First quantum measurement of gravity: What does it mean? (Patreon)

[This is a transcript with links to references.]

The great thing about physics is that it makes the simplest things mindbogglingly complicated.  Take space. What is it?  Well space is whatever is between here and there. Not that complicated. But but. Physicists say, space should have quantum properties. Space itself might be both here and there. And what the heck does that mean? Well, no one knows.

So much about the theory. But we also have experiments in physics,  and this new experiment which I just read about the other day brought us a step closer to figuring out what space is. Or have they? Let’s have a look.

Time is what prevents everything from happening at once, as John Wheeler put it.  And according to John Barrow,  space is what prevents everything from happening in Cambridge. I’m just back from a trip to Cambridge and for all I can tell the only thing that happens there is rain, but let’s not make things more complicated than they need to be.

Both Johns were referring to space and time in Einstein’s  good, old-fashioned theories. Einstein developed his theories before quantum mechanics ruined everything, and this is why they don’t contain stuff like the uncertainty principle or dead and alive cats.

In Einstein’s theories space and time are coordinate grids, basically.  They tell you when and where things happen, like rain in Cambridge, on Monday. Tuesday. Wednesday. You get the idea.

Einstein also told us, however, that space and time react to matter inside by curving.   Trouble is that this responsiveness of space-time doesn’t sit well with quantum physics, not at all.

The issue is that according to quantum physics particles can be in two places at the same time.  And particles have masses that curve space-time. So where do they curve space-time if they’re in two places?  Quantum mechanics doesn’t tell us because it knows nothing about the curvature of space time. And Einstein’s theory can’t tell us because it knows nothing about quantum physics.

 What we do know though is that the way that space works in Einstein’s theory can’t be right, because somehow we need to account for those particles which can be in two places at once.  To sort this out, we need a theory in which, hmm, space can be in two places at the same time, which, erm, makes no sense, and that basically is the problem.

 Since gravity is just the curvature of space and time, a theory that gives them quantum properties is also a theory of  quantum gravity.

Now I have worked on quantum gravity after I finished my PhD 20 years ago,  and back then everyone kept telling me that quantum gravity is basically philosophy because it doesn’t have any measurable consequences.  That’s because gravity is an incredibly weak force, compared to the other interactions.  Think about it, if you pin a magnet to your fridge, then the magnetic force of that tiny thing is stronger than the gravitational pull of the entire planet! And the forces inside atomic nuclei are even stronger, that’s why nuclei stick together, and good thing that they do.

And so the story went we can’t test the quantum properties of space, that’d require enormously high energies, a particle collider the size of the Milky Way,  that’s a common estimate.

But, I’ve tried to tell people, that makes no sense.  Gravity is different from all those forces because it can add up. It doesn’t neutralize like all the other forces do. That’s why we experience it so prominently in daily life. To measure quantum gravitational effects,  you just have to measure the quantum properties of objects that are heavy enough.

I no longer work in the field. That’s a long story, but I am super excited to see that there are now several experimental groups  trying to test quantum gravity.  And not with milky way sized colliders but in the laboratory.

This then brings me to the new experiment, because they have found a new way to measure very small gravitational forces.

You see the issue is that if you take elementary particles like those in the standard model,  electrons, quarks, muons, and all our best friends, their gravitational pull is so tiny we can’t measure it,  so you can’t test quantum gravity with them. If you take something heavy, like a planet,  then you can measure the gravitational field, alright, but you can’t measure its quantum properties.  That’s because normally quantum properties go away the larger the object. Unless you treat them very very carefully.

And that’s what they did in this experiment.  They put a tiny magnet into a superconducting container which is cooled to near absolute zero.  That superconducting container creates a magnetic field, and that keeps the magnet trapped. It levitates in the container,  and because it’s so carefully isolated, it can do quantum things like being in two places at once.

Have a look at their experiment.  It looks a little like the quantum computers at Google and IBM, doesn’t it?  These different levels you see here all noise buffers with these tubes belonging to the different stages of cooling.

 Then they take a fairly heavy weight, of about 2 point 4 kilogram, put it on a wheel, rotate the wheel and move it from one side of the other of this container with the levitating magnet.  The thing is now that the gravitational pull from the moving weight should affect the levitating magnet.  This will make the tiny magnet swing relative to the container with a frequency that depends on the position of the wheel. And that they can measure because it creates a current  for which they have a super-sensitive detector in the container.

The reason they put this weight on a wheel   is that this way they know the effect must come with a particular frequency, and that makes it easier to identify. It stands out against the noise basically.

 And what you can see here is that the motion of the magnet indeed responds  to the wheel being moved. The force that they measured was a tiny 30 attonewton.  That is not the smallest force ever measured. And that also brings me to the question, what do you want to do with it.

Remember that what we want to know is what the quantum properties of space and time are.  For this you need to measure the gravitational field of an object in a superposition of two places.  But this is not what this experiment measures. It measures the gravitational field of the heavy weight  with a quantum sensor, that’s the levitating magnet. This isn’t what you want to measure to test quantum gravity. Yes, it’s something with quantum and something with gravity, but that doesn’t mean it’s quantum gravity.

However, one thing that you can do instead of directly measuring the gravitational pull o a quantum object, is to measure the effect of the gravitational pull of two quantum objects on each other.  In a setup like this that basically becomes a question of how does the thing respond to vibrations, but I’m not sure how you’d extract quantum gravitational effects from that.

So, I still think that the best approach is that pursued by the group of Markus Aspelmeyer pull goes. in Vienna which tries to directly measure the gravitational  pull of small quantum objects by bringing microscopic sensors as close to these objects as possible.  Then again, it’s good to have a variety of experiments and whatever they’ll do next, I’ll let you know, so stay tuned.