Raw Transcript
If science has taught us one thing, it's that our perception of reality can be extremely misleading. The earth isn't flat. Solids don't fill space and no one looks like they do on Instagram. That the future somehow different from the past is another such quirk of human perception that doesn't correspond to reality. At least that's what I think. And today I want to explain why. In a nutshell, the reason is that Einstein's theory of general relativity tells us that the future, present, and past are really all the same thing. They belong together to one common entity, spaceime, which just is. It doesn't come into being. It's there, has always been there, and will always been there in its entirety. In Einstein's theory, you can't separate the past from the present and the future. It's not logically possible. And this theory is experimentally very well confirmed. This is why I think we can't just discard this consequence. It's like insisting that the earth is flat because we can't see the curve instead of denying it because it doesn't agree with our personal experience. We should be trying to understand what it means. This is an argument in three steps. The first step is the question of what we mean when we say that something exists. take the question of whether your feet exist right now. Well, yes, you might say, I can see them. I can wiggle them. They're obviously there. But says the theoretical physicist, light takes time to travel from your feet to your eyes. You only ever see your feet as they were a picoscond or so in the past. And the nerve signals from your feet take almost a second to reach your brain. So you can't possibly know whether your feet exist now. But this just isn't how we think about existence. Most of us are comfortable saying that our feet exist now. Even theoretical physicists and this makes sense because well the light and the nerve signals arrive eventually. The fastest of these signals is light. That's the fastest signal possible. Then you can in retrospective deduce your feet existed a pic a second earlier when the light scattered off them that has now reached your eyes. Now because it's always been like this that the light arrived to confirm your feet were there that fraction of a second earlier. It makes sense to say that your feet exist now. The conclusion of this first step is that we speak of the existence of things elsewhere now. Even though signals from there have not reached us yet because we're confident that these signals will reach us later. So we can reconstruct the earlier moment. The second step is then to factor in Einstein's relativity. Einstein's theories both special and general relativity rest on making time into a dimension similar to space. Of course time isn't the same as space. It's still different most importantly because we can turn around in space but not in time. The other important property of Einstein's theories is that the speed of light in vacuum is always the same. Physicists like to visualize what goes on in space and time by tossing away two dimensions of space so that there's only one dimension of space and one dimension of time. We then draw time on the vertical and the one remaining dimension of space on the horizontal. This is called a space-time diagram. And anyone who moves at a constant velocity makes a straight line. The angle of the line determines the velocity. By convention, we set the speed of light to be a 45° angle. And since we can only move slower than the speed of light, all possible motions are within this region called the light cone. This pretty much exhausts my drawing skills. I know it doesn't look very impressive, but making time into a coordinate has a stunning consequence. Is that there is no universal notion of what it means for two events to happen at the same time. Because coordinates are arbitrary choices. No one is better than the other. And that now holds for time as well. Suppose you just sit here moving straight up with zero velocity and left and right of you at the same distance there are flashlights going off. If you see them flashing at the same time, you'd say, "Well, they were at the same distance from me, so they must indeed have flashed at the same time." The issue is that an observer who moves relative to you sees one flash before the other. So to them they didn't happen at the same time. This observer would say instead that these three events happened at the same time. In general relativity observers who move at different speeds have different notions of simultaneity and therefore different notions of what they mean by now. You might say what's the big deal? It's the same thing with say sound signals. If you move towards one and away from the other, you'll hear one first and the other. Yes. But in the case of sound, the relative motion between you and the sound waves changes. Because the sound travels in a medium, say air, it's got a particular velocity in that medium. If you move relative to the medium, the speed of sound changes. So for sound there's a measurable difference between the two observers. The one moves through the air, the other one doesn't. You can then say objectively the one who doesn't move through air is right about what happens at the same time. But the speed of light doesn't need a medium to travel in and it doesn't change. It's the same for all observers. There is therefore no way to tell which observer is right and which one isn't about what happens at the same time. The only thing you can conclude then is that different observers don't agree on what happens at the same time. This is the second step of the argument. It's that simultaneity isn't universal. It depends on how you move. And that isn't only odd. If you follow this thought, it entirely erodess our intuitive notion of reality. Because now let's look at step three. For an observer who sits still, all events that lie on this horizontal line happen at the