One of the greatest mysteries of our existence is also one of the greatest mysteries of physics: time. We experience time as a passage, with a special moment that we call now. Right now, you are watching this video; half an hour ago, you were doing something else. Whatever you did, there is no way to change it. And what you will do in half an hour depends on you. At least, that is how we perceive time. But what physics tells us about time is very different from our perception. The person who discovered this was none other than Albert Einstein. I know, that guy again. It turns out he knew it all. What did Einstein teach us about the past, present, and future? That is what we will talk about today. The topic we are discussing today is covered in more detail in my new book Existential Physics, which will be published in August. You can find more information about the book at existentialphysics.com. We think of time as something that works the same way for everyone and for all objects. If a second passes for me, a second passes for you, and a second passes for the clouds above. This makes time a universal parameter. This parameter labels how much time passes and also what we all mean by now. Hermann Minkowski was the first to realize that this may not be entirely correct. He noticed that Maxwell's electrodynamic equations make much more sense if one treats time as a dimension, not as a

Then, Minkowski said, we simply combine space with time into a four-dimensional spacetime, and then we can rotate space in time just as we can rotate two directions of space with respect to each other. This naturally explains why Maxwell's equations have the symmetry they do. It has nothing to do with electric and magnetic fields; it comes from the very properties of space and time. I can't draw a flower, let alone four dimensions, but I can manage two straight lines: one for time and the other for at least one dimension of space. This is called a spacetime diagram. If you remain still, then your movement in such a diagram is a straight vertical line. If you move at a constant speed, your movement is a straight line inclined at some angle. So, if you change speed, you are rotating in spacetime. The maximum speed at which you can move is the speed of light, which by convention is usually drawn at a 45-degree angle. In space, we can go back and forth, left and right, or up and down. With time, we can only move forward; we cannot change direction, and there are no entry paths for awkward three-point turns. So, time remains different from space in some aspects. But now that time is also a dimension, it is clear that it is just a label for coordinates; it is not universal. There are many ways to label a two-dimensional space because you can choose your axes however you want. The same

they don't mean much. So, what is the time we are talking about? What does it even mean that time is a dimension? Are there other dimensions? Supernatural ones? Could that explain the strange sounds you've been hearing at night? No. That's a separate issue; I'm afraid I can't help you with that. It was Albert Einstein who understood what this means. If we also want to understand it, we need four assumptions. The speed of light in a vacuum is finite, it is always the same, nothing can go faster than the speed of light, and all points of view of observers are equally valid. This formed the basis of Einstein's theory of special relativity. Oh, and furthermore, observers do not have to exist. I mean, this is theoretical physics, so we are basically talking about theoretical observers. So, if there could be an observer with a certain point of view, then that point of view is equally valid as yours. Who or what is an observer? Is an ant an observer? A tree? How about a dolphin? What do you need to observe to deserve to be called an observer, and with what do you have to observe? Believe it or not, there is actually quite a bit of discussion about this in the scientific literature. We will set aside this interesting discussion and use the word observer in the same way Einstein did, which is a coordinate system. You see, it is a coordinate system that a theoretical observer could use, dolphin or

But if it was good enough for Einstein, it will be good enough for us. So, Einstein's assumption basically means that any coordinate system should be just as good as any other for describing physical reality. These four assumptions sound quite innocent at first, but they have profound consequences. Let's start with the first and third: the speed of light is finite, and nothing travels faster than light. You are probably watching this video on a screen, a phone, or a laptop. Is the screen there now? Unless you are from the future watching this video as a hologram in your space home, I will assume the answer is yes. But a physicist might point out that you actually don't know. Because the light emitted by the screen right now has not yet reached you. Furthermore, if you are from the future watching this as a hologram, make sure to look at me from the right side. It’s my good side. You might have the phone in your hand, but nerve signals are ridiculously slow compared to light. If you couldn't see your hand and someone snatched the phone away, the information that the phone is gone would take several microseconds to reach your brain. So, how do you know your phone is there now? One way to answer this question is to say, well, you don't know, and you really don't know that something exists now, apart from your own thoughts. I think, therefore I am, as Descartes summarized. This isn't wrong; I will

we are discussing things that happen now all the time, and we never worry about how long it takes for light to travel. Why can't we agree on some now and move on? I mean, think about that space-time diagram. Clearly, this flat line is now, so let's agree on this and move forward. Okay, but if this is going to be physics instead of just a diagram, we need to come up with an operating procedure to determine what we mean by now. You have to find a way to measure it. Einstein did exactly that in what he called a Gedankenexperiment, a thought experiment. He said, suppose you place a mirror to your right and another to your left. You and the mirrors are at a fixed distance from each other, so in the space-time diagram, it looks like this. You send a photon to the left and another to the right, making sure that both photons leave you at the same time. Then you wait to see if the photons return at the same time. If they don’t, you adjust your position until they do. Now remember Einstein's second assumption: the speed of light is always the same. This means that if you can send photons to both mirrors and they return at the same time, then you must be exactly in the middle between the mirrors. The final step is then to say that exactly halfway through the time it takes for the photons to return, you know they must be bouncing off the mirror

