Full disclosure, I did this little writeup but thought folks here might find it interesting. It was fun to use dimensional analysis to understand the order of magnitudes involved in graviton...
Full disclosure, I did this little writeup but thought folks here might find it interesting. It was fun to use dimensional analysis to understand the order of magnitudes involved in graviton detection.
Always fun to read comparisons with other forces. I remember reading some works about it and what kind of detector would be needed to distinguish information, and how it would basically be(or...
Always fun to read comparisons with other forces. I remember reading some works about it and what kind of detector would be needed to distinguish information, and how it would basically be(or collapse into) a black hole.
Of course, I can't find it at this moment... but regardless, it makes one think about what it implies for what gravity is. And how insanely weak it is, yet so dominant in the Universe.
Thank you for sharing your write-up! The mathematics behind quantum physics continue to amaze me. I can't quite follow all the derivations, but the complexity and scale of the problem is clear to...
Thank you for sharing your write-up! The mathematics behind quantum physics continue to amaze me. I can't quite follow all the derivations, but the complexity and scale of the problem is clear to me now.
Have scholars speculated upon any additional processes the graviton participates in with higher cross-sections that might be more readily measured? Processes theorized but not yet proven to exist which, if so proven, could be utilized to detect single gravitons? If the limiting factor is the practicality of measurement, then would a different kind of measurement not be ideal? How many ways could there be to detect a particle like this?
I am decidedly not a physicist, but it's interesting to hear about advances in the deep sciences.
My pleasure, thank you for reading. The main problem with detecting gravitons as a scattering process is that the cross-section is always proportional to either G_N or (G_N)^(2) which is tiny as...
Thank you for sharing your write-up!
My pleasure, thank you for reading.
Have scholars speculated upon any additional processes the graviton participates in with higher cross-sections that might be more readily measured?
The main problem with detecting gravitons as a scattering process is that the cross-section is always proportional to either G_N or (G_N)^(2) which is tiny as an interaction strength. As far as alternative theories, there's a whole zoo of model building we could do out there. Dozens of models. One example which comes to mind is the search for the Randall-Sundrum (RS) graviton which is a massive heavy excitation of the graviton that is predicted by some theories involving extra dimensions. We wouldn't see these gravitons as a scattering process because they are heavy and wouldn't propagate easily (same reason W and Z bosons aren't long range) but there is the possibility we produce them in energetic collisions.
The LHC has excluded RS gravitons of mass up to a few TeV, but they could easily be heavier than that.
Maybe you can answer a question I've long had about gravitons... Given that (I believe) it's commonly accepted that gravity is the result of massive objects warping the fabric of spacetime, why...
Maybe you can answer a question I've long had about gravitons... Given that (I believe) it's commonly accepted that gravity is the result of massive objects warping the fabric of spacetime, why would we expect gravitons to exist at all?
From what I understand, the existence of the graviton is a prediction of quantum mechanics, which famously does not play well with the spacetime system described by general relativity. More...
From what I understand, the existence of the graviton is a prediction of quantum mechanics, which famously does not play well with the spacetime system described by general relativity.
More particularly, the predictive power of GR breaks down when it comes to systems on very small scales, but high densities and energies. Conditions like this exist around dense astrophysical objects like black holes and neutron stars, and also in the very first few moments following the Big Bang. Because of this, a theory that explains gravity in a different manner than GR that can be applied to these scenarios is highly desirable, and this "quantum gravity" ususally asks for a graviton. One reason for that is because quantum effects (more accurately, the information about the quantum system) are "quantized", or split into discrete packets, with those packets taking the form of particles.
This is my understanding as an interested layman so I'm happy to be corrected!
This was a really fascinating article to read as well, and though I agree with the other posters in being unable to follow the chain of derivation, the results are very clear. It never occurred to me that there could be such a stark difference in the detactability of quantum particles, but a distance of 85 orders of magnitude is nothing to sniff at!
Thanks for the answer. My understanding, also as a non physicist, is that the principles of quantum mechanics do indeed call for a graviton but most (all?) attempts to create a quantum mechanical...
Thanks for the answer. My understanding, also as a non physicist, is that the principles of quantum mechanics do indeed call for a graviton but most (all?) attempts to create a quantum mechanical theory around gravitons have run into deal breaking issues which is a big part of what led to string theory as an alternative.
And since we can't test any of the string theories, and likely won't be able to for a very long time, we're stuck. My feeling, which of course carries no weight (much like a graviton), is that perhaps the hole where gravitons so neatly fit is occupied by something else entirely.
