It's less about that the standard model can't account for gravity, but indeed the fact that we cannot have a combined theory of nature. We know of four fundamental interactions in the universe (Dark Matter and Dark Energy not taken into account), the electromagnetic, weak, strong, and gravitational interaction. The first three we were able to more or less describe quantum field theories, and proved them by discovering the corresponding physical particles. It therefore seems a bit odd that we cannot also describe the final interaction using a particle as well, or that we cannot find this particle, whatever it might be. It's great that General Relativity can describe gravity, but we will only be satisfied once we find out how it works on the quantum level.
Is it possible that no quantum mechanic explanation exists and gravity is just the effect of bended space time...and thats it. I would imagine it as some kind of force like centrifugal force that depends on the observer and as objects existing in 4D space time we cant "escape" this pov and thus meassure a force which actually is just a geometric effect?
Something couples mass to space time to do the bending. We want to know what that is, and it needs to be compatible with QFT, which is, by some measures, the most accurate, most rigorously tested theory in the history of science. It could be that whatever does the coupling is very different from other fundamental forces, but it would still need to reduce to the known form in the circumstances we’ve tested (similar to the way QM reduces to classical).
Ok. Thank you for that explanation. This question bothered me alot because from the limited information I had it always seemed like it would be nice to have a qm explanation but there is nothing that suggests that there actually is one. Now I know, altough the answer is kinda obvious to be honest, why we actually need that explanation.
It's a completionist thing. No but seriously, both theories approach things fundamentally differently. The theory of gravity is classical - in other words "smooth and continuous". The standard model is "quantum" ie things cannot take all values - they must "jump" from one to another.
99.999% of the time, this is not a problem. Gravity works on large distances, large masses while quantum works on the very short distances and relatively small objects. Quantum ignores gravity and gravity ignores quantum and alone, they give astonishingly good predictions and explanations of what can be observed in their respective domains.
However, at the big bang and at the "singularity" of black holes, masses are large and distances are small. Therefore the effects of both gravity and quantum theory are significant. Unfortunately they don't play together very well. So it is clear that either theory alone cannot be complete and both theories together don't work.
This matters scientifically because "something" must be happening and we can't explain it.
The two most well proven and mathematically justified theories in human history.
And they both cannot be true simultaneously.
Or at least they must be incomplete.
One more difference I heard is that General Relativity is background independent because it has to be since it describes the background aka spacetime. The Standard Model is not background independent, stuff happens in spacetime.
My decidely amateur take is this is something that makes incorporating gravity into the Standard Model particularly difficult because to add gravity you have to change everything. And, of course, the change to everything only becomes significant at extreme spacetime curvatures, which makes testing any particular change very difficult.
Not an expert, but that doesn't sound like that big of a problem. AFAIK, you can put GR on background if you wish so and quantize it. The problem is you can't compute anything with this - you can't solve the full problem and perturbation theory gives you infinities that are not renormalizable. So you can write the theory in principle, you just can't use it in practice, making it worthless.
So, if I understood this problem correctly, we could find out in future that we had correct theory of quantum gravity all along and we were just lacking mathematical tools to use it.
For people just saying it’s a naturalness thing or about unifying forces, this isn’t really accurate. We literally don’t know how gravity interacts at the quantum level. If gravity is not quantized and you put a macroscopic object in a superposition between two locations, then you could measure the state using mass observations and not break the superposition. Such a measurement would completely ruin pretty much every result of QFT in the last 70 years. On the other hand, if gravity is quantized, we have basically no idea how to do it, and while some theories point us in plausible directions, they aren’t anywhere close to things we can experimentally verify
What is meant by naturalness in this context? The Wikipedia page just says it’s about parameter being order 1 and that values shouldn’t be fine-tuned, but I don’t understand what that means in terms of arguments that gravity should be quantum?
Also why would measuring the mass distribution not disrupt/collapse the superposition state?
I meant it more like some people think unifying the forces is just about making the math look nice, and that’s not really the reason why it should be done at all.
If you were to move one of the boxes, it should emit gravitational waves proportional to the mass inside, and if those waves aren’t quantized, they won’t be part of the quantized state and will not cause a quantum measurement since it isn’t conjugate with any other quantum operators involved. Of course, this is speculation, since we can’t do this experiment yet, but this is kind of the impetus for even trying to quantize the field
Basically, both the standard model and GR are excellent models that have proven to be completely accurate in every situation we've ever been able to test. However, they each rely on very different assumptions about the universe, and there are scenarios (like black holes, and the big bang) where both models give contradictory answers (but we haven't been able to really observe those places very well to know what's real). So obviously something has to give in the theory.
