Gravitons?

[Tinker Grey]

OK. A few days ago I watched a video by Sabine Hossenfelder about detecting a quantum of gravity. (Link below)

I thought it was more or less settled that mass warps space. What role would a graviton have if it existed? I get the Higgs (for a peculiar definition of "get") -- it's a field that causes what we experience as mass.

Is mass not sufficient to explain gravity? Apparently, not.

 

[Chesterton]

If mass warps space, we don't know exactly why it does. Maybe gravitons play a role in the "why"?

 

[AlexB23]

OK. A few days ago I watched a video by Sabine Hossenfelder about detecting a quantum of gravity. (Link below)

I thought it was more or less settled that mass warps space. What role would a graviton have if it existed? I get the Higgs (for a peculiar definition of "get") -- it's a field that causes what we experience as mass.

Is mass not sufficient to explain gravity? Apparently, not.

I am hoping for the days we can discover gravitons, cos I remember the year we discovered gravitational waves (it was the fall of 2015, but more talked about in 2016). Light has a force carrier particle, the photon. So, if gravity does have a force carrier, maybe we can use the graviton for something, just as we use photons.

From LIGO (California):

https://www.ligo.caltech.edu/news/ligo20160211

 

[sjastro]

Sabine’s cake analogy isn’t terribly convincing, there are a few good reasons why everyone should bring a cake including gravity.

Firstly the three other forces, electromagnetic, weak and strong forces are mediated by gauge bosons which are predicted by theory and confirmed by experiment.

Secondly the theory predicts these forces are unified at high energies (temperatures).
By 1983 physicists had sufficiently powerful particle accelerators to unify the electromagnetic and weak forces into the electroweak force.
The next step is to unify the electroweak and strong forces into a GUT (Grand Unified Theory) but we are nowhere near the technologies to reach the energies to accomplish this.

Where does quantum gravity fit into this?
If gravitons exist it means quantum gravity was also unified in the very early universe otherwise there are serious problems with BB cosmology.
A quantum gravity theory eliminates the singularity at cosmological time t = 0 which is a product of general relativity which breaks down at small scales.
It is impossible to explain the evolution of the universe if it started off as a singularity as by definition a singularity is indicative the mathematics is seriously wrong!

There are theoretical problems in formulating a quantum gravity theory within the optics of quantum field theories.
Without getting into the mathematical complications, quantum field theories involve point sources making it impossible to renormalize the theory to include gravity.
This is where the much maligned string theory comes into the picture which does not depend on point sources but strings making gravity a renormalizable theory.

 

[Arcangl86]

OK. A few days ago I watched a video by Sabine Hossenfelder about detecting a quantum of gravity. (Link below)

I thought it was more or less settled that mass warps space. What role would a graviton have if it existed? I get the Higgs (for a peculiar definition of "get") -- it's a field that causes what we experience as mass.

Is mass not sufficient to explain gravity? Apparently, not.

There's been a search for a quantumized theory of gravity for a while. As good a job as general relativity does to describe gravitational effects, it's not perfect. In particular, it conflicts with quantum mechanics in a couple of key ways. One is the approach to time. In quantum mechanics time is considered a universe force, constant across all of existence. In relativity, it can be affected by curved space-time, which results from either high velocities or high gravitational energies. The second issue, the one which the graviton would solve if we were to ever experimentally verify it, is that gravity in relativity can't be quantified. If you try, you get gobblygook. But the whole point of quantum mechanics is that energy can only exist in discrete packets, not along a spectrum. It's like the difference between digital and analog computing. But since quantum mechanics does such a good job explaining pretty much everything else in our conception of reality, having gravity not be compatible is a real issue.

 

[Tinker Grey]

Thank you, @sjastro and @Arcangl86. So, graviton theory solves some sticky problems particularly at time = 0. How would one harmonize what one pretty easily (?) sees with GR (mass warps space-time) and the graviton?

If we detect gravitons, would that effectively eliminate GR? Or would gravitons enhance GR? (I mean, obviously Newtonian physics is still useful, but we recognize it as an approximation.)

 

[Hans Blaster]

I am hoping for the days we can discover gravitons, cos I remember the year we discovered gravitational waves (it was the fall of 2015, but more talked about in 2016). Light has a force carrier particle, the photon. So, if gravity does have a force carrier, maybe we can use the graviton for something, just as we use photons.

From LIGO (California):

https://www.ligo.caltech.edu/news/ligo20160211

Some of us remember talking about it before it was announced... (Rumors are so much fun.)

 

[AlexB23]

Some of us remember talking about it before it was announced... (Rumors are so much fun.)

I was a little young back then, but when I was 14 or 15 in the second half of 2014, I was trying a sci-fi plot about a spaceship around ~110 meters long that uses gravity waves to travel FTL at around 410 c between the alien's home world and a spaceport 8 hours away from the home world (3.5 trillion km), as well as to other alien worlds. Earth was not part of the sci-fi world, and was never visited. Sadly, gravity waves travel at the speed of light (1 c). Darn, sci-fi plot failed. I should recycle that ship, and use it in another universe, and where the gravity wave drive malfunctions, sending the ship into the universe that I want to use it in, maybe have it be used in an interstellar battle.

But hey, I am lazy. That sci-fi plot would probably not happen.

 

[Arcangl86]

Thank you, @sjastro and @Arcangl86. So, graviton theory solves some sticky problems particularly at time = 0. How would one harmonize what one pretty easily (?) sees with GR (mass warps space-time) and the graviton?

If we detect gravitons, would that effectively eliminate GR? Or would gravitons enhance GR? (I mean, obviously Newtonian physics is still useful, but we recognize it as an approximation.)

There are attempts to harmonize general relativity and quantum mechanics, but they aren't terribly successful from my understanding. Though there has been success in applying special relativity to quantum systems.

