Thread by @tpierrain

“The mystery of Time has always troubled us. It awakens deep emotions which nourish philosophy and religions" (Carlo Rovelli)

What if Time did not exist?

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2 visions on ‘What is Time?’ has been opposed for many years: that of Aristotle and that of Newton.

For Aristotle, Time is the measurement of change. Things change continually. We call ‘time’ the measurement, the counting of this change.

If nothing changes, there is no time.

For him, Time is what we refer to when we ask ‘when?’ ‘After how much time will you return?’ means ‘When will you return?’ The answer to the question ‘when?’ refers to something that happens.

‘I’ll return in three days’ time’ means that between departure and return the sun will have completed three circuits in the sky. It’s as simple as that.

Newton assumes the exact opposite. He recognize that time defined by Aristotle (relative, apparent, common) exists.

But he also contends that, in addition to this, another time must exist: ‘true’ time that passes regardless, independently of things and of their changes.

Take some time to think about it on your own. Ask yourself:

‘who is right: Aristotle or Newton?’

Warning: Don’t take your intuitions and ideas to be ‘natural’: they are often the products of the ideas of audacious thinkers who came before us and turned it into our common sense.

It seems to you simple and natural, just because you encountered it at school.

It is very likely that you are used to Newton's version of absolute, true, and mathematical time.

The Newtonian time 't' that we find in equations in physics in high school. It is an elegant intellectual construction.

It seems naturel today but it is not an old and natural intuition for humanity. It’s an idea of Newton’s.

The majority of philosophers have in fact responded negatively to this idea.

Legend has it that Leibniz, whose name is still occasionally spelled with a ‘t’ (Leibnitz), had deliberately dropped the letter from his name in accordance with his belief in the nonexistence of the absolute Newtonian time t. 😁

The two interpretations of time (the measure of ‘when’ with regard to events, as Aristotle wanted; the entity that runs even when nothing happens, according to Newton) can be repeated for space.

For Newton, between two things there may also be ‘empty space’.

For Aristotle, it is absurd to speak of ‘empty’ space, because space is only the spatial order of things. If there are no things–their extension, their contacts–there is no space.

One can think: ‘This glass is empty’ (i.e. Newton)
One can think: ‘this glass is full of air’ (i.e. Aristotle)

We can consequently think of the world around us as ‘almost empty’, with just a few objects here and there, or alternatively as ‘completely full’, of air.

‘The answer to all these questions lies in an unexpected synthesis of the apparently contradictory ideas held by these two giants. And, to accomplish this, it was necessary for a third giant to enter the dance.’ (Rovelli)

(and one more confirmation that Dualism is shitty 😉)

According to Rovelli: ‘The synthesis between Aristotle’s time and Newton’s is the most valuable achievement made by Einstein. It is the crowning jewel of his thought.’

Time and space Newton had intuited the existence of, beyond tangible matter, do effectively exist.

They are real.

Time and space are real phenomena. But they are in no way absolute; they are not at all independent from what happens;

This canvas is made of fields.

‘Physicists call ‘fields’ the substances which, to the best of our knowledge, constitute the weave of the physical reality of the world.’ (Rovelli)

Sometimes they may be given exotic names: ‘Dirac fields’ are the fabric of which tables and stars are made.

The ‘electromagnetic’ field is the weave of which light is made, as well as the origin of the forces that make electric motors turn and the needle of a compass point north

Spacetime is the ‘gravitational field’–and vice versa.

It is something that exists by itself, as Newton intuited, even without matter.

But it is not an entity that is different from the other things in the world–as Newton believed–it is a field like the others.

More than a drawing on a canvas, the world is like a superimposition of canvases, of strata, where the gravitational field is only one among others.

Equations describe the reciprocal influences that all the fields have on each other, and spacetime is one of these fields.

Einstein asked himself a question which has perhaps puzzled many of us when studying the force of gravity: how can the sun and the Earth ‘attract’ each other without touching and without utilizing anything between them?

He has imagined that the sun and the Earth do not attract each other directly but that each of the two gradually acts on that which is between them: space and time.

He imagined that the sun & the Earth each modified the space & time that surrounded them, just as a body immersed in water displaces the water around it

This modification of the structure of time influences in turn the movement of bodies,causing them to ‘fall’ towards each other

As a consequence, a mass slows down time around itself.

‘The Earth is a large mass and slows down time in its vicinity. It does so more in the plains and less in the mountains, because the plains are closer to it.

This is why the friend who stays at sea level ages more slowly.’ (Rovelli)

Yeah, time passes faster in the mountains than it does at sea level.

It is not just the clocks that slow down: lower down, all processes are slower.

‘Two friends separate, with one of them living in the plains and the other going to live in the mountains.

They meet up again years later: the one who has stayed down has lived less, aged less, the mechanism of his cuckoo clock has oscillated fewer times.’ (Rovelli)

Lower down, there is simply less time than at altitude.

Things fall downwards because, down there, time is slowed by the Earth.

In a physics laboratory, a clock on a table and another on the ground run at different speeds. Which of the two tells the time?

‘The question is meaningless. We might just as well ask what is most real–the value of sterling in dollars or the value of dollars in sterling. There is no ‘truer’ value; they are two currencies which have value relative to each other.’ (Rovelli)

There is no truer time. There are two times that change relative to each other. Neither is truer than the other.

