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We would hope that the moment that we eternally live in, the “now,” would have a concrete scientific explanation. But the truth is far more complicated, says the relativity of simultaneity.
Jim Al-Khalili explains how the past and future are more fluid than we may think.
JIM AL-KHALILI: My name is Jim Al-Khalili and I'm Emeritus Professor of Physics at the University of Surrey. My book is called On Time, the physics that makes the universe tick. Is the present an illusion? Even in Einstein's special theory of relativity, we can appreciate the concept of a now, an absolute universal present moment simply doesn't make sense. One can imagine two events. I can experience two events that to me seem to have happened at the same time. They are simultaneous events. Now, if I know they took place at an equal distance from me, then even though it's taken some fraction of a second for the light from those two events, the signal to reach my eyes, I can work back with and say, "Well, if I saw them happening at the same time, going back, they must have actually, like flashes of light. Those flashes of light will have happened simultaneously because I'm halfway between them and I saw them happen at the same time." But for another observer moving past me at close to light speed, they will not look as though they've happened at the same time. And this is something that generations of physics students have to learn, something called the relativity of simultaneity, which very clearly shows that what one observer regards as now, as two events happening at some moment in time that they say is happened now, another observer will disagree, say, "No one event happened before the other." In fact, we can even imagine a scenario where I see one event happening, let's call it event A, happening just before event B. For another observer moving very fast relative to me, they may see event B happening before event A. So where is now, if our past and future are mixed up? Now, of course, what we're forbidden from doing is something called the violation of causality. So if event A was the cause of event B, then no one can see B happening before A, you know, if event A is me shooting a gun and event B is someone being shot and falling down, you're not going to see them being shot before I fired the gun, because you can imagine them then stopping me from firing the gun even though they've already been shot. I know that's a rather violent example. I don't tend to use violent examples in physics, but there we go, that might illustrate it. But the fact is if one event can affect the other, then there is an order that you can't mess with. Cause has to come before effect. But if those two events are far enough apart and close enough in time, such that there's not enough time for a light signal to transfer between them, then we say they are not causally connected. And in physics, we talk about them as being space-like separated. So in that case, events A and B can have their order fuzzy. Someone can see A before B, someone else can see B before A. And once you realize this, you realize you cannot pinpoint a universal present moment if we can switch past and future around. So that present moment becomes rather fuzzy, according to relativity theory. And while in relativity theory, we clearly see there's a fuzziness about what we would refer to as a universal now. No such thing exists in relativity. Even in manifest time, our psychological or experienced time, there's a fuzziness about what it means to say now. Apart from the fact that it's ever-changing, you pinpoint a moment in time as now, but it's already in the past by the time you've said it. But the notion that now is a moment is also really not something that we see in psychological time. It's an extended present. It has a thickness to it. To begin with, when an event happens, of course, in relativity, I said an event, events A and B happen, and there's a certain finite time for the light to reach my eyes from the events. There's a further time for that light to enter my eyes, travel to my brain, be processed, and for me to be conscious of that event has taken place. So there's what's called perceptual latency, a delay between an event happening and us being conscious of it. And that can be anything from up to a third or more of a second later. So when is it that we should regard something as happening? When it's happened or when we're conscious of it? There's already that fuzziness there. But let's say that's the moment that something happens. What about when does the now start for us? Well, again, it's rather fuzzy. An example is how we appreciate a piece of music. We don't just hear one note at a time that replaces the previous note because that's gone and it's in our past. No, we experience music as a continuum, and the way we do this is through what's called episodic memories. We are storing memories of events in our brain that are then stitched together in a continuum so that it's not just the present moment note that we are conscious of. We're conscious of some finite time in the past. We've together to give us the music that we appreciate. Added to that, the fact that we anticipate where the music is going in the future, even though we haven't heard it yet. And so what we'd regard as the present now is really an extended period of time that relies on past events that are still stored in our memory that we have access to, even though we are only ever accessing any moment in the past in the present moment. It's still there and it gives us this sense that we're experiencing a flow of time or a finite duration of a present moment. But in physics, there's nothing special about this moment. It's just a point on the time axis in four-dimensional spacetime. [Music] Want to support the channel?