Maybe, but as long as the gravitational field is quantizable, the graviton will exists as the quanta of said field. Maybe it's not a quantum field at all, but there's no evidence for that, either.
Maybe, but as long as the gravitational field is quantizable, the graviton will exists as the quanta of said field. Maybe it's not a quantum field at all, but there's no evidence for that, either.
Thank you for the kind words. It occurs to me I should have perhaps motivated the dimensional analysis better so one could follow why the cross section is proportional to the combination of...
This was a really fascinating article to read as well, and though I agree with the other posters in being unable to follow the chain of derivation, the results are very clear. It never occurred to me that there could be such a stark difference in the detactability of quantum particles, but a distance of 85 orders of magnitude is nothing to sniff at!
Thank you for the kind words. It occurs to me I should have perhaps motivated the dimensional analysis better so one could follow why the cross section is proportional to the combination of constants in each case, but that'd require either some pretty strong (and likely unsatisfying) hand-waving or a crash course into the Feynman rules for quantum fields.
The simplest (not most accurate) explanation that I've heard is that when you quantize things, you get particles. In the case of the graviton, it arises from the quantization of metrics in GR.
The simplest (not most accurate) explanation that I've heard is that when you quantize things, you get particles. In the case of the graviton, it arises from the quantization of metrics in GR.
The other answers you got cover the gist, but here's some more reasons why we expect gravity to be quantized. I'm quoting a reddit comment I made some time ago: Why does gravity have to be quantum...
The other answers you got cover the gist, but here's some more reasons why we expect gravity to be quantized. I'm quoting a reddit comment I made some time ago:
Why does gravity have to be quantum in the first place? Since nobody can figure it out, maybe gravity is just classical. Maybe Einstein got it right the first time and we're just spinning our wheels. The above seems like a reasonable argument, but I want to elaborate on why we should expect gravity to be quantum in nature.
There appears to be four fundamental forces in the universe (five if you include the Higgs, though it's not a gauge boson). Of these, all but gravity, are quantum in nature. It would be strange if all of physics except gravity obeyed the rules of quantum mechanics.
Where quantum mechanics and gravity are both important (most famously black holes), there are calculations that are contradictory, or can only be done approximately. This means that physics must be incomplete. The "Black hole information paradox" says BHs do not preserve information via their decay into Hawking radiation. This is a paradox because quantum mechanics (which predicts Hawking radiation) requires information to be preserved which is a feature called "unitarity." Unitarity is a VERY important feature of quantum theory and we're reluctant to give it up.
Instead of adding quantum fields to curved spacetime, we ask: "Can classical gravitation fields be made from quantum matter?" The simplest form of this "semi-classical gravity" (i.e. G = <T>) where <T> is the "averaged" quantum matter is also inconsistent and leads to incorrect predictions. This doesn't mean gravity can't be classical, but it does mean both classical gravity and quantum mechanics can't be simultaneously true. Either quantum theory or gravity needs to change.
Effective quantum gravity actually works well. If you keep to low energy situations, quantum theory and gravity are actually very compatible. In this domain, you can do everything classical gravity does through the language of quantum theory. Calculating the precession of Mercury using quantized gravity is a graduate student level homework problem. Effective quantum gravity also clearly predicts quantum corrections to Newtonian gravity, though today there's no clear way to measure such corrections.
String theory, despite recent negative buzz and bad PR these days, is a theory of quantum gravity. The problem is well-understood (and well behaved) string theories do not describe our gravity in our universe. We seem to have made math for quantum gravity for somebody else's universe! This is akin to stumbling upon Paris, Kansas instead of Paris, France. It's still Paris however, so maybe there's hope.
Thanks for the thorough answer! Follow up question, or maybe a clarification of my previous question... the fundamental forces describe the interactions of things within spacetime with other...
Thanks for the thorough answer!
Follow up question, or maybe a clarification of my previous question... the fundamental forces describe the interactions of things within spacetime with other things within spacetime yes? Whereas gravity, while it effectively does the same, is the result of the warping of spacetime itself and so it's describing the interactions of things with the base medium, and each other sort of secondarily. It seems intuitive that the physics of gravity wouldn't conform to expectations derived from our understanding of the other forces?
Having typed that I see that there isn't really a clear question, and it borders on philosophical rather than scientific... but feel free to share any related thoughts you have.
