Gravitons and curved space


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Gravitons and curved space

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Jul. 6, 2009, 7:57 am

A little nervouse to post here, as I am not a scientist, but have a question (a bunch actually, but will spare y'all the details). This particular question is about gravity, gravitons, and curved space.

If gravity is a function of the curvature of space, why is there a need for an exchange particle?

If the answer is too much for an LT topic, please recommend a book that might address this.

Jul. 6, 2009, 7:24 pm

It's a good question, since general relativity, which describes gravity in terms of the curvature of spacetime, doesn't need gravitons. It's only when we try to make a theory of gravity consistent with quantum mechanics that a need for quanta (gravitons) arises.

Since quantum gravity is still an open question, I don't know if it would be useful to say much more than that at this point. Similarly, while there are a lot of popular science books discussing quantum gravity, I'm not sure any of them would be especially useful. Perhaps Three Roads to Quantum Gravity, though it's been a while since I read that. Your best bet for more information might be taking this question to Physics Forums, or searching through their archives for similar discussions (such as this one).

This isn't my area of expertise, so corrections are welcome.

Bearbeitet: Jul. 6, 2009, 8:26 pm

a theory of gravity consistent with quantum mechanics that a need for quanta (gravitons) arises.

Thanks for the comment. Very appreciated coming from a graduate student.

I have not seen the issue be addressed in any books I've read. They just leave relativity and seemingly take up a new, unrelated subject.

Without any physics background, I thought, maybe, gravity is not a force and does not need to be unified with the 3.

Thanks for the references, and for the encouragement that the question was not stupid.

Jul. 31, 2009, 1:24 am

daschaich is correct. Graviton is a hypothetical elementary particle that mediates the force of gravity in the framework of quantum field theory. To prove the existence of the graviton, physicists must be able to link the particle to the curvature of the space-time continuum and calculate the gravitational force exerted.

Bearbeitet: Jul. 31, 2009, 8:13 am

#4 Thanks for the comment. daschaich posted a link to PhysicsForums. That is a very good resource for a layperson struggling with the science and math of these questions. I am always a little nervous to ask some questions here, thinking they may be inappropriate for this group.

As far as gravitons, they just do not seem necessary, as far as I can figure with my very limited reading. But I will need to work through more before I can get to QM. Then I can see if I can understand why it is necessary for a gravity theory to be consistent with QM. Right now, it just seems more simple to me if gravity is only a function of geometry that can be, and is usefully, described with F=MA if we throw in the G constant.

Really appreciate the comment. Some of my thinking here is probably wrong, but I will continue to work through it.


PSR191316, after rereading your comment, I thought I may have missed your point initially. Are you saying that the graviton may be the exchange particle that causes the curvature of space and time? Mass does not curve space and time by just existig, but the exchange particle communicates the disturbance? Hence the need for a graviton in relativity?

Bearbeitet: Jul. 31, 2009, 4:48 pm

So far as we know, all of nature is governed by quantum mechanics. It would be very bizarre and disconcerting if gravity were somehow an exception. Moreover, general relativity by itself leads to well-known "singularities", for instance in "black holes" or at the "Big Bang". If a theory predicts some infinite quantity, this is a sign that the theory has exited its domain of applicability and broken down. For reasons you'll see when you get to QM, the microscopic scale at which these singularities arise is precisely where we expect quantum effects to be most important, to the point they can "cure" the singularities. How exactly that happens is what we're still working on figuring out.

And I think "how exactly that happens" is also the point of PSR191316's comment. The key word that jumped out at me there was "link": it's not that gravitons are the unique cause of spacetime curvature (mass and energy are responsible for that, as described by Einstein's equations). Rather, physicists still need to figure out how translate between the quantum language of gravitons and the "classical" language of continuous spacetime curvature, linking the particle and continuum pictures.

Jul. 31, 2009, 8:03 pm

I love to read Einstein's relativity now described as "classical" physics. :) (I realize that I am stretching what you said too far. It just speaks to how far and how fast knowledge is moving.)

