THE BOUNDARIES OF QUANTUM PHYSICS
Jeremias Pfaff speaks to All About Space about what it takes to measure gravity on such a minute scale. The next step? To go even smaller
Why can’t physics describe how gravity works at a subatomic scale?
It might be able to, we just don’t know yet. That’s why we did this experiment, because it’s mainly an experimental issue. Firstly, it’s just really difficult to measure gravity. It’s such a small force; you can imagine the gravity between the two spheres that we measure is 30 billion times smaller than the gravity that Earth pulls on one of the spheres. Just measuring that small force is tricky because you also have to shield it from all the other forces, such as magnetic and electrostatic, that are much stronger. If you can imagine a paperclip, the whole Earth is pulling on it, but just a little magnet is able to pull it up against the gravity of Earth – that’s how much stronger the other forces are. It’s just really difficult to measure on a small scale, so nobody has done it yet.
We know how gravity works. Newton and Einstein were mainly concerned about astrophysics, or the movements of the planets on big scales. However, on a smaller scale the quantum theory came along, and it describes the world in a very different way. We are going in the direction of measuring smaller and smaller objects with this experiment. But when we get to even smaller objects, the position of the object is not defined anymore by quantum physics. You might have heard of this Heisenberg uncertainty, where quantum objects can be at two or more positions at the same time. The theory of gravity from Newton and Einstein just does not account for this kind of thing. If you want to describe gravity, you must know the position of the object at all times. For small things, as far as we know through quantum mechanics, this isn’t true. So what does the gravitational field look like for small things? We have no idea. The answer must be experimental, I think.
Why is it so important to understand gravity on such a small scale?
That’s difficult to answer, because important means different things to different people. I think that there is a human curiosity to understand the world that is surrounding us, and that transcends the scientific community. For me it is just a very curious thing. However, if importance for you is the impact on your daily life or something like that, then I think that a better understanding of our world will at some point also have impact in our daily lives. Einstein, for example, didn’t imagine the navigation systems on your phone, but it wouldn’t work without his theories. At some point I think it will affect our daily lives, but it’s not my main motivation. It is just my curiosity that drives me to understand how gravity works at small scales.
How did you measure gravity on a small scale?
It’s an extremely simple principle. We just have an oscillator-like pendulum, a bit like a ball on a string, and this pendulum is suspended against the Earth’s gravity, so it is free to move. If you now bring another object close to it, that will exert a force on this object. It will feel it and might move a little bit according to this force.
For example, if there were two magnets they might attract or repel. The same is true for any other force, and we wanted to measure gravity. To do this we used a special pendulum, not just a ball on a string, but more like a dumbbell shape with a stiff rod and two gold spheres on the end. This torsion balance idea goes back to experiments more than 200 years old by Henry Cavendish and other scientists at that time, who wanted to figure out the density of the Earth.
What’s cool about the torsional balance we used in our experiment is that it won’t rotate to all the other fields that are exerted by the environment. The horizontal bar with gold spheres on either end will only move when another gold sphere is moved
closer to the horizontal axis of the bar, generating the gravitational force. We move the gold sphere close to the others and can modulate the force, and this will start to oscillate at the same frequency.
When you oscillate the object acting upon the torsion balance, is it easier to pick up a signal?
Yes, definitely, because we can define a certain frequency, and this frequency will show up in our data. If we just had a static signal where we move one sphere close to the other sphere, the pendulum would then slowly turn to this mass. This kind of experiment is also carried out by scientists, but the problem is that noise, such as urban noise, can get in the way of the static signal, whereas the oscillating signal with this frequency will be more dominant compared to the noise.
What challenges did you face? Does the environment influence the measurements?
That’s why the torsional pendulum is so important to us, because if we just had a ball on a string we would have had even more environmental influences. For example, the local tram in Vienna will exert the same force as our tiny gold ball, even though it’s 70 metres [230 feet] away. We could even see the vibrations of the Vienna City Marathon. The cool thing now is we don’t have a linear oscillator, we have a torsional oscillator.
If you imagine a huge city tram 70 metres [230 feet] away, it will not cause the rotation, but it will cause the pendulum to swing. Only the small sphere close to the pendulum will induce the rotation, so this design choice allows for a very good discrimination between local forces very close to this torsion balance and all the other environmental effects. Even if you think about seismic effects, the Earth moving will introduce up-and-down motion, it will introduce side-to-side motion, but it won’t introduce the rotation.
We have also put a huge amount of work into getting rid of surplus charges on the pendulum because the electrostatic force is so much stronger than the gravitational force. There is also a tiny electromagnetic shield between the pendulum and the moving mass.
The experiment also had to be carried out in a vacuum chamber, otherwise the air molecules wouldn’t allow for this torsion motion. The torsion motion is always bouncing against the air molecules, which would dampen the movement of the pendulum, so we have to pump out as much air as possible. Everything has to be super sensitive.
Did you find gravity at this scale acted differently to larger scales?
No, gravity did work as we would have expected by Newton and Einstein. We didn’t see any relevant deviations.
Does this mean you have to go smaller?
Yes, for us this was a proof-of-concept experiment. It’s already smaller than everything that has been done so far for the smallest source mass. But our goal is to go much smaller, and already the next step will be a factor of 1,000 times smaller than this experiment.
This is interesting because a lot of the theories of dark energy or dark matter foretell deviations from Newtonian gravity. If we see these deviations that will of course be super spectacular, because one of these theories then might be right, and might
give clues for us to further develop it. Or if we don’t see any deviations, you can cross off a lot of these theories and invest your time into something more interesting.
Why did you use gold in your experiment?
If you look at gravitational laws, the denser the material, the more mass you can put in a specific size. Gold has a high density, is widely available and you can work with it easily. Making a small sphere out of gold is nice and easy.
There are a few denser materials, like osmium, for example, but it is also harder to get hold of and a lot harder to mould into a sphere. A lighter material would make our spheres bigger, but then they couldn’t get as close, so the force would be weaker. That would make the experiment harder than it already is.
Can your research be used to shed light on other mysteries in our universe, like dark matter and black holes?
That’s what we hope for. A big motivation to do this is getting the first evidence of what the gravitational field of a quantum object could look like and bridge this gap between quantum and classical theory.
But let me stress there is still a long way to go, even with reducing the size of the spheres by a factor of 1,000 for our next experiment.
There are still many more years to go before we can control large enough objects in a quantum way by forcing them to be in two positions at the same time and measuring the gravitational field of this strange thing.
But as you mention dark energy and dark matter, these are also huge mysteries that need explanations. A lot of these theories predict that there are deviations from Newtonian gravity on smaller scales – on smaller mass scales and smaller distance scales. As we get closer, we can look into these theories and provide evidence for or against them. Of course, finding deviation from quantum gravity would be a spectacular result, but also excluding these theories would be helpful to understand mysteries like dark energy and dark matter.
Scientists are also looking at astronomical evidence for clues for dark matter and dark energy theories by working on getting quantum control over larger and larger objects. They are working from one side, getting the quantum world to bigger scales, and we are working from the other side, measuring gravity on smaller and smaller scales. If you want to bridge the gap between quantum theory and gravity, I think this is the way to go.
A big motivation to do this is getting the first evidence of what the gravitational field of a quantum object could look like and bridge this gap between quantum and classical theory