8 ms·

Einstein's description of gravity just got much harder to beat

153 points by dnetesn 6y ago

Taek 6y ago
Actually, it just got easier to beat. What happened was scientists found a new way to measure the effects of gravity that can substantially distinguish between different proposals to unify general relativity with quantum mechanics. The article says it got "harder" because the theory of general relativity wins every measurement, and that means the search space for potential solutions is much smaller - you have more constraints that you need to fit. But this is actually a good thing. Finding the right theory is like looking for a needle in a hay stack. These new measurements are the functional equivalent of removing a huge portion of the hay. Yes, it means there are now fewer places to check, but that's actually what we want. We'll spend less time chasing dead ends because it's easier for us to see they are dead ends sooner.
- timeisweird0 6y ago
  Thinking like an engine “taking relativity apart” and going over its pieces for flaws, finding a piece that’s broken, but needing such a highly customized piece to maintain consistency of the engine and maybe link it to quantum mechanics hasn’t really done away with the possibility we’ll chase thousands of dead ends first. It got easier but “easy” relative ;)
  - raattgift 6y ago
    I don't disagree with your overarching observations, but it's not so much that there are "pieces" of General Relativity to examine, but rather that the most central two features of the theory produce -- inevitably -- several consequences, all of which seem to be supported by observational and experimental evidence. Adapting the central two features generically leads to a universe noticeably different from what we observe. Likewise, the universe we observe appears to demand General Relativity as a framework for describing it accurately. Although there is some history about which empirical results were elevated into postulates of General Relativity, the history of the theory's discovery and development is pretty irrelevant to the two key features of General Relativity as a theory for our universe. They are: the Einstein tensor of curvature is constantly proportional to the stress-energy-momentum tensor of matter, and the universe's spacetime is a Lorentzian manifold with a vanishing torsion tensor. These two theoretical features produce dramatic consequences which are physically testable. The two basic avenues of attack of General Relativity as a theory accurately describing our universe: undermine local Lorentz invariance (if it's wrong then we aren't in a torsion-free 4-dimensional pseudo-Riemannian space), or undermine the strong equivalence principle (specifically the universality of free-fall, which means that inertial mass and (passive) gravitational mass are always identical) which would blow apart the proportionality of curvature to matter. The first line of attack tests the consequence that locally -- within a laboratory, say, whether that's on the ground, on the ISS, or on the moon -- physics which aren't dependent on gravitation are compatible with Special Relativity. Since the Standard Model of Particle Physics is governed by Special Relativity and is extremely sensitive to deviations from it, and is totally silent on the subject of gravitation, tests of the Standard Model of Particle Physics are also probes of this consequence, which is called local Lorentz invariance. Proof that results of Standard Model experiments differ during odd minutes from those obtained during even minutes, or that such experiments produce different results when the ISS is over the northern hemisphere rather than over the southern hemisphere, would call into question universal local Lorentz invariance. These tests happen often, and a lot of attentionis generated whenever a claim is made which violates local Lorentz invariance. Practically all such claims have been found to have been wrong. https://en.wikipedia.org/wiki/Modern_searches_for_Lorentz_violation#Reports_of_alleged_Lorentz_violations https://en.wikipedia.org/wiki/Modern_searches_for_Lorentz_vi... The second avenue is to look for cases where black holes or neutron stars travel on different orbits from ordinary stars, planets, asteroids, people, or flecks of paint falling off spacecraft. Observational evidence favours this part of General Relativity, which implies that the gravitational constant G is truly a universal constant (the same everywhere and at all times and in all systems of matter), and that there is no long range interaction that applies to all mass-energy including the Standard Model and whatever dark matter is. 8 pi G (where c is set to 1) is the constant of proportionality between the tensors mentioned above, and it's not General Relativity if that constant is not the same at all times and in all places. (This exact proportionality also