You probably didn’t know that gravity is pretty strange. But it is. I should know; I’ve spent a significant portion on my life studying it. For a really short version of what I’m doing, you can watch this video (less than 3 minutes) right here!
I’ve been developing a torsion balance experiment to investigate gravitation at distances shorter than a centimeter. I’m sure you’ve got a few questions about that which basically boil down to “What?” and “Why?”
Let’s start with the “Why?”
Like I said, gravity is strange. You probably don’t believe me as, well, it was the first of the fundamental forces we discovered. I mean, Galileo was starting to work this out by dropping things off the Leaning Tower of Pisa, right? Right! And after a while Newton came along and actually wrote down his Universal Law of Gravitation (1687). Which worked amazingly well. Almost too well in fact. Which is why it takes a genius like Einstein to come along and shake things up.
In 1905, Einstein publishes the Theory of Special Relativity. This is basically a complete redesign of mechanics. Time and space start mixing. But this doesn’t even touch gravity yet. A decade later he finally publishes the Theory of General Relativity. Most people just call it GR. This is big. A bunch of little nagging issues start getting resolved because of it. And now, just over a hundred years later, we still keep finding things it predicts.
But sadly there are some issues. First off, there’s another BIG theory that’s pretty well tested: the Standard Model of Particle Physics. This covers the other 3 of the 4 fundamental force: Electromagnetism and the Strong and Weak Nuclear forces. Most physicists just call it the Standard Model. You know that giant collider they built a while back, the LHC (Large Hadron Collider? It is testing that model to insane precision. And it is holding up.
So what’s the issue? These two theories are inherently incompatible. GR is a classical theory. And when you try to mesh it with the quantum world… things don’t work right. Not only that, but gravity is really really weak compared to the other fundamental forces. This is an issue totally separate from the incompatibility and is called the Hierarchy Problem. Some proposed ideas for fixing one of the issues also tries to fix the other.
These ideas include things like extra spacial dimensions. Or new fundamental forces. And some of these ideas would lead to strange things happening on relatively ‘large’ interactions. Like the width of a human hair or larger. But they are so weak that when things happen at places like the LHC that they wouldn’t really show up. Or if they would, many of these effects are so small that we’d need even larger amounts of data than we already have to find them peaking out of the noise.
So that’s where our “What?” comes in. We are building a torsion balance experiment to look at these distances a lot closer.
A torsion balance is a mass of some kind (ours is a disk) that’s suspended by a thin fiber. The fiber is what makes this whole thing work. It is like a spring but instead of being pushed or pulled, it gets twisted. Similar to how a spring scale works, if the mass is pulled or pushed in one direction of rotation, the fibers pushes back. By knowing the strength of the spring and the amount of deflection you can find out the force (or a torque, with the torsion balance). Our experiment consists of a rectangular fiber holding up a think aluminium and quartz disk. It gets very close to a much much larger copper disk that gives a nearly uniform gravitational field.
What’s a uniform gravitational field? You already know, as you basically live in one. The Earth’s gravitational field is fairly uniform at the surface, at least on the scale we are used to. If you ride an elevator up a really tall building, you wont notice a drop off in your weight. In order notice deviations, you’d have to be really large of have very precise measuring devices. Actually, a man by the name Eötvös built a torsion pendulum device specifically for that purpose, because you can find deposits of heavy metals or even oil by looking at those deviations.
So what does that have to do with this? Well, we exploit the idea that gravity is an inverse square law force to create this uniform field. If gravity is how we think it is, then moving this larger disk around shouldn’t affect the smaller one at all. The forces on this disk may change direction or magnitude but will always stay equal on the disk (thus applying equal magnitude but opposite direction torques).
But what if gravity isn’t exactly what we think? What if there are some crazy extra dimensions or other forces? Then when we get to distances near enough on their scale, we should start seeing their effects on the smaller disk. It should start moving in response to the larger disk’s motion .
Now all of this has been grossly simplified, and I’ve glossed over a lot of ways we’ve taken precautions to make sure any other effects don’t come into play. But that’s the gist of it.
Above is a general diagram of the experiment. You can see the large copper disk with some of its support structure, the torsion balance composed of the test mass suspended by the torsion fiber, the vacuum chamber surrounding all of this and outside the chamber is our autocollimating optical lever which allows us to observe the motion of the test mass.