I read this week in reference to Faraday's relative atypical lack of higher formal mathematics training that "by the early 19th century physics had advanced appreciably,..., and one needed a strong background in mathematics to follow this rapidly growing science."

Given where things are and are going, this statement seems even more appropriate for today.

Thanks to both of you for taking time to respond with attempts to speak to my level. I will work some more to get to an approach of understanding of the quantum states. I think I have caught the bug.

I will then return with real questions. Other concepts also escape me, such as substomic symmetry, Hamiltonian and Lagrangian mechanics, string. I will leave the question of quantum behavior of gravity alone for now.

Thanks again so much.

Sept. 5, 2009, 5:43 pm

I hate to bring this back up, but I am still working through this gravity thing.

translate between the quantum language of gravitons and the "classical" language of continuous spacetime curvature, linking the particle and continuum pictures.

I think I am just now getting the point of this statement in #6.

If gravity is a false force, just felt from the effects of perceived acceleration like centrifugal force, then its perceived effects should be continuous.

But physicists are expecting that gravity is discreet in conformity to quantum physics, complete with its own exchange particle. In this case it would not be continuous?

Sept. 11, 2009, 12:32 am

Even though we're using the language of discrete particles, the mathematics describes continuous fields, just as for other quantum field theories.

Just last week I started auditing a course in general relativity. In the first lecture, the instructor emphasized that general relativity can be treated in almost exactly the same way as electromagnetism. Similarly, electromagnetism can be reformulated in geometrical terms, almost exactly in the same way as general relativity.

This may be a useful connection, as it immediately shows that one would no more expect discreteness from gravitons in the gravitational force than from photons in the electromagnetic force.

As an aside, physicists' terminology uses "classical" to refer to anything non-quantum-mechanical, such as general relativity. It's not a value judgment or a suggestion that the theory is dated or obsolete.

Sept. 11, 2009, 3:58 am


Appreciate your patience. I am probably expecting to understand all this, without putting in the effort on the ground work.

I need to develop your patience, and study the basic principles then move up to the more advanced concepts.


Okt. 27, 2009, 6:46 am

Is there any clue or insight to the nature of gravity to be gained from considering the units in the gravitational constant?

Mrz. 15, 2012, 8:22 pm

This is reaching way back to revive an old topic, but I did not want to start another topic on gravity. It has recently dawned on me how remarkable it is that inertial mass is equivalent to gravitation mass.

Mass is the resistance to change in motion.
Mass is the source of the attraction between objects.

It is remarkable first that mass has these two aspects (even definitions?), and also that science has discovered these two qualities of matter which seem to me so different in their natures.

And it is remarkable that it has taken me so long to realize how different these two aspects are, after seeing so often F=MA and F=GMM/r^2.

Mrz. 27, 2012, 9:37 am

Another question about gravitons: if nothing can escape a black hole, how do the gravitons get out?

Mrz. 28, 2012, 12:59 am

Do they get out, or do we just observe the highly-curved spacetime nearby but outside the black hole's event horizon? I have not thought about this carefully, but I believe the latter is the more accurate statement (otherwise it would suggest information flowing the wrong way across the event horizon).

Mrz. 28, 2012, 5:12 am

If they are exchange particles, then they must cross the event horizon to be exchanged? No? Or maybe gravity is not quantitized?

Gravity remains a thing I don't understand? But it is certainly not the only thing. :)

Mrz. 28, 2012, 1:09 pm

How do you "observe the highly-curved spacetime nearby"? I assume "observing" means feeling its pull. What else could it be. If you are attracted to the black hole, then you must be exchanging gravitons, hence they have "gotten out"?

Apr. 4, 2012, 3:33 pm

Sorry for the delay; I've been traveling a lot recently.

