I HAVE been pretty obsessed with space-time this year. Thankfully, I am a theoretical physicist and, in principle, this is my day job. But the more time I have spent returning to my roots with it, the stranger the whole thing has started to seem to me. As I wrote in an earlier column, it is actually pretty hard to explain this strange merger of what are ostensibly familiar, but actually incredibly distinct, concepts.
In your everyday life, you generally experience space and time as separate phenomena. Then along comes the physicist with some thought experiments about clocks, trains and even the soap opera Emmerdale, and we have to revisit the entire universe with a different perspective.
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My work as a scientist is, in some sense, a practice of repetition. I am often going over the same ideas, again and again, but each time I have a slightly different question in mind. This year’s focus on space-time was driven by the difficult question of how to help people have intuition for this concept without resorting to equations.
I also find myself in a regular state of repetition because my research has a tendency at any given time to go more strongly in one of two directions: sometimes it is more general relativity (space-time) and sometimes it is more quantum field theory (particle physics). When I spend a lot of time on the one, the sharpness of my understanding of the other starts to slip. So, I pick up a book to remind myself of the things I thought I knew. Each visit is different from the last, though.
As a case in point, this year, my brain unexpectedly took me in the direction of revising my relationship to the mathematical framework for particle physics – quantum field theory (QFT) – simultaneous to my extended meditation on relativistic notions of space-time. The end product was that I began to yearn for something that drew the two together. And so, for the first time in nearly 10 years, I picked up my books on quantum fields in curved space-times, once a topic I knew so well that I passed an oral examination on it during my PhD.
It felt so otherworldly! I had to remind myself of its significance and value. When we are dealing with particle physics, we need to take quantum mechanics into account, along with special relativity, because particles are very small and often fast-moving.
QFT is the framework that allows us to do both simultaneously. In this picture, we are in a flat space-time (no fun with curvature!), but taking quantum mechanics and the finite speed of light into account simultaneously requires reimagining what, exactly, a particle is. Relativistic quantum mechanics evolves into the mathematical picture known as QFT. In this scenario, the fundamental concept of the particle is replaced by a notion of a field acting on the vacuum.
There is no simple way to intuitively understand a field, in my opinion. But there are examples we can use to try. For instance, temperature as it varies throughout a room is an example of a field. At each point, the temperature is a little bit different. The information together is a field of temperature. Similarly, the sun is exerting a gravitational field on everything in the solar system.
QFT doesn’t take gravity into account because it was developed for scenarios where we don’t need it. And the concept evolved quite distinctly from general relativity for a long time. But eventually, physicists began to wonder how to construct a sense of QFT in a non-flat space-time – a curved space-time where gravity can’t be ignored. This is how QFT in curved space-time was born. Andbecause I have limited space, I will simply share my favourite result from this formulation: an accelerating observer will observe the presence of particles where a non-accelerating observer sees none! The two observers won’t agree on whether they are in an empty vacuum.
This phenomenon is known as the Unruh effect. It is likely that you have never heard of it, but you will have almost certainly heard of a related concept: Hawking radiation. Stephen Hawking realised that when we consider something like the Unruh effect near a black hole’s point of no return – the event horizon – then effectively a black hole could be said to radiate particles.
The Unruh effect is a beautiful, wonderfully strange idea, and it is also the one that underpins the name of this column. For years, I hadn’t thought in any serious way about it, even though there once was a time when I could write out the equations that describe it purely from memory.
The hard lesson I have learned as a physicist is that knowing physics isn’t about having it all memorised, but rather about knowing how to come back to what I thought I knew, and revisit it with a fresh perspective.
Chanda’s week
What I’m reading
I really love Airea D. Matthews’s new poetry collection, Bread and Circus.
What I’m watching
The ending of series 3 of The Morning Show was pretty disappointing.
What I’m working on
Writing two books at once – while keeping up with research – is hard.
Chanda Prescod-Weinstein is an associate professor of physics and astronomy, and a core faculty member in women’s studies at the University of New Hampshire. Her most recent book is The Disordered Cosmos: A journey into dark matter, spacetime, and dreams deferred
