I am very confident that quantum gravity "exists" in some sense. Here is the argument:
In quantum physics labs all over the world we can put stuff in superpositions of being in different places. These objects are currently pretty small, but we can do it. Mostly this happens to electrons right now, but it has been demonstrated with "buckyball" molecules made of about 2000 atoms. These objects have mass, and when they're localised in a single position we have a nice description of what their mass does to space-time around them (General relativity).
On the other hand, when such objects are in superpositions of being in different places they do something to space-time around them, but we have absolutely no clue what.
"Quantum gravity" is just the "something" that they do. We don't know what they do, but they do something.
Them doing nothing (i.e. having no effect on spacetime at all when in superposition) would be a very wild thing to happen. I can't overstate how incredibly unexpected that would be. We believe (pretty strongly) that objects with mass distort space-time when they aren't in a superposition and superposition is a continuous thing. You can be in a superposition with a very high amplitude somewhere and an arbitrarily small amplitude somewhere else. If they suddenly have no impact on space-time when in superposition something very weird happens when you continuously shift from being localised to being in superposition.
Similarly it doesn't really make sense for them to exhibit the same behaviour when in superposition of different positions to when they are not. Imagine we have an object with mass which can either be in position 1 or position 2, say the positions are a meter apart. Classically we have a description of space-time for the situation where the object is in position 1 and a different description of space-time for the situation where the object is in position 2. Now put the object in an equal superposition of positions 1 & 2. If the space-time is not "different from localised buckyball behavior" which of the two choices does it choose? What happens if it chooses one of them and we move some of the weight of the superposition to the other one?
Heh yup looks that way now, along with a priesthood protecting the discourse. Interesting that people like Neil Turock are using their position to right the ship now though.
A better example is dark matter. Numerous people have staked their entire careers upon dark matter, so the emotions are real, more real than the logic they entertain to qualify positions on the matter. Pun intended.
Everyone always misses the corollary to this "groupthink is why we have dark matter" theory which is that academics are very heavily incentivized to try and overturn conventional dark matter because it would be an absolute career-maker to have a credible alternative (and get credit for upturning a widely accepted theory). And in fact lots of people have tried various MOND theories etc but the reason Lambda-CDM persists for so long is because it still fits the data the best.
Dark matter arose because of gaps in the current particle theory. These gaps grow the further out we observe and become significant enough to disqualify gravity as a constant for the formation of galaxies. Dark matter provides an incomplete solution to these gaps just as dark energy is an incomplete stop gap to Einstein’s cosmological constant.
The logic is as follows:
* There are gaps in current models that otherwise invalidates math and disqualifies observed measures.
* Dark matter/energy provide a partial solution that may or may not exist but are sufficient to performed qualified measures across observed space.
* Therefore dark matter/energy provide utility value that for conducting measures that otherwise observationally contradict.
* Therefore dark matter/energy must be because they are qualified as such by the math.
This problem is syllogistic wherein a series of propositions are individually true and linked so therefore the conclusion of such linkage must be true, but it isn’t. In physics truths exist as proofs. The problem isn’t that dark matter exists or not, but that incomplete logic demands it must exist.
Again, it is a confidence problem, that it must exist and anything to the contrary should be censored to hell. People have staked their entire professional reputations on this confidence built on logic, not observation.
The actual physics problem that produces all this mess in the first place is observational limitation. Already the JWST is slowly peeling back some reliance on magic solutions allowing for alternate considerations and better math. Ultimately we will need to observe the universe from outside the solar system and it’s internal distortions.
I don't quite understand the words you're using in your "the logic is as follows" list. (What are "gaps that otherwise invalidates math and disqualifies observed measures"? Really, what are "observed measures"? What does it mean to "performed qualified measures"? etc.)
To a substantial extent, it kinda feels like what you're describing is literally the scientific method. We discover observations that are inconsistent with existing models, so we develop a new model that (within their realm of validity) correctly match the data, and when we continue to find that the new model successfully describes new observations our confidence in the model goes up (and eventually rises to the level of saying "this seems to be a correct description of reality").
