What is spacetime made of? | Hacker News Reader

What is spacetime made of?

scientificamerican.com

56 pointsnyc11158 comments

This article is framed in a way I find epistemologically suspect. It confuses observations, models and truth. Some of its intertitles are meaningless like "Will we ever know the real nature of space and time?".

The article is actually about models in which spacetime is an emergent property of more fundamental structures: string theory and loop quantum gravity. Both face a lack of experimental evidences validating their predictions.

But the question, then, is: is there reality behind the model (or, rather, how close to reality the model is)? The answer lies in how accurate the model is. This is all we can know in this regard.

But knowing if there is reality behind the model is not a scientific question. There is no tool in science which can answer that. The only thing that matters is how well the model predicts what we observe.

One can believe that’s a form of truth but people should be conscious that it is merely a belief.

This is not strictly true. The recent Nobel was such an expedition which was ignored by scientists in the 40s-70s as an esoteric philosophical question that cannot be answered. It is still not “answered” per se but Bell’s formulations leave very few choices for the base reality of the model.

Observe that the model itself worked quite well regardless of the base reality. Knowing that truth doesn’t change any current applications. But who knows where it’ll lead. So knowing which model is successful and which model is real is a very important question that we must try our best to answer. Only in the natural sciences can such a question even be answered.

No matter how good a model is, a simulacrum is still a simulacrum.

Of course not all these models are is a description of the mammalian brains tooling to make prediction. There is not a single reason to believe it’s the real nature of “reality,” whatever that is supposed to mean.

> It confuses observations, models and truth.

Succinctly describes the global zeitgeist.

Perhaps modern idioms are to blame with relation to the copula. We seem to say things are what they are without a qualifier on what type of existence it has.

Scientific American was once like reading well illustrated research journal paper. Today it's the "science" equivalent of People magazine.

I don't think this criticism is fair. For example, the article in the section quoted below clarifies that string theory hasn't been proven experimentally, and is being investigated mathematically, here:

> But string theory is difficult to understand—it lives in mathematical territory that has taken physicists and mathematicians decades to explore. Much of the theory’s structure is still uncharted, expeditions still planned and maps left to be made. Within this new realm, the main technique for navigation is through mathematical dualities—correspondences between one kind of system and another.

Non-majority opinion/Guestimate of reality follows:

I think that spacetime is an emergent phenomenon of something that has no dimensions. Imagine RAM in your computer, inherently there's an address dimension when the CPU needs to read/write data, but the actual physical location doesn't matter (other than latency).

How correlated two bits of matter/energy/spacetime are is a good proxy for distance, except when it isn't. A photon leaves a star, then strikes your eye a billion years later, and those events could be entangled, as far as the photon is concerned, they happened at the same instant in time.

However the laws of physics work, I don't see how there can be any direct mapping between what we see, and the underlying quantum phenomenon, as long as light and quantum superpositions / entanglement, are things.

This kind of terminology soup speculation isn’t helpful. Don’t muddy the waters when you don’t know enough to add anything of value.

The article itself isn’t much better:

If space is made of entanglement, then the puzzle of quantum gravity seems much easier to solve: instead of trying to account for the warping of space in a quantum way, space itself emerges out of a fundamentally quantum phenomenon.

I have a PhD in a closely related field and I'm not sure what you mean--this sentence is well-formed and basically correct, if slightly simplified to appeal to laymen.

“If space is made of entanglement, then it emerges out of a fundamentally quantum phenomenon.”

This makes sense to you?

Its the part that you omitted from your summary that makes the statement nontrivial—the idea that the metric theory (poetically, the warping of space) is only valid as a low energy effective description, and the general case does not have a classical interpretation even in a limit.

To put it another way: the quantum theories that are best understood are the ones that are similar to a classical field theory, but that’s not the case here.

It appears that when I eliminated inessential verbiage, what remained was a tautology.

"I think that spacetime is an emergent phenomenon of something that has no dimensions."

I actually think that's a helpful statement. If you read/watch Bernardo Kastrup or Don Hoffman, this is in line with how they present their primary argument about consciousness being fundamental.

> A photon leaves a star, then strikes your eye a billion years later, and those events could be entangled, as far as the photon is concerned, they happened at the same instant in time.

If a photon does not experience time then I find it challenging to imagine that it could have a perspective at all.

I once read somewhere that if you swap time in spacetime with gravity, then the universe turns crystalline, and lots of stuff makes more sense.

In that way, perhaps photons experience other stuff.

It gets worse. Not only do photons not experience time, they don't experience space either. From a photon's point of view, it's always every "where" it needs to be. Which means there's no distinction between places. Which means space doesn't exist, from a photon's POV.