protects the components of the Einstein curvature tensor: it's not General Relativity if you allow the Riemann scalar ("R") to vary, and there are plenty of alternatives which propose to vary R: these are f(R) gravity theories and most conflict pretty violently with what we observe in the universe (technically General Relativity is an f(R) gravity where f is the identity function). These two postulates of General Relativity produce numerous consequences which are unavoidable, and so there is a wide range of tests of General Relativity that are implicitly tests of these two postulates. These consequences can be described parametrically in such a way that alternatives to General Relativity can have their consequences assessed: deviations from General Relativity in most of these parameters is strongly constrained by observation and experiment, and it is maddeningly difficult to produce an alternative to GR that survives this kind of theoretical analysis. It's easy to shoot down most f(R), f(G), or f(c) theories using a parameterized Taylor-like expansion, for example. It's a pity, because a number of such theories would significantly simplify the description of the hot dense phase of the universe about 13.8 billion years in our past. Worse, so far nobody has found a parameter extracted from General Relativity which is contradicted by observational or experimental evidence, and most tests of such parameters are in accord with General Relativity to better than a fraction of a percent, and in many in some cases to much better than a part in a billion (we have some statistics from the Hulse-Taylor system that constrain to within one part in 10^20 some preferred-frame effects that touch on local Lorentz invariance and the universality of free fall). The problem is that General Relativity itself is mathematically complete and the maths say that gravitational interactions are entirely dependent on the distribution of matter (in the most general sense), and there's lots of detail we don't know about how matter is distributed or how exactly a lot of it works, including in the Standard Model at extremely high energies and also at extremely low energies (e.g. justifying infrared cutoffs on soft photons). Modifying General Relativity as an approach to make extreme-energy particle physics simpler has tended to produce a lot of dead ends.
- anonytrary 6y ago
  It's more like the haystack got smaller but we still don't know where the haystack is. The unknown unknowns got even harder to touch. This is all a good thing.
- bryanrasmussen 6y ago
  Perhaps it should have said: "Einstein's description of gravity just got much less likely to be beaten"
RcouF1uZ4gsC 6y ago
> Despite its successes, Einstein's robust theory remains mathematically irreconcilable with quantum mechanics, the scientific understanding of the subatomic world. Testing general relativity is important because the ultimate theory of the universe must encompass both gravity and quantum mechanics. Maybe the universe just has a giant if statement.
- irrational 6y ago
  Why must the ultimate theory of the universe encompass both? Do we know that there is an ultimate theory of the universe? Could GR and QM be wholly separate with no unifying connection?
  - hvna 6y ago
    The question of if there is an ultimate theory of the universe is one of the major unsolved problems in physics.
  - ordu 6y ago
    > Could GR and QM be wholly separate with no unifying connection? What does this mean? Is it mean that Universe have insufficient rules, so there are phenomena which do not follow any rules? Is it possible to devise an experiment whose results would be totally fundamentally irreproducible? Or you mean that Universe has strict rules, but human's ways to write these rules are flawed and insufficient to grasp in a full a fractal nature of mythical rules of Universe?
  - l33tman 6y ago
    They give different predictions for the same observed event so both can't be true. For example, since there is no gravity in the standard model (the general QM-model encompassing everything but gravity) and gravity obviously is observed, the theories are fundamentally incompatible. It would be a different situations if the theories actually covered different domains of experiments, but neither define such a restriction (and it wouldn't make much sense to either of them).
  - Kednicma 6y ago
jb775 6y ago
During brainstorming sessions, I often think about why/how medieval artists couldn't quite grasp how to depict depth in their paintings (as a lateral thinking technique). Being humans living in the world, they obviously experienced and understood that depth existed all around them, but had a hard time grasping the concept as a whole. I feel like we're kind of in a "medieval depth" phase of gravity/spacetime understanding...it's literally all around us, we know something is there, but we can't quite grasp what it even is we're looking for.
