In 1999, the physicist Julian Barbour published a provocatively titled book called The End of Time. As an undergraduate physics major at the University of Maryland at the time, I recall walking past a classroom one afternoon and seeing a gaggle of physicists gathered to hear Barbour give a seminar on his “revolutionary” new ideas about the nature of time. In Barbour’s new theory, time didn’t exist in any proper sense. It was nothing more than an illusion.
I read his book but never really understood his ideas, or, to the extent that I did understand the ideas, I didn’t understand what was so profound about them. Apparently I wasn’t the only one. It’s been 20 years since Barbour made his claim about the end of time, and as far as I know, he’s failed to convert many physicists to his cause.
Despite the lack of enthusiasm for Barbour’s particular theory of a timeless universe, many physicists actually agree with his prognosis: Something about our conception of space and time is fundamentally flawed. In the last two decades a growing number of physicists—highly respected physicists, physicists with Nobel Prizes—have started to wage a secret war against space and time. For some, the war isn’t actually much of a secret. The renowned physicist Nima Arkani-Hamed, for example, has been on the warpath for years, telling anyone who will listen that “spacetime is doomed.” When Nima says this, it isn’t with the anxiety of Chicken Little, but with the sort of ecstatic rapture of someone who has given up on this life and thrills in looking towards what comes next.
So, what’s going on here? Is spacetime actually doomed? What would that even mean, exactly? And what led physicists to pick this fight in the first place?
Admittedly it seems a bit absurd that you could do away with space and time. What, pray tell, would you put in their place? Ebony and Ivory? Milli and Vanilli?
Let me try to clarify a bit what some physicists mean when they say “spacetime is doomed.” Their eschatological rhetoric aside, they aren’t actually claiming that space and time will soon cease to exist as we know it. No, what these physicists are suggesting is that space and time may not be as fundamental as we think. That there might be a theory of physics where space and time emerge as some higher level phenomena, rather than existing as fundamental structures in the theory.
Consider an analogy from neuroscience. Suppose a neuroscientist runs around saying “consciousness is doomed!” Said scientist probably doesn’t mean that you and I aren’t actually conscious or that we’ll cease to be conscious once we understand some deeper truth. Instead, this scientist is making the classical reductionist argument that scientists have been making for centuries: consciousness isn’t some fundamental aspect of nature, it is an emergent phenomena that arises from the interactions of billions upon billions of neurons. This would mean that you might be able to “explain consciousness” using some more fundamental pieces, like neurons. Or, short of explaining consciousness, you could explain other mental phenomena using a theory of neurons without ever having to invoke consciousness.
But if space and time are emergent phenomena, like consciousness, what the hell do they emerge from? Everything in our experience seems to suggest that space and time are literally as fundamental as you get. Space and time are what’s left when you get rid of everything else, right?
It might help if we stop for a moment and think about what space and time actually do for us. For simplicity, we’ll just focus on space itself for now.
Well, imagine someone put an object down in some completely empty space. What could you say about its location? If the space was totally empty (and large enough), you couldn’t say anything about its location. Because there is nothing to compare its location to.
We can argue about the metaphysics of space, but when it comes to answering practical questions, like where something is located in space, we are forced to concede that space by itself doesn’t really do much for us. It’s the relationship of two or more objects in space—their relative distances, for example, that matter. (And those distances are determined not by space itself, but by other objects, like rulers, that also exist in space).
In this view, space is just a way of organizing some set of objects according to a relationship we call “distance.”
But this begs the question: if space is defined by a particular sort of relationship (distance) between things, might there be other kinds of relationships between things that we could define that make no explicit reference to space (and distance)? And is it possible that some other kinds of relationships are just as “fundamental” (perhaps more fundamental, even) than “spatial relationships”?
If I have particle A and particle B, what kinds of relationships might I consider between them, besides their relative distance? Well, if the particles are electrons we could consider the amount of force they exert on one another. Then you might consider “the degree of force” between particles like electrons as a kind of fundamental relation.
But in classical physics “forces” are intimately related to space and time. All forces are mediated by fields that travel through spacetime. So it can get quite tricky to disentangle forces from these spacetime relationships, at least in a way that would be sensible and useful.
There is somewhere else we could look for a unique kind of relation: quantum mechanics. It turns out that quantum mechanics already has a built in relationship between objects that doesn’t appear to be mediated by space and time in the same way that forces are.
Entanglement is at the heart of so much quantum weirdness. So what is it?
The simplest, though not necessarily most intuitive, answer is that entanglement is a kind of correlation between two or more quantum objects (like particles).
Well quantum mechanics is fundamentally a probabilistic theory of physics—it doesn’t spit out exactly where an electron will be at some time, only the probability that it might be here, or there. And in probability theory, a correlation between two events means that knowing something about one event gives you information about the other event.
Consider flipping two coins. If you flip the first one and it lands heads, this tells you nothing about what will happen when you flip the second coin. These two events are uncorrelated.
However, suppose I told you I have two marbles in my hands: a black one and a red one. If I open one of my hands and show you the black marble and then ask which marble is in the other hand, you know for certain its the red one. In this case the events of guessing which marble I have in each hand are correlated because knowing something about one tells you something about the other.
So far, so good, right? Correlations exist in classical physics and in quantum physics. But, the peculiar features of quantum mechanics end up making quantum correlations much, much stranger.
These coins behave in a sort of crazy way. You flip them up in the air and they keep spinning around until you look at them for a few moments, then they fall down and land heads or tails.
If that wasn’t strange enough, these two coins also always show opposite sides: if one comes up heads, the other will come up tails (and vice versa). These coins are correlated. If you know that one coin came up heads, you know the other one will show tails even if you haven’t looked yet.
I realize this sounds super duper contrived, and it is as far as classical coins go. But there are features of quantum objects like electrons can that behave in this exact manner.
As I said, things can be correlated classically. Correlations are just a probability thing, not a quantum thing. But classical correlations make sense. Suppose I know there’s a red marble and a black marble. In this scenario, I take one marble, without looking at it, and travel a billion miles away. If I then look and see that I have a red marble, I know the other marble I left behind is black. There’s nothing crazy about this correlation because everything was decided upon at the moment I first left. I may not have known my marble was red until I looked later, but it WAS RED when I left.
Consider my quantum coins. They are correlated while they flip in the air, before they ever hit the ground. Before they ever “choose a side.” This means if I flip them both in the air, and somehow take one (still flipping) and move it a million miles away before the coins fall down and land on heads or tails, they remain correlated and will show opposite sides. But unlike the marbles, where I always had a red marble in my hand even if I hadn’t looked yet, the coins haven’t picked a side yet when they get separated.
How the hell can these two coins conspire like that across thousands, millions or billions of miles? Classically, this is impossible.
This is what Einstein called “spooky action at a distance” and it’s one of the reasons he felt quantum mechanics was incomplete.
But for our purposes entanglement is kind of perfect. It’s a relation—in this case a cor-relation—between objects that seems to be completely independent of space and time.
And it’s basically hopeless for me to try and explain it in a couple hundred words. All you need to take away from this whole sidebar is that entanglement is a very well-defined and technically well-understood relationship between things that are independent of space and time.
Now we just need to show how entanglement can be used to replace spacetime as a more fundamental relationship between things in the universe. Or at least, demonstrate that spacetime emerges from something related to entanglement.
And that’s just what we will be doing in Part II.