Reminds me of this quote "A wizard is never late, nor is he early, he arrives precisely when he means to"

I think what you're describing is nonlocality? It's well-described by quantum mechanics, which sits on top of regular old continuous spacetime. Is that not the direct mapping you're talking about?

Stephen wolfram is working on something like this, he wants to model the laws of physics entirely with recursive graph rewiring. I haven’t looked into it, but he says that space and time turn out to be emergent quantities when you look at it like that. He also has things to say about quantum and relativistic effects

https://m.youtube.com/watch?v=CLXyDCISsUY&themeRefresh=1

Stephen Wolfram is interviewed by Sean Carroll here: https://www.youtube.com/watch?v=0bMYtEKjHs0

They discuss the hypergraph model and Sean asks some good, pointed questions.

these folks are doing their best to combine and extrapolate mathematical principles to extremes (an approach which, nota bene, did prove spectacularly successful in explaining huge parts of the universe earlier on) but the results of this exercise has been exceedingly poor for almost half a century now.

at some point we must give up or admit that the nature of the game has changed. In a sense even the tiniest piece of experimental evidence that probes (or at least remotely reflects) the actual inner workings of "quantum" spacetime would be worth more than this enormous cumulative effort to infer what is happening on the basis of abstract symmetries and what not.

there were cool things that came from pure math in the early quantum + relativistic era -- lasers + black holes were math constructs before we thought to build them / look for them in the sky

I think (as an outsider to this field) the 'probing' now has fallen to the quantum computing people. their day job is basically making houses of cards out of entangled particles; if there are tricks to be learned about entanglement, they may be the ones to find them

And superposition. The other part like quantization of field and the wave function … decoherence.

You might be excited by [0]. The idea there is to study a novel phenomenon in strongly-coupled quantum systems that is very complex to see ordinarily, but becomes easy to understand when you look at the same physics from the gravitational perspective. That would be a nontrivial experimental check of AdS/CFT duality. It's effectively a novel form of "quantum teleportation" wherein an excitation is transmitted between systems using the quantum equivalent of a gravitational wormhole [1].

[0] https://arxiv.org/abs/1911.06314

[1] https://arxiv.org/abs/1608.05687

I couple of weeks ago, I watched a really good talk by Nima Arkani-Hamed on the same topic: https://www.youtube.com/watch?v=GL77oOnrPzY that I discovered via https://news.ycombinator.com/item?id=33229614

I found Nima's lecture to be a masterclass on how to explain very advanced physics topics without using unnecessary jargon, or math. Highly recommended for anyone interested in this topic.

Why, luminiferous aether of course!

If we're talking about aether, then it's worth mentioning that the seven elements have been traditionally earth, water, fire, air, aether (magnetic field), space (aka the emptiness) and consciousness.

I wonder if we will bump into something that we are just not able to figure out. Something beyond human comprehension.

I have a highly unpopular opinion that time might be that thing.

I also think that fully comprehending it and being able to influence it is a universal key to how it all works™. And if we did figure it out, we would cease to exist in a recursive manner before we could do much damage.

sigh

not sure why you'd try explaining LQG to a layperson without 'background independence' as a motivating concept

would rather see pop sci invest in explaining the actual concepts to laypeople, rather than their philosophical implications

like give me an interactive intro to witten diagrams as a way into ads-cft + holography

or like make lie groups interesting somehow please

the theoretical physicist they interview in the first paragraph has videos about her specific work (kozsul duality) which go deeper into the concepts https://www.youtube.com/watch?v=VW87-4t-Fsg

Are there any formulas supporting this or that composition of spacetime? Until there's math it's a simple handwaving.

One of the most important papers in this field is [0]. (As is common in science, it built on a long history of nearby ideas, e.g. [1].) An important calculation done around this idea is presented in [2], in which remarkable progress is made on Hawking's black hole information paradox. All of these papers have some amount of mathematical backing, but you'll find plenty of formulas and calculations in [2]!

[0] https://arxiv.org/abs/1306.0533

[1] https://arxiv.org/abs/1005.3035

[2] https://arxiv.org/abs/1908.10996

> General relativity contains solutions in which two distant black holes are connected

I can see these as artifacts of insisting to work in 3D coordinates and dealing with artifacts of projection where it breaks down, instead of working in honest 4D coordinates where formulas are much simpler and everything is smooth.

How would two connected black holes look like in Kruskal–Szekeres coordinates? Where's the firewall in case of Kruskal–Szekeres coordinates? Where's the firewall at all if the event horizon seemingly moves away from you as you approach the wormhole?

