Does the Past Still Exist?

one of the biggest mysteries of our existence is also one of the biggest mysteries of physics time we experience time as passing with a special moment that we call now now you're watching this video half an hour ago you were doing something else whatever you did there's no way to change it and what you will do in half an hour is up to you at least that's how we perceive time but what physics tells us about time is very different from our perception the person who figured this out was none other than albert einstein i

know that guy again turns out he kind of knew it all what did einstein teach us about the past the present and the future that's what we'll talk about today the topic we're talking about today is covered in more detail in my new book existential physics which will be published in august you find more info about the book at existentialphysics.com we think about time as something that works the same for everyone and every object if one second passes for me one second passes for you and one 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 herman minkowski was the first to notice that this may not be quite right he noticed that maxwell's equations of electrodynamics make much more sense if one treats time as a dimension not as a parameter just like a ball doesn't change if you rotate one direction of space into another maxwell's equations don't change if you rotate one direction of space into time so minkowski said we just combine space with time to a four dimensional space time and then we

can rotate space into time just like we can rotate two directions of space into each other and that naturally explains why maxwell's equations have the symmetry they do have it doesn't have anything to do with electric and magnetic fields it comes from the properties of space and time themselves i can't draw a flower let alone four dimensions but i can just about manage two straight lines one for time and the other for at least one dimension of space this is called a space-time diagram if you just stand still then your motion in such a diagram

is a straight vertical line if you move at a constant velocity your motion is a straight line tilts it at some angle so if you change velocity you rotate in space time the maximal velocity 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 forward backward left right or up and down in time we can only go forward we can't make a u-turn and there aren't any driveways for awkward three-point turns either so time is still different from space in

some respects but now that time is also a dimension it's clear that it's just a label for coordinates there's nothing universal about it there are many ways to put labels on a two-dimensional space because you can choose your axes as you want the same is the case now in space-time once you have made time into a dimension the labels on it don't mean much so what then is the time that we talk about what does it even mean that time is the dimension do other dimensions exist supernatural ones that could explain the strange sounds you've

been hearing at night no that's a separate problem i'm afraid i can't help you with it was albert einstein who understood what this means if we also want to understand that we need four assumptions the speed of light in vacuum is finite it's always the same nothing can go faster than the speed of light and all observer's viewpoints are equally valid this formed the basis of einstein's theory of special relativity oh and also the observers don't have to exist i mean this is theoretical physics so we're talking about theoretical observers basically so if there could

be an observer with a certain viewpoint then that viewpoint 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 being called an observer and what do you have to observe with believe it or not there's actually quite some discussion about this in the scientific literature we'll sidestep this interesting discussion and use the word observer the same way that einstein did which is a coordinate system you see it's a coordinate system that the theoretical observer might use

dolphin or otherwise yeah maybe not exactly what the fbi thinks an observer is but then if it was good enough for einstein it'll be good enough for us so einstein's assumption basically means any coordinate system should be as good as any other for describing physical reality these four assumptions sound rather innocent at first but they have profound consequences let's start with the first and third the speed of light is finite and nothing goes faster than light you're probably watching this video on a screen a phone or laptop is the screen there now unless you're from

the future watching this video as a hologram in your space house i'm going to assume the answer is yes but a physicist might point out that actually you don't know because the light that's emitted from the screen now hasn't reached you yet also if you are from the future of watching this as a hologram make sure to look at me from the right it's my good side maybe you hold the phone in your hand but nerve signals are ridiculously slow compared to light if you couldn't see your hand and someone snatch your phone it'll take

several microseconds for the information that the phone is gone to even arrive in 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 really you don't know that anything exists now other than your own thoughts i think therefore i am as descartes sumpted up this isn't wrong i come back to this later but it's not how normal people use the word now we talk about things that happen now all the time and we never worry about how long it takes

for light to travel why can't we just agree on some now and get on with it i mean think back of that space-time diagram clearly this flat line is now so let's just agree on this and move on okay but if this is to be physics rather than just a diagram you have to come up with an operational procedure to determine what we mean by now you have to find a way to measure it einstein did just that in what he called the dump experiment a thought experiment he said suppose you place a mirror to

your right and one to your left you and the mirrors are at a fixed distance to each other so in the space-time diagram it looks like this you send one photo left and one right and make sure that both photons leave you at the same time then you wait to see whether the photons come back 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 if you can send photons to both mirrors and they come back at

the same time then you must be exactly in the middle between the mirrors the final step is then to say that at exactly half the time it takes for the photons to return you know they must be bouncing off the mirror you could say now at the right moment even though the light from there hasn't reached you yet it looks like you've found a way to construct now but here's the problem suppose you have a friend who flies by at some constant velocity maybe in a spaceship her name is alice she's much cooler than you