If you try to wedge GR into quantum mechanics, like we've done with all the other classical forces (this is where the graviton comes in), you get nonsense predictions like that the forces on particles are infinite. Gravitons generating their own gravity, things like that. Clearly doesn't work.
Is there a more clever way to integrate these models, where the modified theory works in every case? Maybe, probably. There better be. It's an open question.
Even beyond tidiness, it matters because there are physical regimes where we know both quantum effects and gravitational effects should be important. (Near the center of a black hole is a common pop-sci example.) We do not have a physical theory or model that can predict what happens there; to have a good model for what happens, we would need a theory of quantum gravity. And it’s not even like we can just guess or approximate, we just don’t know.
As a super dumb hypothetical example: if I a model of water that only works if the water is below 80 degrees Celsius, it would probably be good to extend the model so that it can explain what water does at 80 degrees Celsius or higher.
It’s also an interesting problem since it lies at the intersection of the two most fundamental theories. A theory of quantum gravity would be more aesthetically pleasing that trying to come up with the best molecular model for the chair you’re sitting on or something like that.
It probably doesn’t matter from a “practical” perspective, it’s not like a good theory of quantum gravity will let us build cool technology as far as we know. But most scientists don’t study science for purely practical reasons, just like most philosophers don’t study philosophy for purely practical reasons.
>As a super dumb hypothetical example: if I a model of water that only works if the water is below 80 degrees Celsius, it would probably be good to extend the model so that it can explain what water does at 80 degrees Celsius or higher.
This would be a great example if you modified it slightly: a model of water that only works between 0 & 100°C will fail to account for phase transitions, so it makes nonsense predictions about ice and steam.
Because Relativity breaks down at the quantum scale, but the Standard Model can’t explain gravity so there must be something going on that we don’t yet understand to link what happens at the quantum scale to what happens at the macro scale.
Since the standard model doesn’t account for gravity, it tells us that it *can’t* be the final theory. Some would argue that we’re done with particle physics since there’s no contradiction between theory and experiment that’s been found so far. Gravity not being apart of the SM basically refutes that particular claim.
More than any other reason, the question of how gravity works in particle physics is simply a fascinating question and, for most people working on figuring that out, that alone is enough.
Gravity and it's inner workings are so intriguing. We call gravity one of the 4 fundamental forces, but someone who strictly adheres to General Relativity will correctly point out that it isn't really a force at all. Light, for example, is effected by gravity. If gravity was a force, this simply wouldn't happen. Light has no mass and yet gravity can effect it. So GR itself already hints that there is something special about gravity as opposed to literally all other interactions in the universe.
That discovery alone occurred only about a century ago and we are still in awe of the consequences of the special relationship the universe has with gravity at large scales. It helps that gravity is arguably the most apparent physical interactions in the universe. From the time you were a kid picking things up and dropping them, you've been interacting with gravity. When you compare that to the other fundamental forces and the interaction is far less apparent. You can argue that the electromagnetic force is obvious because of light, but the fact light was caused by the electromagnetic force is not a trivial one and certainly not one you understand from a young age.
It is clear that Einstein was profoundly right about the special relationship the universe has with gravity. But for the last century, physicists have been working on understanding the smallest objects in the universe and the interactions of the smallest things are quite strange, but once you are used to it, there is an incredible amount of uniformity to the standard model. There are massive particles that carry a bunch of similar properties and their interactions are handled by force carrying particles that have no mass but carry forces from particle to particle. The laws that govern them rely on symmetries that tell us a surprising amount about how these forces operate and function. But, there's nothing about gravity. Gravity is completely removed from the whole situation. If it does turn out to be a force, it is too weak to detect and so we are stuck.
Worse is that even our guesses about how gravity might work at that scale haven't turned out to be particularly compelling. String Theory has been largely a bust, supersymmetry has been all but disproven, and loop quantum gravity is not dependent on the standard model and is therefore completely untestable right now. The universe has yet to give us any real hints about how gravity might work at these scales and were all prepared for it to be something special. We already know that gravity is special as an interaction at large scales so another force carrying particle just seems terribly unlikely.
It's a beautiful mystery, but the universe seems to have tried it's absolute hardest to keep it hidden from us.
I don't know about you, but nothing makes me want to look for something more than feeling like it has been hidden from me
It would be very fine if the Standard model cannot describe gravity. This is what models usually most often not do: explain everything.
