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Comment by mabbo | Hacker News Reader

> “We were stunned by how well the size of the ring agreed with predictions from Einstein’s Theory of General Relativity,"

And ten thousand physicists sighed disappointingly.

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Gravitational physicist here. Every time we look in a new place and see that GR still works, it is pretty amazing.

Part of this is the fact that everyone in the field works very hard to find a crack in GR's armor, and so far it has resisted everyone.

The other part is that GR has a romantic beauty to it, both in structure and in predictions. Each time GR matches reality in a new context, the feeling is akin to watching a beautiful sunrise on a summer's morning. You've seen it before, but darn if it isn't pretty.

It seems the common expectation is that GR is more likely to be the theory that needs to be modified the most - or more or less tossed out and replaced with a quantum theory of gravity. Is it possible that GR is just "right" and QM needs to be modified to fit into it?

Whenever we learn more about the connections between GR and the rest of physics, the early corrections are likely to appear as "GR+new stuff" or "QM+new stuff". Both theories have enough predictive power that we'll hang on to them as effective theories for the rest of humanity's existence.

That said, the mathematical fundamentals of GR do not directly incorporate notions of the uncertainty principle. That fact alone, I believe, means that GR is an incomplete description of space/time/physics.

Just to cause trouble, since we're straying far far from today's presentations (except arguably UCL's Z. Younsi in the ongoing ESO Q&A panel). :-) Can we write down the uncertainty principle in covariant form? Maybe! There is active theoretical work ongoing (e.g.) <https://arxiv.org/abs/2110.15951v2> ("... a geometric formalism for the generalised uncertainty principle which is covariant and connects features of the underlying geometry with the deformation of canonical commutator relations ... [and] an elegant interpretation for the standard dispersion relation p^2=−m^2: it describes flat spacetime in the Milne coordinates, with rest mass m giving the measure of geodesic length from origin").

As I understand it, quantum physicis essentially predicts that the randomness of quantum fields effectively disappears at scales of just micrometers, let alone at the scale GR works on. How is the uncertainty principle supposed to affect bodies at the scale of light years?

The most likely explanation is that we have no clue, just like every other point in history. Science is always new.

Everyone is different, but most scientists seek areas where experiment doesn’t match the predictions. It’s the only way we learn.

One such instance is the W Boson Anomaly: https://vm.tiktok.com/ZTdpwrmoj/?k=1

If the equations were right, the experimental error would be zero (modulo uncertainty bars). Instead, we’re quite confident the standard model is wrong (or “incomplete” to phrase it diplomatically).

As an aside, this is also a great example of TikTok turning a corner. I now have 184 educational videos saved, along with dozens of science videos. I learn more on TikTok than any other source now, which I didn’t expect. There’s an avid physicist community, and I made friends with someone who works at CERN. https://twitter.com/simoneragoni?s=21&t=xIkxhA--TzKDWA5XN3ve... Get ready for TikTok to become the new Wikipedia within a decade.

> Get ready for TikTok to become the new Wikipedia within a decade.

I've also learned a lot from TikTok, where short form content has led to 30 second tutorials that leave 30 minute YouTube equivalents in their dust. My learning topics include smartphone photography, wood working, and knots. I was also surprised at how informative it can be.

It'll be interesting to see how TikTok's content moderation compares against Wikipedia. We won't have notability wars, but there is massive scope for disinformation and banal wrongness.

> As an aside, this is also a great example of TikTok turning a corner. I now have 184 educational videos saved, along with dozens of science videos. I learn more on TikTok than any other source now, which I didn’t expect.

I too have been surprised with the amount of genuinely educational and interesting material on TikTok. It won't replace Wikipedia but it's a great companion to it.

Back when Trump was making a big stink about TikTok I didn't understand it, because my feed was almost exclusively high quality short form educational content from physicists, doctors, linguists, etc. It's a great format for people who want to share their knowledge with the public in a way that is easy to digest, but do not have time to script and edit 15+ minute videos.

We can't even measure the gravitational constant to more than about 4.5 significant digits. Most other natural constants are in the 7-12 digit range and improving along with our technology. Measurement of big G hasn't convincingly progressed since the vacuum tube era.

We have a lot more to learn about gravity, especially at laboratory scale and smaller.

As a laboratory-scale experimentalist, I heartily agree, but must also point out the existence of compelling precision equivalence principle tests.

While we can only measure G to ~10 ppm, the equivalence principle has been tested at the 10^{-14} level (and ~10^{-9} at meter-scales). The EP is the property that makes gravity really special and simultaneously the thing that makes gravity hard to test.

The core assertion that all things fall the same way is axiomatic to GR and has been very well tested. We have everything left to learn about gravity, but at the same time, GR has held up far better than it "should" have against a battery of really great experiments.

> the feeling is akin to watching a beautiful sunrise on a summer's morning. You've seen it before, but darn if it isn't pretty.

