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Katie Mack, theoretical astrophysicist and assistant professor of physics at North Carolina State University.

N-/Handout

Katie Mack is a theoretical astrophysicist, assistant professor of physics at North Carolina State University and the author of The End of Everything (Astrophysically Speaking).

“I’ve been trying to wrap my head around what it means to live in a universe where space and time are not fundamental.” I say this to Nima Arkani-Hamed, a theoretical physicist studying the deepest foundations of the cosmos and its workings, as we walk the grounds of the Institute for Advanced Study in Princeton, N.J. The institute is Einstein’s old stomping ground and widely recognized as one of the world’s most esteemed academic havens for serious deep thought. It’s the spring of 2019 and we’ve been wandering the institute’s woods for half an hour, pestering toads and discussing the latest developments in particle physics. This is the topic I’ve been trying to work my way toward because, honestly, I’m more than a little disturbed by it.

I’ve been reading for months about new developments in particle physics theory that seem to point to the idea that space and time as we think of them aren’t real – that there’s a deeper, abstract, mathematical reality to the universe and space and time are just what we perceive, because we are weird limited creatures who can’t get to the heart of existence, to what’s really out there. I entertain the possibility that this framing is just the sort of thing theorists say to make their results sound cooler, so I half expect Mr. Arkani-Hamed to correct me or brush off my burgeoning existential crisis. Instead he just laughs and says, “Join the club!”

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Physicists don’t usually talk like this. For the most part, we take space and time to be the background against which everything else happens. Which is not to say space and time themselves can’t bring us surprises. Before Einstein came along with his relativity theory, we thought both space and time were not only real, but immutable: Time ticks along the same for everyone, space is just the giant invisible grid through which we move. But relativity forced us to adjust our thinking to include the possibility that both space and time are malleable and intertwined, affecting each other in counterintuitive ways. How you move through space affects time, and space itself can change shape as time passes. We now think of space and time tied up into a kind of universal, stretchy fabric we call space-time. We’ve even been able to detect ripples in space-time, caused by black holes spiralling in and colliding with each other in other galaxies. So why the sudden suggestion that it’s not what the universe is made of?

The notion that space and time aren’t fundamental has been floating around theoretical physics for decades, but I first heard about it a few years back, in connection with some hot new ideas in particle physics. The short version is that when you calculate the expected results of a certain kind of particle collision, there’s a way to do so that assumes space and time exist, and a way that involves some weird abstract geometrical object but doesn’t use the concepts of space and time at all. Bizarrely, you sometimes get the same answer, faster and more easily, in the latter case. Which is a little alarming when you think about it, because what it seems to be indicating is that those notions of “space” and “time” that you wrote into your equations were just dead weight. Perhaps there is a deeper structure to the universe that is more properly described by that new geometry. Perhaps space and time don’t really exist at all.

To me, it’s an unsettling notion, and it throws me off balance when I try to comprehend that what I think of as the fabric of my universe might be a convenient illusion built upon something else – something more powerful and fundamental and untouchable that I can’t fully perceive. And right now, in this moment (if it even is a moment), this has a strange resonance.

For months now, we’ve all been trying to build some kind of stable existence on top of a foundation that is suddenly less solid than we would have liked to believe. Many of us find our universes dramatically confined, as we walk our tiny circles around our allotted patches of space, feeling time stretch and squeeze and loop back upon itself as one month lasts four years and another is over in a week. Our view of the past is blurry and indistinct; the future completely obscured. Our nearest friends and family are now impossibly distant; strangers at the grocery store alarmingly close.

Theoretical physics can’t solve this one for us. When it comes to everyday life, there’s little that theoretical physics can do for us, practically speaking. But what it can do – what it does better than almost anything else – is offer up a different perspective. Because it turns out that reality is a sliding scale. While some physicists look at their equations and say, “space-time isn’t real,” others say, “it’s real, but it’s not fundamental.” This is the argument I hear from cosmologist Sean Carroll when I ask him about this, a few months after my chat with Prof. Arkani-Hamed. The existence of a deeply hidden structure shouldn’t stop us from stating with confidence that the universe around us does fully and completely exist. Just because a chair is built out of molecules I can’t see doesn’t mean that I’ll fall through it if I try to sit down. Our world is, for all intents and purposes, what we experience of it.

Possibly the best way to think of space-time is as an emergent phenomenon – something that isn’t automatically written into the structure of reality, but appears in our universe anyway, not so much as an illusion, but as an unexpected circumstance we happen to find ourselves in. Writing it off as unreal doesn’t make it go away, after all. Acknowledging that there’s more going on than we see, and certainly more than we can control, might give us the insight we need to work with what we have. We physicists will keep trying to figure it all out, of course. We’ll be doing this in our own separate spaces, gathering our ideas in a grid of Zoom meeting boxes, shifting our schedules to adapt to time zones as needed. Maybe none of it feels quite as real as it used to. But we hold out hope that something new and powerful might, nonetheless, emerge.

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