Quantum Beginner Question

[mozilla]

Hi,

 

I have a question. Apologies as I have no background in science/physics and have just started so my language may be a bit elementary. I only know what i have read.

 

Now to the question ...

 

I understand general relativity specifies that gravity is the curvature of space which causes the earth to orbit the sun. That makes sense in explaining the attraction of the planets.

 

However quantum mechanics specifies that gravity is due to the exchange of virtual particles known as gravitons.

 

So how can two sub atomic particles exchange energy that causes them to be attracted to each other? One particle releases a graviton that another absorbs. How can that cause attraction? If anything would the second particle be disturbed from the energy absorption?

 

I guess the same question applies to the attraction of the electromagnetic force .

 

Thanks in advance!

[Klaynos]

Hi,

 

I have a question. Apologies as I have no background in science/physics and have just started so my language may be a bit elementary. I only know what i have read.

 

Now to the question ...

 

I understand general relativity specifies that gravity is the curvature of space which causes the earth to orbit the sun. That makes sense in explaining the attraction of the planets.

 

However quantum mechanics specifies that gravity is due to the exchange of virtual particles known as gravitons.

 

There's currently no quantum theory of gravity, so I doub't anyone will really be able to answer many questions.

 

So how can two sub atomic particles exchange energy that causes them to be attracted to each other? One particle releases a graviton that another absorbs. How can that cause attraction? If anything would the second particle be disturbed from the energy absorption?

 

Due to the lack of a quantum theory of gravity, it'll be easier if we think of electromagnetism here.

 

So you've a 1q charged particle and a -1q charged particle, they exchange virtual particles and move closer together... How can this possibly be? We could talk about negative momentum exchange but that's just making pretty pictures. The real answer is that it's quantum mechanics, it's something that doesn't work well in our heads when compared to the classical world we're all used to, it's just how stuff it.

 

I guess the same question applies to the attraction of the electromagnetic force .

 

Thanks in advance!

 

I know I've not really answered your question, but there isn't really a good classical way of explaining it.

[ajb]

It is true that there is no full quantum theory of gravity, but we can think of general relativity as an effective theory and calculate things to tree level. So in that respect, curvature of space-time and gravitons are not competing ideas. Just as the (classical) electromagnetic field is consistent with the notions of photons, i.e. particles associated with quantising the EM field.

 

I think discussing GR to tree level is outside of what is needed here.

 

Virtual particles are really only a way to describe a mathematical expansion of what is know as the generating functional. So I would not take the idea of exchange particles as meaning much more than that. From this generating functional using well known methods you can calculate the S-matrix, which describes the scattering of particles.

 

Now, it depends on the nature of these virtual particles as to whether the force is attractive, repulsive or can be both as in electromagnetism. Principally, it depends on their spin. If I can remember, I will give you a list of spins and if the force is attractive or repulsive.

[Pete]

I understand general relativity specifies that gravity is the curvature of space which causes the earth to orbit the sun. That makes sense in explaining the attraction of the planets.

General Relativity (GR) doesn't say that. GR states that tidal gradients and spacetime curvature are the same phenomena.

However quantum mechanics specifies that gravity is due to the exchange of virtual particles known as gravitons.

 

So how can two sub atomic particles exchange energy that causes them to be attracted to each other? One particle releases a graviton that another absorbs. How can that cause attraction? If anything would the second particle be disturbed from the energy absorption?

I see no problem in reconciling GR and QM in this respect. By the way its quantum field theory (QFT) that speaks of gravitons.

 

Pete

[mozilla]

Thanks for the responses .

 

From your responses I think I am starting to understand that the issue might be more in the way I am thinking about the problem. I guess classical mechanics is not a good way to visualise any type of quantum theory. I need to expand my classical mind

 

Although we have no full quantum theory of gravity we accept the experimental results of the attraction of particles that give us an insight to the different world on a quantum scale (Charges and spin causing attraction).

 

 

I see no problem in reconciling GR and QM in this respect. By the way its quantum field theory (QFT) that speaks of gravitons.

 

Pete

 

Does this mean QM and GR or both are compatible theories in explaining forces including gravity? I was under the impression that they both are unable to be correct. GR breaks down at the big bang and QM violates some of the fundamental principles of GR such as quantum entanglement information transferring faster than the speed of light.

 

Cheers,

Mozilla

[Pete]

Does this mean QM and GR or both are compatible theories in explaining forces including gravity?