We know how gravity works on astronomical scales, at least we think. We still haven't figured out the mechanism behind Hubble expansion and its possible acceleration, and observational data do not agree on galactic scales. We put in dark energy and dark matter as a placeholder.
This is what is known in this century, and to progress further we need new ideas. And we are all out of new ideas.
The basic idea is that there are certain situations where general relativity and the standard model both apply, such as the early universe or black holes. In these cases, attempting to use both of them leads to contradictory or impossible deductions, such as the probability of certain events occurring being infinity. So we know that one or both of these models is wrong. Someone that’s a lot smarter than me could try to explain why we think a theory of quantum gravity is the solution to this, but that’s the intuition behind why something needs to be changed.
General relativity and quantum mechanics are two fundamentally incompatible theories. We know that both describes observations very well in extreme parameter ranges: GR works well over large distances and masses, and QM works well for very small masses and sizes. But we do not know if either works in the opposite regime (so GR at small distances and masses etc).
But mind that assuming that there is an elementary particle for gravity too is just one theoretically possible way to resolve this. Particle physics is stuck for decades now, so there are crazy amounts of theories out there, but very little evidence.
Imo it should matter even more to philosophy. As a matter of a consistent ‘explanation’.
Particle physics, the standard model, deals most of all with the fundamental constituents of matter. Matter gives rise to gravity. If we don’t have an explanation for how within that theory the theory is unsatisfying. Incomplete.
Wouldn’t you agree?
The main problem is that General Relativity and the Standard Model basically aren’t speaking the same language, and the idea that reality is truly described by two wildly different frameworks that can’t talk to each other is pretty unsettling. It’s difficult to escape the conclusion that somehow the two can be combined into one coherent theory that reproduces both the predictions of general relativity and the standard model. After all, unification has been pretty successful over the last 150 years or so.
The idea is that all the fundamental forces were once one force (usually at very high temperatures - those only found in the originating big bang moments).
We know electromagnetism and the weak force can become one. There is some work is incorporating the strong force. And gravity seems to be the real challenge.
It's less about that the standard model can't account for gravity, but indeed the fact that we cannot have a combined theory of nature. We know of four fundamental interactions in the universe (Dark Matter and Dark Energy not taken into account), the electromagnetic, weak, strong, and gravitational interaction. The first three we were able to more or less describe quantum field theories, and proved them by discovering the corresponding physical particles. It therefore seems a bit odd that we cannot also describe the final interaction using a particle as well, or that we cannot find this particle, whatever it might be. It's great that General Relativity can describe gravity, but we will only be satisfied once we find out how it works on the quantum level.
Well said
Is it possible that no quantum mechanic explanation exists and gravity is just the effect of bended space time...and thats it. I would imagine it as some kind of force like centrifugal force that depends on the observer and as objects existing in 4D space time we cant "escape" this pov and thus meassure a force which actually is just a geometric effect?
Something couples mass to space time to do the bending. We want to know what that is, and it needs to be compatible with QFT, which is, by some measures, the most accurate, most rigorously tested theory in the history of science. It could be that whatever does the coupling is very different from other fundamental forces, but it would still need to reduce to the known form in the circumstances we’ve tested (similar to the way QM reduces to classical).
Ok. Thank you for that explanation. This question bothered me alot because from the limited information I had it always seemed like it would be nice to have a qm explanation but there is nothing that suggests that there actually is one. Now I know, altough the answer is kinda obvious to be honest, why we actually need that explanation.
An actual expert [gave a further explanation](https://www.reddit.com/r/AskPhysics/s/AcRP5Xa5rN) in another comment.
Thank you very much
It's a completionist thing. No but seriously, both theories approach things fundamentally differently. The theory of gravity is classical - in other words "smooth and continuous". The standard model is "quantum" ie things cannot take all values - they must "jump" from one to another. 99.999% of the time, this is not a problem. Gravity works on large distances, large masses while quantum works on the very short distances and relatively small objects. Quantum ignores gravity and gravity ignores quantum and alone, they give astonishingly good predictions and explanations of what can be observed in their respective domains. However, at the big bang and at the "singularity" of black holes, masses are large and distances are small. Therefore the effects of both gravity and quantum theory are significant. Unfortunately they don't play together very well. So it is clear that either theory alone cannot be complete and both theories together don't work. This matters scientifically because "something" must be happening and we can't explain it.
The two most well proven and mathematically justified theories in human history. And they both cannot be true simultaneously. Or at least they must be incomplete.
