It’s kind of amazing to me that in a 100 years a whole lot of extremely smart, and very focused, people couldn’t arrive to a verifiable way of bringing together QM and GR. There’s something wrong with this situation.
In particular the bottleneck is often the math, rather than the inability to understand physical reality. Earlier thinkers grasped at what Newton said, but couldn't get very far without the scaffolding of calculus. General relativity required complex non-Euclidean geometry and would have been simply impossible even a few decades prior.
In this case, the bottleneck has been a deliberate refusal to work on the underpinnings of QM. The Copenhagen "Interpretation" refuses to consider what a measurement is, and creates an artificial separation between a quantum system and the system taking the measurement. For sixty years, adherents were told to "shut up and calculate" rather than ask questions like: Isn't the apparatus taking the measurement also a quantum system? Why should there be two sets of rules, one when a measurement is made and one when a measurement does not occur? What is it about measurements that cause this? What mechanism is going on in the universe that causes it to appear to function in the classical, emergent way, the world of Newton and Relativity, rather than in superpositions and spooky action at a distance?
Part of the reason little progress has been made is that it was considered career-ending in physics to work on those questions, because some of them dive into philosophy (What is knowledge? What is real? What is emergent?) Throughout the latter half of the twentieth century graduate students were counseled away from any work related to the field. Oppenheimer famously organized the shunning of Bohm for daring to publish (at Einstein's encouragement) on the subject. Hugh Everett was driven out of academia altogether.
Hard problems can be solved when people actually work on them. Often the problem is other people preventing the work. Thankfully this attitude seems to be easing thanks in part to the efforts of people like Sean Carroll and Lee Smolin.
But an interpretation is just that - an interpretation, which does not have any effect on the equations (or does it?), so if it's a new math that is needed for QM and GR to start looking like two ends of the same elephant, then that's what it'll take.
Bohm and Everett proposed theories that are mathematically distinct. Today there are families of theories (Objective Collapse, Hidden Variables, and Everettian Mechanics) that are mutually exclusive.
One of Sean's papers, in fact, proposes a way to falsify Everettian mechanics through experiment (though the experiment would be exceedingly difficult in practice).
Copenhagen was "just an interpretation" but serious work on the subject is actually advancing work on new theories (with distinct mathematical models), not just hand-waving and dismissal.
Maybe it's a hard problem, harder than a few techies on HN are willing to give credit for. I mean if you think it can't be that big of a deal, maybe you could give us a solution to the problem of motion in gague theories of matter, to help us out?
Could you ELI5 what the "problem of motion" is supposed to be? I've read the abstract, the first one and a half pages and the conclusion of the paper you linked, and I'm still confused what problem exactly you are seeking a solution to (or in fact demanding from GP).
I'm saying "you", not "author", because the paper's author seems to be interested in a very specific, somewhat niche question, which is studying the equation of motion of test particles (at rest) in alternative theories of gravity and in the situation where, in addition, the test particle is charged and interacting with a fixed gauge field. (One needs to be very careful with the term "gauge" here because the author confusingly uses it for both, the matter gauge theory / gauge group and the "gravitational gauge" group, i.e. coordinate invariance.)
This question might be interesting to a few select people but there is certainly no "problem of motion in gauge theories of matter" at large, at least not in the way you portrait it.
I mean, for classic gravity / General Relativity, one expects that, depending on whether the particle is charged or not charged, the equation of motion reduces to:
- the geodesic principle – i.e. the hypothesis that (uncharged) test particles at rest move along geodesics.
- a Lorentz-force-type law for gauge-charged test particles that (only) interact with a (fixed) gauge field and are otherwise at rest.
But both are quite well-established I'd say:
- The geodesic principle can actually be rigorously derived from the Einstein field equations for a large class of matter or situations[0]. Given this body of evidence, it's rather likely it's a mathematical theorem and does not actually need to be assumed as an axiom of General Relativity.
- The Lorentz law can already be derived[1] from the special-relativistic Lagrangian of the matter field and its coupling to the gauge field (where both fields are obviously classic, not quantum).
As for the latter, sure, strictly speaking the special-relativistic derivation (i.e. on a flat background) can only be a "local" one in light of General Relativity. In a fully relativistic derivation one should instead consider a curved background, i.e. the Einstein-Maxwell action (or a generalization thereof for arbitrary gauge fields). But then again – given the evidence for the geodesic principle – we know the Lorentz force must come from the interaction of the particle with the (fixed) gauge field (not gravity) and that interaction is largely "understood" – with the usual fine print that:
- forces are a classical concept but particles are actually quantum and there is backreaction (so the Lorentz force can only be the lowest-order term, anyway),
- obviously we don't really know how quantum fields work on curved backgrounds / in conjunction with General Relativity. Then again, we don't know how to make quantum fields mathematically rigorous on a flat background to begin with. So there is no point in asking for mathematical rigorisity in the context of deriving the Lorentz force from first principles when much larger issues would need to be tackled first.
