matter with weak charge, and the strong force only couples to fields/matter with color charge. If there's a particle, you can bet it's effected by gravity.
• Gravity is incompatible with modern Quantum Field Theory, the basis of the Standard Model. The other three forces have descriptions in terms of quantum fields. In particular, General Relativity has been proven to be non-renormalizable. A successful quantum field theory of gravity has yet to be developed, though there are a number of well-known approaches to solve this problem -- String Theory and Loop Quantum Gravity being the most recognizable.
• Gravity is a geometric force. The other three forces are described in terms of fields on a background spacetime. With gravity, the field is spacetime itself. This property gives rise to the equivalence principle, which is why heavy objects and light objects will fall at the same rate. This may in fact turn out to be less of a distinction than you might think. Kaluza-Klein type theories can also describe the other forces/matter in terms of geometry, though they require extra spatial dimensions. For example, gravity + adding a fourth spatial dimension in a particular way gives rise to electromagnetism, with electric charge being the conserved momentum through the extra dimension.
• Gravity is much much much weaker than the other three forces. I've heard this example used before: a tiny fridge magnet is strong enough to overcome the force of the entire Earth pulling down on it. You too; you can walk around the surface of the planet with relative ease, lift objects, etc. Ever tried to pull apart two strong magnets?

 

I'm not entirely sure what you mean by "absolute lower limit." I'm not aware of any such thing in QFT. If two masses are sufficiently small, you can indeed treat the gravitational interaction between them as zero. This is what is done in quantum theory -- the masses involved are so small and gravity is so weak that it is completely negligible.

Edited January 11, 2015 by elfmotat

[swansont]

As I understand things, the 3 fundamental forces that conform to quantum theory have an absolute lower limit, based somehow on the Planck constants; nothing can have a lower energy than the Planck limit, no particle can have a dimension smaller than the Planck length, etc.

 

The planck energy and mass are macroscopic, so no, this is not true. QM works quite well with smaller values.

[Sensei]

Electromagnetism only couples to fields/matter with charge.

Not entirely true.

When (charge neutral) photon passes through region with strong magnetic field, its polarization is rotating.

From not polarized light, we can have linear polarized light on demand (when current flows through electromagnet and creates external magnetic field).

http://en.wikipedia.org/wiki/Faraday_effect

http://en.wikipedia.org/wiki/Faraday_rotator

 

 

If two masses are sufficiently small, you can indeed treat the interaction between them as zero. This is what is done with modern quantum theory -- the masses involved are so small and gravity is so weak that it is completely negligible.

Compare differences between Hydrogen and Deuterium spectral lines.. and then with Tritium.

The only difference between them is mass in nucleus, caused by additional neutrons.

[elfmotat]

Not entirely true.

When (charge neutral) photon passes through region with strong magnetic field, its polarization is rotating.

From not polarized light, we can have linear polarized light on demand (when current flows through electromagnet and creates external magnetic field).

http://en.wikipedia.org/wiki/Faraday_effect

http://en.wikipedia.org/wiki/Faraday_rotator

 

I wouldn't exactly call that an "interaction" by any usual definition of the word. Superposition is not an interaction.

 

 

Compare differences between Hydrogen and Deuterium spectral lines.. and then with Tritium.

The only difference between them is mass in nucleus, caused by additional neutrons.

 

I didn't say "mass and gravity don't exist," I said you can treat the gravitational interaction as being zero if the masses involved are sufficiently small. Which you can.

[MigL]

I would take issue with one of the things mentioned by other members.

Gravity itself is not necessarily geometric.

The best model we currently have, GR, is geometric.

[swansont]

Not entirely true.

When (charge neutral) photon passes through region with strong magnetic field, its polarization is rotating.

From not polarized light, we can have linear polarized light on demand (when current flows through electromagnet and creates external magnetic field).

http://en.wikipedia.org/wiki/Faraday_effect

http://en.wikipedia.org/wiki/Faraday_rotator

 

That's not a direct interaction — the coupling is via the medium.

 

Compare differences between Hydrogen and Deuterium spectral lines.. and then with Tritium.

The only difference between them is mass in nucleus, caused by additional neutrons.

 

But that's not due to gravity.

[mothythewso]

Agreed about the Planck mass and Planck energy; mea culpa.

mothythewso

[Commander]

 

The reason that people want to quantize gravity is to unify it with quantum mechanics.

 

 

What is the need to unify gravity with quantum mechanics ? What compulsion ??

[Strange]

What is the need to unify gravity with quantum mechanics ? What compulsion ??

To explain what happens in extreme conditions such as black holes, the early universe and probably also things like supernovae and neutron stars.

 

Apart from that, there is no real "need" but it is what scientists do: try and learn more...

