Battlefield 3 Unlocks the Secrets of the Universe

[freyar]

And, just because, I'd like to point out that the post you're replying too actually said, "And since I haven't heard explanations lately for where all the universe's dark matter is hiding..."

The funny bit is that the actual question of *where* it is is part of the reason for thinking it exists at all! We can observe motions in the universe, and see that they are not consistent with the visible matter, and thus deduce the presence of matter that isn't glowing (is "dark") in those places. We know *where* it is (as in, say, the halo of galaxies) much better than we know *what* it is.

Yeah, good point!

 

[freyar]

Right. A meta-question of mine is "does this model break down under what we already know?" The follow-up is "does this model change something?" Where it might change something is that it's a different take on "many worlds," in that the different worlds have some impetus to bend a bit, and agree with each other. Sort of like if Schroedinger's cat lives in one universe and dies in the other, then universes parallel to the "live" universe are somehow influenced toward the cat surviving.

Sure, there is never a time when we disagree on what is going on in the universe - if we are in the same universe. When two observers, moving relative to each other, disagree on when something happens or what time it is, that would be disagreement. But do they have the same client, or are they using different clients?

Not sure what you mean by "universes parallel to the 'live' universe," I'm afraid. And it's very hard to say who has what client, since we don't have a (mathematical) definition of what that would really mean in quantum physics. There may in fact be subtle experimental differences between the orthodox interpretation of quantum mechanics and the "many worlds" interpretation (I do know of some pretty reputable people working on the subject, though not many), but it's never that two observers would disagree at any point. And there's no global "host computer" resolving things. In many worlds quantum mechanics, everything happens through usual interactions --- normal quantum physics.

Sure. Relative to you, they have past-future ordering. The different-clients idea suggests, though, that there are more than one person; there's more than one observer. See, now I'm wondering: does the state of science today assume that there is only one observer (since all events are consistent and cannot be disagreed upon), and that one observer is "ME?" Or to put it another way, do we still think that the sun revolves around us?

Those two events have past-future ordering relative to me. But because of that there is no way to change observers to reverse the ordering, according to the math. It just can't be done. So they have past-future ordering according to everyone.

As Umbran says, relativity is a lot about how what different observers see is related. An important point is about figuring out what every observer always agrees on! But it's the shocking differences that tends to get a lot of the attention.

 

[freyar]

Quite true. But, until such time as it gives us new testable predictions, it is not giving us much actual knowledge of the physical universe. String theories have advanced the fields of mathematics far more than those of the physical sciences.

Yes and no. We have learned a lot about what physical theories are even possible from string theory. Particle physicist (not string theorist) Matt Strassler wrote a really lovely series of blog posts about this (nine in all, I think), starting here. Well worth reading all of those.

It is untested. It has remained untested for a long time. For decades, some very bright minds have been barking up the tree, and no sign of a squirrel. I begin to think that maybe we should try other trees, because these really aren't getting us anywhere. There's probably a good argument to be made that continued fascination with this untested category of theory is getting in the way of finding a theory that does make testable predictions - that's not so much saying it is unscientific, as it is saying that it is unfruitful science.

Moving forward as if it were tested science would be unscientific. Right now, it is more like mathematical noodling about to see if there's something there we might use. So far, I am unimpressed with it as a physical model. Some very pretty math in there, though.

String theory as a theory of everything is guilty as charged. However, it seems people -- scientists, as well as non-scientist EN World members and the rest of society -- are genuinely interested in understanding what a complete theory of quantum gravity might be like. Every charge you levy against string theory is just as true for any theory of quantum gravity (or theory of everything) for exactly the same reasons --- the energy scales needed to make such tests are beyond human technology for the foreseeable future. And, by and large, the indirect tests people come up with for any other theory of quantum gravity look a lot like the tests for string theory. String theory has what is both the advantage and disadvantage of being mathematically rich, rigorous, and precise (when we have the skill to do it properly).

So I guess my question is, do you think it is unfruitful scientifically to think about quantum gravity or theories of everything? If so, that's just something we can disagree about. But if you think those are valuable questions, then it seems a bit silly to pick out string theory as being unfruitful, since it's been a lot more productive than other theories.

Well, yes, but that's kind of like saying that Special Relativity reduces to Newtonian mechanics for low speeds. Matching up with already measured cases is a prerequisite. There were many versions of string theories that got tossed out because it was found they *didn't* reduce to the known particle physics - I know of several such efforts that were stillborn and never published for this reason.

Actually, no, not at all. You seem not to have understood what I was saying, and I'll take the blame for a poor explanation and try again. What you're saying is applicable to string theory as a theory of everything. Any theory of our universe must surely reduce to what we already know, of course (well, or just be a toy model of some particular aspect). But, since string theory is a theory of quantum gravity, it can also describe other universes very different from ours. Hold that thought for now.

Something that happens to be true from time to time in physics is that two very different looking theories end up actually being the same. One way to think about it is that there is really one theory, but the two theories we know are different projections of that one theory, like the shadows being projections of the blocks on the cover of Godel, Escher, Bach. The shadows look different, but they represent the same thing.

One of the great surprises of theoretical physics at the end of the 20th century is that theories of particle physics that are similar to the theory of the strong nuclear force in our universe are actually the same as string theories in special universes that look nothing like ours at all! So when people were doing new nuclear physics experiments 10 or so years ago and not understanding some results, it was string theorists who were able to provide an explanation, since nuclear physics in our world is roughly similar to string theory in a very different type of universe. I'll repeat for emphasis: string theory as a calculational tool was the first way to understand real experiments in nuclear physics, which were at that time otherwise baffling.

And it's important to note that this is all true even if string theory is not the description of quantum gravity in our universe at all. It's a totally separate question.