the light from there hasn't reached you yet. It seems that you have found a way to construct now. But here is the problem. Suppose you have a friend who flies at a constant speed, perhaps in a spaceship. Her name is Alice; she is much cooler than you, and you have no idea why she agreed to be your friend. But here she is, accelerating in her spaceship from left to right. As we saw earlier in your space-time diagram, Alice moves in a straight, slanted line. She does exactly what you do: she places mirrors on both sides, sends out photons, and waits for them to return, then she states that when half the time has passed, that is the moment when the photons hit the mirrors. Except that this is clearly not right from her perspective. Because the mirrors on her right are in the direction of her flight, it takes longer for the light to reach them than it does for the mirrors on the left, which are moving towards the light. She would say that the photon going to the left clearly hits the mirror first because the mirror is moving towards it. From your perspective, she simply doesn’t realize that when the photons return to Alice, the exact opposite happens. The photon coming from the left takes longer to return, so the net effect cancels out. What Alice says happens now is clearly not what you think happens now. For Alice, on the other hand, you are the one moving in relation to her

than the speed of light, lo cual es imposible. Así que, en este caso, hay un espacio entre esos dos eventos que no se puede cruzar. Sin embargo, desde la perspectiva de diferentes observadores, estos eventos pueden parecer que ocurren en diferentes momentos. Es decir, uno podría ver un evento sucediendo antes que el otro, mientras que otra persona podría verlos en el orden inverso. Esto lleva a una conclusión fascinante: la simultaneidad no es absoluta, sino que depende del observador y de su movimiento relativo. Así que, en el mundo de la relatividad, no hay una única línea de tiempo que todos compartan. En cambio, hay múltiples líneas de tiempo que dependen de cómo se mueven y dónde están los observadores. Esto puede parecer complicado, pero es una de las características más sorprendentes de nuestra comprensión del universo.

Light cannot be sent from one to another. Schematically, this means that if you connect the two events, the line has an angle of less than 45 degrees with respect to the horizontal. The previous construction with the mirrors shows that for either of these two events, there is always some observer for whom those two events occur at the same time. You just have to imagine that the mirrors fly through the events and the observer flies directly in the middle. Then you adjust the speed until the photons hit both events at the same time. Well, two causally disconnected events occur simultaneously for some observer. Now take any two events that are causally connected. For example, eating too much cheese at dinner and then feeling terrible the next morning. Find some event that is not causally connected to either of the two. Let’s say this event is a supernova exploding in a distant galaxy. There are always observers for whom the supernova and your cheese dinner are simultaneous, and there are observers for whom the supernova and your next morning are simultaneous. So, let’s put all of that together. If you feel comfortable saying that something, anything, exists now that is not here, then according to Einstein's fourth assumption, this must be the case for all observers. But if all the events you believe happen now exist and all the other observers say the events occur at the same time as those events, then all events exist now. Another way to say it is that all

This is called the block universe. It is just there. It does not arise, it does not change. It simply sits there. If you find that difficult to accept, there is another possibility of consistently combining a notion of existence with Einstein's Special Relativity. Everything I just said comes from assuming that you are willing to say that something exists now, even if you cannot see it or experience it in any way. If you are willing to say that only things that are here and now exist, then you do not get a block universe. But perhaps that is even harder to accept. Another option is to simply invent a notion of existence and define it as a particular portion in spacetime for each moment in time. This is called slicing, but unfortunately, it has nothing to do with pizza. If it had any observable consequences, that would contradict the fourth assumption made by Einstein. So it is in conflict with Special Relativity, and since this theory is extremely well confirmed experimentally, this would almost certainly mean that the idea is in conflict with observation. But if you just want to define a now that has no observable consequences, you can do so. Although I am not sure why you would want to. Quantum mechanics does not change anything about the block universe because it remains compatible with Special Relativity. The update of the wave function measurement, which I talked about in this previous video, occurs faster than the speed of light. If it could be observed,

Some people have argued that, since quantum mechanics is indeterministic, the future cannot exist in the block universe, and therefore there must also be a special moment of now that divides the past from the future. And maybe that's true. But even if that were the case, the previous argument still applies to the past. So yes, it is true. As far as we currently know, the past exists in the same way that the present does. So you thought this was a video about special relativity, but I've been talking again about quantum mechanics. In fact, I now have a complete course on quantum mechanics on Brilliant that complements my videos on the topic. Brilliant is an amazing tool for learning with courses on a wide variety of subjects in science and mathematics. Our new course will provide you with an introduction to superpositions and entanglements, the uncertainty principle, non-commutativity, and Bell's theorem. Then you can build on your knowledge with Brilliant's courses on quantum objects and quantum computing, or linear algebra, or wherever you want to go after this. Like all their courses, our new course is interactive and will challenge you with questions so you can check your understanding immediately. I had a lot of fun working with the Brilliant team on this, and I hope you enjoy it too. To support this channel and learn more about Brilliant, go to Brilliant dot org slash Sabine and sign up for free. The first 200 subscribers who use this link will receive a

Thank you for watching us. We'll see you next week.

Does the Past Still Exist?

As simple as possible, but not any simpler! Science and technology updates and summaries. No hype, no spin, no tip-toeing around inconvenient truths. News each Wednesday, new topical video each Saturday.

Sabine Hossenfelder has a PhD in physics. She is author of the books "Lost in Math: How Beauty Leads Physics Astray" (Basic Books, 2018) and "Existential Physics: A Scientist's Guide to Life's Biggest Questions" (Viking, 2022).