Maybe if I understood the exact meaning of your words here, I'd understand how what you're talking about differs from that and what your concerns are. But as I've said elsewhere, I really don't see where this is a "confidence problem", and I've never had reason to believe that alternatives are "censored to hell". There are people who've built entire careers publishing papers studying MOND, after all. It just doesn't seem to work as well as dark matter (Lambda-CDM) to explain the copious data that we now have.
And I'll definitely push back (again) against calling the case for dark matter "an observational limitation". These are quantitative, positive observations that demand some model to explain them. The specific quantitative amounts and distributions of dark matter necessary to explain galaxy rotation curves also turned out to be the same specific details necessary to explain gravitational lensing observations. As new observations have come in over the years, the case for dark matter has gotten stronger. (That's why it's become a dominant belief among experts.) So I genuinely don't have any idea what you're getting at with those comments.
I’m no specialist of the question, but from what I got, ok the standard model doesn’t match what we can observe in astronomic features, unless we suppose some untrackable heavy weight is there though not measurable with our contemporary tools.
So the basic paths are, we admit there is indeed some entity like this, or we need a new model that can also encompass these observations without new object type.
MOND (Modified Newtonian Dynamics) is a minority (not mainstream) theory in physics, that maybe Newton's law of gravity doesn't work the same over galactic distances. After all, we have only tested the law in length scales smaller than our own solar system.
You also have to know, that the observed rotation speeds of galaxies do not fit with estimated amount of visible stars (and their mass) in the galaxy. We basically can not explain why galaxies rotate at the speeds they do.
MOND suggests that the shape of the gravity law changes, over very large distances.
The mainstream explanation is Dark Matter: Galaxies have more mass than we can see, and that makes them rotate differently that what their number and mass of stars would suggest.
Lambda-Cold-Dark-Matter: Lambda is the name of the cosmological constant, cold dark matter is about the simplest possible model for the unseen dark matter: It is cold (no fast moving particles, meaning no particles like photons that move at relativistic speeds), dark (we cannot see it), and it is matter (provides gravity).
This is the traditional, mainstream cosmological model in its simple form.
I don't mean this unkindly, but I am always so baffled when I see how common this take is. I mean, sure, say this about string theory (the field I was trained in): fine. I don't agree, but I see the point.
But dark matter? You think we wanted this "weird particle or something very like it that we persistently can't find" situation? No! The reason that the overwhelming majority of cosmologists and astrophysicists pretty solidly believe that dark matter exists (after a whole lot of skepticism when it was first proposed) is that there is very strong evidence for it from at least three radically different sets of experimental data. [Patterns in how galaxies rotate (the original), gravitational lensing to locate and quantify mass in galaxy clusters (including cluster collisions like the "Bullet Cluster"), and the overall history of cosmological evolution.] And not only do all three lines of evidence indicate "dark matter exists", but as I understand it, they all independently indicate a consistent amount of dark matter (relative to the observable ordinary matter).
People have tried to come up with modified theories that eliminate the need for dark matter for years and years (once they gave up suggesting the observations themselves might be faulty): it would be so much more satisfying, especially in light of our continued failure to find any sort of "stuff" that the dark matter might be made of. But as I understand it, every modification of basic theory that's so far been proposed to give a different explanation of one of those pieces of evidence has had nothing at all to say about the other two. And while maybe you could come up with a different theory modification to account for each line of evidence separately, at that point the coincidence starts to seem far less plausible even than "particle we can't find". ("Our otherwise solidly established theories are wrong in three very specific but unrelated ways, each tuned to match the effects we'd see from the exact same amount of dark matter.")
And yet somehow, the notion has taken root among a whole bunch of non-experts that this is all somehow a conspiracy or a group delusion or a stubborn self-interested refusal to consider other answers. I'll never pretend that science is perfect or immune to groupthink! But for goodness sake, aim that skepticism in some direction where the observational evidence is less overwhelming. (And before you just pick another field to go after, maybe first think carefully about what factors led you to disagree with the experts on this one where it really, really wasn't justified.)
The problem is not group think or conspiracies as you believe. I suspect you believe that must be the counter-position due to emotional investment and low empathy.