My understanding is that gravitons, and the quantization they represent, will not be capable with space-time at all. But because general relativity describes large scale events so well, I imagine it will never go away. Quantum would be the fundamental basis of reality, with classical mechanics and electrodynamics for day-to-day stuff and realivitlistic mechanics for cosmic scale applications.

 

[Hans Blaster]

I was a little young back then, but when I was 14 or 15 in the second half of 2014, I was trying a sci-fi plot about a spaceship around ~110 meters long that uses gravity waves to travel FTL at around 410 c between the alien's home world and a spaceport 8 hours away from the home world (3.5 trillion km), as well as to other alien worlds. Earth was not part of the sci-fi world, and was never visited. Sadly, gravity waves travel at the speed of light (1 c). Darn, sci-fi plot failed. I should recycle that ship, and use it in another universe, and where the gravity wave drive malfunctions, sending the ship into the universe that I want to use it in, maybe have it be used in an interstellar battle.

Gravitational waves. Gravity or buoyancy waves are waves in media where gravity (buoyancy) is the restoring force. Surface waves on the lake are Gravity waves.

But hey, I am lazy. That sci-fi plot would probably not happen.

That's the point of sci-fi.

 

[sjastro]

Thank you, @sjastro and @Arcangl86. So, graviton theory solves some sticky problems particularly at time = 0. How would one harmonize what one pretty easily (?) sees with GR (mass warps space-time) and the graviton?

If we detect gravitons, would that effectively eliminate GR? Or would gravitons enhance GR? (I mean, obviously Newtonian physics is still useful, but we recognize it as an approximation.)

Newtonian gravity expressed as a field theory is a first order approximation for GR, the Einstein field equations reduce to LaPlace and Poisson equations for Newtonian gravity for weak gravitational fields when external matter is absent or present in the gravitational field respectively.

Not only is the singularity a problem for GR but below the Planck length of 10⁻³⁵ metres, quantum fluctuations dominate where space-time does not have the smooth continuous characteristics as modelled in GR.
GR will not be eliminated as it is a first order approximation for quantum gravity, the challenge is to recover the Einstein field equations at scales greater than the Planck length and as a low energy limit for quantum gravity.

 

[Bradskii]

If anyone needs a heads up on basic gravitational theory, my avatar will gladly supply it. He starts around the 6 minute mark.

 

[AlexB23]

Gravitational waves. Gravity or buoyancy waves are waves in media where gravity (buoyancy) is the restoring force. Surface waves on the lake are Gravity waves.

That's the point of sci-fi.

Yeah, I get those two confused sometimes. I meant gravitational waves. Gravitational waves are limited to the speed of light. Seems that there is no way of getting around that speed limit.

But yes, sci-fi is meant to bend physics sometimes. Now, maybe for thanksgiving, my folks can make some graviton-flavored gravy. If quarks have flavors, who know what flavors gravitons have.

 

[sjastro]

If anyone needs a heads up on basic gravitational theory, my avatar will gladly supply it. He starts around the 6 minute mark.

As a human interest story Feynman being from the old school of quantum physics along with Paul Dirac - Wikipedia were vehemently opposed to renormalization which was brought up in post #4.

 

[sjastro]

Another point I should have made is not only GR a first order approximation for quantum gravity but also is GUT (Grand Unified Theory) where the strong and electroweak forces are unified.

This adds to the complexity as there are number of candidate GUTs under consideration.

As mentioned in a previous post we do not have the technology for unifying the strong and electroweak forces.
In theory energies of 10¹⁶ GeV are required, the best we have is the LHC reaching levels of 14 x 10³ GeV which is approximately one trillionth of the required energy.
At this stage GUT's can be evaluated such as by searching for evidence of proton decay, the existence of magnetic monopoles and right handed neutrinos which are predictions of the theory.

 

[Sif]

Yeah, I get those two confused sometimes. I meant gravitational waves. Gravitational waves are limited to the speed of light. Seems that there is no way of getting around that speed limit.

But yes, sci-fi is meant to bend physics sometimes. Now, maybe for thanksgiving, my folks can make some graviton-flavored gravy. If quarks have flavors, who know what flavors gravitons have.

Reminds me of the scene in Star Trek IV: The Voyage Home when Spock is "re-calibrating" his mind:

"Adjust the sine wave of this magnetic envelope so that anti-neutrons can pass through it but anti-gravitons cannot"

 

[Hans Blaster]

Reminds me of the scene in Star Trek IV: The Voyage Home when Spock is "re-calibrating" his mind:

"Adjust the sine wave of this magnetic envelope so that anti-neutrons can pass through it but anti-gravitons cannot"

Give Alex a chance. He's working his way there. At ST III right now...

 

[AlexB23]

Reminds me of the scene in Star Trek IV: The Voyage Home when Spock is "re-calibrating" his mind:

"Adjust the sine wave of this magnetic envelope so that anti-neutrons can pass through it but anti-gravitons cannot"

Woo, I am gonna watch Star Trek IV sometime this week. Just pretty much completed Star Trek III: The Search for Spock this morning. I will watch out for that quote.

Now, if only Star Trek could talk about muons more. Apparently, if a muon were to ever decay into a photon instead of a neutrino, that means the Standard Model would need to be revised. I am hoping that we can reach a day to show if the Standard Model is still valid, or disprove it.

 

[sjastro]

Give Alex a chance. He's working his way there. At ST III right now...

I hope he avoids ST V at all costs.

 

[AlexB23]

I hope he avoids ST V at all costs.

Oh oh, is Star Trek V that bad? It can't be any worse than Star Trek Discovery, with that mushroom powered engine/drive.

Spore Drive activated (Duration = 1 min 20 sec):