illuminating science

The European Space Agency announced today a new test of the final not-quite-verified predictions of general relativity - gravitomagnetism, or as its more commonly known frame dragging (which is actually a subset of gravitomagnetism). It’s based on the idea in general relativity that mass deforms space, and that’s what produces gravity. The old analogy is placing a bowling ball on a trampoline - it warps the space around it, and so a golf ball placed nearby will roll towards it. Of course, that’s just an analogy, but the theory is much the same idea, at least mathematically. Frame dragging builds on this and says that as body moves and particularly rotates, it can “drag” space with it, affecting other objects in a way we wouldn’t expect from our traditional picture of gravity. It’s something akin to rotating a spoon in a jar of honey - honey far away from the spoon will be dragged around by that motion, albeit slowly (but again, this is just ana analogy!) Experimental evidence from satellites suggests strong agreement between general relativity and experiment, although the big test will be the results from Gravity Probe B which will hopefully be not too long away. In both these cases, because the frame dragging effect is so minisculy small, long term experiments in orbit were needed to observe the effects.

This recent experiment, by Martin Tajmar, looked at effects near to a rapidly spinning and accelerating superconducting disk. They placed acceleration sensors near it and detected forces which they believe are due to the gravitomagnetic effects of the rotating disk. What’s most surprising, however, is that the effects they measured were one hundred million trillion times larger than Einstein’s General Relativity predicts (NB: I can’t find this in their papers - just the article). The question, of course, is whether you really believe their data or, alternatively, their analysis. They say that they performed over 250 experiments and argued and checked their results over three years, but at the same time, for general relativity to be wrong would be an incredibly surprising result, given its great success. Their signal to noise ratio (from their paper) is 3.3, which seems quite low (as in, their signal is only three times larger than the noise they’d expect from the experiment, possibly making it hard to extract the true data) but they approach this cautiously, and agree that it needs to be repeated.

But at the same time, that would be an incredible breakthrough in our understanding of the universe. Yes, I know that sounds odd, but in some ways we will learn more from this experiment if it disgrees with general relativity (and hence sparking huge amounts of research and funding) than if it confirms what we already “knew”. I think many (most?) scientists suspect that general relativity isn’t the complete truth, but the thinking is usually that the answer will appear at the highest energies of your particle accelerators, rather than in a “simpler” lab setting. As they claim, it would open up a whole new way of experimenting with the quantum/GR boundary. I think that, if I had a choice, I’d be hoping that general rel doesn’t work out here - it would be quite exciting!

Incidentally, this result came from (and overshadowed) their attempts to explain the discrepency between experiment and theory of the mass of the “Cooper pairs” in superconductors (Niobium, mainly), these being the pairs of electrons which enable current to flow without resistance. I’m just not sure from the article whether they succeeded with this or not - though looking at their paper seems to support it. At the very least, the trends they observed for the gravitomagnetic effects are apparently in agreement.

If you’d like more info, you can check out either the above paper on the mass of Cooper pairs, or this paper where they discuss the implications of these experiments for the mass of gravitons, particles which are theorised to transmit the gravitational force. Their main result there is that the current values of the cosmological constant (related to “dark energy”) and gravitons predicts massive gravitomagnetic fields, which aren’t observed (but how this fits with his statements that the observed fields are much larger than general relativity, I don’t know!) They argue that it can be explained by allowing the local “energy density” (literally what it sounds like - how much energy you pack into one spot) controls the “vacuum energy”, meaning that we could engineer and control the vacuum using coherent matter (like superconductors, etc). Unfortunately, I’m in no position to debate these conclusions!

The papers are relatively readable, at least in the introduction and conclusion (and provided you don’t care about the maths!) but I have to admit that some of it sounds slightly far fetched. I won’t say crackpotty - perhaps this is just how all cutting-edge fundamental physics papers look to the non-expert. But certainly the first line of their last paper is a hard sell - “It is well known that the mass of the photon and graviton in vacuum must be non-zero…of about 10^-69kg” Woah!