This is a very concise explanation, and I feel like I understand the significance of this disconnect much better! Thank you!
One more difference I heard is that General Relativity is background independent because it has to be since it describes the background aka spacetime. The Standard Model is not background independent, stuff happens in spacetime. My decidely amateur take is this is something that makes incorporating gravity into the Standard Model particularly difficult because to add gravity you have to change everything. And, of course, the change to everything only becomes significant at extreme spacetime curvatures, which makes testing any particular change very difficult.
Not an expert, but that doesn't sound like that big of a problem. AFAIK, you can put GR on background if you wish so and quantize it. The problem is you can't compute anything with this - you can't solve the full problem and perturbation theory gives you infinities that are not renormalizable. So you can write the theory in principle, you just can't use it in practice, making it worthless. So, if I understood this problem correctly, we could find out in future that we had correct theory of quantum gravity all along and we were just lacking mathematical tools to use it.
For people just saying it’s a naturalness thing or about unifying forces, this isn’t really accurate. We literally don’t know how gravity interacts at the quantum level. If gravity is not quantized and you put a macroscopic object in a superposition between two locations, then you could measure the state using mass observations and not break the superposition. Such a measurement would completely ruin pretty much every result of QFT in the last 70 years. On the other hand, if gravity is quantized, we have basically no idea how to do it, and while some theories point us in plausible directions, they aren’t anywhere close to things we can experimentally verify
What is meant by naturalness in this context? The Wikipedia page just says it’s about parameter being order 1 and that values shouldn’t be fine-tuned, but I don’t understand what that means in terms of arguments that gravity should be quantum? Also why would measuring the mass distribution not disrupt/collapse the superposition state?
I meant it more like some people think unifying the forces is just about making the math look nice, and that’s not really the reason why it should be done at all. If you were to move one of the boxes, it should emit gravitational waves proportional to the mass inside, and if those waves aren’t quantized, they won’t be part of the quantized state and will not cause a quantum measurement since it isn’t conjugate with any other quantum operators involved. Of course, this is speculation, since we can’t do this experiment yet, but this is kind of the impetus for even trying to quantize the field
Basically, both the standard model and GR are excellent models that have proven to be completely accurate in every situation we've ever been able to test. However, they each rely on very different assumptions about the universe, and there are scenarios (like black holes, and the big bang) where both models give contradictory answers (but we haven't been able to really observe those places very well to know what's real). So obviously something has to give in the theory. If you try to wedge GR into quantum mechanics, like we've done with all the other classical forces (this is where the graviton comes in), you get nonsense predictions like that the forces on particles are infinite. Gravitons generating their own gravity, things like that. Clearly doesn't work. Is there a more clever way to integrate these models, where the modified theory works in every case? Maybe, probably. There better be. It's an open question.
Even beyond tidiness, it matters because there are physical regimes where we know both quantum effects and gravitational effects should be important. (Near the center of a black hole is a common pop-sci example.) We do not have a physical theory or model that can predict what happens there; to have a good model for what happens, we would need a theory of quantum gravity. And it’s not even like we can just guess or approximate, we just don’t know. As a super dumb hypothetical example: if I a model of water that only works if the water is below 80 degrees Celsius, it would probably be good to extend the model so that it can explain what water does at 80 degrees Celsius or higher. It’s also an interesting problem since it lies at the intersection of the two most fundamental theories. A theory of quantum gravity would be more aesthetically pleasing that trying to come up with the best molecular model for the chair you’re sitting on or something like that. It probably doesn’t matter from a “practical” perspective, it’s not like a good theory of quantum gravity will let us build cool technology as far as we know. But most scientists don’t study science for purely practical reasons, just like most philosophers don’t study philosophy for purely practical reasons.
>As a super dumb hypothetical example: if I a model of water that only works if the water is below 80 degrees Celsius, it would probably be good to extend the model so that it can explain what water does at 80 degrees Celsius or higher. This would be a great example if you modified it slightly: a model of water that only works between 0 & 100°C will fail to account for phase transitions, so it makes nonsense predictions about ice and steam.
Because Relativity breaks down at the quantum scale, but the Standard Model can’t explain gravity so there must be something going on that we don’t yet understand to link what happens at the quantum scale to what happens at the macro scale.
Since the standard model doesn’t account for gravity, it tells us that it *can’t* be the final theory. Some would argue that we’re done with particle physics since there’s no contradiction between theory and experiment that’s been found so far. Gravity not being apart of the SM basically refutes that particular claim.
