All my life, they've been hawking string theory; what feels like 40-odd years of breathless articles in New Scientist and on "Horizon". I don't really understand why it's still funded.
> “It’s hard to say really where you should draw the boundary around and say: This is string theory; this is not string theory,” said Douglas Stanford, a physicist at the IAS. “Nobody knows whether to say they’re a string theorist anymore,” said Chris Beem, a mathematical physicist at the University of Oxford. “It’s become very confusing.”
I knew some string people who easily transitioned into other fields (like condensed matter) because the work was similar. I don't personally see it as a huge waste of time, and it's like the least expensive thing to fund.
</humour>
I'd also like to point out that the actual total funding for string theory is extremely tiny compared to scientific research funding in general since it is a theoretical discipline with very few practitioners. If you want to attach inefficiencies in scientific funding, the LHC is a much better target. I'm a physicist and have an interest in theory, and even I think that we spend too much money on particle accelerators given what we stand to learn from them and the other pressing problems the human race has. I'd be delighted to see all that money re-allocated to large scale public works building solar panels and infrastructure or just giving it to poor people.
https://en.wikipedia.org/wiki/Pareto_principle
Even if string theory is just pure math with no base in reality, it still can be useful (any math domain for that matter).
Almost all the surplus we have could be traced back to our discoveries in physics leading to machines.
People lived much much poorer lives without modern physics discoveries.
The expects return on physics is so great we should find ways to afford more long shot experiments not less.
One of the most fundamental scientific facts is that quantum theory (the most precisely tested theory in human history) and general relativity are incomplete. There must be a bridge. And we have no idea what that bridge is. It's been this way for over a century; the lifespan of string theory is not so long in comparison. Until we find a way to falsify it, we have to keep trying, don't we?
We do know that mathematical frameworks cannot be at the same time totally complete and internally consistent. Would it be a stretch to assume descriptions of our physical reality could have the same restriction? General relativity and Quantum mechanics are relatively complete in describing our physical reality, however they are not consistent with each other. Perhaps if we ever find a description that is consistent, it won’t be complete. Perhaps grand unified theories are simply a mathematical impossibility.
How is the boundary between GR and QM different from the boundary of psychology and sociology?
In contrast to condensed matter vs the standard theory (QM basically), QM vs GR has fundamental incongruities, since both theories make claims about what happens at the same scale. Only one (or most likely neither) can be correct at the event horizon and center of a black hole.
Outliers in the latter aren't necessarily indicative of anything wrong with theory in general, while even a single outlier in anything in the former is indicative of an incomplete model.
There are various results in physics which should be predictable via both GR and QM independently. The results should be the same as both models are supposed to be describing the same thing, so it follows that there should be some sort of gradual transition as one set of effects gradually comes to dominate over the other. Otherwise we'd see a single point in the data where QM stops being accurate and GR takes over, but despite investigating so many different scales, we have not seen any such cutoff point.
I'm aware of my own behavior as an individual being influenced by social context, is that not the kind of bleed over you might look for? Maybe you're referring to specific concepts I'm not actually even understanding.
> either you describe the individual or you describe a group
In gravitational physics, with respect to a flow in a dynamical system (an example is galaxies in an expanding universe) we can use a Lagrangian observer (e.g., one galaxy, drifting along with the flow, tracing out a pathline/worldtube that depends on features like its mass-evolution and proper motion within a cluster of galaxies) or a Eulerian observer (e.g. a notional observer with no spatial motion at all, watching alllll the galaxy clusters jiggle, swirl, turn, and age a little differently in relation to her). One can convert observations of each type of observer to the other in a rigorous mathematical procedure, since they are just two (families of) the infinity of different observers allowed by even just Special Relativity. See e.g. <https://en.wikipedia.org/wiki/Lagrangian_and_Eulerian_specif...> for more detail.
>> There are boundaries between where GR and QM are predictive
> this feels alien to me
You can do both quantum matter and classical General Relativity in one of several effective field theories, which I'll return to below.
GR and relativistic quantum field theories (QFT) purport to make accurate predictions in strong gravity, which one only finds deep within black holes (i.e., not on our side of any horizon), but they make very different predictions in that regime pretty generically. Generically in the sense that choosing different behaviours of particle-particle interactions (and self-interactions) do not really move the needle on GR's prediction of a collapse to a core of infinite density. However, in various approaches which convert GR's classical gravitational waves into large number of gravitons, one can write down a matter QFT in which charged particles' self-interaction can lead to a degeneracy pressure (a repulsive force) that increases at higher particle energies such that they overwhelm gravitational collapse at very high but finite density in black holes of arbitrary mass.
In weak gravity, like we have in our solar system, QFTs allow us to prepare significant masses in superpositions of (spatial) position. General relativity does not allow for such superpositions. We are approaching lab-testability, with results from sensitive accelerometers allowed to point at tiny superposed masses.