That's beautifully worded. It brings up the respect scientists can have for one another's work (in the best case scenarii, let's skip the bad seeds for a moment).

can't GR be a micro macro scale solution where perhaps that's why the theories do not work out?

How long does it take to train to fully appreciate this like you do, if that is even possible for someone in a full time job in an unrelated field.

How long is a piece of string? It depends...

I majored in physics, and at least half of the enjoyment is seeing how pure and beautiful the theory is, how it all works out somehow, while wiping tears away from your eyes at the library.

(That's counterbalanced with the tears shed over your midterm grades.)

> romantic beauty to it, both in structure

I have been thinking about this off and on since reading your comment. Is it really the structure that is beautiful, or is it that we can convert a large wall of symbols[3] into about eight easily memorized ones? That is, is it the notation that is beautiful, rather than the calculations it describes?

Relatedly:

> ... and in predictions. Each time GR matches reality in a new context, the feeling is akin to watching a beautiful sunrise on a summer's morning.

But how does one get predictions?

If you use white chalk on a blackboard to write down the constraints, boundary conditions, a realistic stress-energy (like, oh, anything satisfying Klein-Gordon or the way Weinberg writes down Belinfante), and so forth, to actually capture a realistic physical system in our universe (with actual broken global symmetries), the chalkboard's albedo sure gets higher. I'm certain you know this, and within an hour or two would go reaching for Etk/Cactus or even NRPy+/SENR. :-) And I haven't even gotten into things like Israel Junction conditions and other approaches to more-than-one-source problems.

I agree that conceptually GR is (or really, the EFEs are) amazing, and that it's a nice sandbox (oh the fun you can have in a (-,+) or (+,+,+,+,+,+,+,+,+,+,+,+,...,+) spacetime! Or Misner's mixmaster! Or lots of the entries in <https://www.cambridge.org/core/books/exact-solutions-of-eins...>), but when it comes to doing (and especially intuiting[1]) actual physics maybe familiarity breeds frustration or something. It is wonderful that the "sandbox" can, with effort and additional details (NS EoS, say), grind out astrophysical, cosmological, and even laboratory observables.

> darn if it isn't pretty

Darn if it isn't impressive how much work (including observation/experiment) went into the matching. :-)

- --

[1] Is it sharp enough to cut paper? Your answer to the question about the "sharpness" (in a different sense) of the event horizon got me wondering about extended objects, like what happens to a tissue-paper space capsule on a hyperbolic orbit grazing the point of no return, if a tiny corner of the space capsule is allowed to dip below it? Does it tear? Does it drag the rest of the capsule in, even though a good sneeze could rip its structure apart? Is the answer <https://en.wikipedia.org/wiki/Mu_(negative)> because of the nature of the horizon?

I started scribbling on this, then decided I really wanted to pretend the Weyl curvature tensor away so started thinking of really really massive BH -> Rindler approach (hoping Egan [2] had done the work already), abandoned that (because I don't think it works in general -- binding energy even in tissue paper might raise a "bump" (and thus tidal forces) on the BH horizon but I don't see how that works for the Rindler horizon, and I don't really trust intuitions built on staticity), and so on.

So, maybe I'm being a bit dumb at the moment, but a simple "what if?" turned into an "I have no idea" in spite of reasonable working knowledge of GR and some ideas about how to get the idea (with a sinking feeling that anything I arrive at is going to annoy me even more than not knowing at all, especially when trying to make sense of "no drama").

I dunno: something so hard to work with doesn't (to me) evoke romance. It's more like the feeling of summoning a cat who really doesn't want to come until your pspspspsing arrives at an acceptable-to-the-cat approximation of cat food being dispensed.

[2] http://www.gregegan.net/SCIENCE/Rindler/RindlerHorizon.html ("a constantly accelerating observer in flat spacetime trailing an object behind them")

[3] For anyone who has stumbled upon this rant and who has never seen anything like the full horror, check the tip of the iceberg at <https://profoundphysics.com/einstein-field-equations-fully-w...> (eyeballing just that page, things look reasonably OK; I have not really looked at anything else on that site).

[4] Oh, "Contritutions" (sic). <http://einsteintoolkit.org/guidelines.html> I'm starting to feel better now.

Any evidence supporting general relativity at these scales is presumably also evidence in favour of dark matter actually being matter, as opposed to gravitational theory needing modification. Seems like we can still learn quite a bit from ruling out alternatives, even if it's not the quick answer we might like.

Why is that the case?

Well maybe it's more evidence against MOND-like theories explaining dark matter, but those already have little buy-in nowadays.

The Schwarzschild radius of a 4M solar mass black hole like Sagittarius A* is only (lol) about 7M km. Most of the MOND theories I've seen are about fall off curves at very large distances (ie. galatic scales) being more complex than GR suggests, so I'm not sure this is really proof either way. (And I say this as not a MOND adherent).

Just curious but why?

They want an exciting disruption to current theory.

Basically they're bored/want mysteries to solve.

tbf, there is already a mystery to solve. This just means we have no new information to help us solve it.