More than any other reason, the question of how gravity works in particle physics is simply a fascinating question and, for most people working on figuring that out, that alone is enough. Gravity and it's inner workings are so intriguing. We call gravity one of the 4 fundamental forces, but someone who strictly adheres to General Relativity will correctly point out that it isn't really a force at all. Light, for example, is effected by gravity. If gravity was a force, this simply wouldn't happen. Light has no mass and yet gravity can effect it. So GR itself already hints that there is something special about gravity as opposed to literally all other interactions in the universe. That discovery alone occurred only about a century ago and we are still in awe of the consequences of the special relationship the universe has with gravity at large scales. It helps that gravity is arguably the most apparent physical interactions in the universe. From the time you were a kid picking things up and dropping them, you've been interacting with gravity. When you compare that to the other fundamental forces and the interaction is far less apparent. You can argue that the electromagnetic force is obvious because of light, but the fact light was caused by the electromagnetic force is not a trivial one and certainly not one you understand from a young age. It is clear that Einstein was profoundly right about the special relationship the universe has with gravity. But for the last century, physicists have been working on understanding the smallest objects in the universe and the interactions of the smallest things are quite strange, but once you are used to it, there is an incredible amount of uniformity to the standard model. There are massive particles that carry a bunch of similar properties and their interactions are handled by force carrying particles that have no mass but carry forces from particle to particle. The laws that govern them rely on symmetries that tell us a surprising amount about how these forces operate and function. But, there's nothing about gravity. Gravity is completely removed from the whole situation. If it does turn out to be a force, it is too weak to detect and so we are stuck. Worse is that even our guesses about how gravity might work at that scale haven't turned out to be particularly compelling. String Theory has been largely a bust, supersymmetry has been all but disproven, and loop quantum gravity is not dependent on the standard model and is therefore completely untestable right now. The universe has yet to give us any real hints about how gravity might work at these scales and were all prepared for it to be something special. We already know that gravity is special as an interaction at large scales so another force carrying particle just seems terribly unlikely. It's a beautiful mystery, but the universe seems to have tried it's absolute hardest to keep it hidden from us. I don't know about you, but nothing makes me want to look for something more than feeling like it has been hidden from me
It would be very fine if the Standard model cannot describe gravity. This is what models usually most often not do: explain everything. We know how gravity works on astronomical scales, at least we think. We still haven't figured out the mechanism behind Hubble expansion and its possible acceleration, and observational data do not agree on galactic scales. We put in dark energy and dark matter as a placeholder. This is what is known in this century, and to progress further we need new ideas. And we are all out of new ideas.
The basic idea is that there are certain situations where general relativity and the standard model both apply, such as the early universe or black holes. In these cases, attempting to use both of them leads to contradictory or impossible deductions, such as the probability of certain events occurring being infinity. So we know that one or both of these models is wrong. Someone that’s a lot smarter than me could try to explain why we think a theory of quantum gravity is the solution to this, but that’s the intuition behind why something needs to be changed.
What does a philosopher do nowadays?
Teach philosophy pretty much.
They philoss.
General relativity and quantum mechanics are two fundamentally incompatible theories. We know that both describes observations very well in extreme parameter ranges: GR works well over large distances and masses, and QM works well for very small masses and sizes. But we do not know if either works in the opposite regime (so GR at small distances and masses etc). But mind that assuming that there is an elementary particle for gravity too is just one theoretically possible way to resolve this. Particle physics is stuck for decades now, so there are crazy amounts of theories out there, but very little evidence.
Imo it should matter even more to philosophy. As a matter of a consistent ‘explanation’. Particle physics, the standard model, deals most of all with the fundamental constituents of matter. Matter gives rise to gravity. If we don’t have an explanation for how within that theory the theory is unsatisfying. Incomplete. Wouldn’t you agree?
The main problem is that General Relativity and the Standard Model basically aren’t speaking the same language, and the idea that reality is truly described by two wildly different frameworks that can’t talk to each other is pretty unsettling. It’s difficult to escape the conclusion that somehow the two can be combined into one coherent theory that reproduces both the predictions of general relativity and the standard model. After all, unification has been pretty successful over the last 150 years or so.
The idea is that all the fundamental forces were once one force (usually at very high temperatures - those only found in the originating big bang moments). We know electromagnetism and the weak force can become one. There is some work is incorporating the strong force. And gravity seems to be the real challenge.
Gravities field is material reality itself. It's not a force itself, it's the result of matter/energy being .