Opinions on Boltzmann brain arguments constraining modern multiverse theories

post by Dr_Manhattan · 2012-08-10T18:05:52.417Z · score: 1 (6 votes) · LW · GW · Legacy · 14 comments

Leading inflationary multiverse theorist Andrei Linde (and others) gives significant enough credence to the Boltzmann brain problem to tweak some of the parameters of his theory (here for example http://arxiv.org/abs/hep-th/0611043). Are there more elegant ways to deal with the issue that LWs are aware of that would not require changing physics to accommodate? 


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comment by shminux · 2012-08-10T21:16:25.237Z · score: 3 (3 votes) · LW · GW

Are there more elegant ways to deal with the issue that LWs are aware of that would not require changing physics to accommodate?

Please note that no currently confirmed physics (specifically, the Standard Model of Particle Physics) needs to be changed. What is under discussion is a number of string theory models. All such models have supplied plenty of testable predictions, and all these predictions have been invariably falsified. The BB problem arises as a consequence of trying to salvage what is clearly not a description of the observed Universe.

comment by Mitchell_Porter · 2012-08-11T09:39:36.411Z · score: 3 (3 votes) · LW · GW

In a sense, the string situation is even worse than that: there is no agreement on how to approach cosmology in string theory. All these landscape papers are based on ansatze that just guess how the big picture works.

The Boltzmann brain problem arises because the volume of eternally inflating de Sitter space is vastly bigger than the non-inflating regions, big enough for an exponentially unlikely entity to fluctuate into existence. But de Sitter space is quantum-mechanically unstable, and how to do quantum gravity there is probably the outstanding issue in that subject right now.

One possibility that Linde examines is that de Sitter space is unstable enough that it transitions to something else before it can become vast. The best toy model of de Sitter space right now is (in my opinion) dS/CFT duality in Vasiliev gravity theories, which are like a truncated version of string theory, and there's very recent work on the instability there. So that might end up providing the answer.

My general response to the original post is therefore that you shouldn't attach too much significance to papers on Boltzmann brains. They are a bit like papers on the Drake equation: they make as many assumptions as they need to make, in order to reach a conclusion, but those assumptions aren't very well-founded. It's the hard technical work on quantum gravity in de Sitter space which will decisively clarify the issue.

comment by Dr_Manhattan · 2012-08-11T13:29:29.450Z · score: 1 (1 votes) · LW · GW

Does this generally lower your estimate of multiverse theories (vs single universes) or are initial arguments solid (there seems to be significant consensus in that direction), but further details are questionable?

comment by Dr_Manhattan · 2012-08-11T13:57:23.067Z · score: 0 (0 votes) · LW · GW

Interesting, I just came across the discussion here http://physics.stackexchange.com/questions/32979/why-arent-we-boltzmann-brains-in-an-infinite-universe where you gave the argument from immediate disintegration. This is what I've been thinking also (some theist friends of mine were attempting to "prove" that multiverse theories are crazy based on BB).

comment by Mitchell_Porter · 2012-08-11T15:00:17.098Z · score: 1 (1 votes) · LW · GW

Boltzmann brains and multiverse are logically distinct issues. A single universe with an eternal de Sitter future potentially has this problem. I regard any theory that predicts a Boltzmann brain majority as wrong, but it's not clear how generic a prediction this is for eternal inflation, or even whether eternal inflation is the right cosmology (e.g. for the test case, Vasiliev gravity).

comment by Zaine · 2012-08-10T21:23:40.566Z · score: 2 (2 votes) · LW · GW

... and all these predictions have been invariably falsified.

Would you mind providing some links to a more detailed summary of how they were falsified, please? or alternatively personally summarizing a more detailed link?

Thank you, if no.

comment by shminux · 2012-08-10T21:56:03.129Z · score: 9 (9 votes) · LW · GW

Falsified predictions:

  • All string theories require more than 3 spatial dimensions, all on equal footing. (Plenty of workarounds have been proposed, like compactification, to work around this obvious and glaring issue.) This has been falsified time and again, most recently by the LHC not generating microscopic black holes.

  • Superstring theories predict "superpartners" for all known particles, none have been seen so far. The proposed workarounds center on "fixing" superpartner masses to be too heavy to be observable. Such moving goal posts are a standard approach.

  • Multiple possible universes with wildly different physical laws, ours being one of the least likely ones (hence the hyper-anthropic Boltzmann Brain discussion)

The Not Even Wrong blog has addressed the issue a number of times. Here is an example.

String theory remains popular mainly out of desperation: no one has any better model at this point. Plus some of its ideas, like holography and duality, ring true, even if they aren't literally true. Additionally, some of the math developed by the string theorists ended up being quite useful for some real-world calculations. None of which justifies taking Boltzmann Brains seriously.

comment by Luke_A_Somers · 2012-08-13T14:01:45.053Z · score: 1 (1 votes) · LW · GW

Such moving goal posts are a standard approach.

When you're talking about a theory as insanely vague as string theory, this isn't exactly an independent problem - it's just something that comes along with it being insanely vague. Not that that isn't a problem, but accusing them of moving the goalposts doesn't fit.

Their goalposts were already spread all over the field. We've just discovered that many of them weren't the set that the ref is watching... if any are.

comment by [deleted] · 2012-08-12T23:05:05.201Z · score: 1 (1 votes) · LW · GW

This has been falsified time and again, most recently by the LHC not generating microscopic black holes.

Is that the case? I'm not an expert, but I'm under the impression that the LHC was predicted to generate microscopic black holes only for somewhat large compactified dimension size (i.e. millimetres), and even string theorists wouldn't have thought of that as particularly likely.

comment by shminux · 2012-08-13T04:53:59.157Z · score: 0 (0 votes) · LW · GW

Right, I should have phrased it better. It puts tighter constraints on extra dimensions. The string theory itself says nothing about the expected size, so the tighter the constraints, the less likely string models are correct.

comment by Zaine · 2012-08-11T15:01:42.476Z · score: 0 (0 votes) · LW · GW

Thank you!

,,, no one has any better model at this point.

Recently I was directed towards A Universe From Nothing by Lawrence M. Krause, which I was told puts forth a map that is better than string theories at describing the everything that string theories try to sketch. Have you heard of it?

comment by shminux · 2012-08-12T00:54:33.678Z · score: 3 (3 votes) · LW · GW

This review by Sean Carroll, who is an expert in the subject matter, seems to analyze the book and the reviews pretty nicely.

Nothing about modern physics explains why we have these laws rather than some totally different laws, although physicists sometimes talk that way — a mistake they might be able to avoid if they took philosophers more seriously

comment by Dr_Manhattan · 2012-08-10T23:47:15.790Z · score: 1 (1 votes) · LW · GW

How do string theories relate to inflation models? They seem orthogonal, but I'm a total layman in the area.

comment by shminux · 2012-08-11T04:42:21.086Z · score: 1 (1 votes) · LW · GW

I'm not an expert, either (I only ever took one grad course in string theory). From what I know, you are correct, the two are independent, though there are some string-based proposals for inflation or something like it. For example:

an interesting proposal for pre-Big Bang cosmology where the stringy Universe starts out flat, cold and very large instead of curved, hot and very small. This early Universe is unstable and starts to collapse and contract until it reaches the self dual point, where it heats up and starts to expand to give the expanding Universe we observe today.