Starts With A Bang! » String Theory Ethan Siegel's blog/video blog about Cosmology, the Universe, and everything else Sat, 04 Apr 2009 20:12:38 +0000 en Is the Universe a Giant Hologram? Wed, 11 Feb 2009 22:21:39 +0000 ethan Some days the questions I get are easy, and some days I get questions from our longtime reader, Ben. This past week, there have been reports all over the news that our world may be a giant hologram. Let’s take a look at what’s going on.

In Hanover, Germany, there’s an experiment called GEO600. These are two perpendicular lasers, and they shoot out for thousands of feet, get reflected, and come back to their original location to make an interference pattern.

Now the reason this is important is because gravitational waves cause ripples in space in a certain way. These perpendicular lasers are particularly sensitive to what gravitational waves do, and the interference pattern will shift in a very particular way if gravitational waves pass through them. This is the same idea that’s behind the upcoming LISA mission.

Now, GEO600, like every laser interferometer we’ve ever built, has not seen any evidence for gravitational waves. But it has seen something that it can’t explain, and that’s always interesting for an experiment.

It found some extra noise, above and beyond what can be predicted/explained by things like the vibrations of the Earth, temperature fluctuations, or instrumental noise. What does this look like? Whenever you do your experiment, you do your best to understand what noise you expect to see, and then you look for deviations from this. GEO600 saw something like this:

So there are two possibilities now: either there’s a source of noise they haven’t figured out, or something physically interesting and novel is causing this. Now, historically, whenever experiments are done, it’s almost always unexpected noise that causes something like this to happen. But once in awhile, there really is a new effect that we have going on.

It’s very important to state, clearly and unambiguously, before we go any further, that this may simply turn out to be noise. This may not be a physical effect at all, and that no other similar experiments (such as LIGO) see these effects.

But if it is a physical effect, Craig Hogan of Fermilab has come up with an extremely interesting possible explanation. He says that this excess noise could be a sign that our Universe has an extra dimension to it. How does this work? Let’s think of a hologram:

A hologram has all the information you could ever want about the dimensions of a 3-D object, but it has it all in two dimensions. For instance, you could tell some object’s (or someone’s) length, width, and depth just from looking at a 2-D hologram. All of the information is encoded in there.

Well, our Universe may be the same exact way. We know about our 3 space dimensions and our 1 time dimension. But we may have more dimensions of space than we know about; many interesting theories have them. One possible consequence is that these extra dimensions could cause extra “blurring” of our 3 regular space dimensions at very small lengths.

Now, this is very interesting, because the noise we see in the GEO600 experiment causes the laser light to blur on scales of about 10-16 meters and below. This is smaller than the size of a single proton, but amazingly, our technology is sensitive enough that we can detect it! But is this blurring due to extra dimensions? Let’s see what the people connected with the experiment say about Craig Hogan’s idea:

However Danzmann is cautious about Hogan’s proposal and believes more theoretical work needs to be done. “It’s intriguing,” he says. “But it’s not really a theory yet, more just an idea.” Like many others, Danzmann agrees it is too early to make any definitive claims. “Let’s wait and see,” he says. “We think it’s at least a year too early to get excited.”

The longer the puzzle remains, however, the stronger the motivation becomes to build a dedicated instrument to probe holographic noise. John Cramer of the University of Washington in Seattle agrees. It was a “lucky accident” that Hogan’s predictions could be connected to the GEO600 experiment, he says. “It seems clear that much better experimental investigations could be mounted if they were focused specifically on the measurement and characterisation of holographic noise and related phenomena.”

So it looks like this is worth further investigation, but it’s way too early to draw any definitive conclusions. But it’s a possibility, and for something as grand as this, for something that would forever change the way we view our Universe, I think it’s worth investigating further, and so does the entire GEO600 team.

What do you think?

Trying to Understand Gravity Mon, 29 Sep 2008 21:26:14 +0000 ethan Sure, gravity sounds like a pretty simple idea, now that we’re used to it. But, how does it work?

Think about it for a minute. What is gravity? It’s the idea that anything at all, with any mass or energy at all, in the whole Universe, is attracted to everything else with mass or energy in the Universe. This is true for familiar things that are near, but not touching Earth,

and it’s also true for things that are on (and touching) the surface of the Earth,

and it’s even true for objects that have nothing to do with the Earth at all:

But how does this work? Or in other words:

How can two things that don’t come into contact with one another exert a force on one another?

Newton didn’t know the answer to this, and he made up the phrase “action-at-a-distance” to explain it.

Nice try, wig-boy. I know that giving something a fancy name doesn’t actually explain what it is! (As an aside, that’s exactly what we’ve done now, over 300 years later, with dark energy.) So we go from wig-boy to wall-socket licker:

And this time, we actually get a deeply profound answer: all objects with mass and energy are connected through spacetime. So the Earth bends space around it, and that’s why things that are closer to the Earth are more attracted to it.

This same principle works with everything, including the gravity from the Sun, and even light from distant stars! If you look up at the sky near the Sun during a solar eclipse, you will find that stars are out of position, because the Sun’s gravity even bends starlight!

And so now, 400 years after we tried to answer the question of how gravity works, we realize that we still don’t have an answer for what happens at very small distances. This is what people working on quantum gravity are trying to discover, and honestly, this is the big hope of people who work on string theory: that strings will solve this problem.

Will it? It hasn’t so far, although no other solution looks promising. Any ideas as to how gravity really works? Hopefully, we’ll find one theory that explains it successfully for both strong and weak fields, and for small and large distances.

On String Theory from a String Theorist Fri, 18 Apr 2008 17:01:36 +0000 ethan Bret Underwood, a friend of mine from my time in Madison, WI, saw my post on String Theory, and took issue with my statement that it wasn’t testable. I’m still standing behind what I said, but let’s address what Bret has to say.

