I Why are we looking for a graviton?

[revnice]

The current explanation of gravity is that it's the warping of spacetime by the prescence of a body. So why are we looking for a graviton?

Thanks - rev

 

[QuarkyMeson]

If nature is quantum, gravity should have a quanta; consistency says those quanta must be massless spin-2 particles that couple to the stress-energy tensor, which in the classical limit is just GR.

So far, there is no evidence for gravitons and the bounds on where we could find them has gotten tighter with indirect experiments. (Direct experiments wouldn't work as far as I know.)

This isn't the only way to quantize gravity, there exists plenty of other attempts without spin 2 massless particles (Throw away Lorentz symmetry and you can do silly things, I think this has been basically ruled out though by observations, weird Emergent-gravity or massive graviton approaches, etc.) and so far no ones figured it out. That is the limit of my knowledge on the topic.

For more, this thread is an interesting read:

K

I Thread 'Are gravitons necessary?'

May 23, 2024

If gravity is not a force, but rather curvature of spacetime, are gravitons necessary to couple with gravitational field?

• KevinMcHugh
• Field Graviton Quantum
• Replies: 18
• Forum: Special and General Relativity

 

[PeroK]

The current explanation of gravity is that it's the warping of spacetime by the prescence of a body. So why are we looking for a graviton?

Thanks - rev

GR makes sense on a macroscopic scale. The assumption is that the effect of a massive body like a planet must be the sum of the effects of all its constituent elementary particles

But, elementary particles are described by QM and they individually behave in ways that are incompatible with the equations of GR.

So, it's assumed there must be a way to derive these equations of GR (the warping of spacetime, as you call it) from.purely QM calculations.

That's the search for a quantum theory of gravity. To provide a QM foundation for GR.

 

[Ibix]

We know there are problems with general relativity because singularities keep dropping out of the maths. So we strongly suspect that it's only an approximation to a more complete theory.

We strongly suspect that such a theory must be a quantum theory because the sources of gravity are quantised. You can do things with quantised matter like put it into superposed states, which leads to superposed states for things like its electric field (if it is charged). That's fine because we have a quantum theory that includes electromagnetism, but there is no way to feed a superposed state into the equations of a classical field theory like general relativity. Yet we can make superposed matter states. So we need the successor theory of gravity to be able to handle such things, which means it pretty much has to be a quantum field theory.

 

[martinbn]

We know there are problems with general relativity because singularities keep dropping out of the maths. So we strongly suspect that it's only an approximation to a more complete theory.

People say this often, but why a singularities a problem!

 

[phinds]

People say this often, but why a singularities a problem!

Singularities show up in the math as infinities and when translated to the physical world, that those math infinities translate to "OK, this cannot be physically possible, so we don't really know what the H... is going on when the math gets down to this level."

In other words, the math is just a model. The usefulness of a model is its ability to accurately describe the world that it is modeling. When the model throws out something that can't be real in the world, we do NOT assume it's the world's fault, we assume that our model has a limitation.

 

[martinbn]

Singularities show up in the math as infinities and when translated to the physical world, that those math infinities translate to "OK, this cannot be physically possible, so we don't really know what the H... is going on when the math gets down to this level."

In other words, the math is just a model. The usefulness of a model is its ability to accurately describe the world that it is modeling. When the model throws out something that can't be real in the world, we do NOT assume it's the world's fault, we assume that our model has a limitation.

Not quite, they show up as incompleteness of geodesics (or some other curves). For example the tip of a cone is a singular point, but no one thinks that there is a problem with cones.

 

[martinbn]

Coming back on topic

So why are we looking for a graviton?

Who exactly is looking for a graviton? They way you ask it seems that you think that every theoretical physicist is looking for it.

But since it is a possibility, why shouldn't some be trying that approach?

 

[PeroK]

Coming back on topic

Who exactly is looking for a graviton? They way you ask it seems that you think that every theoretical physicist is looking for it.

But since it is a possibility, why shouldn't some be trying that approach?

In fact, some physicists are only really thinking about ways that a graviton might be detected. E.g. through a quantized interaction between an atom and a gravitational wave.

 

[Ibix]

People say this often, but why a singularities a problem!

If you use latitude and longitude to give directions, there's a singularity at the poles - all directions are south at the north pole, so you can only say "go south" and that does not define a unique direction. The mathematics fails you.