To a certain extent, we already see that it does defy expectations by being difficult (or impossible) to quantize in a manner as satisfying as say electromagnetism. But a different point is that...
It seems intuitive that the physics of gravity wouldn't conform to expectations derived from our understanding of the other forces?
To a certain extent, we already see that it does defy expectations by being difficult (or impossible) to quantize in a manner as satisfying as say electromagnetism.
But a different point is that you don't have to formulate general relativity as spacetime curvature (the geometrical interpretation). It is perfectly alright to consider a background of special relativity on which the gravitational field acts. From this perspective, gravity is then an additional dynamics field like any other (electromagnetism, electroweak, etc.) living on top of special relativity.
The flipped perspective is to LEAN into the geometrical interpretation. Gravity is the spacetime curvature of our base reality, but so are the other forces! The electromagnetic field tensor F is a curvature tensor for the gauge field we call the electromagnetic field in the same way the Reinmann curvature tensor R is for spacetime. From this perspective, you write all the interactions (gravity, electromagnetism, electroweak, strong, etc.) as geometric curvature. Our reality is then not just spacetime, but spacetime plus all gauge fields. Why give spacetime a preference when mathematically it is a dimension just like any of the others?
That's interesting. Brings to mind string theory, with the math coming first and the reality it describes being wholly hypothetical (for now). Because it's the set of dimensions we experience, and...
The flipped perspective is to LEAN into the geometrical interpretation
That's interesting. Brings to mind string theory, with the math coming first and the reality it describes being wholly hypothetical (for now).
Why give spacetime a preference when mathematically it is a dimension just like any of the others?
Because it's the set of dimensions we experience, and which we are most invested in understanding :) It would be fitting if it turned out we can't understanding the universe from our 4D centric viewpoint.
That's not a really a compelling case. We can't see radio, but it's just as much a legit form of light as any other wavelength. Our human experience is quite limited and is often a poor source for...
Because it's the set of dimensions we experience
That's not a really a compelling case. We can't see radio, but it's just as much a legit form of light as any other wavelength. Our human experience is quite limited and is often a poor source for physics intuition. In contrast, there's a lot of similarities between the degrees of freedom that arise from spatial dimensions and the degrees of freedom in the gauge fields, including the fact curvature in both are defined analogously. You might then naturally ask; are they actually separate things?
We already expanded Euclidean 3-space to include time as a dimension from relativity:
xi = (x,y,z)
xμ = (x,y,z,t)
We're then tempted to include the photon Aν gauge field.
Xa = (xμ,Aν)
We can add even more of the fields until we have a unified geometry of all interactions. Now, with all this said, the "project" of doing this in physics in incomplete and success is not guaranteed, but it's a cool idea. All theories of extra-dimensions (including String Theory) start with thoughts like these.
Agreed, that's what I was getting at re: 4D centric viewpoint. We made a similar mistake with geocentricity and then heliocentricity. Also though, I think it's safe to say that there's more...
Our human experience is quite limited and is often a poor source for physics intuition
Agreed, that's what I was getting at re: 4D centric viewpoint. We made a similar mistake with geocentricity and then heliocentricity.
Also though, I think it's safe to say that there's more practical value in understanding the universe we can interact with (radio waves included).
It will be ironic, then, if it turns out we've arrived at a point where we can't progress our understanding of the universe solely through observing the bits we can interact with.
but it's a cool idea
Definitely. The idea that adding more dimensions can create a more elegant and unified understanding of the universe, like Einstein did, is very compelling. It was the first thing that came to mind when I encountered string theory.
I have to admit my heart kind of fell in love with kaluza Klein theory back when I first learned about it several years ago and that love has persisted since. I'm always on the lookout for a...
I have to admit my heart kind of fell in love with kaluza Klein theory back when I first learned about it several years ago and that love has persisted since. I'm always on the lookout for a reasonable theory that adds more dimensions.
Full disclosure, I did this little writeup but thought folks here might find it interesting. It was fun to use dimensional analysis to understand the order of magnitudes involved in graviton detection.
Damn those highly reactive neutrinos, swamping out the signal, isn't a phrase often uttered!
Always fun to read comparisons with other forces. I remember reading some works about it and what kind of detector would be needed to distinguish information, and how it would basically be(or collapse into) a black hole.
Of course, I can't find it at this moment... but regardless, it makes one think about what it implies for what gravity is. And how insanely weak it is, yet so dominant in the Universe.
What gravity has going for it, is that all charges are always positive and attractive.
