Why do we need a quantum theory of gravity?

[Alan Grayson]

If such a theory could be constructed, it would have particles to manifest excited states, called gravitons. But for a BH, gravitons generated by its mass couldn't escape, so they couldn't function as force carrying particles as in other quantum field theories. We'd still need Einstein's GR to account for the gravitational "force" via curvature of space-time. So what would a quantum theory of gravity buy us? Why do we need it? AG

[Brent Meeker]

On 7/23/2020 8:03 PM, Alan Grayson wrote:
> If such a theory could be constructed, it would have particles to
> manifest excited states, called gravitons. But for a BH, gravitons
> generated by its mass couldn't escape, so they couldn't function as
> force carrying particles as in other quantum field theories.

That's nonsense.  Gravitons are linearized solutions of the weak field
equations and you're saying they can't escape from a region of infinite
curvature...see the problem?

> We'd still need Einstein's GR to account for the gravitational "force"
> via curvature of space-time. So what would a quantum theory of gravity
> buy us? Why do we need it? AG

We need it because Einstein's equation has classical field variables on
the left and quantum field densities on the right.

Brent

[Alan Grayson]

On Thursday, July 23, 2020 at 10:30:52 PM UTC-6, Brent wrote:

On 7/23/2020 8:03 PM, Alan Grayson wrote:
> If such a theory could be constructed, it would have particles to
> manifest excited states, called gravitons. But for a BH, gravitons
> generated by its mass couldn't escape, so they couldn't function as
> force carrying particles as in other quantum field theories.

That's nonsense.  Gravitons are linearized solutions of the weak field
equations and you're saying they can't escape from a region of infinite
curvature...see the problem?

LC wrote that gravitons can't escape from a BH. He may have meant

the weak field solutions. But why can the others, if they exist, escape 

from a region of infinite curvature. AG 

> We'd still need Einstein's GR to account for the gravitational "force"
> via curvature of space-time. So what would a quantum theory of gravity
> buy us? Why do we need it? AG

We need it because Einstein's equation has classical field variables on
the left and quantum field densities on the right.

On the right side is the stress-energy tensor. What is quantum about this? Didn't

AE abhor quantum theory?  AG 

Brent

[Lawrence Crowell]

On Thursday, July 23, 2020 at 10:03:36 PM UTC-5 agrays...@gmail.com wrote:

If such a theory could be constructed, it would have particles to manifest excited states, called gravitons. But for a BH, gravitons generated by its mass couldn't escape, so they couldn't function as force carrying particles as in other quantum field theories. We'd still need Einstein's GR to account for the gravitational "force" via curvature of space-time. So what would a quantum theory of gravity buy us? Why do we need it? AG

The way you state this illustrates considerable confusion and in these threads I and others have indicated how to think of this. This does not involve gravitons coming out of black holes. You have repeated this error a number of times.

A weak low energy quantum gravitation is easy to derive. The low energy limit of gravitation is linear because terms in the curvature involving the square of connection terms are much smaller. This makes gravitation and gravitational waves linear. Quantization is not much different from quantizing electrodynamics in QED. The gravitational waves detected by the LIGO are long wavelength and with small amplitude. There should be signatures of gravitons there which would be linear. As the wavelength shortens the energy increases and as this approaches TeV and higher energy the nonlinear terms become appreciable. The nonlinear feature of gravitation, and that it is an exterior fibration so the field correlates direction with the quantum wave, means this is a nonlinear quantum mechanics, which is a contradiction of quantum mechanics.  

LC

[Alan Grayson]

On Friday, July 24, 2020 at 4:38:20 AM UTC-6, Lawrence Crowell wrote:

On Thursday, July 23, 2020 at 10:03:36 PM UTC-5 agrays...@gmail.com wrote:

If such a theory could be constructed, it would have particles to manifest excited states, called gravitons. But for a BH, gravitons generated by its mass couldn't escape, so they couldn't function as force carrying particles as in other quantum field theories. We'd still need Einstein's GR to account for the gravitational "force" via curvature of space-time. So what would a quantum theory of gravity buy us? Why do we need it? AG

The way you state this illustrates considerable confusion and in these threads I and others have indicated how to think of this. This does not involve gravitons coming out of black holes. You have repeated this error a number of times.

