The (somewhat ridiculous?) holographic principle

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Frenetic Pony
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The (somewhat ridiculous?) holographic principle

Postby Frenetic Pony » Tue Jan 31, 2017 9:54 am UTC

Was looking at another new "holographic principle/the universe is a 2d hologram/etc." paper: https://journals.aps.org/prl/abstract/1 ... 118.041301 nee https://phys.org/news/2017-01-reveals-s ... verse.html and was yet again struck by the fact that all it seems to do is be yet another "we re-wrote all the maths to fit the exact same data we've always had, and that makes no new testable predictions whatsoever, but isn't it exciting??!"

So, perhaps I'm misunderstanding something. But to me it's just another multiple worlds/QM interpretation/yada yada that, when really, actually pressed to say that it predicts a testable difference from the standard model, like say explaining dark matter or something, all proponents of it will just look a bit sheepish and go "well, I mean. But it's really neat isn't it?" Anyone know more about it as such?

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Re: The (somewhat ridiculous?) holographic principle

Postby doogly » Tue Jan 31, 2017 3:19 pm UTC

I think you get some stronger stuff, like the bekenstein bound for entropy becomes exact, and you're forced into the firewall nonsense.

Which is to say that the actual novel predictions are of things that seem worse than evinced.
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Re: The (somewhat ridiculous?) holographic principle

Postby Tub » Fri Feb 03, 2017 12:56 am UTC

I'm not sure if the goal of the work is to make predictions. If the idea is that this 2d-thing and that 3d-thing are equal, then there's not going to be an experiment that'll tell you whether the universe is actually 2d or 3d - any experiment that could distinguish between the two is a failure of the holographic principle.

It's an interesting way to poke holes in our assumptions, though, and it's another hint that space might not be fundamental. We need to understand quantum gravity, and for that we need to understand space, and the holographic principle may help us on the way. Sometimes, it's not about testable predictions, but about new ideas and perspectives. I wouldn't dismiss it just yet..

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Re: The (somewhat ridiculous?) holographic principle

Postby Tchebu » Mon Feb 06, 2017 7:40 pm UTC

I second what Tub is saying and would like to expand on it a bit.

Regarding the holographic principle specifically, it is not a model by itself, it's a statement about the theoretical structure of gravity theories. It's the statement that a clever reshuffling of the degrees of freedom in a theory of gravity results in another theory that doesn't explicitly contain gravity and that lives in one dimension lower than the original gravity theory. As Tub has pointed out to say that it doesn't make predictions is missing the point, because it's not even in the right category of things for that.

This is a specific case of a more general "phenomenon", known as duality. There are many other examples of theories, which look somewhat different if you just list their particle content and interactions, but that can be transformed into each other through a clever reshuffling of their degrees of freedom. It's basically a fancier and more abstract version of changing reference frames in a mechanics problem. Except you're not just changing the position and velocity of some reference point, you're changing what we choose to consider as the fundamental ingredients of the theory. The fact that this is possible indicates that there is no fact of the matter as to what the fundamental ingredients are, just like our ability to change reference frames indicates that there's no fact of the matter as to who is moving and who is at rest. I don't think the significance of such conceptual achievements can be overstated and I'm semi-expecting philosophers to make a big deal of this at some point in the future.

The special thing about holography is that it's a duality between theories with and without gravity, giving a potential avenue for addressing the conceptual problems with gravity by translating it into a non-gravitational language, that we have a much better theoretical grasp of.

As a more general comment, I'd like to point out that there are two broad categories of things that theoretical physicists do. One is to construct specific models and work out their experimental predictions. People who do this are probably actively collaborating with experimentalists or at least directly use their output data, but don't actually deal with the details of specific experiments, apparatus, etc. The other category is the study of the mathematical and conceptual structure of physical theories in general. These are the people who cause experimental apparatus to malfunction within a 15 meter radius just by being there. This second kind of work is basically an exploration of the space of possible internally consistent theories, and an attempt to identify their fundamental assumptions, identify various classes of theories etc. This then allows for a top-down approach to model building. This is also the kind of work that could lead to new conceptual breakthroughs in our understanding of existing theories as well as approaches to formulating new ones.

The arguments about "theory so-and-so is not science" often boil down to the statement that activities in the latter category are not in the former. My personal feeling on the topic is "no shit..." This is the case with string theory, this is also the case with all the newer ideas about quantum gravity, including the holographic principle. This state of affairs is not surprising given the scarcity of experimental data on these topics, so we have no choice but to engage in abstract exploration of "theory-space" and top-down model building. On top of that we pretty much know that the problems with quantizing gravity is not one that data is going to solve, even if we had it (although it would be helpful for inspiration). It's primarily a conceptual problem, because it's not that we can't construct a theory that fits the data, it's that we can't construct a theory at all, because the whole toolkit that gave us the "standard model" simply breaks down.
Our universe is most certainly unique... it's the only one that string theory doesn't describe.

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Re: The (somewhat ridiculous?) holographic principle

Postby Tub » Tue Feb 07, 2017 11:11 pm UTC

Ah, a post from Tchebu. Though rare, they're always insightful. Thanks for taking the time to write.

I took this as an excuse opportunity to watch smart people talk on youtube, so let me quote this audience question:
Leonard Susskind | Lecture 2: Black Holes and the Holographic Principle, around 1h19m (warning: audio is horrible)

<audience member> It's been a beautiful talk, thank you very much, but: what experiment can you do?
<Leonard Susskind> There is none.


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