Mind you, I don't know relativistic quantum mechanics so take what I say on this with a large grain of salt. However from what I understand there doesn't seem to be any problem in this respect, hence the graviton is still lurking around. Its common for people to say that there is no such thing as a gravitational force in GR etc. The truth is that there are two classes of forces. One is known as inertial forces (like the gravitational force) while the other is sometimes referred to as poderomotive forces (like the Lorentz force). Just because has a frame dependant existance and the other doesn't doesn't mean that one should be considered "real" and the other shouldn't. Thinking otherwise can make one question whether gravitons exist or not. What one should really be asking is whether the observation of gravitons is dependant on the observer. A similar question comes up when one considers the fact that wether a charged particle is observer to radiate or not depends on the obsever's frame of reference.

I was under the impression that they both are unable to be correct.

Why?

GR breaks down at the big bang ...

Since when??

..and QM violates some of the fundamental principles of GR such as quantum entanglement information transferring faster than the speed of light.

QM does not violate quantum entanglement, it is actually consistent with it.

And there have been reports of FTL signals and there are several articles on FTL using quantum entanglement that are in the physics literature.

 

Pete

[Klaynos]

FTL communication with quantum entanglement is nearly always (if not always) a missunderstanding of what quantum entanglement is.

 

I'm not sure on this, but the singularity predicted at the Big Bang, surely that indicates that GR has broken down?

[Pete]

FTL communication with quantum entanglement is nearly always (if not always) a missunderstanding of what quantum entanglement is.

Please elaborate. What is this missunderstanding of which you speak? Researchers who write such articles are not as naive as that statement seems to make them out to be.

 

In the mean time you might want to consider reading the following articles. I haven't read more than the abstracts to date (been too busy on other projects) so if you believe you've found an error in one or more of them please post it. Thank you.

 

Can EPR-correlations be used for the transmission of superluminal signals? P. Mittelstaedt, Ann. Phys (Leipzig) 7 (1998), 7-8, 710-715

Abstract. In a compound quantum system with EPR-like correlations a measurement of one subsystem induces instantaneously changes of the subsystem, irrespective of the relative distance of the two subsystems. We consider several arguments which were put forward in recent years in order to show that these nonlocal effects cannot be used for superluminal communication. It turns out that arguments mentioned above are merely plausible but not really stringent and convincing. This means that the question in the title of this paper is still open.

 

Superluminal signal velocity, G. Nimtz, Ann. Phys (Leipzig) 7 (1988), 7-8, 618-624

Abstract. It recently has been demonstrated that signals conveyed by evanescent modes can travel faster than light. In this report some special features of signals are frequency band limited. Evanescent modes are characterized by extraordinary properties: Their energy is negative, they are not directly measurable, and the evanescent region is not causal since the modes traverse this region instantaneously. The study demonstrates the necessity of quantum mechanics in order to understand the superluminal velocity of classical evanescent modes.

 

Faster than Light?, Raymond Y. Chiao, Paul G. Kwiat and Aephraim M. Steinberg, Scientific American, August 1993

Abstract - Experiments in quantum optics show that two distant events can influence each other faster than any signal could have traveled between them.

 

Pete

[Klaynos]

My reference was more to pop sci articles.

 

The misunderstanding is normally to do with the entangled particles either continuing in their entangled state AFTER an interaction, or that there is some communication between the two particles when one is observed. Experiments have shown this is not the case. I don't have the time to find references right now, but in the last 5 years there has been some quite conclusive stuff about it.

[Pete]

I don't have the time to find references right now, but in the last 5 years there has been some quite conclusive stuff about it.

Physicists always argue about what is really "conclusive" or not. Experiment is the true test.

 

I look forward to when you have the time.

 

Pete

[ajb]

I cannot remember the details for exchange fermions, but for bosons we have;

 

spin-1 repulsive

spin-0 and spin-2 attractive

 

Higher spins are ruled out by Weinberg-Witten theorem.

 

Now, EM is mediated by the spin-1 photon, but we have attractive and repulsive forces as we have both positive and negative charges.

 

The spin-0 graviton is ruled out experimentally. Basically, it does not reduce to general relativity and does not agree with how light bends in a gravitational field. As we appear to only have positive mass gravity is attractive.

 

As for exchange fermions, I will have to consult the literature. I have seen similar statements for them but I cannot recall them.