I rarely write much about it in my research papers, but every piece of science I have ever done either assumes the correctness of general relativity – our most fundamental theory for explaining gravity – or assumes it is a fantastic approximation of a more correct theory. When I and others in my field write up our research, we rarely say it out loud. But general relativity lurks everywhere in physics – from the way it allows us to navigate with the global positioning system to how it helps us launch and use telescopes for studying planets outside our solar system.
General relativity successfully, and quite beautifully, theorises why gravity is and what gravity does. There is no part of the visible universe untouched by this force – not even light, even though it has no mass and we wouldn’t expect it to experience gravity.
Isaac Newton’s 1687 law of gravity, and his notions of absolute space and time, turned out to be incorrect. General relativity, established by Albert Einstein in 1915, invites us to rethink the fundamental nature of reality, and it also gets used in nearly every single area of physics and every area of astronomy. Many university classes in these fields tend to start by teaching Einstein’s special relativity, which theorises that space and time cannot be separated and must instead be thought of as a unified entity, space-time.
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Practically speaking, this means we measure distances and times differently. The ruler used to make measurements in Newtonian physics doesn’t work when we take the universal speed limit – the finite speed of light – into account. This is the stage at which students are introduced to the mathematical concept we use to measure distances, the metric. The metric, which abstracts the everyday notion of a ruler, was lurking in our Newtonian calculations, but we never had to discuss it. Special relativity is the first time we have to acknowledge it.
General relativity, colloquially known to physicists as GR, builds on the unification of space-time by taking gravity into account. Its most memorable lesson is that space-time curves: the metric that we met in special relativity must allow for the possibility that our ruler bends and isn’t straight. That curvature manifests as a force, gravity. Where there is more bending, we see stronger gravitational effects. In other words, as I wrote in my first book, space-time isn’t straight.
General relativity is like a puzzle where the pieces fit perfectly and if we change even one it stops working
When I was a student, I was very focused on understanding the calculational tools involved. We had to abstract space and time for the first time, to understand the general mathematical idea of a “space”. When I learned enough to put the pieces together, I was astonished by the beauty of the theory. General relativity is like a puzzle where the pieces fit together perfectly and where, if we changed even one, the whole thing would stop working.
Despite all the writing I have done, I still find it hard to describe the sensation this evokes for me. This is our marvellous universe! It works in this wonderful way and we have been able to figure it out. How delightful!
I was so enthusiastic about general relativity that I did my doctorate studying how measurements of the speed of space-time’s expansion might inform our effort to merge GR with another fundamental theory, quantum mechanics (see our special, page 28). At the time, I understood, in mathematical terms, the challenges of merging these two theories. Modifying general relativity without ruining what makes it special seems virtually impossible, and the same is true of quantum physics.
After I completed my doctorate, I took almost 15 years off from trying to reckon with this conflict. When I returned to it recently as I worked on my next book, The Edge of Space-Time, I found that general relativity ages like fine wine – and so does the quantum gravity problem. What I more deeply understand now are the conceptual challenges involved. General relativity may eschew absolute space and time, but it still calculates with certainty. Quantum mechanics, however, deals in probabilities and comes with guaranteed uncertainty. And yet, somehow, a world that feels certain – at least physically, if not politically – emerges for us to live in. I have a better gut feeling now for why this disagreement is so fascinating and inspiring.
I am so grateful that, at an early stage, I learned the fundamentals that would allow me to grow old with this problem. It is a reminder that thinking about our material conditions is more than worrying about money and whether leaders will make social policies that serve all of us. Our material conditions are also the result of being in space-time, being scientists and community knowledge holders, and being lifelong learners who never see our relationship with space-time as complete.
Chanda’s week
What I’m reading
I have quite enjoyed Victoria Adukwei Bulley’s gorgeous poetry collection Quiet.
What I’m watching
Medical drama The Pitt is very good – and very sad.
What I’m working on
I have two students finishing their PhDs this month!
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
