Are We Free? What Relativity Reveals About Time

Do we have free will? It is one of the oldest questions out there, and normally it is argued by philosophers, theologians and drunk people. Physicists on the other hand, have had the answer sitting right in front of them for over a century. It’s called relativity , and the answer is already in the equations.

The reason is simple. Free will is a question about the future. If I say “I am free to choose”, what I really mean is that the future is open: there is not yet a fact about what I will do tomorrow, and I am the one who will create that fact. So before we can talk about free will, we need to answer a more basic question: is the future real? Does it “exist” right now, in the same way the chair I’m sitting on exists, or is it just a space of possibilities waiting to be filled?

In this article we first look at the three main ways of answering that question. Then we look at what Einstein’s relativity and finally we come back to free will and see what is left of it.

Three Pictures of Time

1. Presentism: only the present is real

This is the “common sense” view, and it’s probably the one you hold without ever having thought about it. The past is gone: Julius Caesar and the dinosaurs were real, but they don’t exist anymore. The future is not yet: tomorrow’s breakfast doesn’t exist at all, it’s just a possibility. The only thing that is real is now, a single, universal, razor-thin slice of time that sweeps forward, bringing things into existence and immediately destroying them behind it.

   past                    NOW                   future
 (gone, not real)          |                 (not yet, not real)
 . . . . . . . . . . . . . # . . . . . . . . . . . . . . . . .
                           |
                    the only real slice,
                    moving forward --->

For free will, presentism is the most comfortable view: the future literally does not exist, so it is completely open. Nothing is written anywhere, because there is nowhere to write it.

Before even getting to physics, presentism has a funny problem: it declares that “now” is the only real thing, but that “thing” nobody can really experience it.

The reason is that everything you perceive is in the past. Light from the Sun is about 8 minutes old when it reaches your eye, light from the screen you are reading is a few nanoseconds old. In addition, your brain needs time to process it. Signals from the retina take tens of milliseconds to reach the visual cortex, and building a conscious perception out of them add more delays.

So the presentist has an awkward position: the only real thing in the universe is a moment that no conscious being ever has direct access to. Everything we experience is in the past, so strictly speaking, already “not real” by the presentist’s own definition.

2. The growing block: past and present are real, the future is not

The second view tries to fix something odd about presentism. If the past is not real, what makes a statement like “Caesar crossed the Rubicon” true? There has to be something out there it refers to.

The growing block theory (proposed by C. D. Broad in 1923) says: the past is real, and it stays real. Everything that has ever happened is permanently part of reality. The present is like a shockwave that creates reality, and the future does not exist yet. As time passes, new slices of reality are added on top, and the block grows.

   past (real, fixed)      NOW        future
 ######################### # . . . . . . . . . . . . . .
 #########################  |          (not yet real)
 #########################  |
                    the growing edge --->

For free will this is also a comfortable view, maybe even the most intuitive one: the past is fixed and can’t be changed (which matches our experience), and the future is open, created slice by slice at the edge. Our choices happen exactly at that edge.

3. Eternalism: past, present and future are all real

The third view is the strange one. Eternalism says that all moments of time are equally real. The dinosaurs, you reading this sentence, and the machines taking over our galaxy all exist, just at different times, in the same way that Rome and Tokyo exist at different places (and they are both real). “Now” is not a special, objective property of one slice of the universe; it is just the label each of us gives to the moment we happen to be in, exactly like “here” is the label we give to the place we happen to be.

   past                    "now"                  future
 ######################### # ###########################
 ######################### # ###########################
 #########################   ###########################
     all equally real: a 4D block of space-time

This is the block universe: the whole history of the universe as a single four-dimensional object. Nothing “comes into existence” or “goes out of it”. Things just are, at their location in space and time.

For free will this looks like terrible news. If tomorrow already exists, then what I will do tomorrow is already “there”. How can I be free to choose something that is already in the block?

Now let’s see what physics has to say about those different time pictures.

Time Concepts in Relativity

Whose “now”?

Look carefully at the first two views. Both of them need a very specific ingredient: a single, universal NOW. Presentism needs it because it is reality. The growing block needs it because it is the edge where reality grows. In both cases, there must be a single clock that synchronize the events in the universe are happening “right now”.