That's only a singularity in coordinates and you can solve the problem by picking a different coordinate system should you ever need to give directions from the pole. Or even just stand there pointing and saying "thattaway". Singularities in the underlying theory are more problematic because you can't simply pick a better mathematical model. There isn't one. And we can't fudge it from experience because we don't have any. So we end up with a theory that doesn't work in some places.

For example, in Schwarzschild spacetime you can predict what will happen as you fall in to the black hole. You reach the singularity in a fairly short time. But then we cannot say what will happen next. There is no "next" in the model - anything reaching the singularity is just gone from the model and we cannot predict consequences for that matter or anything it might interact with thereafter - which is a failing in a model.

 

[phinds]

Not quite, they show up as incompleteness of geodesics (or some other curves). For example the tip of a cone is a singular point, but no one thinks that there is a problem with cones.

But that singular point does NOT show up in the math as an infinity, it shows up as a zero so of course it's not an issue.

 

[revnice]

Thanks to all! I'm not as good as I need to be to understand this. I'm aware of the terms you guys used but it's prettymuch a case of 'Here lie dragons.' Would it be fair to say that a graviton is implied because that would be consistent?

 

[Ibix]

Would it be fair to say that a graviton is implied because that would be consistent?

More that a new theory of gravity can't solve the problems we have with the current one unless it's a quantum theory.

 

[martinbn]

But that singular point does NOT show up in the math as an infinity, it shows up as a zero so of course it's not an issue.

What is zero and not infinity in the example of the cone? Suppose the cone is tip up, like a party hat. If you slice it horizontally you get a circle of some radius, say $r$. The curvature of that circle is $\kappa = \frac1r$. As your slices go up towards the tip the circles get smaller and the radius goes to zero, so the curvature goes to infinity.

 

[martinbn]

If you use latitude and longitude to give directions, there's a singularity at the poles - all directions are south at the north pole, so you can only say "go south" and that does not define a unique direction. The mathematics fails you.

As you say that is not a good example, there are no singularities here. The maths doesn't fail me. I am not using it well.

That's only a singularity in coordinates and you can solve the problem by picking a different coordinate system should you ever need to give directions from the pole. Or even just stand there pointing and saying "thattaway". Singularities in the underlying theory are more problematic because you can't simply pick a better mathematical model. There isn't one. And we can't fudge it from experience because we don't have any. So we end up with a theory that doesn't work in some places.

I don't understand this. How does the theory not work? It works perfectly well. It even predicts the singularities. Which are not part of the space-time so you cannot say the theory doesn't work in some places. There are no such places. It works in all places and times.

For example, in Schwarzschild spacetime you can predict what will happen as you fall in to the black hole. You reach the singularity in a fairly short time. But then we cannot say what will happen next. There is no "next" in the model - anything reaching the singularity is just gone from the model and we cannot predict consequences for that matter or anything it might interact with thereafter - which is a failing in a model.

As you say there is no next, so the question is not well posed. The singularities might be unexpected and in conflict with intuition build on other models, but they are not problem. This happens all the time in relativity.

Think about it this way there is no mathematical inconsistency, and there is no observational evidence either way. So you dismiss them, and say that they are a problem that needs to be resolved in a more accurate theory.

 

[Demystifier]

The current explanation of gravity is that it's the warping of spacetime by the prescence of a body. So why are we looking for a graviton?

The current explanation of sound is that it's a change of density of the medium. So why are we looking for a phonon? (Which, by the way, is found.)

 

[Ibix]

As you say that is not a good example, there are no singularities here. The maths doesn't fail me. I am not using it well.

I didn't say it was a bad example, just that it was possible to work around that particular singularity by adopting a different convention. That you can adopt a different mathematicsl model doesn't mean that you don't have a failure of the original one.

The difference between a coordinate singularity like the geographic pole or the Schwarzschild event horizon and a singularity like the Schwarzschild $r=0$ singularity is that you can't work around the latter because there is no alternative mathematical model describing the same thing in alternative terms. Both have the property that you can't predict what happens when you reach them.

Think about it this way there is no mathematical inconsistency, and there is no observational evidence either way. So you dismiss them, and say that they are a problem that needs to be resolved in a more accurate theory.

That's exactly what I'm saying. I just gave a bit more explanation around why we suspect singularities are a problem: they are places you can reach in finite time, and beyond which you cannot predict with our current models.