The problem is misplaced confidence. The accuracy of opinions/predictions is almost always inversely proportional to the confidence of the claimant. If there were observational evidence then that would alone be a sufficient qualifier and overstated confidence wouldn’t be necessary. It’s bad logic that I explained here: https://news.ycombinator.com/item?id=38203818
> The accuracy of opinions/predictions is almost always inversely proportional to the confidence of the claimant.
I recognize the sort of phenomenon you're pushing back against here, and there's legitimate value in consciously trying to counterbalance the all too human tendency to nod along to overconfident proclamations of truth.
But even so, the claim you're making here is absurd when treated as a general rule. Expertise is a real thing that people can have. Yes, there are countless places where a genuine expert will express caution and give carefully qualified statements about what is "true", and that's good. It's especially common for topics close to the frontiers of human knowledge. But there are also a whole lot of places where an expert will be willing to say "it's basically safe to take ____ as true", and where if asked they'll be able to back that up with copious explanations and evidence.
If I tell you that tomorrow morning the sun will rise in the east rather than the west, I will say it very confidently, and I will be right. If I tell you as a physicist that this bowling ball I'm about to drop from the second floor window will accelerate toward the ground at a rate very close to 9.8 m/s^2 (maybe only a smidge less due to air resistance, given the short time involved), I will be right. You trust your life every day to countless engineers who have at some point said, "The design and fabrication process we have implemented has involved enough rigor and enough quality control and enough margin of safety that I'm willing to stake my career on it being safe." And the vast majority of the time, for each essential bolt in your car and each fuel line on your airplane and each support beam in your office building, they are right.
So here, in the case of dark matter, the community of astrophysicists and cosmologists has assembled over the past 50-ish years a tremendous array of evidence from multiple independent directions that all seems to paint a consistent picture of some type of matter with quite specific properties. Many of them were highly skeptical at first, and there are still some actively looking for radically different explanations. But the vast majority of those experts have considered all of this positive evidence for "dark matter" and have become convinced that something very close to the Lambda-CDM model must be more or less true.
It's not at all clear to me what more you want from them. I mean, heck, the gravitational lensing results (especially as applied to something like the Bullet Cluster) could be considered a successfully tested prediction of the dark matter model, since those measurements weren't even possible until well after most of the community had become reasonably convinced that dark matter was real. The fact that we haven't yet answered the question "how does dark matter fit with the rest of our understanding of particle physics and astronomy?" is fascinating, but the existence of open questions doesn't on its own mean that all that observational data somehow doesn't count!
Sigh. The cosmological model came into being after several years of academic discussions and finally crystallized around 1978, I believe. Plans for the Hubble didn’t even exist then. All people knew is what they could see from Earth surface and it was fine until later observations became absurdly contradictory comparing the math to known physics from agreed upon physics almost a century earlier. So… you solve for the math. You aren’t going to get a better cosmological model by observing distant galaxies from Earth surface. The only other options are abandoning the entirety of the cosmological model without a replacement or abandoning Einstein physics, which is worse.
That still does not make something entirely without evidence thrust into existence. It certainly doesn’t explain the level of emotional investment. Your only arguments supporting your position are purely social conduct: expertise, agreement, confidence, and apparent sadness. These are not observations. They are not physics proofs. They aren’t even measurements. They are the equivalent of the church calling Galileo a heretic. You call my opinion absurd only because it, according to your own words, inconveniences you, god forbid.
Angela Collier did a nice video on YT debunking this, about why dark matter is not a theory, but rather a name for a discrepancy in observations. So it's not a social construct, the discrepancy is real.
Do you mean that the weight of their intangible emotions largely exceed the measurable observations they showcase in their scientific decisions? How unbearable!
In quantum physics labs all over the world we can put stuff in superpositions of being in different places. These objects are currently pretty small, but we can do it. Mostly this happens to electrons right now, but it has been demonstrated with "buckyball" molecules made of about 2000 atoms. These objects have mass, and when they're localised in a single position we have a nice description of what their mass does to space-time around them (General relativity).
On the other hand, when such objects are in superpositions of being in different places they do something to space-time around them, but we have absolutely no clue what.
"Quantum gravity" is just the "something" that they do. We don't know what they do, but they do something.