We're kind of at a dead end between both leading theories. A counterexample to either of them would mean that we got it wrong somewhere, and might help us figure out where, potentially unlocking more later.

You wouldn't be surprised if your door opened just as usual when you put your key in the lock. The day it starts jamming however, you get very interested.

Side note: Einstein didn't believe in black holes, at least initially.

Black holes are a solution to the general relativity equations, but Einstein thought they were just a mathematical quirk, and that in real life, they wouldn't have been able to form.

So ever time someone says "Einstein was right" when talking about black holes, then no. He was right about general relativity, but he was wrong about the existence of black holes.

Many papers/authors postulating something different to what Einstein did?

GR and quantum field theory don't work together, so at least one of them is wrong, and it would be good to figure out which and how.

Thanks. Can you please point me to some popular science article that explains why the two do not agree with each other?

Brian Greene's "The Elegant Universe" does a very good, completely non-technical, job of explaining the conflict between the two theories. It then goes on to explain why String Theory is one possible solution to the problem.

Perfect! Thank you.

There's also a Nova documentary version if parent would prefer to watch something: https://www.pbs.org/wgbh/nova/series/the-elegant-universe/

I'll add an additional recommendation: Stephen Hawking - The Theory Of Everything

Here are a few that might help:

https://gizmodo.com/why-cant-einstein-and-quantum-mechanics-...

https://www.theguardian.com/news/2015/nov/04/relativity-quan...

https://en.wikipedia.org/wiki/Problem_of_time

https://physics.stackexchange.com/questions/387/a-list-of-in...

For a good general source if you want to learn a lot more from a good popular science point of view (but one that takes great care to not accidentally mislead by oversimplification as often happens with popular science) about general relativity and quantum mechanics, try PBS Space Time, especially after Dr. Matt O'Dowd took over as host and main writer.

https://www.youtube.com/c/pbsspacetime

I liked Sean Carroll's take on this with Steven Strogatz in the recent episode of Joy of Why.

https://www.quantamagazine.org/where-do-space-time-and-gravi...

tl;dr they don't conflict with each other, they conflict with a third assumption that particles have zero size. GR suggests they would be black holes, and those have mathematical difficulties. String theory is an attempt to model particles of finite size and thus eliminate black holes.

You're saying this team has proven neither of them wrong in that their findings agree with GR, is that correct? Essentially, nothing new has been learned about these two theories?

GR and QFT essentially have to meet at black holes and this is where you’d expect to find an anomaly with ought to help open doors to new physics, but what we see here is that this observation seems to still obey GR just fine and a a closer look will be necessary to find any anomaly.

It might just not be possible to see anything odd without having a black hole right there to study, but there’s always hope that the next new observation will provide a clue as the previous ones repeatedly have not.

Why would that matter at this scale ?

Black holes are extreme enough to be one of the places where both can matter.

Scale was not the right word - maybe I meant distance or resolution. Does QM make a prediction that could be falsified at this resolution?

so at least one of them is wrong

This is a semantic nitpick, but I don’t think it’s useful to think of these theories as “wrong”. They are both models that make predictions about physical phenomena that, when tested, are extremely accurate. They provide incomplete and inconsistent predictions of what happens at the very edges of physical reality, and they need reconciled.

Sometimes physicists like the theories--especially the battle-tested ones--proven wrong so that those theories needs to be refined or reevaluated. Something related to the joy of problem solving.

Quantum Physics and General Relativity make different predictions in specific circumstances which are beyond our ability to test to figure out which one is wrong. They can't both be perfectly correct. But in every test we can make, both seem to model reality really well.

The best place to look for new data where we might find reality disagreeing with either model is in the extreme parts of the universe, like black holes. If there was even a hint in this photo that General Relativity wasn't perfectly accurate, we might be able to take the discrepancy and build a new model that solves the disagreement.

Whoever does that gets a Nobel Prize and has their name as immortalized as "Einstein".

Relatedly, before general relativity, people already knew something was wrong with Newton's theory of gravity from observing the orbit of Mercury, which didn't follow the theory accurately. At that time Mercury, being so close to the Sun, was an extreme condition that showed the flaws in the theory.

The fact that general relativity continues to hold up to every observation we can make is remarkable.

I'd be willing to bet that the discrepancy is something that we generally consider invariant but in reality should be a ratio between two things that are very large, but get bigger or smaller at _slightly_ different rates depending on the scale or distances you're working with.

Granted, I only ever got as far as E&M physics in college so I could be way off here but that scenario has turned up so many times in history.

Could it happen that the universe simply never provides us with the required evidence? Eg if there's an anomaly but only when something that never happens happens. Like a large enough mass/charge/etc or some flavor of unlikely coincidental occurrence, like an eclipse is a bit of a coincidence.

I've heard it said as "The universe is under no obligation to make sense to us". It's possible that the events where the two theories diverge will always be out of reach for us to observe.

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