One observes highly-curved spacetime through the radiation it emits or affects. Most prosaically, this is electromagnetic radiation: radio waves, light, x-rays or gamma rays. Electromagnetic radiation can be either redirected or redshifted by passing through highly-curved spacetime. The redirection is known as "gravitational lensing" and is a very powerful and popular technique to search for the presence of objects that aren't visible in their own right. This includes even objects with very small gravitational effects, such as planets orbiting distant stars. Electromagnetic radiation can also be generated from the energy in astrophysical systems; examples of this include quasars, pulsars and gamma ray bursts, and I believe some of these phenomena are related to black holes.

In addition, black holes are objects that can have very large gravitational effects, to the point that it may be possible to observe gravitational radiation that they emit directly. On the large scale that we may hope to observe in the future, this gravitational radiation consists of gravitational waves, and processes such as the formation of a black hole produce very distinctive patterns of gravitational waves. Just as electromagnetic waves are described in terms of photons on the quantum level, a quantum description of these gravitational waves involves gravitons. As you may know, quantizing gravity is not as easy as quantizing electromagnetism, but at a more practical level, gravity is so enormously weaker than electromagnetism that there is essentially no hope of ever observing individual gravitons.

In my previous, off-the-cuff comment, I alluded to a very deep result in general relativity that information cannot escape a black hole, where "escaping" means passing through the event horizon from the inside to the outside. I'm afraid I'm not able to explain this result, or where it comes from, in any intuitive way (and quantum effects introduce their own wrinkles which I don't even fully understand). However, we can understand the consequences: if we interacted directly with gravitons or gravitational waves that had crossed the event horizon ("gotten out of the black hole"), we on the outside of the event horizon would have obtained information about the interior of the event horizon, which is forbidden.

We can only ever learn three properties of a black hole (from studying the highly-curved spacetime nearby but outside the event horizon): its mass, its electric charge and its angular momentum. (These quantities determine its size and shape, so the size and shape are really the same properties discussed in different terms.) This is somewhat whimsically called a "no hair theorem". In practice we can't actually learn any of these properties with existing technology; this is the best we could ever do in theory, again neglecting potential and poorly-understood quantum complications.

Did that make any sense at all? These are great questions, so keep them coming if anything I wrote isn't clear.

Bearbeitet: Apr. 4, 2012, 6:49 pm

Always pleased daschaich to read your comments. Thanks.

I still don't understand why general relativity does not end the discussion about gravitons. If gravity is just warped space, and objects are simply going straight along a curved path, then why the need for exchange particles.

The man did not feel any force when he fell off the scaffold until he hit the ground, right?

I am still trying to catch back up to work on understanding the simultaneity question. I had to go back to F=MA again, more accurately back to T=Iα.

Apr. 4, 2012, 11:02 pm

The discussion about gravitons is really independent of general relativity itself. General relativity is a classical (i.e., non-quantum) field theory, very much comparable to Maxwell's classical electrodynamics. Classical electrodynamics does not include any concept of photons, just as general relativity does not include any concept of gravitons. Both of these theories discuss the dynamics of fields: the electromagnetic field and the spacetime metric ("curvature"), respectively.

Photons and gravitons only come into the picture when one tries to make those two classical theories of fields consistent with quantum mechanics. The generic result of this procedure is a quantum (i.e., non-classical) field theory. Quantum electrodynamics was developed mainly in the 1930s and 1940s, and has been very successful ever since. There is at present no fully-developed and self-consistent theory of quantum gravity. A way to appreciate why the development of a quantum field theory of gravity is much trickier than the case of electromagnetism is to note that quantum electrodynamics deals with the behavior of electromagnetic fields living in a static, unchanging spacetime. A quantum field theory of gravity, in contrast, would need to deal with the behavior of spacetime itself, living in... itself! This is a much trickier picture to wrap your mind around.

Reading your question, I think the central issue you are getting at is why quantum mechanics requires there to be these photons and gravitons in the first place. You note that one can discuss gravitational forces (at the classical level) in terms of Einstein's equations for the metric field, that is the curvature of spacetime, with no need to mention gravitons. In exact analogy, one can discuss electromagnetic forces (at the classical level) in terms of Maxwell's equations the electromagnetic field, with no need to mention photons.

