Physicists can't find "now" anywhere in the universe | Jim Al-Khalili

0:00 My name is Jim Al-Khalili and I'm Emeritus

0:03 Professor of Physics at the University of Surrey.

0:05 My book is called On Time, the physics that makes the universe tick.

0:14 Is the present an illusion?

0:17 Even in Einstein's special theory of relativity,

0:21 we can appreciate the concept of a now,

0:25 an absolute universal present moment simply doesn't make sense.

0:32 One can imagine two events.

0:36 I can experience two events that to me seem to have happened at the same time.

0:42 They are simultaneous events.

0:43 Now, if I know they took place at an equal distance from me,

0:49 then even though it's taken some fraction

0:52 of a second for the light from those two events,

0:55 the signal to reach my eyes, I can work back with and say, "Well,

0:58 if I saw them happening at the same time,

1:01 going back, they must have actually, like flashes of light.

1:04 Those flashes of light will have happened simultaneously

1:06 because I'm halfway between them and I saw them

1:09 happen at the same time." But for another

1:11 observer moving past me at close to light speed,

1:14 they will not look as though they've happened at the same time.

1:18 And this is something that generations of physics students have to learn,

1:23 something called the relativity of simultaneity,

1:26 which very clearly shows that what one observer regards as now,

1:32 as two events happening at some moment in time that they say is happened now,

1:37 another observer will disagree, say,

1:40 "No one event happened before the other." In fact,

1:44 we can even imagine a scenario where I see one event happening,

1:48 let's call it event A, happening just before event B.

1:52 For another observer moving very fast relative to me,

1:57 they may see event B happening before event A.

2:02 So where is now, if our past and future are mixed up?

2:08 Now, of course, what we're forbidden from doing

2:12 is something called the violation of causality.

2:15 So if event A was the cause of event B,

2:18 then no one can see B happening before A, you know,

2:22 if event A is me shooting a gun

2:25 and event B is someone being shot and falling down,

2:28 you're not going to see them being shot before I fired the gun,

2:31 because you can imagine them then stopping me

2:33 from firing the gun even though they've already been shot.

2:36 I know that's a rather violent example.

2:38 I don't tend to use violent examples in physics,

2:41 but there we go, that might illustrate it.

2:44 But the fact is if one event can affect the other,

2:47 then there is an order that you can't mess with.

2:50 Cause has to come before effect.

2:52 But if those two events are far enough apart and close enough in time,

2:59 such that there's not enough time for a light signal to transfer between them,

3:03 then we say they are not causally connected.

3:06 And in physics, we talk about them as being space-like separated.

3:11 So in that case, events A and B can have their order fuzzy.

3:17 Someone can see A before B, someone else can see B before A.

3:22 And once you realize this, you realize you cannot pinpoint

3:27 a universal present moment if we can switch past and future around.

3:33 So that present moment becomes rather fuzzy, according to relativity theory.

3:37 And while in relativity theory, we clearly see there's a fuzziness about what

3:43 we would refer to as a universal now.

3:46 No such thing exists in relativity.

3:48 Even in manifest time, our psychological or experienced time,

3:52 there's a fuzziness about what it means to say now.

3:57 Apart from the fact that it's ever-changing,

4:00 you pinpoint a moment in time as now,

4:03 but it's already in the past by the time you've said it.

4:09 But the notion that now is a moment is

4:12 also really not something that we see in psychological time.

4:18 It's an extended present.

4:20 It has a thickness to it.

4:23 To begin with, when an event happens, of course,

4:26 in relativity, I said an event, events A and B happen,

4:30 and there's a certain finite time for the light

4:35 to reach my eyes from the events.

4:39 There's a further time for that light to enter my eyes, travel to my brain,

4:44 be processed, and for me to be conscious of that event has taken place.

4:50 So there's what's called perceptual latency,

4:53 a delay between an event happening and us being conscious of it.

4:58 And that can be anything from up to a third or more of a second later.

5:05 So when is it that we should regard something as happening?

5:10 When it's happened or when we're conscious of it?

5:14 There's already that fuzziness there.

5:16 But let's say that's the moment that something happens.

5:21 What about when does the now start for us?

5:27 Well, again, it's rather fuzzy.

5:29 An example is how we appreciate a piece of music.

5:33 We don't just hear one note at a time that replaces

5:37 the previous note because that's gone and it's in our past.

5:40 No, we experience music as a continuum,

5:42 and the way we do this is through what's called episodic memories.

5:47 We are storing memories of events in our brain that are then stitched together

5:52 in a continuum so that it's not just

5:55 the present moment note that we are conscious of.

5:59 We're conscious of some finite time in the past.

6:03 We've together to give us the music that we appreciate.

6:08 Added to that, the fact that we anticipate

6:10 where the music is going in the future, even though we haven't heard it yet.

6:15 And so what we'd regard as the present now is really

6:19 an extended period of time that relies on past events that are still

6:24 stored in our memory that we have access to, even though we

6:28 are only ever accessing any moment in the past in the present moment.

6:32 It's still there and it gives us this sense that we're experiencing

6:37 a flow of time or a finite duration of a present moment.

6:41 But in physics, there's nothing special about this moment.

6:44 It's just a point on the time axis in four-dimensional spacetime.

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