00:00Time would only be a tour of prestidigitation.
00:03Ah yes, we are used to what time flows like a car, always turned to the future.
00:09It creates causes and effects, aligning events one after the other, like dominoes.
00:15Sometimes it stretches and becomes so slow that it feels like it's going to stop.
00:20It is focused on the present moment, because it is the only one that really matters.
00:25But there is a new theory.
00:27It could be that it does not exist at all.
00:33We generally think that space has three dimensions.
00:36The top and the bottom, the left and the right, the front and the back.
00:40And everyone thought that time was a separate thing,
00:43just a universal clock that goes tic-tac in the background, and that is the same for everyone.
00:48That's when Einstein appeared.
00:50He decided to surprise people in 1905 by introducing his theory of restricted relativity.
00:56According to this theory, time is also a dimension, like the others,
01:01and they are all linked in one and the same spacetime.
01:04On this basis, it turned out that time can change depending on the speed at which you move.
01:10Let's now imagine two twins.
01:13The twin A stays on Earth, the twin B goes into space aboard a spaceship,
01:18moving at a speed close to the speed of light.
01:22Let's say 90% of the speed of light.
01:25The twin B travels to a distant star, then returns to Earth.
01:29For the twin B, the journey seems to have lasted only a few years.
01:33But when he returns to Earth, it turns out that the twin A has aged much more.
01:37He discovers that 20, 30 years or more have passed on Earth.
01:41All this is due to a phenomenon called temporal dilation.
01:46This phenomenon occurs when we reach speeds close to those of light,
01:50which slows down time for the traveler compared to someone who stays in the same place.
01:59Ten years later, Einstein proposed his theory of general relativity.
02:03It was then that we learned that time changes not only depending on our speed,
02:08but also depending on gravity.
02:10At the time, Newton thought that gravity was a magical force
02:14that made objects attract each other.
02:17But according to Einstein's theory, it is not a force, but rather a side effect.
02:22Imagine space as a huge piece of fabric.
02:25When you throw a heavy ball on a fabric, it bends.
02:29Now, if you throw small balls all around, they will naturally approach the heavy ball.
02:36In the same way, massive objects such as stars and planets
02:40make space-time curve around them.
02:43When a very heavy object is in space-time, matter curves around it.
02:47This curvature tells objects how to move.
02:51This is what we call gravity.
02:53This idea gave rise to all kinds of interesting predictions
02:57that proved to be accurate later on.
03:00Curvature of light under the effect of gravity,
03:03black holes and time dilation around these black holes,
03:06as in the movie Interstellar.
03:08This theory revolutionized our vision of things.
03:11But we may be at the dawn of another revolution.
03:17Some philosophers have a crazy idea.
03:19They say that the things we believe about time
03:22are just ideas in our heads, and not facts of the physical world.
03:25This idea, that goes along with the traditional vision of time,
03:28is called the Bloch universe.
03:30It relies on Einstein's theory of relativity,
03:33but by extending it and complicating it.
03:35Imagine the universe as a huge sandwich.
03:38This sandwich contains all the moments of time,
03:41past, present and future, at the same time.
03:43Each layer or slice of the sandwich represents a different moment in time.
03:47And all these layers exist simultaneously,
03:50and not one after the other.
03:52In this model of our world,
03:54the past, the present and the future are all equal.
03:57Unlike our daily experience,
03:59where the past no longer exists,
04:01where the future has not yet taken place,
04:03and where only the present is real,
04:05the Bloch universe says that each point in time
04:08is as real as any other point.
04:10Imagine, your birth, the present moment
04:13and what you will do in ten years,
04:15all exist together in this block.
04:17According to this theory,
04:19there is no real temporal flow.
04:21We are used to the fact that time flows from one moment to the next,
04:24even with Einstein's theory.
04:26But in the Bloch universe,
04:28the impression that time flows
04:30is more related to the way we experience events,
04:32when we only remember how time itself works.
04:35It's like reading a magic book,
04:37all the pages exist at the same time,
04:39but you can only live the story page by page.
04:44This implies frightening things,
04:46like fate,
04:47and the question of whether the free will really exists.
04:50If everything is already planned,
04:52does what we do matter?
04:54For example,
04:55the fact that you clicked on this video
04:57was already predetermined in the Bloch universe.
05:00Every choice you made was an illusion.
05:03You are only a program that follows a code,
05:05but that is able to understand the consequences.
05:08This would explain why we order malibu
05:11at midnight,
05:12while knowing that it is a bad idea
05:14and that we are going to bite our fingers.
05:16But some scientists claim
05:18that the flow and direction of time
05:20are unrealistic,
05:22because the cause and effect exist well.
05:25If your birth,
05:26your present moment
05:27and your gray future all existed
05:29at the same time,
05:30you would be able to visit your past at any time
05:33and you would already know what your future looks like.
05:36It could therefore be that all this debate
05:38is simply based on the way
05:40we reconstruct our past stories
05:42and not on the improbable proof
05:44that the future is already done.
05:47There is also an endless battle
05:49between general relativity
05:51and quantum mechanics,
05:52which only adds spice to the debate.
05:55On the one hand,
05:56general relativity plays with time,
05:58as if it were an extensible model.
06:01The flow of time can stretch
06:03or compress depending on speed
06:05or gravity.
06:06On the other hand,
06:07in quantum mechanics,
06:08time is not something
06:10that can be manipulated or modified.
06:12It is a permanent scene
06:14on which the particles
06:15and forces of the universe act.
06:17These two domains have been confronting each other for a long time
06:19and scientists are trying to bring them together
06:21in a coherent theory of everything that exists.
06:24Some even try to merge them
06:26in a logical way,
06:28as with the theory of strings.
06:30The theory of strings may seem frightening,
06:32but it simply says that the entire universe,
06:34with all its particles and all its atoms,
06:36is made up of extremely thin vibrating strings.
06:40Each string can vibrate in different ways,
06:42like the strings of a guitar,
06:44and in many dimensions.
06:46The way these strings vibrate
06:48determines the type of particle they will form,
06:50like an electron or a quark.
06:52Another theory,
06:54called loop quantum gravitation,
06:56claims that space and time
06:58are made up of tiny loops.
07:01Whether they are strings or loops,
07:03they would unfortunately be so tiny
07:05that we do not yet have
07:07a sufficiently powerful equipment to observe them.
07:09Therefore,
07:11as long as we have not tested these theories,
07:13they will not be proven.
07:15What is interesting is that
07:17these two ideas do not really need time
07:19to explain the functioning of the universe.
07:21We consider that objects
07:23and chairs are real,
07:25even if physics does not describe them directly.
07:27We say that they emerge
07:29from the tiny particles that make up the universe.
07:31But time,
07:33we do not know exactly
07:35what it could emerge from.
07:37Whatever the answer,
07:39everything will be fine for you.
07:41If it turns out that time does not really exist,
07:43you could say that everything is useless.
07:45But think about this.
07:47We continue to live our lives,
07:49to experience causality,
07:51to make sense of everything around us.
07:53The impression that we have chosen
07:55to order food at midnight
07:57is extremely tenacious.
07:59So, is the fact that everything can be predetermined
08:01really important?
08:03Anyway,
08:05we are wrong about many things
08:07that we thought were certain.
08:09At present,
08:11the theory of the block universe
08:13and our classical conception of the world
08:15are very far from each other.
08:17But over time, yes,
08:19everything will be fine.
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