In Newton’s world that is not a problem. Time is absolute: there is one clock for the whole universe, and “now” is the same for everybody, everywhere. But Newton is wrong. In Relativity, two observers moving relative to each other disagree about which events are happening at the same time. And, neither of them is wrong. Each observer has their own equally valid definition of “now,” even though those definitions don’t match.

The disagreement is tiny for everyday distances, but it grows with distance. This is shown in the famous Andromeda Paradox. Alice and Bob are walking past each other on a street. Alice walks one way, Bob walks the other, each at a normal walking speed.

So at the moment they pass each other on the street, Alice’s “now” and Bob’s “now” in the Andromeda galaxy are more than a week apart. Imagine that in Andromeda an alien council is debating whether to launch an invasion fleet toward Earth. In Alice’s “now”, the council is still debating: the decision has not been made yet. In Bob’s “now”, the fleet has already left.

Now ask the presentist: which one is real? Is the launch of the fleet part of reality “right now” or not? For Alice it’s in the future, so it’s not real. For Bob it’s in the past (or present), so it is. They are standing in the same spot, at the same moment, a few centimetres apart, and relativity says both descriptions are equally valid.

Of course, neither of them can see Andromeda’s present, and nothing about the fleet can reach them for 2.5 million years. That’s not the point. The point is that presentism claims there is a fact about what exists, and relativity says that fact would depend on which direction you are walking. A theory where reality changes when you turn around on the sidewalk is not a very good theory of reality.

From “real for me” to “real, period”

This argument was made rigorous in the 1960s by Rietdijk and Putnam, and it goes roughly like this:

Events in my “now” are real (this is the very minimum any view of time must accept).
Bob is right here next to me, and he is as real as I am. So the events in his “now” are real too.
But Bob’s “now” includes events that are in my future (the launched fleet).
So some events in my future are real.

And it doesn’t stop there. By choosing observers with different speeds and directions, you can tilt the “now” slice in any direction (up to the speed of light). Chain enough of them together and you can reach any event in the universe, past or future. If “real” is not allowed to depend on who is asking, then every event is real. That is exactly eternalism.

If we reject the eternalism suggested by relativity, we face a extreme alternative: you are the only observer who truly exists, and only the reality you experience is real. That’s a pretty steep price to pay. Introducing even another single observer bring you back to eternalism.

Einstein himself accepted this. When his lifelong friend Michele Besso died, he wrote to Besso’s family:

“… People like us, who believe in physics, know that the distinction between past, present and future is only a stubbornly persistent illusion.”

So, What About Free Will?

If the future is already “there”, is free will dead? Well it depends. A few things are worth separating.

The block is not the same as determinism. Determinism means that if you knew the exact state of the universe and the laws of physics, you could, in principle, predict everything that happens next. The block universe says something different: that all events exist. These are independent claims. You could have a block universe full of genuinely random quantum events, where the future could never have been predicted from the past, and yet, viewed “from outside”, it is all there. So, the block tells you that there is a fact about tomorrow.

The block is not fatalism. Fatalism says “whatever you do, the outcome will be the same, so why bother”. That is not what the block says. In the block, your decisions are real events, sitting inside your own world-line, and they really do cause what comes after them. Think of a film: the last scene is already on the reel, but it is there because of everything the characters did in the previous scenes. Remove the decisions, and you get a different film.

What dies is a particular idea of freedom. The block universe challenges the idea that we create the future from scratch, as if we were standing in front of a blank canvas, free to paint whatever we want. In relativity, there is no blank canvas waiting to be filled in.

Whether this is enough to call it “free will” is another question. If our choices are already part of the structure of space-time, in what sense could we have done otherwise? Are we really free? I think that’s up to you to decide. Personally, I find very fascinating the idea that we are simply four-dimensional patterns in a block of space-time!

But could quantum mechanics offer a way out? Could the randomness at the heart of quantum physics give us back some of the freedom that relativity seems to take away? That’s a question worth exploring, and I will save for a “future” (O_o) article.

For now, one thing is hard to ignore: if relativity is right, as every experiment so far suggests, the future may not be a blank page waiting to be written. It may simply be a page we haven’t read yet.

I hope this article has given you a new perspective on how you think about time.