You previously stated that gravitons cannot escape BH's.  Do you stand by this claim? AG

[Lawrence Crowell]

On Friday, July 24, 2020 at 5:49:23 AM UTC-5 agrays...@gmail.com wrote:

On Friday, July 24, 2020 at 4:38:20 AM UTC-6, Lawrence Crowell wrote:

On Thursday, July 23, 2020 at 10:03:36 PM UTC-5 agrays...@gmail.com wrote:

If such a theory could be constructed, it would have particles to manifest excited states, called gravitons. But for a BH, gravitons generated by its mass couldn't escape, so they couldn't function as force carrying particles as in other quantum field theories. We'd still need Einstein's GR to account for the gravitational "force" via curvature of space-time. So what would a quantum theory of gravity buy us? Why do we need it? AG

The way you state this illustrates considerable confusion and in these threads I and others have indicated how to think of this. This does not involve gravitons coming out of black holes. You have repeated this error a number of times.

You previously stated that gravitons cannot escape BH's.  Do you stand by this claim? AG

Yep!

LC

[Alan Grayson]

So if a full quantum theory of GR must be non-linear and thus in contradiction with QM, do you conclude it can't exist? AG  

LC

[Brent Meeker]

On 7/24/2020 3:49 AM, Alan Grayson wrote:

On Friday, July 24, 2020 at 4:38:20 AM UTC-6, Lawrence Crowell wrote:

On Thursday, July 23, 2020 at 10:03:36 PM UTC-5 agrays...@gmail.com wrote:

If such a theory could be constructed, it would have particles to manifest excited states, called gravitons. But for a BH, gravitons generated by its mass couldn't escape, so they couldn't function as force carrying particles as in other quantum field theories. We'd still need Einstein's GR to account for the gravitational "force" via curvature of space-time. So what would a quantum theory of gravity buy us? Why do we need it? AG

The way you state this illustrates considerable confusion and in these threads I and others have indicated how to think of this. This does not involve gravitons coming out of black holes. You have repeated this error a number of times.

You previously stated that gravitons cannot escape BH's.  Do you stand by this claim? AG

Of course that's true.  If a double neutron star, which would be a source of gravitational radiation, fell into a super-massive black hole, its gravitational radiation would go futureward into the "singularity", not out.

But gravitons are the linearized infinitesimal wave solutions of gravitational perturbation.  The mass of the neutron double star would still add to the mass of the black hole and expand its event horizon.  From the standpoint of a distant observer, the gravitational effect of the (double star + black hole) is the same whether the former is inside the latter or just nearby.

Brent

A weak low energy quantum gravitation is easy to derive. The low energy limit of gravitation is linear because terms in the curvature involving the square of connection terms are much smaller. This makes gravitation and gravitational waves linear. Quantization is not much different from quantizing electrodynamics in QED. The gravitational waves detected by the LIGO are long wavelength and with small amplitude. There should be signatures of gravitons there which would be linear. As the wavelength shortens the energy increases and as this approaches TeV and higher energy the nonlinear terms become appreciable. The nonlinear feature of gravitation, and that it is an exterior fibration so the field correlates direction with the quantum wave, means this is a nonlinear quantum mechanics, which is a contradiction of quantum mechanics.  

LC

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[Alan Grayson]

On Friday, July 24, 2020 at 4:38:20 AM UTC-6, Lawrence Crowell wrote:

On Thursday, July 23, 2020 at 10:03:36 PM UTC-5 agrays...@gmail.com wrote:

If such a theory could be constructed, it would have particles to manifest excited states, called gravitons. But for a BH, gravitons generated by its mass couldn't escape, so they couldn't function as force carrying particles as in other quantum field theories. We'd still need Einstein's GR to account for the gravitational "force" via curvature of space-time. So what would a quantum theory of gravity buy us? Why do we need it? AG

The way you state this illustrates considerable confusion and in these threads I and others have indicated how to think of this. This does not involve gravitons coming out of black holes. You have repeated this error a number of times.

What error are you referring to? I was just POSTULATING that IF a quantum theory of gravity is possible, gravitons would exist but couldn't escape a BH and thus couldn't function as force carrying particles analogous to photons for QED. We'd still need Einstein's theory of gravity based on curvature of space-time to explain the gravity field external to a BH. So what would be gained from such a quantum theory? I have no problem with gravitons existing in a weak field approximation of GR, and this being a linear quantum theory. AG

[Lawrence Crowell]

On Friday, July 24, 2020 at 1:46:54 PM UTC-5 agrays...@gmail.com wrote:

On Friday, July 24, 2020 at 4:38:20 AM UTC-6, Lawrence Crowell wrote:

On Thursday, July 23, 2020 at 10:03:36 PM UTC-5 agrays...@gmail.com wrote:

If such a theory could be constructed, it would have particles to manifest excited states, called gravitons. But for a BH, gravitons generated by its mass couldn't escape, so they couldn't function as force carrying particles as in other quantum field theories. We'd still need Einstein's GR to account for the gravitational "force" via curvature of space-time. So what would a quantum theory of gravity buy us? Why do we need it? AG

The way you state this illustrates considerable confusion and in these threads I and others have indicated how to think of this. This does not involve gravitons coming out of black holes. You have repeated this error a number of times.