[mothythewso]

>:DI have to ask. If gravity isn't truly a force as fundamental forces are understood, if it's an artifact, a result of the curvature of space-time, why does it have to obey the speed of light limit. Also, what I meant by saying "absolute lower limit" is that since space-time is quantized, smaller than 1 quantum of anything obeying QT is impossible.(Sorry for the wording). Finally, the Planck length and Planck time are absolute lower limits; a smaller value is essentially meaningless, in any theory we now have, at least in my understanding. It's sort of like speculating on the physics extant before the beginning of physics, the properties of the pre-universe, the quantum foam, say. It's fun, nobody can say you're wrong, but neither can anyone say you're right. Kind of like super-strings, imhp

[swansont]

>:DI have to ask. If gravity isn't truly a force as fundamental forces are understood, if it's an artifact, a result of the curvature of space-time, why does it have to obey the speed of light limit. Also, what I meant by saying "absolute lower limit" is that since space-time is quantized, smaller than 1 quantum of anything obeying QT is impossible.(Sorry for the wording). Finally, the Planck length and Planck time are absolute lower limits; a smaller value is essentially meaningless, in any theory we now have, at least in my understanding. It's sort of like speculating on the physics extant before the beginning of physics, the properties of the pre-universe, the quantum foam, say. It's fun, nobody can say you're wrong, but neither can anyone say you're right. Kind of like super-strings, imhp

 

"in any theory we now have" is the key phrase here. I'm not aware of any theory that actually says that the Planck length and time quantize space and time (do you have a citation?), just that we need a quantum theory of gravity to work at those scales.

[mothythewso]

I didn't say that the Planck length or Planck time "quantized" anything; I said they are the lowest values of those measurable physical attributes, and smaller values have no physical meaning, granted as we NOW understand physics. However, if time is quantized, I suppose the Planck time could be considered as the quantum of time.

As I understand things, the photon is the quantum of electromagnetic radiation, radiation is emitted in discrete, quantized values. Isn't there a least value for a quantum of electromagnetic radiation? Some value below which there is no reality? After all, there is no "half" quantum, the quantum value doesn't approach 0 as lower limit.

[swansont]

I didn't say that the Planck length or Planck time "quantized" anything; I said they are the lowest values of those measurable physical attributes, and smaller values have no physical meaning, granted as we NOW understand physics. However, if time is quantized, I suppose the Planck time could be considered as the quantum of time.

As I understand things, the photon is the quantum of electromagnetic radiation, radiation is emitted in discrete, quantized values. Isn't there a least value for a quantum of electromagnetic radiation? Some value below which there is no reality? After all, there is no "half" quantum, the quantum value doesn't approach 0 as lower limit.

 

But you actually stated that time and space are quantized (no "if"). So even if your other statements didn't imply that, do you have a citation for accepted physics that says that space and time are quantized?

 

Energies are quantized in some systems, and the energy of emitted EM radiation at some arbitrary frequency comes in discrete bundles. But in principle any value of frequency is possible.

[Strange]

Isn't there a least value for a quantum of electromagnetic radiation?

 

No. It is not quantized in the same way as, say, electric charge.

[mothythewso]

(But you actually said time and space are quantized. (no 'if'). So even if your other statements didn't imply that, do you have a citation for accepted physics that says that time and space are quantized?)

 

I was a biology major, for god's sake. But:

1) "Introduction of a quantum of time(chronon) and its consequences for quantum mechanics", Roy A. H. Farias, Erasmo Recami, 27 Jun 1997, Cornell University Library

2) Entry in Wikipedia, Quantum Spacetime> "...the concept of 'quantum spacetime' is a generalization of the usual concept of spacetime..."

3) "Quantum Time", Mark Lawrence, July 13, 2011

 

Agreed, these are not "citations", I shouldn't have said space-time IS quantized, but some people a whole lot smarter than I seem to think so. Also, M-Theory, (which I do not accept), the cutting edge of physics (or metaphysics, take your choice), certainly deals with the quantization of space-time, tho I truly can't wrap my brain around strings and 26 dimensions.

[elfmotat]

(But you actually said time and space are quantized. (no 'if'). So even if your other statements didn't imply that, do you have a citation for accepted physics that says that time and space are quantized?)

 

I was a biology major, for god's sake. But:

1) "Introduction of a quantum of time(chronon) and its consequences for quantum mechanics", Roy A. H. Farias, Erasmo Recami, 27 Jun 1997, Cornell University Library

2) Entry in Wikipedia, Quantum Spacetime> "...the concept of 'quantum spacetime' is a generalization of the usual concept of spacetime..."

3) "Quantum Time", Mark Lawrence, July 13, 2011

 

Agreed, these are not "citations", I shouldn't have said space-time IS quantized, but some people a whole lot smarter than I seem to think so. Also, M-Theory, (which I do not accept), the cutting edge of physics (or metaphysics, take your choice), certainly deals with the quantization of space-time, tho I truly can't wrap my brain around strings and 26 dimensions.

 

Those are theoretical models, some of which have problems, and none of which are accepted physics.

[swansont]

Untested hypotheses are not mainstream, accepted physics.

[mothythewso]

Swansont said at 02:03 today

Untested hypotheses are not mainstream, accepted physics.

 

When I was a lowly undergrad, quarks weren't accepted physics.

In 1800, Newtonian physics reigned supreme.

And why do physicists refer to "string theory" when not only is it not tested, it's inherently untestable at this time. Shouldn't it be referred to at best as "string hypothesis" or "string conjecture"? Can or should string "theory" be open for discussion on this forum? Cronons?

I admit I've indulged in speculation in this thread, and I apologize if it's not appropriate.

[swansont]

 

When I was a lowly undergrad, quarks weren't accepted physics.

 

But physicists did the experiments and got the evidence, so that changed.

 

If you want to discuss string theory, go right ahead and open up a thread. (The naming falls under the general observation that anything with "theory" as the last word in the official title really isn't a theory.)

 

As far as engaging in speculations goes, the main issue is whether you are presenting a new model or asking about established physics. Thus far you seem to be reasonably receptive to correction, so as long as that continues, we're good.

[mothythewso]

Cool