- ry454 6y ago
  Here's a possible explanation that I'd seen somewhere. Up until very recently, humans had had weak ability to think, especially abstract thoughts, and the phrase common today "just think about it", would be meaningless just a few centuries ago. Perhaps the concept of depth in those times would be similar in complexity to 4-dimensional structures today.
  - roomey 6y ago
    Plato? Socrates? Pythagoras? I was an artist for a while. It wasn't that artists couldn't depict depth. Fashion, religious instructon, and to a lesser extent materials, is what decided which paintings and frescoes lasted the test of time.
  - imtringued 6y ago
    That's completely wrong. Humans were just as smart 3000 years ago as they were today. The difference between today and back then is that people were much poorer and didnt receive an education. Their jobs were farming which didnt require things we consider essential like reading or writing which meant they stay unchallenged for their entire life. When you look at the rich elite or rich cities in the past then you see how small the difference between us and the people 3000 years ago is. They were just as smart but there were fewer of them. Nowadays everyone is educated and "rich". You can still find people who are just as "unchallenged" as a medieval peasant today. Look in rural places of countries like china.
    - 6y ago
mantap 6y ago
Can some knowledgable person articulate why physicists are so sure that QM and GR are incompatible?
- computerfriend 6y ago
  Rather it is the belief that they fundamentally must be compatible that is driving this research. Currently we don't know how to answer questions like "what is the gravitational field around a photon?".
  - brabel 6y ago
    Photons have zero mass so we know them to produce zero gravity, no?
    - mantap 6y ago
      As I understand it (not a physicist), according to GR the Einstein field equations equate the curvature of spacetime to the stress-energy tensor, of which mass is just one component. Photons, having energy, should produce a gravitational field.
    - pja 6y ago
      Energy gravitates. Photons have energy, therefore photons gravitate. Photons only have zero /rest mass/, not zero total mass.
      - brabel 6y ago
        Photos by definition don't have a rest mass because they are never at rest. They must always travel at the speed of light, which means they don't experience time, so rest does not even make sense for a photon.
        jlokier 6y ago
        That makes sense in English, however "rest mess" is also a technical and mathematical term, and in that terminology it has the value zero for photons.
galaxyLogic 6y ago
I don't quite get it they say relativity and Quantum Mechanics are incompatible. Does that mean they give different predictions?
- computerfriend 6y ago
  No. They cover different domains, so their predictions within those domains are consistent (and non-overlapping). It is combining them that is the problem.
- lazyjones 6y ago
  GR assumes reality is continuous, QM assumes it’s discrete.
  - drdeca 6y ago
    QM doesn’t assume space is discrete. It does deal with a number of discrete quantities though. Are you sure that’s where the conflict is? I think people have done QM with a given curved spacetime, just, where the spacetime manifold isn’t decided based on the distribution of matter and whatnot? Like, I think that’s where the ideas of Hawking radiation come from?
  - mhh__ 6y ago
    Quantum mechanics is not inherently discrete. We don't know whether the planck length is fundamental or not, but current models are in terms of continuous spatial and rotational degrees of freedom (in general). The eigenvalues often are quantized, however.
- T-A 6y ago
  It means we don't know how to quantize GR. Unlike the standard model of particle physics, it's a classical theory, i.e. a set of equations which tell you deterministically, without any of the inherent uncertainty of QM, how a given initial configuration will evolve. This becomes a problem when you try to include GR in even simple experimental setups which expose QM's non-classical behavior, like the Stern-Gerlach experiment [1]. What does gravity do before the path chosen by the particle (which is gravitationally attractive) is measured? [1] https://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experiment https://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experime...
sidcool 6y ago
It's intuition defying how weird relativity is and so true.
tus88 6y ago
> Despite its successes, Einstein's robust theory remains mathematically irreconcilable with quantum mechanics This is exaggeration, they are not badly in conflict with each other, we just have not found a way to unify them, i.e. explain gravity through QM.
jdthedisciple 6y ago
I dont get it. Isnt Newtonian Physics the connection between QM and GR?

