American Scientist

Is there a fundamental unit of space, and hence a baseline graininess to the universe? If so, that limit caps the total possible amount of information the universe can store. It also has a weirder implication: The three-dimensional reality we perceive might be an illusion—a projection of space similar to a hologram, which is actually encoded in two dimensions. As bizarre as those ideas may sound, they are actually testable. Aaron Chou, a physicist at the Fermi National Accelerator Laboratory in Batavia, Illinois, is the lead scientist and project manager for the Holographic Interferometer, or Holometer for short. Chou explained to Managing Editor Fenella Saunders how the instrument may help answer some of these questions.

What is a holographic interferometer and what are you trying to find?

It’s two 40-foot-long laser interferometers, which make extremely precise measurements of the relative positions of different objects, in particular the relative positions of the mirrors inside these devices. Interferometers provide the best resolution of any instrument because they use billions and billions of photons, so you can make that measurement over and over again. We’re using about 10²² photons per second. We’re resolving positions to about 1,000 times smaller than the size of an atomic nucleus.

We are trying to see if there is any ultimate limit in the precision that one can possibly make in this kind or any other kind of measurement. There are some ideas originating from gravitational physics and quantum mechanics that imply that the information storage capacity of the space-time itself that we live in is finite, that it’s just like in a hard drive or a memory stick, there’s a maximum amount of information that you could possibly pack in. If you try to measure it better than that precision, you won’t be able to because space doesn’t have any more digits to give you.

Can you describe how this limit relates to the 3D nature of the universe?

The information content of all the matter that you throw into a black hole would end up being stored on the surface of the black hole. The bizarre thing about black hole physics is the information content is not proportional to the volume of the black hole, but rather just the surface area. This is called the holographic principle; it’s an analogy to a hologram in which you store an apparently 3D image on a 2D surface. But if you scatter light off the surface in a particular way, the pattern of the scattered light apparently reconstructs the three-dimensional image. This is similar to what’s believed to happen when objects fall into black holes, that somehow the three-dimensional information of the object that fell in gets transcribed and encoded on the two-dimensional surface of the black hole.

How does that affect the information storage limit of the universe?

Say you start with a situation where you don’t have any black holes at all but you have a bunch of memory sticks, and you say well, gee, I don’t have enough storage here in the memory sticks in my backpack, and I need more storage capacity. I’m going to go to buy a bunch more memory sticks, cram them all in my backpack and then I’ll have more storage capacity. Eventually what happens is if you’re super strong, like Hulk strong, you cram all those memory sticks in, and the density of all that matter inside your backpack gets so large that you form a black hole. You might say, well, that wasn’t very good, but no matter, I’ll go buy some more memory sticks. But then you see to your dismay that instead of being able to cram more information into this backpack-shaped black hole, the black hole grows, so the actual density of information storage you have in your backpack stays the same. That’s kind of what we mean when we say that it could be that space-time itself has some maximum information storage capacity. Once you reach the black hole limit, if you try to cram in more information, it just takes up more space.

How would this limit be reflected in the Holometer measurements?

There’s a prediction that if space runs out of digits, it gives you the same kind of error in the measurement of the two devices that are situated close to each other. Your measurements start being correlated at that point for no reason.

If you do find this limit, does that mean that 3D is a construct?

If it turns out to be true that the information is stored on two-dimensional surfaces like in a hologram, rather than in three-dimensional volume, I think that it would be a very interesting curiosity, and maybe it would lead us down to other paths of thought and study. I don’t think it really affects our everyday three-dimensional lives. One could ask if you find it disturbing that all the instructions for constructing a person or an animal or a plant could actually be encoded in one dimension using an alphabet based on four different letters.

What’s your timeline for figuring out whether there is a limit?

We have recently commissioned our detectors to be operating at full sensitivity, so we are beginning to collect data. We’re expecting to have reportable results on a one-year time scale. Any time you operate an instrument at greater sensitivity than you have ever done before, you’re going to find all sorts of problems, so at that point you enter in a long debugging phase to make sure that if you do see something that looks a little bit odd, that you really figure out what it is, so if we do see this unexpected limit on the precision of our measurements, that we can confidently draw some sound scientific conclusions from it.

How do you handle the pressure that you might not find anything?

One of my favorite professors from graduate school described basic research to us as it’s like going out for a midnight swim in the ocean. Maybe by chance you’re going to bump into something floating out in the water, but most likely it’s just going to be perfectly clear waters. But that doesn’t prevent you from wanting to go out for a midnight swim. A lot of the fun and the excitement is in the process itself. I personally think of each experiment I work on as a lottery ticket. The probability that I’m actually going to find anything in any particular experiment is pretty small. But on the other hand, it’s not vanishing. All of these experiments I work on have a very good theoretical motivation. So if we do find something, life is really good.