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Comment by grabcocque | Hacker News Reader

Something I always felt 20 years ago when doing my physics degree, and now 20 years later when no measurable progress has been made down the road of unification feels even more appropriate:

General Relativity and Quantum Mechanics are two astonishingly successful theories. They don't really have any massive gaps waiting to be filled. The belief that there must be a way to unify them always seems to me to stem from the human desire for tidiness, not any sound epistemological reasoning about how physical laws behave.

We want the theories to be unifiable because we're human. Sadly the universe doesn't much seem to care what we want.

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General Relativity and Quantum Mechanics are two astonishingly successful theories. They don't really have any massive gaps waiting to be filled.

Individually, you're entirely right about this: both general relativity (GR) and quantum field theory (QFT) are beautiful, self-consistent theories that describe things with tremendous accuracy in the domains where each is relevant. But nevertheless, there are fundamental inconsistencies between them, or at least, scenarios that we absolutely do not understand today.

That's why so many people are interested in black hole physics, as one example: those are objects whose gravitational properties can only be understood using general relativity, but which have profoundly complicated and interesting behaviors related to quantum field theory and particle physics as well. In that context, GR and QFT really do seem to contradict one another, as the famous black hole information paradox illustrates. (Rigorous theorems in relativity say that information crossing an event horizon is lost forever; the postulates of quantum mechanics require that every process in physics be "unitary" and thus cannot possibly destroy information.) So it's simply not internally consistent to postulate a universe simultaneously obeying the rules of both GR and QFT as we understand them today.

The universe definitely has no obligation to cater to our human aesthetic preferences, but I think we're justified in assuming that it doesn't actively contradict itself, either.

We want the theories to be unifiable because we're human.

That's a pretty neat theory you have there, but I don't think the reality is as tidy as that! You must want it to be neat because you're human. :)

For a very long time, nobody could figure out how to unify earth-bound mechanics (objects falling to the ground, remaining at rest until pushed, etc) and celestial mechanics (objects moving endlessly in not-quite-circular patterns). Most people were happy with this distinction, as it didn't cause any practical problems and fit well with religious ideas.

As far as I know (but maybe somebody can correct me) there wasn't really a major push to unify the mechanics of earth and space until Newton figured out universal gravitation.

Both Hooke and Huygens both pioneered many ideas in universal gravitation, but the key difference as I understand it wasn't that Newton had a better theory but rather a better tool: calculus

This is a really important point. Numberphile had an interview with mathematician Terrence Tao recently [1] and he was asked if he works on big problems like the Riemann Hypothesis. His response was that we just don't have enough mathematical tools to fully tackle the problem. However when new tools become available he and many other mathematicians go through a list of problems and ask if any breakthroughs are possible.

GUT Physics is also similar. M-Theory was a breakthrough when Witten combined many disjointed string theories together. Recently Arkani's Amplituhedron [2], based on advances in algebraic geometry, is able to calculate some previously complicated QFT integrals using simple algebra, hinting at underlying symmetries.

1: https://www.youtube.com/watch?v=MXJ-zpJeY3E 2: https://en.wikipedia.org/wiki/Amplituhedron

> They don't really have any massive gaps waiting to be filled.

Except the Standard Model is at best incomplete, at worst flawed, considering for instance galaxy rotation discrepancy (i.e. a need for dark matter possibly super-symmetrical, or modified formulas etc). Obviously black hole physics, expansion of space... There are many open questions, hard problems still unsolved. There's even a nice wiki article about "unsolved problems in physics".

However I very much appreciate your point and think it's something to keep in mind when building models.

(Disclaimer: physics postdoc working in an unrelated area of research, not a domain expert.)

I'm assuming you mean GR and the Standard Model (which is a very long way away from QM). If so, there's a number of known issues with the SM, but that's not really what unification is about. Unification is about the simple fact that given the four known fundamental interactions we have a theory that (largely) accounts for three of them and a theory that accounts for one, but the two theories are mutually incompatible. That's an issue because whenever we encounter phenomena where gravity and at least one of the other fundamental interactions are simultaneously relevant, we have no established theory to describe them. Such phenomena, while comparatively rare, exist and are quite important to our understanding of the universe. Hence the long standing desire to produce a unified theory.

Well, maybe they don't need to be unifiable - but GR and the standard model are outright contradictory! We live in one universe, so there must be one true story about how it is.

I always thought it was more that the techniques used to describe interactions in the SM simply don't work when asked to describe gravity.

So we have a detailed theory of particle interactions for the three forces, and a very small stub of a dead end which blows up to infinity in a useless way for hypothetical gravitons.

From a very native hand-wavey view, I'm not sure this is a surprise. GR describes a rather lovely and smooth view of spacetime, while the SM assumes spacetime just works transparently and effortlessly, and quantum lumpiness somehow happens inside it, with a few relativistic adjustments.

It looks possible they're both different ways to think about information, causal relationships, and correlations. There's enough weirdness around quantum ontology to suggest that neither information/state nor causality are so trivially simple they can be taken for granted.

> We live in one universe

I think this may be an oversimplification.

Nevertheless, GR and QFT describe the same universe.

    We live in one universe

That remains to be proved.

What needs to be proven is that [if] we live in the only universe. That we live in one universe [where both GR and SM are correct] is not, as far as I know, being debated.

Except in Many Worlds. :)

Side question - if MW were true, could it lead to universes where either/both of GR and SM were very different?

Are there any constraints on theory classes? Is it possible to go past the usual ideas of MW (different physical constants, or maybe some trivial historical differences) to imagine universes with completely different logical/mathematical/causal bases?

It doesn't fit with the overwhelming patterns of nature to have a different set of rules for when you're dealing with really big stuff vs. really small stuff.

And that doesn't even get into the fundamental issues with time and gravity. The quantum world depends on a steadily ticking clock, but general relativity tells us that when an astronaut carries a clock to the ISS and then brings it home, it has fallen behind a surface clock.

20 years is a short time for unification theories. How long in the existence of humans did it take to unify electricity and magnetism ? ;)

By the way, I'm happy people are still searching, just in case...

That's stupid. What happens when both GR and QM become important as in the vicinity of a black hole or in the early universe? There should be a way of answering these questions and without unifying QM and GR we cannot.

A perfectly reasonable point which can be made without insulting the OP

> We want the theories to be unifiable because we're human.

It could be a cultural thing, going back to Plato at least. But perhaps the whole of Existence cannot be reduced to a simple formula.

Perhaps at deeper levels there will be a sort of federation of theories, each with its own domain, interfaced somehow but not overlapping much.

It's human nature to seek ultimate explanations. We just aren't satisfied with working theories that don't answer the deepest questions. One way or the other, the effort will bear fruit I think, even if it's by nothing else than to show us that there is no ultimate law out there.

> One way or the other, the effort will bear fruit I think

Or even if it's just to give us agriculture, mass production, electricity, sabremetrics and iPhones.

...Ok... given that please explain AdS/CFT correspondence.

As I've seen it explained in prior threads, AdS/CFT correspondence is a powerful tool in string theory that let you match theories that are inaccurate in our universe to those that operate on a model of spacetime that is inaccurate in our universe.

That's both true, and misleading. It is accurate in our universe to certain limits, which is true for literally every theory we have.

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