153 points by dnetesn 6y ago

Taek 6y ago

Actually, it just got easier to beat. What happened was scientists found a new way to measure the effects of gravity that can substantially distinguish between different proposals to unify general relativity with quantum mechanics. The article says it got "harder" because the theory of general relativity wins every measurement, and that means the search space for potential solutions is much smaller - you have more constraints that you need to fit. But this is actually a good thing. Finding the right theory is like looking for a needle in a hay stack. These new measurements are the functional equivalent of removing a huge portion of the hay. Yes, it means there are now fewer places to check, but that's actually what we want. We'll spend less time chasing dead ends because it's easier for us to see they are dead ends sooner.

timeisweird0 6y ago
Thinking like an engine “taking relativity apart” and going over its pieces for flaws, finding a piece that’s broken, but needing such a highly customized piece to maintain consistency of the engine and maybe link it to quantum mechanics hasn’t really done away with the possibility we’ll chase thousands of dead ends first. It got easier but “easy” relative ;)
- raattgift 6y ago
  I don't disagree with your overarching observations, but it's not so much that there are "pieces" of General Relativity to examine, but rather that the most central two features of the theory produce -- inevitably -- several consequences, all of which seem to be supported by observational and experimental evidence. Adapting the central two features generically leads to a universe noticeably different from what we observe. Likewise, the universe we observe appears to demand General Relativity as a framework for describing it accurately. Although there is some history about which empirical results were elevated into postulates of General Relativity, the history of the theory's discovery and development is pretty irrelevant to the two key features of General Relativity as a theory for our universe. They are: the Einstein tensor of curvature is constantly proportional to the stress-energy-momentum tensor of matter, and the universe's spacetime is a Lorentzian manifold with a vanishing torsion tensor. These two theoretical features produce dramatic consequences which are physically testable. The two basic avenues of attack of General Relativity as a theory accurately describing our universe: undermine local Lorentz invariance (if it's wrong then we aren't in a torsion-free 4-dimensional pseudo-Riemannian space), or undermine the strong equivalence principle (specifically the universality of free-fall, which means that inertial mass and (passive) gravitational mass are always identical) which would blow apart the proportionality of curvature to matter. The first line of attack tests the consequence that locally -- within a laboratory, say, whether that's on the ground, on the ISS, or on the moon -- physics which aren't dependent on gravitation are compatible with Special Relativity. Since the Standard Model of Particle Physics is governed by Special Relativity and is extremely sensitive to deviations from it, and is totally silent on the subject of gravitation, tests of the Standard Model of Particle Physics are also probes of this consequence, which is called local Lorentz invariance. Proof that results of Standard Model experiments differ during odd minutes from those obtained during even minutes, or that such experiments produce different results when the ISS is over the northern hemisphere rather than over the southern hemisphere, would call into question universal local Lorentz invariance. These tests happen often, and a lot of attentionis generated whenever a claim is made which violates local Lorentz invariance. Practically all such claims have been found to have been wrong. https://en.wikipedia.org/wiki/Modern_searches_for_Lorentz_violation#Reports_of_alleged_Lorentz_violations https://en.wikipedia.org/wiki/Modern_searches_for_Lorentz_vi... The second avenue is to look for cases where black holes or neutron stars travel on different orbits from ordinary stars, planets, asteroids, people, or flecks of paint falling off spacecraft. Observational evidence favours this part of General Relativity, which implies that the gravitational constant G is truly a universal constant (the same everywhere and at all times and in all systems of matter), and that there is no long range interaction that applies to all mass-energy including the Standard Model and whatever dark matter is. 8 pi G (where c is set to 1) is the constant of proportionality between the tensors mentioned above, and it's not General Relativity if that constant is not the same at all times and in all places. (This exact proportionality also protects the components of the Einstein curvature tensor: it's not General Relativity if you allow the Riemann scalar ("R") to vary, and there are plenty of alternatives which propose to vary R: these are f(R) gravity theories and most conflict pretty violently with what we observe in the universe (technically General Relativity is an f(R) gravity where f is the identity function). These two postulates of General Relativity produce numerous consequences which are unavoidable, and so there is a wide range of tests of General Relativity that are implicitly tests of these two postulates. These consequences can be described parametrically in such a way that alternatives to General Relativity can have their consequences assessed: deviations from General Relativity in most of these parameters is strongly constrained by observation and experiment, and it is maddeningly difficult to produce an alternative to GR that survives this kind of theoretical analysis. It's easy to shoot down most f(R), f(G), or f(c) theories using a parameterized Taylor-like expansion, for example. It's a pity, because a number of such theories would significantly simplify the description of the hot dense phase of the universe about 13.8 billion years in our past. Worse, so far nobody has found a parameter extracted from General Relativity which is contradicted by observational or experimental evidence, and most tests of such parameters are in accord with General Relativity to better than a fraction of a percent, and in many in some cases to much better than a part in a billion (we have some statistics from the Hulse-Taylor system that constrain to within one part in 10^20 some preferred-frame effects that touch on local Lorentz invariance and the universality of free fall). The problem is that General Relativity itself is mathematically complete and the maths say that gravitational interactions are entirely dependent on the distribution of matter (in the most general sense), and there's lots of detail we don't know about how matter is distributed or how exactly a lot of it works, including in the Standard Model at extremely high energies and also at extremely low energies (e.g. justifying infrared cutoffs on soft photons). Modifying General Relativity as an approach to make extreme-energy particle physics simpler has tended to produce a lot of dead ends.
anonytrary 6y ago
It's more like the haystack got smaller but we still don't know where the haystack is. The unknown unknowns got even harder to touch. This is all a good thing.
bryanrasmussen 6y ago
Perhaps it should have said: "Einstein's description of gravity just got much less likely to be beaten"