Thank you for sharing your write-up! The mathematics behind quantum physics continue to amaze me. I can't quite follow all the derivations, but the complexity and scale of the problem is clear to me now.
Have scholars speculated upon any additional processes the graviton participates in with higher cross-sections that might be more readily measured? Processes theorized but not yet proven to exist which, if so proven, could be utilized to detect single gravitons? If the limiting factor is the practicality of measurement, then would a different kind of measurement not be ideal? How many ways could there be to detect a particle like this?
I am decidedly not a physicist, but it's interesting to hear about advances in the deep sciences.
My pleasure, thank you for reading.
The main problem with detecting gravitons as a scattering process is that the cross-section is always proportional to either G_N or (G_N)^(2) which is tiny as an interaction strength. As far as alternative theories, there's a whole zoo of model building we could do out there. Dozens of models. One example which comes to mind is the search for the Randall-Sundrum (RS) graviton which is a massive heavy excitation of the graviton that is predicted by some theories involving extra dimensions. We wouldn't see these gravitons as a scattering process because they are heavy and wouldn't propagate easily (same reason W and Z bosons aren't long range) but there is the possibility we produce them in energetic collisions.
The LHC has excluded RS gravitons of mass up to a few TeV, but they could easily be heavier than that.
Maybe you can answer a question I've long had about gravitons... Given that (I believe) it's commonly accepted that gravity is the result of massive objects warping the fabric of spacetime, why would we expect gravitons to exist at all?
From what I understand, the existence of the graviton is a prediction of quantum mechanics, which famously does not play well with the spacetime system described by general relativity.
More particularly, the predictive power of GR breaks down when it comes to systems on very small scales, but high densities and energies. Conditions like this exist around dense astrophysical objects like black holes and neutron stars, and also in the very first few moments following the Big Bang. Because of this, a theory that explains gravity in a different manner than GR that can be applied to these scenarios is highly desirable, and this "quantum gravity" ususally asks for a graviton. One reason for that is because quantum effects (more accurately, the information about the quantum system) are "quantized", or split into discrete packets, with those packets taking the form of particles.
This is my understanding as an interested layman so I'm happy to be corrected!
This was a really fascinating article to read as well, and though I agree with the other posters in being unable to follow the chain of derivation, the results are very clear. It never occurred to me that there could be such a stark difference in the detactability of quantum particles, but a distance of 85 orders of magnitude is nothing to sniff at!
Thanks for the answer. My understanding, also as a non physicist, is that the principles of quantum mechanics do indeed call for a graviton but most (all?) attempts to create a quantum mechanical theory around gravitons have run into deal breaking issues which is a big part of what led to string theory as an alternative.
And since we can't test any of the string theories, and likely won't be able to for a very long time, we're stuck. My feeling, which of course carries no weight (much like a graviton), is that perhaps the hole where gravitons so neatly fit is occupied by something else entirely.
Maybe, but as long as the gravitational field is quantizable, the graviton will exists as the quanta of said field. Maybe it's not a quantum field at all, but there's no evidence for that, either.
Thank you for the kind words. It occurs to me I should have perhaps motivated the dimensional analysis better so one could follow why the cross section is proportional to the combination of constants in each case, but that'd require either some pretty strong (and likely unsatisfying) hand-waving or a crash course into the Feynman rules for quantum fields.
The simplest (not most accurate) explanation that I've heard is that when you quantize things, you get particles. In the case of the graviton, it arises from the quantization of metrics in GR.
The other answers you got cover the gist, but here's some more reasons why we expect gravity to be quantized. I'm quoting a reddit comment I made some time ago:
Why does gravity have to be quantum in the first place? Since nobody can figure it out, maybe gravity is just classical. Maybe Einstein got it right the first time and we're just spinning our wheels. The above seems like a reasonable argument, but I want to elaborate on why we should expect gravity to be quantum in nature.
There appears to be four fundamental forces in the universe (five if you include the Higgs, though it's not a gauge boson). Of these, all but gravity, are quantum in nature. It would be strange if all of physics except gravity obeyed the rules of quantum mechanics.
Where quantum mechanics and gravity are both important (most famously black holes), there are calculations that are contradictory, or can only be done approximately. This means that physics must be incomplete. The "Black hole information paradox" says BHs do not preserve information via their decay into Hawking radiation. This is a paradox because quantum mechanics (which predicts Hawking radiation) requires information to be preserved which is a feature called "unitarity." Unitarity is a VERY important feature of quantum theory and we're reluctant to give it up.