The existence of those solutions is independent of a choice of coordinates. (Nothing in GR is ever allowed to depend on coordinates, in the end!) The existence of the second asymptotic region is easiest to see from the Penrose diagram of the Schwarzschild black hole. A derivation of this that uses Kruskal coordinates is in [0], see Figure 3.3 for the result.

[0] https://scholar.harvard.edu/files/yshi/files/physics_211r_-_...

Thank you for this insightful link. I'm still not sure why one would need to explain that (such as an "ERP pair") instead of just the topology of space being what it is in this specific case. As far as I know these models fail to produce any interesting prediction and even explain all the known facts in their terms.

One would need to find out which stellar evolution paths would lead to black holes being linked by a wormhole. This assumes no magic happening beyond the event horizon and the evolution of matter inside it following already known laws.

Memory addresses?

Site down?

Spacetime is the Ether, in my opinion. This was tested, but never proved/disproved: https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_exper...

Here's a paper on it: https://www.researchgate.net/publication/283669822_Connectio...

The upshot is a charged shell will modify the acceleration/inertia of particles inside the shell, according to Weber's Electrodynamics: https://en.wikipedia.org/wiki/Weber_electrodynamics

Well, the concept of ether/aether had many forms and some of them were definitely disproved. The wiki link you gave basically confirms the absence of aether at increasingly smaller scales.

> Spacetime is the Ether, in my opinion. This was tested, but never proved/disproved

Am I missing something ? because you just linked an article that clearly disproves your first assertion.

... and followed it by what looks like a poorly written research article by a self described "independent physicist" (https://nitter.net/tavi_balaci) with only three published articles and all kinds of red flags.

Yeah, I hear you.

> After all this thought and preparation, the experiment became what has been called the most famous failed experiment in history.

We accept Maxwell's equations as the truth, not Weber. I'm not saying this alternate approach is true, it's just my opinion we missed something, collectively.

The paper is likely poorly written, but it's something I found while researching this a few years ago. Just sharing it for others to see. Again, my comment isn't claiming one way or the other...

Your'e missing Phlogiston. It surrounds us and penetrates us. It binds the galaxy together.

Probably emotions. An endless torrent of the stuff. Like your favorite song, minus the words and notes.

Which is then interpreted as sights, sounds, smells, thoughts (we love those), feelings, time, space, things, pizza, money, etc.

Scientific American's reputation is in the toilet: https://www.math.columbia.edu/~woit/wordpress/?p=13152

I don't recommend wasting your time.

Middle management idiocracy has taken over to “create jobs” everywhere.

Dont they kinda have to, to avoid falling into problems with the DEI crowd?

ad majorem Dei gloriam

Exactly. There’s no law forcing me me to chip into healthcare. Tacit admission in the US I don’t have to care any other meat bag exists; why should I care what they have to say?

Peter Woit has strong opinions but they're often not representative of the physics community as a whole. In my opinion, which as a physicist myself I think matters as much as Woit's (i.e. not much), Scientific American is variable in quality, but I wouldn't dismiss the article before reading it.

This article does a pretty good job of summarizing a range of modern ideas about spacetime. I see that it introduces laymen to some of the important names in the field, including van Raamsdonk and Maldacena, and presents their important and influential work in an accessible way.

No paywall version: https://web.archive.org/web/20221124171006/https://www.scien...

There's also nonsense like this: https://www.scientificamerican.com/article/physicists-need-t...

https://scottaaronson.blog/?p=6202

https://www.scientificamerican.com/article/nasa-needs-to-ren...

Is this what the kids call "cancel culture"? I'm judging the quality of the OP article, not the sum total of everything that they've ever published. The politics of the editors of Scientific American may not align perfectly with my own, but it's still worth engaging with their content when it's thoughtful, and I think this article is an example of that.

Edit: actually that's not true, I just wrote that SA's overall quality is "variable". So yes, I already conceded not everything they publish is a winner. And for what it's worth, I agree that the piece they published on E. O. Wilson was in very poor taste.

No, it's not "cancel culture". If you're not an expert in the area, you need to know how trustworthy the publication is. As it stands, there are indicators that it's not trustworthy.

[edit] Sure, the article might have some value. I can accept that. Thank you.

I have a PhD in physics. I gave my assessment of the article in the first reply above. You're welcome to disagree, but since it seems you're mostly interested in rejecting the article on the basis of who published it instead of the content, perhaps this is a dead end.

Edit: appreciate the edit :)

It’s what the kids call click bait of the subspecies known as “trolling for controversy.” It comes in two flavors: “woke” and “anti-woke.” In all cases the goal is to get you to click so someone can sell ads.

At some point in the past SA contradicted some point of dogma. Now their output is forever picked over for evidence supporting a case for, yes, "cancelling them".

That's how they do it these days.

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