RcouF1uZ4gsC 6y ago

> Despite its successes, Einstein's robust theory remains mathematically irreconcilable with quantum mechanics, the scientific understanding of the subatomic world. Testing general relativity is important because the ultimate theory of the universe must encompass both gravity and quantum mechanics. Maybe the universe just has a giant if statement.

irrational 6y ago
Why must the ultimate theory of the universe encompass both? Do we know that there is an ultimate theory of the universe? Could GR and QM be wholly separate with no unifying connection?
- hvna 6y ago
  The question of if there is an ultimate theory of the universe is one of the major unsolved problems in physics.
- ordu 6y ago
  > Could GR and QM be wholly separate with no unifying connection? What does this mean? Is it mean that Universe have insufficient rules, so there are phenomena which do not follow any rules? Is it possible to devise an experiment whose results would be totally fundamentally irreproducible? Or you mean that Universe has strict rules, but human's ways to write these rules are flawed and insufficient to grasp in a full a fractal nature of mythical rules of Universe?
- l33tman 6y ago
  They give different predictions for the same observed event so both can't be true. For example, since there is no gravity in the standard model (the general QM-model encompassing everything but gravity) and gravity obviously is observed, the theories are fundamentally incompatible. It would be a different situations if the theories actually covered different domains of experiments, but neither define such a restriction (and it wouldn't make much sense to either of them).
- Kednicma 6y ago

jb775 6y ago

During brainstorming sessions, I often think about why/how medieval artists couldn't quite grasp how to depict depth in their paintings (as a lateral thinking technique). Being humans living in the world, they obviously experienced and understood that depth existed all around them, but had a hard time grasping the concept as a whole. I feel like we're kind of in a "medieval depth" phase of gravity/spacetime understanding...it's literally all around us, we know something is there, but we can't quite grasp what it even is we're looking for.