Instead of adding quantum fields to curved spacetime, we ask: "Can classical gravitation fields be made from quantum matter?" The simplest form of this "semi-classical gravity" (i.e. G = <T>) where <T> is the "averaged" quantum matter is also inconsistent and leads to incorrect predictions. This doesn't mean gravity can't be classical, but it does mean both classical gravity and quantum mechanics can't be simultaneously true. Either quantum theory or gravity needs to change.
Effective quantum gravity actually works well. If you keep to low energy situations, quantum theory and gravity are actually very compatible. In this domain, you can do everything classical gravity does through the language of quantum theory. Calculating the precession of Mercury using quantized gravity is a graduate student level homework problem. Effective quantum gravity also clearly predicts quantum corrections to Newtonian gravity, though today there's no clear way to measure such corrections.
String theory, despite recent negative buzz and bad PR these days, is a theory of quantum gravity. The problem is well-understood (and well behaved) string theories do not describe our gravity in our universe. We seem to have made math for quantum gravity for somebody else's universe! This is akin to stumbling upon Paris, Kansas instead of Paris, France. It's still Paris however, so maybe there's hope.
Thanks for the thorough answer!
Follow up question, or maybe a clarification of my previous question... the fundamental forces describe the interactions of things within spacetime with other things within spacetime yes? Whereas gravity, while it effectively does the same, is the result of the warping of spacetime itself and so it's describing the interactions of things with the base medium, and each other sort of secondarily. It seems intuitive that the physics of gravity wouldn't conform to expectations derived from our understanding of the other forces?
Having typed that I see that there isn't really a clear question, and it borders on philosophical rather than scientific... but feel free to share any related thoughts you have.
To a certain extent, we already see that it does defy expectations by being difficult (or impossible) to quantize in a manner as satisfying as say electromagnetism.
But a different point is that you don't have to formulate general relativity as spacetime curvature (the geometrical interpretation). It is perfectly alright to consider a background of special relativity on which the gravitational field acts. From this perspective, gravity is then an additional dynamics field like any other (electromagnetism, electroweak, etc.) living on top of special relativity.
The flipped perspective is to LEAN into the geometrical interpretation. Gravity is the spacetime curvature of our base reality, but so are the other forces! The electromagnetic field tensor F is a curvature tensor for the gauge field we call the electromagnetic field in the same way the Reinmann curvature tensor R is for spacetime. From this perspective, you write all the interactions (gravity, electromagnetism, electroweak, strong, etc.) as geometric curvature. Our reality is then not just spacetime, but spacetime plus all gauge fields. Why give spacetime a preference when mathematically it is a dimension just like any of the others?
That's interesting. Brings to mind string theory, with the math coming first and the reality it describes being wholly hypothetical (for now).
Because it's the set of dimensions we experience, and which we are most invested in understanding :) It would be fitting if it turned out we can't understanding the universe from our 4D centric viewpoint.
That's not a really a compelling case. We can't see radio, but it's just as much a legit form of light as any other wavelength. Our human experience is quite limited and is often a poor source for physics intuition. In contrast, there's a lot of similarities between the degrees of freedom that arise from spatial dimensions and the degrees of freedom in the gauge fields, including the fact curvature in both are defined analogously. You might then naturally ask; are they actually separate things?
We already expanded Euclidean 3-space to include time as a dimension from relativity:
We're then tempted to include the photon Aν gauge field.
We can add even more of the fields until we have a unified geometry of all interactions. Now, with all this said, the "project" of doing this in physics in incomplete and success is not guaranteed, but it's a cool idea. All theories of extra-dimensions (including String Theory) start with thoughts like these.
Agreed, that's what I was getting at re: 4D centric viewpoint. We made a similar mistake with geocentricity and then heliocentricity.
Also though, I think it's safe to say that there's more practical value in understanding the universe we can interact with (radio waves included).
It will be ironic, then, if it turns out we've arrived at a point where we can't progress our understanding of the universe solely through observing the bits we can interact with.
Definitely. The idea that adding more dimensions can create a more elegant and unified understanding of the universe, like Einstein did, is very compelling. It was the first thing that came to mind when I encountered string theory.
I have to admit my heart kind of fell in love with kaluza Klein theory back when I first learned about it several years ago and that love has persisted since. I'm always on the lookout for a reasonable theory that adds more dimensions.
For anyone curious: Kaluza-Klein Theory. The years immediately following GR must have been heady in physics.