Theoretical physicist Anastasia Borschevsky and experimental physicist Steven Hoekstra. Foto Reyer Boxem

A fertile mix of theory and practice

Opposites solve the mystery of matter

Theoretical physicist Anastasia Borschevsky and experimental physicist Steven Hoekstra. Foto Reyer Boxem
Experimental physicist Steven Hoekstra likes to work with his hands. Theoretical physicist Anastasia Borschevsky creates computer models. But theory and practice need to work together to solve the burning question of how matter can even exist.
By Laura Nederveen
10 November om 16:57 uur.
Laatst gewijzigd op 22 November 2020
om 16:16 uur.
November 10 at 16:57 PM.
Last modified on November 22, 2020
at 16:16 PM.

There was no way that Anastasia Borschevsky was ever going to work in a lab. She knew early on in her studies that she was made to be a theorist. She prefers to create computer models of atoms and molecules to predict the behaviour of electrons. 

There was no way that Steven Hoekstra was ever going to sit behind a computer to do calculations. He knew early on that he wanted to work with his hands. ‘I like to do experiments and to play around. Making sparks and experimenting with lasers and seeing what happens if I do this or that.’ 

Experimental physicist Hoekstra and theoretical physicist Borschevsky are both group leaders at the Van Swinderen Institute for particle physics at the UG. For years, they have been working on an issue that occupies many physicists: how is it possible for matter to even exist?

Tunnel vision

Sure, they’re both physicists. However, theoretical physics and experimental physics are completely different disciplines. Theorists create models, while experimentalists, as the name suggests, do physical experiments. Over the past 150 years, theories and experiments have become so complicated that no one physicist can study both at the same time. 

We live in our own bubble. That can lead to tunnel vision and more limited ideas

Steven Hoekstra

 But the two realised that this has its disadvantages. If you don’t theoretically test the outcomes of your experiments or field-test your theories, you can easily get off track. ‘If the interaction between the two disciplines only takes place through scientific articles, it goes really slowly’, says Hoekstra. ‘Besides, those articles leave out so much information. They only describe the successes.’ 

In your own field, you’re always surrounded by like-minded people. ‘We live in our own bubble. That can lead to tunnel vision and more limited ideas, although you might not realise it.’

Lost matter

This can make it really difficult to find solutions for complex issues, like the one that says that, according to the accepted standard model of particle physics, matter shouldn’t be able to exist. Hoekstra: ‘The Big Bang created both matter and antimatter. When these two collide, the particles disappear and their energy is converted into light.’ 

However, this also means that over time, all matter should disappear. So now the question remains: why hasn’t this happened yet? ‘The only way this could happen is if the distribution of matter and antimatter was uneven from the start’, Hoekstra continues. ‘If there is more matter than antimatter, in the end we’re left with that surplus of matter.’

And indeed, as far as we can tell, the universe mainly consists of matter. Any antimatter that is created quickly disappears as it collides with matter. 

New theories 

But this explanation doesn’t cover all the bases. According to the standard model of particle physics, it’s possible there was a small discrepancy between matter and antimatter, but not nearly enough to explain what we’re currently observing in the universe, says Hoekstra. ‘The standard model is generally accepted to be incomplete.’

We’ve had to learn a whole new language in order to talk about each other’s expertise

Steven Hoekstra

So theorists have come up with dozens of new theories in an attempt to unite the standard model with reality. Borschevsky: ‘We want to find out which theory might be correct and which ones we can rule out.’ 

Testing these theories means working together, which they do at the Van Swinderen Institute. Together with a few other researchers, Hoekstra and Borschevsky regularly hold group meetings. ‘Seven group leaders, seven PhD students, two post-doctoral students, a few master students… There are like twenty people in these meetings.’ 

It’s a diverse team, consisting of theorists and experimentalists. ‘We all have a different way of looking at the same thing. There are a lot of different perspectives in one room’, says Hoekstra. ‘It really stimulates us. The experiments get a new boost, and the theorists have a new direction to go into.’


It’s not always easy. Each discipline has its jargon and a specific way of reasoning. ‘You have to be able to understand what other people are doing’, says Hoekstra. ‘We’ve had to learn a whole new language in order to talk about each other’s expertise.’

Occasionally, the disciplines each used a different formula to describe the same phenomenon. ‘One time, two PhD students spent weeks figuring out how to combine the two’, says Borschevsky.

Then again, the partnership is leading to interesting new insights. Once, the experimental students asked the theorists why they didn’t include any error bars in their graphs. Borschevsky: ‘A calculation only leads to one value. If you repeat it and you get a different value, something is wrong.’ 

However, the calculations had been based on certain assumptions. If you include the uncertainties and calculate the various scenarios, you end up with a margin after all. So Borschevsky has adjusted her process: ‘I now evaluate uncertainties in the methods, which I didn’t do before. But eventually you have to trust the calculations, that is the key point for a good collaboration.’

Dipole moment

The pair is currently studying one possible solution to the matter issue. This theory supposes that not only matter is unevenly distributed across the universe, but that electrons also have an uneven charge. If the asymmetry exists in electrons, the researchers posit, the creation of matter during the Big Bang may have been asymmetrical as well.

I now evaluate uncertainties in the methods, which I didn’t do before

Anastasia Borschevsky

This ‘dipole moment’ has already been observed in whole molecules. In those cases, there is a larger charge on one side of the molecule than on the other, because one of the atoms is ‘pulling’ more on the (negatively charged) electrons. But it’s never been observed in a fundamental particle like an electron. Even if anyone does find it, the actual value will be so little that it’ll be impossible to measure it directly.

But that’s where theory and practice come together for a potential solution, say Hoekstra and Borschevsky. Hoekstra and his team built a machine in which he ‘captures’ barium monofluroide (BaF). BaF is a very small molecule, consisting of only two atoms. However, it does have sixty-five protons, sixty-five electrons, and almost as many neutrons. ‘You could never calculate exactly what they do; it’s much too complex’, says Borschevsky.

She uses quantum mechanic models and powerful computers to predict the particles’ behaviour. ‘The molecule is inside your computer’, Hoekstra tells Borschevsky.

Exact measurements

Hoekstra cannot do without these predictions. ‘I need them to calibrate my experiment.’ He uses lasers to cool down the BaF molecules to such an extent that they almost stop moving entirely. This gives him more time to do exact measurements. The models also help with interpreting the results, which means Hoekstra can make assumptions about a few electrons on the basis of measurements of entire molecules. 

I need experimental values to adjust my theory

Anastasia Borschevsky

Conversely, Borschevsky can’t do without Hoekstra’s experiments. Borschevsky: ‘I need experimental values to check whether the computer models match reality, and to adjust my theory.’ 

‘We have now characterised the molecules, through the combination of theory and experiment’, says Hoekstra. ‘We are pushing the accuracy of the theoretical methods and the sensitivity of the measurements.’

They are now able to properly cool down their molecules, and they have found out how fast BaF returns to its regular energy level after the experiment. 

‘In the next period we are going to integrate these components to form the final experiment, which two years from now will give the first results’, says Hoekstra. 

Borschevsky nods: ‘Because of our joint investigations, we now know how to perform this challenging experiment.’