I don’t understand your argument above for why string theory is untestable. In fact, it seems to me you just outlined the best possible case for string theory! What you said above is that if I have a string theory construction of a phenomenon (say, the Standard Model or Inflation), which uses a set of parameters X, and makes some predictions, then I can find another set of parameters Y that gives a different set of predictions. Wonderful! This means I can determine the parameters (either X or Y) by making measurements, and rule out models and parameter sets!

I don’t think that’s wonderful at all; I think that’s a very dangerous analogy. Why? Because string theory has too much “wiggle room” to be scientific. String theory isn’t at all predictive in this sense. How many possible values for the string vacua are there at this point? The typical estimate that people cite is about 10500, according to the string landscape. For comparison, the number of subatomic particles in the entire Universe is somewhat less than 1091. There are far fewer than 10500 parameters describing our entire Universe, so people argue that one of those models is likely to match with the Universe. And when we find out which one it is, then we can figure out everything.

But that isn’t a scientific theory to me, not as I understand science. A scientific theory makes definitive predictions that are unique. But string theory doesn’t do that; at least, not for any prediction that I know. Most models of string theory predict a negative cosmological constant, for instance. But we don’t say that this means string theory is wrong, we say that this rules out those models, because we see a positive one. String theory gives you a 10-dimensional Brans-Dicke theory of gravity; we observe a 4-dimensional one with a Brans-Dicke parameter of infinity. But somehow, we don’t say string theory is wrong, we say that we just need to get rid of 6 of those dimensions and make that parameter be infinite. We don’t say how, we just say somehow. Most models of string theory predict small tensor modes, but most models of inflation do, too. What does string theory predict that’s unique to string theory? Other than “everything is made of strings,” I don’t know of one. Until there is one, and one that can be tested, I can’t be comfortable calling it a scientific theory.

The nightmare scenario for me is the following: suppose I describe a phenomenon (Standard Model or Inflation) in string theory with a set of parameters X, which gives some predictions. Then I consider another different set of parameters Y that give the same predictions. Thus, predictions are not unique, so I cannot distinguish models!

But what’s even the point of distinguishing models if there’s no observation I can make or experiment I can do that validates string theory? Maybe I can find one (if you’re lucky) or more than one (if your nightmare comes true) set of parameters that agree with all the laws of physics in our Universe. But what can you tell me that I don’t already know? That’s what I mean by untestable.

I’m not saying that people who are interested in this shouldn’t work on it. But I’m saying that if string theory is going to bill itself as being science as opposed to mathematics, it needs to address the issue of “what does it predict that nothing else predicts?” I don’t know of any test that’s ever been devised, even in principle, that can test it. Do you?

FYI: I downloaded a talk by Michael R. Douglas at Rutgers, a string theorist who’s actually optimistic about string theory being a predictive theory. He asks the question: “Are there testable predictions of string theory?” He says yes initially, but then admits the following:

…none of the ideas which have been suggested so far are guaranteed signatures of string theory.

Yikes. Sorry, Bret, but that means I place it into the category of an untestable hypothesis at the present time. If you can’t validate it and you can’t falsify it, it isn’t yet a good scientific theory.

Q & A: On String Theory Mon, 14 Apr 2008 17:08:29 +0000 ethan Over the past few months, I have been asked a number of questions about String Theory and the Universe, including from readers Benhead and Mastery Mistery. But now Jamie, whom I’m going to marry later this year, has been asking me about it, and so it’s time to write something about the scientific topic of String Theory. (Send in your questions now, because I’ll answer them all this week if there’s enough interest.) Let’s start with this pair of questions:

String theory has been around for over 20 years, and so far, there is not one shred of experimental or observational evidence in support of this theory. Is it even a scientific theory at this point, and why do people still care about this at all?

First off, this hypothesis is that all the particles of matter that we now call “fundamental,” such as quarks, electrons, neutrinos, and photons, are actually different vibrational modes of the one truly fundamental thing: a string. The theory says that some strings are open like a jumprope, and some strings are closed like a loop, and the different vibrations make up everything that we see today.

Let’s get to the first part: is it even a scientific theory at this point? Well, part of a scientific theory is that you have a hypothesis, and we’ve got that. The next part is that you have to either devise a way to experimentally test the theory or observationally test the theory. This is where the alarm bells go off. String theory has so many free, unconstrained parameters (literally, hundreds) that as far as being able to make scientific predictions as to the outcomes of various experiments or observational tests, it has never been able to definitively make one. Why not? Because for every value of these parameters (known as string vacua) that I can choose that predicts one thing definitively, there are other values I can choose that will predict the opposite. Since we don’t know what rules these string vacua follow, we can’t make predictions. All we can do is rule out some range of values for some parameters. So at best, what we’ve got with string theory right now is an untestable hypothesis, but nobody’s going to fund you if they ask you what you work on and you say “I work on the Untestable Hypothesis of Strings.” But that’s what it is. Or, to quote xkcd:

So this brings us to the second part: why do people still care? Well, as far as understanding how the Universe works, we’ve got the quantum world on one hand (left), and gravity on the other (right). We don’t know how gravity works on quantum scales. I’ll say it again in a different way, because that is the really important reason behind all of this. We know how gravity works on terrestrial scales and up, but experimentally, we only know how gravity works on scales down to about a tenth of a millimeter. What happens to gravity on atomic or subatomic scales? Not only don’t we know experimentally, but we don’t have a theory for it, either. String theory, it is argued, is the only way we know of to approach this problem. And it is a self-consistent mathematical framework for approaching this problem. And that’s its value, and that’s why people care.

But is that enough? What do you think?