However, in regimes far from (non-negligibly) gravitating superpositons and strong gravity, GR and QFT are usefully (and possibly fully) compatible. We get good results in astrophysics from semi-clasical gravity, where the classical curved spacetime of General Relativity couples with the expectation value of QFT matter (i.e., we average out some quantum weirdness and justify this by the weak gravitational effects of the "lumpy" weirdness being practically impossible to measure; superpositions and ultra-high-energy/ultra-high-denisty systems might be too lumpy).
We also get good results from perturbative quantum gravity and canonical quantum gravity, for example. Neither of these latter two is really classical General Relativity so they can deal with the gravitation of superposed matter (otherwise they give for all practical purposes the same answers as semiclassical gravity). These approaches do not work in strong gravity, however. Essentially they become calculationally intractable or they crash into unresolved problems splitting spacetime into space and time (in order to do time-dependent quantum mechanics).
Do you happen to know of any promising upcoming experiments in particular? Or any groups who are at the forefront of such research endeavors?
HN user ISL <https://news.ycombinator.com/user?id=ISL> is probably au fait with recent quantum gravimetry experiments.
The Müller group at UC Berkeley came to my mind. They did a recent paper <https://arxiv.org/abs/2210.07289>.
Gavin Morley's group at Warwick University is doing work in the area <https://warwick.ac.uk/fac/sci/physics/staff/academic/gmorley...>. He has what looks like a useful bibliography on the subject too: <https://warwick.ac.uk/fac/sci/physics/staff/academic/gmorley...>.
Finally, while not really related to your question (except that greater-precision gravimetry is likely to mean smaller superposed masses are useful), https://www.nature.com/articles/s41586-021-04315-3 is extremely cool, and I wish it could be sent back into the heyday of https://en.wikipedia.org/wiki/Time_Team . (ETA: Müller's group, similarly, https://arxiv.org/abs/1904.09084 .)
I take it you're referencing Godel's theorems here, but "consistent" and "complete" have rather technical (and somewhat limited) meanings within that context, so it's not clear to me how they'd usefully map onto the potential relationship between QM and GR?
This property is completely irrelevant to a theory like QM or GR - it is only relevant for a system that aims to be a universal foundation for mathematics (a formal language in which any mathematical statement whatsoever could be precisely formally encoded, and then proven or disproven).
QM is not a theory about small particles: it is a theory that describes the evolution of any object whatsoever. It just happens to be completely wrong for large objects. And even if we accepted that there is some objective cutoff points between "small objects" and "large objects" (the "objective collapse" interpretation), it would still be wrong, because it predicts effects like gravitational lensing don't exist.
Conversely, GR is also a theory about any size of object; and it is also completely wrong when tested on small objects, as it predicts effects like the self-interference of particles don't exist.
And again, even if some kind of objective boundary existed between the domains of GR and QM existed, that would need to be incorporated into the maths of both of them, and questions about behaviors close to that margin would arise.
But we use gravitational lensing, and the amount of lensing is predicted by GR?
I don't understanding this statement. Is there some additional affect around lensing we should see if quantum gravity is a thing?
GR does of course predict gravitational lensing. However, if you used the formulae of GR to compute the motion of photons passing through a double-slit experiment, the solution would show two different bright spots, corresponding to the two slits; in reality, we see an interference pattern. So GR is also wrong.
By definition, a (correct) theory of quantum gravity would predict both gravitational lensing and the way photons behave in a dobule-slit experiment (otherwise, we would say that either the theory is wrong, or it is not a theory of quantum gravity). However, no such theory exists today, at least none that doesn't contradict other observations.
> no; I'm saying that if you apply QM as it exists today to describe the motion of light beams around the sun, you will not get any effect similar to gravitational lensing
falls down because we can describe such an effect purely quantum mechanically.
Also, I think you should be put to proof with respect to a claim against quantum perturbative or canonical methods in the solar-mass lensing regime in which perturbation theory works great for classical GR, taking into account all sorts of beyond-leading-order (classical) effects like noncircularity, backreaction, helicity, you name it. The sun is a fixed enough background that it's a linearized gravity problem. What exactly in "QM as it exists today" breaks (or is broken by) this?
> ... if you used the formulae of GR to compute the motion of photons passing through a double-slit experiment ... GR is also wrong.
How exactly does taking the fully Lorentz-invariant QED or Standard Model to local Lorentz-invariance (with the radius of curvature significantly larger than the laboratory experiment) break the picture? We are nowhere near needing to consult Birrell & Davies.
What do you think needs doing here if "you used the formulae of GR", beyond solving the EFEs and the geodesic equations for the whole (region of) spacetime, and then wondering what geodesic any given photon will couple to? What do you think the scale of the correction from Minkowskian geodesics will be? And how much of that do you impute to the apparatus?