The key is that these fields have ripples, and in a quantum world, these ripples are quantized. (In this discussion, I'm borrowing heavily from explanations that Matt Strassler has written on his Web site, in particular this article.) The quantum ripples in the electromagnetic field are called photons; those in the spacetime metric are called gravitons. While we don't have a fully self-consistent theory of quantum gravity, the quantization of the spacetime metric is a very generic requirement for any reconciliation of gravity with quantum mechanics. A theory of quantum gravity would need to deal with the behavior of gravitons (quantum ripples in the spacetime metric), living in... themselves!

You focus on the role of photons and gravitons as "exchange particles" of the electromagnetic and gravitational forces, respectively. All this amounts to is the identification of photons and gravitons as (quantum) ripples in the electromagnetic field and spacetime metric. In both of these cases, large-scale phenomena can be described in the corresponding classical language -- Maxwell's equations governing the behavior of electromagnetic fields, and Einstein's equations governing the warping of space and objects simply going straight along a curved path. It's only when one enters the quantum regime (which is tremendously more accessible for electromagnetism than for gravity, because gravity is so much weaker) that precisely these processes are described in terms of photons and gravitons, respectively.

Bearbeitet: Apr. 5, 2012, 7:57 am

Is the idea of space as abstract and constructed as the idea of a field - a helpful conceptual tool?

I am sure that question is almost too vague to answer.

Apr. 5, 2012, 9:37 pm

More than helpful, the idea that spacetime (or more precisely the metric describing spacetime) is a field is really a central concept of general relativity. You may have seen that Einstein's equations are often called the Einstein field equations, because that's exactly what they are: equations describing the behavior of a field. Whenever we say something about spacetime being "curved" or "warped", for instance, we are implicitly treating it as a field.

Bearbeitet: Apr. 12, 2012, 10:55 pm

OK, I read through this once. Now putting the link here for safe keeping, to be reviewed again maybe later when I am better ready.

Very interesting though the presence and treatment of pervasive fields, which I was about to put away as helpful conceptualizations and useful models.

Apr. 13, 2012, 2:21 am

In case it isn't clear, the subject of that blog post is not what we've been discussing in this thread. I don't object to the link being here; I just don't want anybody to be confused by trying to find a... link (sorry).

Mai 16, 2012, 12:13 pm

I would like to add a thought that I believe is too often overlooked. We revere Einstein. His word is THE word. That perception has, perhaps, led to faulty acceptance of concepts that are NOT correct in an absolute sense but only relativistically. One of the above comments mentioned that the Theory of Gen Rel leads to singularities and a brewakdown of mathematics. How true, think wormholes, white holes etc. The curvature of space time is a mathematically correct model but not necessarily reality., Is space-time REALLY curved? To postulate the curving ST around a home run while ST curves around the hot dog in the bun held by the spectator who catches the ball while he's held down in his seat is absurd. Lots of curving occurring in different directions all at once! Quantum mechanics rules and we must look at gravity as such while understanding that for now the math describes ST as a curvature. BUT IT'S NOT LITERALLY CURVED

Mai 16, 2012, 1:00 pm

I think current scientific understanding is that space time is literally curved by mass.

Mai 19, 2012, 4:25 am

I think current scientific understanding is that space time is literally curved by mass.

This is correct, though I would refine it to state that spacetime is literally curved by energy, one form of which is mass.

24 raises a number of very important issues, the first of which is why we should take general relativity seriously in the first place. This has nothing to do with Einstein. We don't accept that spacetime is curved by energy because Einstein said so and he was a smart guy (even though he did and he was). Rather, the predictions of general relativity have been validated by a long series of experimental and observational tests.