So again, what "problem of motion" exactly would you like to see solved?
> It’s kind of amazing to me that in a 100 years a whole lot of extremely smart, and very focused, people couldn’t arrive to a verifiable way of bringing together QM and GR. There’s something wrong with this situation.
What's wrong about it? If humans evolved, there's no good reason to think that even the smartest human has the mental capacity to do something like that, or to do it quickly. Science will hit a wall determined by human limits, and it's quite possible that limit has already been hit in some areas.
There are a lot of popular fairy tales that portray humans as universal understanders (with no limits as long as they Science™ hard enough), but they seem kind silly to me.
Part of the reason is for the past 100 years we've been teasing out all the details of these theories and verifying them. We've only recently discovered where the incompatibility of these theories present problems - and now we're tackling them. What I'm really saying is there hasn't been a whole lot of people trying to bring together QM and GR, not to the level that's going on at present. For those who had been trying to reconcile the two theories they were working with theories in progress, theories who's impacts were just being understood. As another commenter mentioned these things take time to bake!
I think, that's because they are too focused trying to bring together 2 things that don't fit. Apparently they aren't smart enough to take a couple of steps back, and start over. That is not something that is encouraged in the scientific community. Failure is rarely getting any attention, and even frowned upon, only success is relevant. Yeah, I agree, that's just amazing and terribly wrong.
EDIT: I mean, who does revisit long established facts everybody just agrees upon, without checking, because they are long established?
In fundamental physics, that's literally what it's all about. We know some of our assumptions must be wrong, because we know the theories we have conflict. That's been perfectly clear since the time of Einstein and Bohr. One thing that has been hugely helpful has been data such as from the big particle colliders, but that data is getting harder and harder to get as the required energies have gone through the roof.
The thing is there is no shortage of theories, or whole classes of theories, the problem is getting the data we need to verify and refine them.
Lol the arrogance in this comment. “The hundreds/thousands of people who work on theoretical physics are just too dumb to step back and take a look at the big picture.”
I don't think it's arrogance, I think you have far too much faith in large groups of people.
Of the 4.4 million software developers in the US, most of them produce really bad code and have a lot of wrong ideas about development and don't think twice about what they're doing.
Same with the medical field - rife with dysfunction and malpractice.
Pretty much all fields work like this as far as I can tell. It sometimes takes hundreds of years for new correct ideas to be actually accepted and integrated into the mainstream knowledge/practice in any given field. Most people have giant egos and are resistant to change.
I've held this belief for a long time. We tend to think we have the brainpower but we just need the understanding. I think it's far more likely that we have nowhere near the brainpower required to understand the true nature of reality.
There is an argument to be made that there could be a phase transition when it comes to the ability of understanding things.
When you manage to write down your thoughts so future generations can build on prior work, and when you manage to formalize problems by abstract notation (math), and when you gain the capability of breaking down complex problems into small solvable chunks, then maybe, and I want to stress maybe, there is no limit to understanding things.
On the other hand, there also may be problems with irreducible complexity, so it really could be either way.
Perhaps, but I'm partial to the interpretation that we simply don't understand quantum mechanics well enough to explain it to, if not a dog, children in high school.
Note that we do teach classical mechanics (to a point) in high school, even though it was the cutting edge of physics at some point.
It is really amazing, and they even discuss it in the podcast. What the interviewee basically says is that quantum mechanics is the reality of how things work, and that classical physics, including general relativity, is an approximation of how QM works in large scales. In the end he says one day we may be able to think of an experiment to measure the things the theories don't agree upon. He guesses this may come from experiments around Lorentz invariance.
What I wish someone would do is a double slit experiment, where the path of the photon is long enough to be affected by gravity. Maybe then there would be some clues to quantum gravity in the interference pattern.
It took like 200,000 years to get to F=ma, then 200 to get to relativity. I think we're moving at a breakneck pace. It's just a really tough nut to crack.
He does call for a radical reformulation of which math to study, as he wants to avoid some issues of arithmetic, infinity, and incompleteness.
Maybe his discrete, computable mathematics (which is merely alluded to) is the temporary step back we need to move forward again.
Although removing the continuum from QM seems questionable. AFAIK (which isn’t much) continuous change is a feature of QM. It’s one reason quantum computation has an advantage. No 0 or 1, but a continuous range of states somehow.
