Does the Past Still Exist?
Summary
TLDRCe script vidéo explore la nature mystérieuse du temps, une notion centrale de l'existence humaine et de la physique. Einstein a révolutionné notre compréhension du temps avec sa théorie de la relativité restreinte, introduisant l'idée que le temps n'est pas universel mais dépend du observateur. Le script discute de la 'relativité de la simultanéité', où le 'maintenant' d'un observateur peut différer de celui d'un autre en mouvement, et introduit le concept de l'univers bloc, où le passé, le présent et le futur existent tous de la même manière. Le script invite également à réfléchir sur la compatibilité de la mécanique quantique avec ces idées, laissant le spectateur avec une perspective altérée sur la réalité.
Takeaways
- 🕰️ Le temps est l'un des plus grands mystères de notre existence et de la physique.
- 👀 Notre perception du temps comme quelque chose qui passe est différente de celle de la physique.
- 🧠 Einstein a changé notre compréhension du temps avec sa théorie de la relativité restreinte.
- 🌐 Hermann Minkowski a proposé que le temps devrait être traité comme une dimension et non comme un paramètre.
- 🔄 La combinaison de l'espace et du temps en un espace-temps 4D permet de comprendre la symétrie des équations d'électrodynamique de Maxwell.
- 🚀 La vitesse de la lumière dans le vide est finie et aucune chose ne peut aller plus vite que la lumière.
- 🌌 Le concept de 'maintenant' est relatif et dépend du point de vue de l'observateur.
- 🤔 La notion d'observateur en physique théorique n'implique pas nécessairement une entité vivante mais plutôt un système de coordonnées.
- 🔄 L'idée que le passé, le présent et le futur existent de la même manière est connue sous le nom d'univers bloqué.
- ⚛️ La mécanique quantique est compatible avec la relativité restreinte et ne change rien à l'univers bloqué.
Q & A
Quel est le mystère principal lié au temps mentionné dans le script?
-Le mystère principal lié au temps est que notre expérience du temps comme une succession de moments distincts (passé, présent, futur) est très différente de la manière dont la physique le décrit, en particulier selon la théorie de la relativité restreinte d'Einstein.
Pourquoi Hermann Minkowski a-t-il considéré le temps comme une dimension plutôt qu'un paramètre universel?
-Minkowski a remarqué que les équations d'électrodynamique de Maxwell prennent plus de sens si l'on considère le temps comme une dimension, similaire à un axe spatial, permettant ainsi des rotations entre les axes spatiaux et le temps, ce qui explique la symétrie des équations.
Que signifie le diagramme espace-temps et comment est-il utilisé pour illustrer le mouvement?
-Un diagramme espace-temps est un outil conceptuel utilisé pour visualiser le mouvement dans un espace à trois dimensions où l'axe vertical représente le temps et les autres axes représentent l'espace. Un mouvement à vitesse constante est représenté par une ligne droite inclinée, tandis qu'un changement de vitesse implique une rotation dans l'espace-temps.
Quelle est la différence entre le temps et l'espace dans le contexte de la théorie de la relativité restreinte?
-Dans la théorie de la relativité restreinte, le temps est considéré comme une dimension de l'espace-temps, mais contrairement à l'espace, on ne peut pas revenir en arrière dans le temps; il est unidirectionnel.
Quels sont les quatre postulats fondamentaux de la théorie de la relativité restreinte d'Einstein mentionnés dans le script?
-Les quatre postulats sont: 1) La vitesse de la lumière dans le vide est finie et constante; 2) Aucune chose ne peut aller plus vite que la lumière; 3) Tous les points de vue des observateurs sont également valides; 4) Les observateurs ne doivent pas nécessairement exister physiquement, ils peuvent être des systèmes de coordonnées théoriques.
Comment le script explique-t-il la notion d'« maintenant » (now) dans le contexte de la relativité restreinte?
-Le script explique que la notion d'« maintenant » dépend du point de vue de l'observateur et n'est pas universelle. Grâce à un gedankenexperiment d'Einstein, on peut construire un « maintenant » pour un observateur en utilisant des miroirs et des photons, mais cet « maintenant » peut varier d'un observateur à un autre en fonction de leur mouvement relatif.
Pourquoi la notion de 'relativité de la simultanéité' est-elle importante dans la physique?
-La relativité de la simultanéité est importante car elle montre que ce qui est simultané pour un observateur ne l'est pas nécessairement pour un autre en mouvement relatif, ce qui remet en question la notion commune d'un 'maintenant' universel.
Que signifie le 'block universe' et comment est-il lié à la théorie de la relativité restreinte?
-Le 'block universe' est la notion selon laquelle le passé, le présent et le futur existent tous de la même manière, formant un bloc statique de l'espace-temps. Cette idée découle de la théorie de la relativité restreinte qui ne permet pas de distinguer un 'maintenant' universel pour tous les observateurs.
Comment le script aborde-t-il la question de l'existence dans le contexte de la physique quantique?
-Le script mentionne que la physique quantique, bien qu'indéterministe, est compatible avec la relativité restreinte et ne change rien à la notion de 'block universe'. La mise à jour du vecteur d'état dans la mécanique quantique se produit plus rapidement que la lumière, mais comme elle n'est pas observable, elle ne crée pas de contradiction avec la relativité restreinte.