and you have no idea why she's agreed to be friends with you but here she is speeding by in her spaceship left to right as we saw earlier in your space-time diagram alice moves on a tilted straight line she does the exact same thing as you places mirrors to both sides sends photons and waits for them to come back and then says when half the time has passed that's the moment the photons hit the mirrors except that this clearly isn't right from your point of view because the mirrors to her right are in the direction

of her flight so the light takes longer to gather than it does to the mirrors on the left which move towards the light you would say that the photon which goes left clearly hits the mirror first because the mirror is coming at it from your perspective she just doesn't notice because when the photons go back to alice the exact opposite happens the photon coming from the left takes longer to get back so the net effect cancels out what ellis says happens now is clearly not what you think happens now for alice on the other hand

you are the one moving relative to her and she thinks that her notion of now is right and yours is wrong so who's right probably alice you might say because she's much cooler than you she owns a spaceship after all maybe but let's ask einstein here is where einstein's fourth assumption comes in the viewpoints of all observers are equally valid so you're both right or to put it differently the notion of now depends on the observer it is observer dependent as physicists say you're now is not the same as my now if you like technical

terms this is also called the relativity of simultaneity these mismatches in what different observers think happens now are extremely tiny in everyday life they only become noticeable when relative velocities are close to the speed of light so we don't normally notice them if you and i talk about who knocked at the door right now we won't misunderstand each other if we'd zipped around with nearly the speed of light however referring to now would get very confusing this is pretty mind-bending already but wait it gets wider let us have a look at the space-time diagrams again

now let us take any two events that are not causally connected this just means that if you wanted to send a signal from one to the other the signal would have to go faster than light so signaling from one to the other isn't possible diagram diagrammatically this means if you connect the two events the line has an angle less than 45 degrees to the horizontal the previous construction with the mirrors shows that for any two such events there is always some observer for whom those two events happen at the same time you just have to

imagine the mirrors fly through the events and the observer flies through directly in the middle and then you adjust the velocity until the photons hit both events at the same time okay so any two causally disconnected events happen simultaneously for some observer now take any two events that are causally connected like eating too much cheese for dinner and then feeling terrible the morning after find some event that isn't causally connected to either let's say this event is a supernova going off in a distant galaxy there then always observers for whom the supernova and your cheese

dinner are simultaneous and there are observers for whom the supernova and your morning after are simultaneous let's then put all those together if you're comfortable with saying that something anything exists now which isn't here then according to einstein's fourth assumption this must be the case for all observers but if all the events that you think happen now exist and all other observers say that the events that happen at the same time as those events exist then all events exist now another way to put this is that all times exist in the same way this is

called the block universe it's just there it doesn't come into being it doesn't change it just sits there if you find that somewhat hard to accept there is another possibility to consistently combine a notion of existence with einstein's special relativity all that i just said came from assuming that you're willing to say something exists now even though you can't see or experience it in any way if you're willing to say that only things exist which are now end here then you don't get a block universe but maybe that's even more difficult to accept another option

is to simply invent a notion of existence and define it to be a particular slice and space time for each moment in time this is called a slicing but unfortunately it has nothing to do with pisa if it had any observable consequences that would contradict the fourth assumption that einstein made so it's in conflict with special relativity and since this theory is experimentally extremely well confirmed this would almost certainly mean the idea is in conflict with observation but if you just want to define a noun that doesn't have observable consequences you can do that though

i'm not sure why you would want to quantum mechanics doesn't change anything about the block universe because it's still compatible with special relativity the measurement update of the wave function which i talked about in this earlier video happens faster than the speed of light if it could be observed you could use it to define a notion of simultaneity but it can't be observed so there's no contradiction some people have argued that since quantum mechanics is indeterministic the future can't already exist in the block universe and that therefore there must also be a special moment of

now that divides the past from the future and maybe that is so but even if that was the case the previous argument still applies to the past so yeah it's true for we currently know the past exists the same way as the present so you thought this is a video about special relativity but then i've been talking about quantum mechanics again indeed i now have an entire course about quantum mechanics up at brilliant which accompanies my videos on the topic brilliant is an amazing tool for learning with courses on a large variety of topics in

science and mathematics our new course will give you an introduction to superpositions and entanglement the uncertainty principle non-commutativity and bell's theorem and you can then build up your knowledge with brilliance 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 right away i had a lot of fun working with brilliant's team on this and i hope you'll enjoy our new course to support this channel and learn more

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