Caspar Jacobs investigates what physical theories actually tell us about the world
ERC grant image: Nehir Aksel
Physical theories are full of dimensions, constants and units, and yet nobody knows how these concepts relate to reality. Assistant Professor Caspar Jacobs has been awarded an ERC Starting Grant to change that.
‘Generally speaking, you could say that theories in physics describe the world,’ Jacobs begins. ‘However, with regard to some elements in those theories, one might wonder whether they are actually part of that description or whether they merely structure it, just as full stops and commas do in written texts.’
A forgotten debate
Ever since quantum mechanics became widely accepted, most researchers have tended to say that these elements are indeed not descriptive – or to refrain from commenting on the matter at all. ‘That is partly because the theory is so mysterious,’ explains Jacobs. ‘“I’m not going to concern myself with what this means, as long as it works”. How that quantum world worked and what it meant turned out to be too strange, too difficult to fathom. I think that’s why other discussions, such as those about dimensions, have also faded into the background.’
A shame, says Jacobs. He wants to use his new project to test the hypothesis that concepts such as dimensions and units are indeed descriptive. ‘In physics, taxonomies are used to classify physical quantities into categories,’ he explains. ‘Mass, for example, is a different type of physical quantity to distance. Where it gets interesting is that some physical quantities are linked to others. Speed, for instance, relates to distance, because you calculate it as distance divided by time. Volume, on the other hand, is distance to the third power.’
A different classification
These categories can also be organised differently. Jacobs: ‘You could also say that time is equal to distance. You would then measure how long something takes by looking at how much distance a particle travels. Speed would then no longer be distance divided by time, and so the two would no longer be related to one another. Although, in a sense, it is a matter of convention which taxonomy you use, we see that one choice yields a better result than the other. That could suggest that one classification is closer to the way the world actually works than the other. That is why I think we need to take a closer look at this.’
Dimensions and constants
When his project begins in January, Jacobs will first focus on dimensions. ‘This is a system for classifying quantities such as mass, charge and duration,’ he explains. ‘I want to lay the groundwork for a clearer theory. Until now, philosophers have mainly looked at each quantity in isolation: how does mass work? How does charge work? I want to incorporate dimensions into that picture, so that it also becomes clear how these quantities are interconnected.’
As soon as Jacobs is joined by a PhD student and a postdoc in the summer of 2027, he intends to delve into the second phase of the project with them, which concerns constants. ‘These are terms that appear in the laws of nature. For example, there is the constant speed of light, or Planck’s constant. The strange thing about these constants is that they do not seem to correspond to anything in the real world. Where m stands for the mass of an object in the real world, G is a constant that appears only in the law. As a philosopher, I find that strange. Shouldn’t such a constant correspond to something in reality? We will therefore examine a number of examples of constants in various theories. This also immediately links back to dimensions, because the system of dimensions I choose influences which constants appear in the law of nature. Ultimately, I would like to write a book on this subject, and the PhD student will, of course, defend a thesis.’