Stopping molecules

Standing still is progress

RUG professor Steven Hoekstra is finishing up the molecular decelerator that he built himself. In his search for the shape of electrons, he uses the machine, which is four-and-a-half metres long, to make super fast molecules stand still. That might not sound very exciting, but it could have major implications for how we see the universe.
By Thereza Langeler / Photo by Reyer Boxem / Translation by Sarah van Steenderen

Physics professor Steven Hoekstra and his team are putting the finishing touches on the molecular decelerator they built themselves. It will be able to bring super fast molecules to a standstill.

When the molecules have stopped moving, Hoekstra can take precise measurements. He will be studying the shape of electrons contained in the molecules.

It is possible that electrons have a different shape than was previously thought: they may be elliptical rather than round.

If that is true, that means the standard model of particle physics will need to be added to. This model is the theory that describes the behaviour and characteristics of all matter.

Stopping the molecules requires an advanced technique. Electrical fields gradually slow down the molecules.

This is a technically and theoretically difficult process. Hoekstra has been working on it for years. But that is part of the job, he feels. ‘Running into problems is only normal when you’re trying to do something that’s never been done before.’

Reading time: 6 minutes (1,184 words)

The machine, built by the professor of atomic and molecular physics, would not be out of place in a Star Wars film. It is a four-metre long steel contraption, covered in tubes and gauges, pumps and panels with buttons. At the back, which is normally hermetically sealed, we catch a glimpse of the intricate ring construction within. One can almost envision the machine shooting lasers while ominous orchestral music plays.

But this machine is not a prop from a science fiction film. It is a particle decelerator, also known as a Stark Decelerator. It has been specifically designed to bring barium fluoride molecules to a standstill. This is not an easy job, because in their natural state at room temperature, the molecules travel through space at 300 metres per second – roughly 1,000 kilometres per hour.

The device is currently under construction: another 50 centimetres still need to be added. ‘We can only decelerate the molecules very gradually’, Hoekstra explains. If they are stopped too abruptly, it would have the same effect as when a driver, not wearing his seatbelt, crashes his car into a tree while driving at top speed.

Stretched electrons

That means the decelerator needs to be of a certain length for the experiment to succeed. ‘We calculated that length at four-and-a-half metres. So far, our calculations have pretty much matched what’s happening inside the machine. So yes, we’re fairly certain that we will succeed.’

Hoekstra and his colleagues at the Van Swinderen Institute for Particle Physics and Gravity have built the decelerator themselves. They will also attach the final part themselves. The lab is full of nuts and bolts, and a manual is tacked onto a notice board. There are 21 numbered steps, and step 22 is ‘done’.

Once that step is finished, the machine will be ready to make the lightning-fast barium fluoride molecules come to a stop. Stopping the molecules is done with a clear goal in mind. Hoekstra wants to perform extremely precise measurements of the molecules. To be more exact: he wants to measure the electrons in those molecules. That is because there is a possibility that those electrons are not round, as has been the assumption among physicists, but slightly stretched.

In a manner of speaking, that is. ‘We’re talking about something abstract, something invisible, here. Strictly speaking, electrons don’t have a shape at all. But round versus stretched is a good way to visualise it.’

Small cracks

The asymmetry is basically due to how the electrical charge is distributed over the electrons. It is probably slightly unequal. Hypothetically, there could be a slightly larger positive charge on the one side, and a slightly larger negative charge on the other. Officially, this is called the ‘electron’s electric dipole moment’ (eEDM).

Hoekstra suspects that it is because of that eEDM that the electrons are elliptical rather than a perfect sphere. So what, you might think. But these small electrons are very important. If they do turn out to have this eEDM, it would mean that particle physics’ standard model would have to be expanded.

‘Symmetry is very important in that standard model’, says Hoekstra. ‘It is a kind of theoretical construction that encompasses all the particles and the forces between those particles. We should, in principle, be able to explain everything we observe using the standard model of particle physics. ‘

Emphasis on ‘in principle’. Because in reality, the theoretical construction is showing some small cracks, for example where it concerns the shape of electrons. But there is an even larger crack in the theory, and it is that crack that Hoekstra is researching: the relationship between matter and antimatter.

In a flash

‘Each particle has an antiparticle with opposite characteristics. The negatively charged electron, for example, has a positively charged antiparticle, called the positron’, Hoekstra explains. ‘If a particle and an antiparticle collide, there is a flash and both particles disappear.’

According to the standard model, the big bang should have created practically equal amounts of matter and antimatter, but that cannot be right, because that would have only created light. It would seem, then, that the standard model is incorrect in this matter.

Some people have proposed additions to the standard model: bits of theory to spackle over the cracks. Some of these theories do correctly predict the matter/antimatter ratio and leave room for an electron that is elliptical rather than round. In other words: if Hoekstra and his colleagues find an asymmetrical electron, they will know for certain that something is missing from the standard model.

‘It’s very exciting research’, says Hoekstra. He is not the only who feels this way. Starting in January 2017, the research programme began using a 2.7 million euro subsidy from the Foundation for Fundamental Matter Research (FOM). ‘It’s about the Big Questions. How does the world work? For a lot of people, that’s the ultimate motivation to start studying physics: to figure out how the world works.’

Completely new

With big questions come big challenges. For Hoekstra, the enormous speed of the molecules present the biggest challenge. In order for him to take very accurate measurements, the molecules have to stand still. And that is easier said than done. It took Hoekstra years to get where he is now, years during which he had to solve all kinds of theoretical and technical problems.

For example, existing deceleration methods were insufficient. ‘We can fairly easily make atoms stop moving by cooling them down with lasers’, Hoekstra says, who obtained his doctorate by decelerating atoms. ‘But a molecule has a much more complicated energy structure than an atom. While that means it’s more versatile, it also means that laser cooling is ineffective.’

Therefore, Hoekstra and his colleagues had to use something else to slow down their molecules: electrical fields. ‘It enables us to exert force on molecules, just like a magnetic field exerts force on magnets.‘ The electrical fields gradually counter the molecules movements until they have come to a stop.

What Hoekstra and his colleagues are doing is completely new. ‘That makes it very exciting, but it’s also complex. There’s a lot of theory involved.‘ On top of that, Hoekstra and his team faced technical problems with the decelerator, which they had to fix before they could continue construction.

Goal in sight

‘In reality, research can sometimes be a bit slower than we’d like’, says Hoekstra, in typical Groningen understated fashion. ‘We’ve got one particular goal in sight, but we can only get there one step at a time.’

However, that does nothing to dampen his spirits. ‘Figuring things out, always running into problems and solving them, that’s what lies at the heart of science. If you’re discouraged by every single complication, you shouldn’t be a scientist. We’re trying to do something that’s never been done before. Running into problems is only natural.’

The larger decelerator will be ready for use in a few weeks. After that, Hoekstra can put a few molecules in there and – if everything goes according to plan – bring them to a standstill for the very first time. Until then, he and his colleagues will continue building the machine bit by bit – until they can finally put a check mark behind step 22.


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