Probably the most famous of these tests is the global positioning system (GPS), which is very sensitive to the curvature of spacetime caused by the earth. If the earth did not curve spacetime as general relativity predicts, GPS would not work. Based on these numbers, if these general relativistic effects were neglected (with everything else still done correctly), then the system would drift by almost ten miles every day.

Speaking of neglecting general relativistic effects, let's consider the (valid) observation that curved spacetime appears "absurd" at the everyday level of baseballs and hot dogs. One of the first things you should learn about general relativity (curved spacetime) is that it correctly reproduces newtonian gravity (flat spacetime) at the everyday level. "One of the hardest things to learn in physics is when subtle effects matter and when they don’t." In this case, the curvature of spacetime is a subtle effect that does not matter. (And, for that matter, the quantum mechanics mentioned in 24 matters even less.)

Since it was an earlier comment of mine that mentioned singularities in the context of general relativity, let me conclude by clarifying their significance. I would not say that general relativity "leads to singularities". Rather, the appearance of singularities (in predictions from any theory) is a sign that you are trying to use that theory outside of its domain of applicability. In the case of general relativity, singularities appear when spacetime becomes so highly curved that quantum mechanical effects are likely to be important (for instance in black holes or shortly after the Big Bang). If you'll pardon the vernacular, the singularities simply indicate that by trying to use classical general relativity in such contexts, you're doing it wrong.

Mai 19, 2012, 7:15 am

Thanks for that link about the effect of relativity on GPS. That is a very interesting and helpful article, pointing to a real every day impact and every day experience of the validity of theory.

Mai 19, 2012, 8:33 am

Is it reasonably correct to understand matter as a form of energy with low entropy? Or is that jumbling up different concepts?

Mai 19, 2012, 3:10 pm

Is it reasonably correct to understand matter as a form of energy with low entropy? Or is that jumbling up different concepts?

That's jumbling up different concepts. The relation between matter and energy in particular can cause many sorts of confusion, which Matt Strassler has discussed at length, allowing me to give my keyboard a break.

Mai 19, 2012, 3:33 pm

I will have to go through his blog thoroughly. I thought everything was mass or energy (or perhaps given equivalency - everything was energy). Actually, that very thought - everything is energy - had been in my mind a couple times this week.

Need to go back for a brain rewrite apparently.

Apr. 27, 2014, 9:35 am

This is not intended to bring an old topic, but rather to discuss the recent discovery of gravity waves. I did not find a topic on the discovery.

I am still struggling with gravity as a force and as a geometry, and still struggling with GR.

Any comments on the impact of this discovery as it relates to confirming cosmological theories, the nature of gravity, General Relativity.

Mai 2, 2014, 1:03 pm

The detection of low-multipole B-mode polarization in the cosmic microwave background is definitely significant. It may well be the biggest discovery of the year (sorry). The BICEP2 ("Background Imaging of Cosmic Extragalactic Polarization") result still needs independent confirmation, but (unlike certain other recent "discoveries") I'm not aware of any specific reason to strongly suspect problems.

Regarding prospects for independent confirmation, I've heard conflicting rumors about when the Planck Collaboration plans to release their updated results, and about what they might be able to say based on the amount of data they have. I'm still hoping to hear something interesting over the summer, but I wouldn't be surprised if we had to wait until next year.

Regarding the impact of this result (if it holds up) on general relativity, I'm most excited by the indication that general relativity remains valid at much smaller distance scales (equivalently, much larger energy scales) than we have ever been able to study before.

Based on my previous posts, you can probably guess where I'm going to send you for further comments and information:
* BICEP2: New Evidence Of Cosmic Inflation!
* If It Holds Up, What Might BICEP2′s Discovery Mean?
* Did BICEP2 Detect Gravitational Waves Directly or Indirectly?

Mai 2, 2014, 3:28 pm

Thanks David.

Very glad to see you comment here again, or still. : )

The whole thing is exciting to me, because of the confirmed predictions and the implications for cosmology.

I had read several of Prof Strassler's posts. Unfortunately, I am still not quite able to follow and to understand very well his articles.