In a sibling post I talk about how this is not a problem in psychology, and ask what the difference is. This answers that question kind of well, the difference is that in psychology/sociology the scopes are well defined. We know what a population is and apply sociology to it, and we know what an individual human being is, and apply psychology to it.
With quantum mechanics and general relativity the scope is supposed to be the cosmos, but both theories fail on some places in the cosmos. So either the theories are wrong, or the scopes are wrong. I’m leaning towards the latter.
Well, no. That's not really possible here. The scopes can't be adjusted. The theories are wrong. They both must explain all physical phenomena, but neither does, therefore a more accurate theory exists which we have not found yet.
I just don't know if there's much value in trying to make an analogy to psychology here. Currently it seems to be getting in the way of understanding.
A similar problem actually exists in psychology vs sociology, though it seems you choose to ignore it. When you want to study the behavior of two human beings (say, a married childless couple), do you apply sociology or psychology? How about a married couple with children? An extended family?
Also, psychology has to explain how an individual human behaves inside of a population, and sociology has to explain how the behavior of groups emerges out of the individual psychologies of their constituents. If they don't do this, then either one or both theories must be wrong. To take an extreme silly example, if psychology predicted that no person would never choose to kill themselves for the sake of another; but sociology predicted groups of people regularly have members sacrificing themselves for the group - then one of the theories must be wrong, you can't just say "we apply one theory when talking to a man and another when talking about the group".
It would be troubling to think that physical reality was inconsistent.
The fact is, we have no falsifiable theories that can unite GR and QM. Should every theory be abandoned that doesn't quickly lead to a resolution? No, clearly not. So the question is what kind of criteria we could use to determine that string theory is a dead end or is otherwise stifling true progress.
And that's pretty much what I was trying to ask previously... is ST actually sucking all the air out of the room? I'm a layperson and not just going to assume that hundreds of experts have blown their careers doing pointless calculations on a theory that "obviously" isn't worth the resources put into it. But the comment I replied to seemed to be making that assumtpion,
By that same token, we should keep looking for the fountain of eternal youth in El Dorado until it can be conclusively proven it doesn't exist.
A theory which is not falsifiable is not a scientific theory, at least in principle, and it is hard to tell why it should be entertained for quite so long.
Not at any cost of course, but the falsifications gathered e.g. in trying to disprove the string theory might also help us figure out what is actually going on.
How long did it take to go from Principia to General Relativity?
Maxwell's equations were first formulated in 1865, the "patch" to Newton's theories was formulated soon after (the luminiferous aether), the Michelson-Morley experiment proving the patch did not work was run in 1887, and the theory of special relativity was proposed in 1905 - so it took about 20 years at most between Newtonian mechanics being conclusively proven to contradict experiments and a new theory becoming proeminent. And special relativity was quickly replaced with general relativity (just 10 years later), because, despite its success, its limitations were immediately apparent.
Conversely, the electric field is just a potential. It's not made up of anything, it just describes how charged particles interact if they are in a particular place in space-time.
I see what you did there.
Also, Sabine Hossenfelder was trained as a particle physicist and she doesn't like it either. She explains why it's like that and how it's bad and what they should be doing instead (like actively trying to reconcile the contradictory theories of gravity and quantum mechanics). She has changed career to professional youtuber https://en.wikipedia.org/wiki/Sabine_Hossenfelder
Isn't that exactly what string theory is trying to do?
If someone is actually trying to get to the bottom of some contradictions in physics experiments I don't think she would be against this. I think she has observed that string theory has failed at it for so long and maybe some other methods can get a chance for funding instead of spending a hundred billion dollars on string theory and ten thousand dollars on other methods.
What causes political parties to prioritize their donors over their constituents?
What causes corporations to disregard the safety and health of the public if the worst they'll face is a fine for negligence?
These questions, while superficially unrelated, all point to the same underlying mechanism in the nature of societies.
A lot of people have spent a lot of their time developing this theory
Kepler was convinced the planetary orbits around the sun were circular and could be described with the five Platonic solids. His theory was testable, and when he measured it against observation, it failed. He could have persisted, modified his theory, and continued on the wrong path, but instead, he discarded his theory and discovered elliptical orbits with his three laws of planetary motion.
Kepler was a scientist.
So walking away from string theory years ago, in my mind, would make you a mathematician.
It’s fun to talk about scientists as these staunch defenders of their theories and say “science advances one funeral at a time” but the reality of the situation is that sometimes scientists are faced with the realization that their entire life’s work is in ruins. It’s hard to imagine a more severe test of character than that. Harder still to imagine how one might pick up the pieces and move on to something else and still be able to contribute to science in a meaningful way.