What is time?
Is the flow of time real or an illusion?
The flow of time is an illusion, and I don’t know very many scientists and philosophers who would disagree with that, to be perfectly honest. The reason that it is an illusion is when you stop to think, what does it even mean that time is flowing? When we say something flows like a river, what you mean is an element of the river at one moment is in a different place of an earlier moment. In other words, it moves with respect to time. But time can’t move with respect to time—time is time. A lot of people make the mistake of thinking that the claim that time does not flow means that there is no time, that time does not exist. That’s nonsense. Time of course exists. We measure it with clocks. Clocks don’t measure the flow of time, they measure intervals of time. Of course there are intervals of time between different events; that’s what clocks measure.
So where does this impression of flow come from?
Well, I like to give an analogy. Suppose I stand up, twirl around a few times, and stop. Then I have the overwhelming impression that the entire universe is rotating. I feel it to be rotating—of course I know it’s not. In the same way, I feel time is flowing, but of course I know it’s not. And presumably the explanation for this illusion has to do with something up here [in your head] and is connected with memory I guess—laying down of memories and so on. So it’s a feeling we have, but it’s not a property of time itself.
And the other thing people contemplate: They think denying the flow of time is denying time asymmetry of the world. Of course events in the world follow a directional sequence. Drop an egg on the floor and it breaks. You don’t see eggs assembling themselves. Buildings fall down after earthquakes; they don’t rise up from heaps of rubble. [There are] many, many examples in daily life of the asymmetry of the world in time; that’s a property of the world. It’s not a property of time itself, and the explanation for that is to be sought in the very early universe and its initial conditions. It’s a whole different and perfectly respectable subject.
Is time fundamental to the Universe?
Time and space are the framework in which we formulate all of our current theories of the universe, but there is some question as to whether these might be emergent or secondary qualities of the universe. It could be that fundamentally the laws of the universe are formulated in terms of some sort of pre-space and time, and that space-time comes out of something more fundamental.
Now obviously in daily life we experience a three-dimensional world and one dimension of time. But back in the Big Bang—we don’t really understand exactly how the universe was born in the Big Bang, but we think that quantum physics had something to do with it—it may be that this notion of what we would call a classical space-time, where everything seems to be sort of well-defined, maybe that was all closed out. And so maybe not just the world of matter and energy, but even space-time itself is a product of the special early stage of the universe. We don’t know that. That’s work under investigation.
So time could be emergent?
This dichotomy between space-time being emergent, a secondary quality—that something comes out of something more primitive, or something that is at the rock bottom of our description of nature—has been floating around since before my career. John Wheeler believed in and wrote about this in the 1950s—that there might be some pre-geometry, that would give rise to geometry just like atoms give rise to the continuum of elastic bodies—and people play around with that.
The problem is that we don’t have any sort of experimental hands on that. You can dream up mathematical models that do this for you, but testing them looks to be pretty hopeless. I think the reason for that is that most people feel that if there is anything funny sort of underpinning space and time, any departure from our notion of a continuous space and time, that probably it would manifest itself only at the so-called Planck scale, which is [20 orders of magnitude] smaller than an atomic nucleus, and our best instruments at the moment are probing scales which are many orders of magnitude above that. It’s very hard to see how we could get at anything at the Planck scale in a controllable way.
If multiple universes exist, do they have a common clock?
The inter-comparison of time between different observers and different places is a delicate business even within one universe. When you talk about what is the rate of a clock, say, near the surface of a black hole, it’s going to be quite different from the rate of a clock here on Earth. So there isn’t even a common time in the entire universe.
But now if we have a multiverse with other universes, whether each one in a sense comes with its own time—you can only do an inter-comparison between the two if there was some way of sending signals from one to the other. It depends on your multiverse model. There are many on offer, but on the one that cosmologists often talk about—where you have bubbles appearing in a sort of an inflating superstructure—then there’s no direct way of comparing a clock rate in one bubble from clock rates in another bubble.
What do you think are the most exciting recent advances in understanding time?
I’m particularly drawn to the work that is done in the lab on perception of time, because I think that has the ability to make rapid advances in the coming years. For example, there are famous experiments in which people apparently make free decisions at certain moments and yet it’s found that the decision was actually made a little bit earlier, but their own perception of time and their actions within time have been sort of edited after the event. When we observe the world, what we see is an apparently consistent and smooth narrative, but actually the brain is just being bombarded with sense data from different senses and puts all this together. It integrates it and then presents a consistent narrative as it were the conscious self. And so we have this impression that we’re in charge and everything is all smoothly put together. But as a matter of fact, most of this is, is a narrative that’s recreated after the event.
Where it’s particularly striking of course is when people respond appropriately much faster than the speed of thought. You need only think of a piano player or a tennis player to see that the impression that they are making a conscious decision—“that ball is coming in this direction; I’d better move over here and hit it”—couldn’t possibly be. The time it takes for the signals to get to the brain and then through the motor system, back to the response, couldn’t work. And yet they still have this overwhelming impression that they’re observing the world in real time and are in control. I think all of this is pretty fascinating stuff.
In terms of fundamental physics, is there anything especially new about time? I think the answer is not really. There are new ideas that are out there. I think there are still fundamental problems; we’ve talked about one of them: Is time an emergent property or a fundamental property? And the ultimate origin of the arrow of time, which is the asymmetry of the world in time, is still a bit contentious. We know we have to trace it back to the Big Bang, but there are still different issues swirling around there that we haven’t completely resolved. But these are sort of airy-fairy philosophical and theoretical issues in terms of measurement of time or anything being exposed about the nature of time.
Then of course we’re always looking to our experimental colleagues to improve time measurements. At some stage these will become so good that we’ll no doubt see some peculiar effects showing up. There’s still an outstanding fundamental issue that although the laws of physics are symmetric in time, for the most part, there is one set of processes having to do with the weak interaction where there is apparently a fundamental breakdown of this time-reversal symmetry of a small amount. But it seems to play a crucial role and exactly how that fits into the broader picture in the universe. I think there’s still something to be played out there. So there’s still experiments can be done in particle physics that might disclose this time-reversal asymmetry which is there in the weak interaction, and how that fits in with the arrow of time.