same time. And by the first step of the argument, we reconstruct what we mean by now from signals that arrive later. So this entire line is what exists now in this moment and then this is the next moment and so on. Suppose that this first observer says if this is now then everything on this line is also now. And this observer says if this is now then this is also now. But there could be an observer at any distance from you. Each of whom has their own now. None of those nows is any better than any other. So this means every moment your past, present or future is now for someone somewhere. If you take these three steps together, then you arrive at the following conclusion. Once you identify that what exists with a present moment, then Einstein's theory forces you to accept that all moments exist in the same way. This perplexing consequence of Einstein's relativity has been dubbed the block universe by physicists. In this block universe, the past still exists and the future already exists. They are all there and our fourdimensional spaceime have always been there and will always be there. In case you think this sounds pretty crazy, it's actually a standard argument that physics professors throw at undergrad students and then just let the students cope with the consequences because how do we make sense of this? What does it mean that the past, present, and future all exist the same way? Yes, I've spent three decades trying to wrap my head around this. I think that for one thing it means that our past selves still exist the same way as this present self. It's just that our past selves are all disconnected from each other. They can't communicate. And our future selves also already exist. But doesn't this also mean that we can't change anything about the future? Depends on what you mean by change. The future will be whatever is the consequence of today. But these consequences depend on what you and the matter around you do. This means that the future does depend on what you will do. It depends on what information you take away from this video and how this affects your thoughts and your actions. The opposite is also the case by the way. The present depends on the future in the same way that the future depends on the present. It's just that we don't think about it this way. Einstein's theories really are strange, and I don't think we've yet entirely appreciated how strange they are. I explained the block universe in more detail in my book, Existential Physics, but since I wrote the book, I've come to a different conclusion about the role of quantum physics in this argument. You see, so far what I told you was all Einsteinian physics, which doesn't know anything about quantum physics. If you take into account quantum physics, the argument becomes more complicated because quantum physics has an indeterministic unpredictable element which is what happens the moment one makes a measurement. And the entire problem of quantum physics is that we don't know just what a measurement is. Now I used to think that this means it's only after you've made a measurement that the past becomes fixed so to say. There seems to be a difference between the past and the future in quantum mechanics. But I now think that this unpredictability in quantum physics goes in both directions of time. What I mean is that if you take all the information of a particle that's in the wave function, then you can't predict the outcome of a measurement of say the position of a particle. You can only predict the probability of measuring the particle in one place or another. But that also goes the other way around. Once you've measured the particle, you can't say exactly what the wave function was. You can only say it was more likely to have been this instead of that. And this sort of indeterminism is entirely compatible with the block universe. It's not deterministic, okay? But it still doesn't tell you which moment is now. To put this differently, there's no such thing as a now in the mathematics of quantum physics. Not unless you actually change something about the maths. This is why I think the block universe is how nature really works and that our perception of the present moment being somehow special is just false. Like the idea that the earth is flat. Maybe this video more than anything explains why I've always been about fit for academia. Because what really is this? Is it philosophy? Is it physics? It's neither here nor there. Yet for me, questions like this are the reason I studied physics because I want to understand how the universe works. And how about you? Let me know in the comments. That the future already exists can sound a little defeist, like it doesn't matter what we do anyway. But this would be the wrong message to take away from this video. It matters a lot what decisions you make, whether those were predetermined or not. And since this is a fresh and new year, I have a suggestion for a simple decision that'll bring more meaning to your life. It's to support Planet Wild, who've been sponsoring this video. I've been working with Planet Wild for more than 2 years now. If you don't know them, Planet Wild is basically crowdfunding for nature. Every month, their community funds a new nature protection mission, and they document everything on YouTube, so you can see what your contribution makes possible. It's why I became a member myself. In their most recent mission, they fought ocean plastic in India. It's become one of my favorites. If you want to start the year doing something that matters, join Planet Wild. Contribute any amount that feels right. And if you're one of the next 100 people to sign up using my code, Zabina41, I'll cover your first month. All you need to do is scan the code or click the link in the description. And don't worry, you can cancel anytime. If you want to see them in action, watch their mission fighting ocean plastic. Thanks for watching. See you tomorrow.