What error are you referring to? I was just POSTULATING that IF a quantum theory of gravity is possible, gravitons would exist but couldn't escape a BH and thus couldn't function as force carrying particles analogous to photons for QED. We'd still need Einstein's theory of gravity based on curvature of space-time to explain the gravity field external to a BH. So what would be gained from such a quantum theory? I have no problem with gravitons existing in a weak field approximation of GR, and this being a linear quantum theory. AG

It is not the case that gravitons come out of a black hole to intermediate a force between it and some other mass. From the perspective of an exterior observer all mass-energy and quantum fields that make up a black hole are on the event horizon or just above. This is why I got into the whole Tortoise coordinates and so forth. I will have to leave it here I think.

LC

[Bruce Kellett]

On Sat, Jul 25, 2020 at 9:13 AM Lawrence Crowell <goldenfield...@gmail.com> wrote:

It is not the case that gravitons come out of a black hole to intermediate a force between it and some other mass. From the perspective of an exterior observer all mass-energy and quantum fields that make up a black hole are on the event horizon or just above. This is why I got into the whole Tortoise coordinates and so forth. I will have to leave it here I think.

Why does the perspective of an exterior observer have preferred status? This is an absurdly intstrumentalist/positivist idea. What matters is the objective reality, not what you might chance to see from some perspective or the other. The idea of all the mass-energy residing on the stretched horizon is just so much positivistic twaddle......

Bruce

[Brent Meeker]

On 7/24/2020 4:13 PM, Lawrence Crowell wrote:

On Friday, July 24, 2020 at 1:46:54 PM UTC-5 agrays...@gmail.com wrote:

On Friday, July 24, 2020 at 4:38:20 AM UTC-6, Lawrence Crowell wrote:

On Thursday, July 23, 2020 at 10:03:36 PM UTC-5 agrays...@gmail.com wrote:

If such a theory could be constructed, it would have particles to manifest excited states, called gravitons. But for a BH, gravitons generated by its mass couldn't escape, so they couldn't function as force carrying particles as in other quantum field theories. We'd still need Einstein's GR to account for the gravitational "force" via curvature of space-time. So what would a quantum theory of gravity buy us? Why do we need it? AG

The way you state this illustrates considerable confusion and in these threads I and others have indicated how to think of this. This does not involve gravitons coming out of black holes. You have repeated this error a number of times.

What error are you referring to? I was just POSTULATING that IF a quantum theory of gravity is possible, gravitons would exist but couldn't escape a BH and thus couldn't function as force carrying particles analogous to photons for QED. We'd still need Einstein's theory of gravity based on curvature of space-time to explain the gravity field external to a BH. So what would be gained from such a quantum theory? I have no problem with gravitons existing in a weak field approximation of GR, and this being a linear quantum theory. AG

It is not the case that gravitons come out of a black hole to intermediate a force between it and some other mass. From the perspective of an exterior observer all mass-energy and quantum fields that make up a black hole are on the event horizon or just above. This is why I got into the whole Tortoise coordinates and so forth. I will have to leave it here I think.

LC

Besides that, the static field is mediated by virtual gravitons...which can travel back in time, if you were trying do a Feynman diagram of gravitational attraction.

Brent

[Brent Meeker]

I agree.  The question would be "Residing there when?"  The only interest in the distant observers viewpoint is to compare it to what we distant observers observe.

Brent

[Bruce Kellett]

A while ago you, Brent, made an observation that seemed to me to be the perfect answer to all thoughts of black hole complementarity -- we could call it "bent stick complementarity". When you partially submerge a straight stick in water, it appears bent to the external observer; but when you take it out of the water it appears straight. You can't have it both in the water and out of the water at the same time, so the two observations are complementary. Complementarity would say that, from the point of view of the exterior observer, the stick is in fact bent when it is partially immersed in water; and it is in fact straight when it is wholly out of the water. No observer can see both situations at the same time -- they are complementary.

Similarly for the observations of the observer exterior to the black hole and the observer who plunges through the horizon..... The exterior view is entirely illusory.

Bruce

[Alan Grayson]

IIRC, LC introduced tortoise coordinates in an objection to my claim that the gravitational field of the BH at the center of our galaxy can be observed, or inferred, from the rotation rate of stars near our galactic core.  Is this observation not sufficient? AG

[Alan Grayson]

As far as I can tell, the advantage of tortoise coordinates is that they obfuscate the relevant issues.  Still waiting for some argument why a quantum theory of gravity is needed. TIA, AG