Should it be space-time or spacetime – and why does it matter anyway?

One of the most awkward parts of writing a book is that, eventually, authors have to ask people for “blurbs” – the endorsements that you see on the cover of a book, encouraging you to buy it. I am now in this phase with The Edge of Space-Time. I’m writing to people and asking them to read my book and send me some nice words about it, in exchange for nothing but my goodwill.

It’s a bit nerve-wracking, but it can also be interesting. One person – whose endorsement will become public later – sent in their blurb with a query: why did I choose “space-time” with a hyphen rather than “spacetime”?

This sounds like it’s “just grammar”, but something else is lurking. When we talk about space or time separately, we have some intuitive sense of what we are talking about. Space-time – or spacetime – is relatively new to the scientific vocabulary. Even though, throughout history, many cultures have had unified concepts that don’t separate space and time, as a profession, physics conceived of them as distinct for longer than we have understood them to be unified.

And from my perspective as a science communicator, it is still an extremely difficult concept to explain. Even trying to describe “space” feels challenging. While writing this, I thought about saying that space is the place where motion happens. But in a sense, motion also happens in time. I could say that space is where things live, but again, I suppose a similar statement could be made about time. I also thought about saying space is a site of geography, but that sounds both very academic and tautological. It just means space is the site of space-type things.

To really talk about space and time is to rely on an intuition I presume you have. So let’s say that space has three dimensions that we move about in, and time has one dimension that we move in but only ever in one direction.

But, as Albert Einstein famously pointed out, these aren’t really separate phenomena. Especially when moving close to the speed of light, observers moving at different speeds won’t necessarily agree on when events happen. They may also disagree about the size of objects. To really grasp all this would require us to measure space-time, rather than space or time separately. This doesn’t necessarily feel natural, but it is the best way to make sense of how things work.

In this context, it is worth thinking whether space-time is a merger of two familiar phenomena or something new. This is where the grammar comes in as a reflection of our scientific sensibilities, at least for me.

My last name contains the first hyphen I ever met: my parents decided I should carry both of their last names. So to me, Prescod-Weinstein means I am a Prescod and a Weinstein.

The hyphenated version of space-time can be read as signalling that the phenomenon we are describing is both space and time. Alternatively, dropping the hyphen to render it as “spacetime” could suggest that we are talking about something completely new. Something that has features of space and time, but is a third, separate thing.

So, are we dealing with something that is space and time – a space-time – or are space and time just approximations for the completely different concept of spacetime? I am a bit of a fence-sitter. My first book, the blurb-writer pointed out, didn’t use the hyphen, which raises the question of why I made the switch.

The easy answer is that the title of the new book appears in the early pages of a classic textbook about cosmology, The Large Scale Structure of Space-Time by Stephen Hawking and George F. R. Ellis. At the very start of the first chapter, Hawking and Ellis argue that solving the equations that describe the universe “involves thinking about the edge of space-time in some sense”. Two pages later, they say that singularities, places where the equations break down (such as the centre of a black hole), can be thought of as “representing part of the edge of space-time”.

In my book, I have a chapter where I explain what kinds of equations they are talking about and the importance of boundaries and edges in physics. And since I was going to borrow this phrase for the title of my book, I had to commit and spell things the way they did.

That’s the easy answer. But more deeply, I don’t know which side I am on. I guess, like my blurb-writer, I lean towards spacetime without the hyphen. But I have no idea how much of that is force of habit from typing out the subtitle of my last book so many times. Scientifically, I think I am inclined to say that space-time is space and time – and also a third thing, spacetime. This may be a very quantum-mechanical answer, but I want it to be both simultaneously!

Chanda’s week

What I’m reading

I’m super hyped about the new Charlie Jane Anders novel, Lessons in Magic and Disaster.

What I’m watching

I just caught up on the sci-fi series Invasion, and it’s really interesting.

What I’m working on

I’m onboarding a new postdoctoral researcher into my group.

Chanda Prescod-Weinstein is an associate professor of physics and astronomy at the University of New Hampshire. She is the author of The Disordered Cosmos and the forthcoming book The Edge of Space-Time: Particles, poetry, and the cosmic dream boogie