Quelle est la conclusion finale du script sur la nature du temps?
-La conclusion finale du script est que, selon notre compréhension actuelle et la théorie de la relativité restreinte, le passé, le présent et le futur existent de la même manière, ce qui suggère que le temps peut être perçu différemment selon les observateurs et les contextes.
Outlines
🕰️ Le mystère du temps et la physique
Ce paragraphe introduit le concept de temps comme l'un des plus grands mystères de l'existence humaine et de la physique. L'auteur mentionne que notre perception du temps est différente de celle offerte par la physique, et cite Albert Einstein comme étant un des pionniers dans la compréhension de la nature du temps. Le temps est généralement perçu comme unité, avec un 'maintenant' particulier, mais Einstein et Hermann Minkowski ont proposé que le temps est un quatrième dimension, fusionnant l'espace et le temps en un 'espace-temps' 4D. Cette idée remet en question la notion universelle du temps, suggérant que le 'maintenant' peut être différent selon l'observateur.
🚀 La relativité de la simultanéité
Dans ce paragraphe, l'auteur explique comment la vitesse de la lumière, qui est finie et constante, affecte notre perception du 'maintenant'. Il utilise un exemple d'expérience mentale d'Einstein pour démontrer comment l'idée de 'maintenant' peut varier selon l'observateur. L'auteur discute de la relativité de la simultanéité, où le 'maintenant' d'un observateur peut ne pas correspondre à celui d'un autre observateur en mouvement relativement à lui. Cela soulève des questions sur la validité des coordonnées spatiales et temporelles et comment elles sont interprétées par différents observateurs.
🌌 L'univers bloc et l'existence du temps
Le paragraphe explore l'idée de l'univers bloc, où tous les événements passés, présents et futurs existent de la même manière. L'auteur utilise des diagrammes d'espace-temps pour montrer comment des événements qui ne sont pas connectés de manière causale peuvent être perçus comme simultanés par certains observateurs. Il discute également des implications philosophiques de cette perspective, où le passé, le présent et le futur pourraient tous exister de manière équivalente, ce qui remet en question la notion d'un 'maintenant' unique.
🎓 La mécanique quantique et l'espace-temps
Enfin, le dernier paragraphe touche brièvement la mécanique quantique et son interaction avec la théorie de la relativité restreinte. L'auteur mentionne que, malgré l'indéterminisme de la mécanique quantique, elle est compatible avec la relativité restreinte et ne change pas la perspective de l'univers bloc. Il conclut en annonçant un cours sur la mécanique quantique disponible sur Brilliant, un outil d'apprentissage interactif qui couvre de nombreux sujets scientifiques et mathématiques.
Mindmap
Keywords
💡temps
💡Einstein
💡espace-temps
💡relativité spéciale
💡observateur
💡vitesse de la lumière
💡relativité de la simultanéité
💡univers bloc
💡quantum mechanics
💡Brilliant
Highlights
Time is a mystery in both human experience and physics.
Einstein's theory challenges the conventional perception of time.
Hermann Minkowski proposed time as a dimension similar to space.
Space-time diagrams visualize the movement of objects through space and time.
The speed of light is the maximum velocity in the universe.
Time is not universal; it is relative and depends on the observer.
Einstein's theory of Special Relativity is based on four key assumptions.
Observers in physics are theoretical and can be any coordinate system.
The speed of light is constant and finite, with no object exceeding it.
The concept of 'now' is not absolute and varies with the observer's motion.
Einstein's thought experiment with mirrors and photons defines 'now'.
The relativity of simultaneity shows that 'now' is observer-dependent.
The block universe concept suggests all times exist equally.
Quantum mechanics does not contradict the block universe theory.
The past, present, and future may all exist in the same way.
The block universe implies that time does not flow or change.
The concept of 'now' can be defined without observable consequences.
Transcripts
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 dot 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”.
Hermann 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 4 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 tilted 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 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 respect. 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 a
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 it,
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 observers’ 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 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 side-step
this, erm, 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 a 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
are probably watching this video on a screen, a phone or laptop. Is the screen there now?
Unless you are 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 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 snatched your phone,
it’d 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 summed it up.
This isn’t wrong – I’ll 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
to 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 Gedankenexperiment, a “thought experiment”.
He said, suppose you place a mirror to your right and one to your left. You and the mirrors
are at fixed distance to each other, so in the space time diagram it looks like this.
You send one photon 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
space-ship. Her name is Alice, she is 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 space-ship 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 get there 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’s 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 left takes longer to get back, 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 relative to her. And she thinks that
her notion of “now” is right and yours is wrong.
So who is 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 forth 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. Your “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 every-day
life. They only become noticeable when relative velocities are close by 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 wilder. 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. 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 are 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 are 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 the events that happen at the same time as those events,
then all events exist “now”. Another way to put it 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 are willing to say something exists now even though you can’t see
or experience it in any way. If you are willing to say that only things exist which are now
and 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 in space-time for each moment in time. This is called a “slicing” but unfortunately
it has nothing to do with pizza. If it had any observable consequences, that
would contradict the fourth assumption 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 “now” 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 all 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 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 right away.
I had a lot of fun working with Brilliant’s team on this,
and I hope you’ll enjoy it too. To support this channel and learn more about Brilliant,
go to Brilliant dot org slash Sabine and sign up for free.
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Thanks for watching, see you next week.
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