But there are not just two times. Times are legion: a different one for every point in space. There is not one single time; there is a vast multitude of them.

Let me repeat that: there is one time for every point in space.

Every phenomenon that occurs has its ‘proper time’ (the appropriated term in physics) , its own rhythm.

Einstein has given us the equations that describe how proper times develop relative to each other. He has shown us how to calculate the difference between two times.

10 years before understanding that time is slowed down by mass, Einstein had realized that it was slowed down by speed.

For everything that moves, time passes more slowly.

For this effect to become perceptible, one must move very quickly. It was first measured in the 1970s, using precision watches on aeroplanes.

‘Proper time’ depends not only on where you are and your degree of proximity to masses; it depends also on the speed at which you move.

‘It’s a strange enough fact in itself, but its consequences are extraordinary. Hold on tight, because we are about to take off’ (Rovelli)

‘Now’ Means Nothing

What is happening ‘now’ in a distant place?

Imagine, for example, that your sister has gone to Proxima b, the recently discovered planet that orbits a star at approximately four light years’ distance from us.

What is your sister doing now on Proxima b?

The only correct answer is that the question makes no sense.

It is like asking ‘What is here, in Peking?’ when we are in Venice. It makes no sense because if I use the word ‘here’ in Venice, I am referring to a place in Venice, not in Peking. (Rovelli)

Indeed, if you look at your sister, you are receiving light that travels from her to your eyes.

Therefore, you are not quite seeing what she is doing now but what she was doing... 4 years ago.

You can’t even say that what your sister is doing now is what she will be doing four years after the moment that you see her through the telescope.

This does not work: 4 years after you have seen her through the telescope, in her time, she might already have returned to Earth and could be (yes! this is really possible!) ten terrestrial years in the future. But ‘now’ cannot be in the future…

The truth of the matter is that we need to give up asking the question.

There is no special moment on Proxima b that corresponds to what constitutes the present here and now.

Our ‘present’ does not extend throughout the universe. It is like a bubble around us.

(Rovelli)

How far does this bubble extend?

It depends on the precision with which we determine time. If by nanoseconds, the present is defined only over a few metres; if by milliseconds, it is defined over thousands of kilometres. (Rovelli)

As humans, we distinguish tenths of a second only with great difficulty; we can easily consider our entire planet to be like a single bubble where we can speak of the present as if it were an instant shared by us all.

This is as far as we can go.

The idea that a well-defined now exists throughout the universe is an illusion, an illegitimate extrapolation of our own experience.

There is no such thing as “the same moment” definable in the universe.’

The ‘present of the universe’ is meaningless.

(Rovelli)

Here is the structure of spacetime that Einstein understood when he was 35 years old

(physicists have the habit of drawing every event as a light cone with its past, its future and a part of the universe that is neither past nor future, just as every person has forebears)

Light travels along the oblique lines that delimit these cones. This is why we call them ‘light cones’.

Near to a black hole, the lines converge towards it, like this:

This is because the mass of the black hole slows time to such a degree that, at its border (called the ‘horizon’), time stands still.

If you look closely, you will see that the surface of the black hole is parallel to the edges of the cones.

So, in order to exit from a black hole, you would need to move (like the trajectory marked in black in the following diagram) towards the present rather than towards the future! (Rovelli)

This is impossible. Objects can only move towards the future, as in the trajectories shown in the diagram in white.

This is what constitutes a black hole: an inclination of the light cones towards the interior, marking a horizon, closing off a region of space in the future from everything that surrounds it. (Rovelli)

But let’s get back to the synthesis that Einstein found between Aristotle’s conception of time and Newton’s.

With a tremendous beat of his wings, Einstein understands that Aristotle and Newton are both right.

Newton is right in intuiting that something else exists in addition to the simple things that we see moving and changing.

True and mathematical Newtonian time exists; it is a real entity; it is the gravitational field.

But Newton is wrong in assuming that this time is independent from things–and that it passes regularly, imperturbably, separately from everything else.

For his part, Aristotle is right to say that ‘when’ and ‘where’ are always located in relation to something.

But this something can also be just the field, the spatio-temporal entity of Einstein, because this is a dynamic and concrete entity, like all those in reference to which, as Aristotle rightly observed, we are capable of locating ourselves.

Success, as ever, is destined to be short-lived–even great success. Einstein writes the equations of the gravitational field in 1915, and barely a year later it is Einstein himself who observes that this cannot be the last word on the nature of time and space...

...because of the existence of quantum mechanics. The gravitational field, like all physical things, must necessarily have quantum properties...
(Rovelli)

I’ll end this thread without spoiling all the wonderful content of this incredible book from @carlorovelli where you will find a mix of Philosophy, Physics, Epistemology, Poetry...

But the rest of the book also explains the time on the quantum level (basically, time does not exist) and a lot more.

Highly recommended! 🤩🤩🤩

Yesterday after having finished my twitter thread about time, I’ve re-watch (and re-enjoyed 😄) Interstellar from Christopher NOLAN.

Awesome movie, and great illustration of the impact of gravity on time.

SPOILER ALERT: I’ve also discovered an interesting theory about the ending of the movie which totally makes sense IMO.

konbini.com/fr/cinema/theo…

@threadreaderapp can you please unroll this too?