ry454 6y ago
Here's a possible explanation that I'd seen somewhere. Up until very recently, humans had had weak ability to think, especially abstract thoughts, and the phrase common today "just think about it", would be meaningless just a few centuries ago. Perhaps the concept of depth in those times would be similar in complexity to 4-dimensional structures today.
- roomey 6y ago
  Plato? Socrates? Pythagoras? I was an artist for a while. It wasn't that artists couldn't depict depth. Fashion, religious instructon, and to a lesser extent materials, is what decided which paintings and frescoes lasted the test of time.
- imtringued 6y ago
  That's completely wrong. Humans were just as smart 3000 years ago as they were today. The difference between today and back then is that people were much poorer and didnt receive an education. Their jobs were farming which didnt require things we consider essential like reading or writing which meant they stay unchallenged for their entire life. When you look at the rich elite or rich cities in the past then you see how small the difference between us and the people 3000 years ago is. They were just as smart but there were fewer of them. Nowadays everyone is educated and "rich". You can still find people who are just as "unchallenged" as a medieval peasant today. Look in rural places of countries like china.
  - 6y ago

mantap 6y ago

Can some knowledgable person articulate why physicists are so sure that QM and GR are incompatible?

computerfriend 6y ago
Rather it is the belief that they fundamentally must be compatible that is driving this research. Currently we don't know how to answer questions like "what is the gravitational field around a photon?".
- brabel 6y ago
  Photons have zero mass so we know them to produce zero gravity, no?
  - mantap 6y ago
    As I understand it (not a physicist), according to GR the Einstein field equations equate the curvature of spacetime to the stress-energy tensor, of which mass is just one component. Photons, having energy, should produce a gravitational field.
  - pja 6y ago
    Energy gravitates. Photons have energy, therefore photons gravitate. Photons only have zero /rest mass/, not zero total mass.
    - brabel 6y ago
      Photos by definition don't have a rest mass because they are never at rest. They must always travel at the speed of light, which means they don't experience time, so rest does not even make sense for a photon.
      - jlokier 6y ago
        That makes sense in English, however "rest mess" is also a technical and mathematical term, and in that terminology it has the value zero for photons.

galaxyLogic 6y ago

I don't quite get it they say relativity and Quantum Mechanics are incompatible. Does that mean they give different predictions?

computerfriend 6y ago
No. They cover different domains, so their predictions within those domains are consistent (and non-overlapping). It is combining them that is the problem.
lazyjones 6y ago
GR assumes reality is continuous, QM assumes it’s discrete.
- drdeca 6y ago
  QM doesn’t assume space is discrete. It does deal with a number of discrete quantities though. Are you sure that’s where the conflict is? I think people have done QM with a given curved spacetime, just, where the spacetime manifold isn’t decided based on the distribution of matter and whatnot? Like, I think that’s where the ideas of Hawking radiation come from?
- mhh__ 6y ago
  Quantum mechanics is not inherently discrete. We don't know whether the planck length is fundamental or not, but current models are in terms of continuous spatial and rotational degrees of freedom (in general). The eigenvalues often are quantized, however.
T-A 6y ago
It means we don't know how to quantize GR. Unlike the standard model of particle physics, it's a classical theory, i.e. a set of equations which tell you deterministically, without any of the inherent uncertainty of QM, how a given initial configuration will evolve. This becomes a problem when you try to include GR in even simple experimental setups which expose QM's non-classical behavior, like the Stern-Gerlach experiment [1]. What does gravity do before the path chosen by the particle (which is gravitationally attractive) is measured? [1] https://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experiment https://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experime...

sidcool 6y ago

It's intuition defying how weird relativity is and so true.

tus88 6y ago

> Despite its successes, Einstein's robust theory remains mathematically irreconcilable with quantum mechanics This is exaggeration, they are not badly in conflict with each other, we just have not found a way to unify them, i.e. explain gravity through QM.

jdthedisciple 6y ago

I dont get it. Isnt Newtonian Physics the connection between QM and GR?