Thread by @tpierrain

More than 25 years after my last physics course at university, I picked up my old textbook to refresh myself on quantum physics & thermodynamics this WE.

Thread to come about What is Time (episode 2).

Ok. Since some of you have requested me in PM to continue with the topic of time, I’ll take some time to zoom on the conclusion of ‘the Order of Time’ (Carlo Rovelli’s great book).

In particular the part where he explains that Time doesn’t really exist ;-)

But before jumping into his conclusion on ‘what is time’, we must first make some reminders about 2 main branches of physics: thermodynamics and quantum mechanics.

Quantum mechanics is the branch of physics relating to the very small. 

While classical mechanics was not really useful to describe how things move at the scale of atoms and electrons,

quantum mechanics provides a completely different (and counter intuitive) model that fits better to describe how small objects and particles are existing and moving: probabilities.

But let's see how we got there.

Before quantum physics, Rutherford's model of the atom (1911) described a positively charged nucleus containing the majority of the atom's mass, and separated by a vacuum, electrons spinning around like planets around a star.

It was the planetary model of the atom.

But this fairly simple model carried some contradiction with reality.

Indeed, according to the classical theory of electromagnetism, if an electric charge is subjected to an acceleration, there is emission of electromagnetic radiation.

And here, despite the supposed centripetal acceleration of the electron around the nucleus, we couldn’t observe any emission of radiations around the atoms...

Also, the energy of the electron was supposed to decrease gradually in that model, due to those emissions, and to gradually approach the nucleus to finally fall on it...

The contradiction between Rutherford's model and the observed reality was too obvious.

Indeed, atoms are stable and their lifetimes seemingly unlimited. They emit radiation only if they are first excited, the spectrum of this radiation is discontinuous.

We had to find a better theory and Niels Bohr resolved all those contradictions in 1913, reusing also the quantum theory imagined a few years before by Max Planck and Albert Einstein. 🤯

Characteristics of Bohr’s new model of atom was:

- the energy of the electron is quantified, that is to say that it can only take certain determined values, also called energy levels

...

- to each of the possible values ​​for the energy of an electron corresponds to a stable circular trajectory on which the electron does not radiate and therefore does not lose energy

- the changes of energy of an electron can only be carried out by discontinuous jumps from one level to another

- in the absence of external excitation, an electron is permanently on the lowest possible energy level.

Bohr also included to his model the Quantum theory:

- the exchanges of energies between matter & radiation (absorption or emission) can only take place in whole multiples of a minimum quantity of energy: a quanta

- the value of a quanta depends on the frequency of the radiation

The quantum model of the atom proposed by Bohr in 1913 had 2 aspects: one geometric (stable trajectories), the other energetic (quantified levels).

In 1924, Louis de Broglie improved the quantum model of the atom by replacing Bohr’ stable trajectories of electrons by the wave nature of the electrons. 🤯

With de Broglie, electrons may now be considered as waves. In the wave model of the atom, electrons are not assigned a definite trajectory.

We consider that we can only know the probability of the presence of each of them at a point in the space surrounding the nucleus.

Indeed, according to Quantum mechanics particles have a certain chance of being at a point A, another chance of being at point B and so on.

Contrary to classical mechanics, one can never make simultaneous predictions of conjugate variables such as position and momentum, to arbitrary precision.

The state of a system at a given time is described by a complex wave function.

De Broglie also introduced the concept of Wave–particle duality.

The Wave–particle duality is the concept in quantum mechanics that every particle or quantum entity (like electrons) may be described as either a particle or a wave.

At least this is how it was (and still) learned in France. According to the physicist Etienne Klein, the concept of wave-particle is a total non-sense.

The wave-particle duality is ultimately a step in our understanding of quantum phenomena, ultimately an absurd thing.

This is even something that made it more difficult to understand Heisenberg's principle of indeterminacy (still unfortunately translated as "principle of uncertainty") a few years later, in 1927.

“A quantum particle is never an object as described by classical physics.

The quantum revolution is first of all an ontological revolution. That is, it changes the nature of the objects it claims to describe. "(Etienne Klein)

The very notion of speed and position for a quantum field makes no sense.

The uncertainty principle becomes a principle of indeterminacy which says that when you make a measurement of a quantum object to obtain one of its classical properties, for example the position: you do not have the right to say that this position pre-existed to measure.

This is the measure that forced the particle to take a position.

Even today in some French textbooks, things are said in the name of the "uncertainty" principle which are dated (1927) and which do not take into account what we have learned since ... (Etienne Klein)

According to Carlo Rovelli, there are: « 3 fundamental discoveries that quantum mechanics has led to: granularity, indeterminacy and the relational aspect of physical variables. »

GRANULARITY is the most characteristic feature of quantum mechanics, which takes its name from this: ‘quanta’ are elementary grains.

A minimum scale exists for all phenomena. For the gravitational field, this is called the ‘Planck scale’.

The second discovery made by quantum mechanics is INDETERMINACY: it is not possible to predict exactly, for instance, where an electron will appear tomorrow.

Between one appearance and another, the electron has no precise position, as if it were dispersed in a cloud of probability. In the jargon of physicists, we say that it is in a ‘superposition’ of positions.

RELATIONS:
Indeterminacy is resolved when a quantity interacts with something else.

In the interaction, an electron materializes at a certain point. For example, it collides with a screen, is captured by a particle detector...

or collides with a photon–thus acquiring a concrete position, producing one of the luminous dots that goes into the making of a TV image.

But there is a strange aspect to this materialization of the electron: the electron is concrete only in relation to the other physical objects it is interacting with.

With regard to all the others, the effect of the interaction is only to spread the contagion of indeterminacy.

Concreteness occurs only in relation to a physical system: this, I believe, is the most radical discovery made by quantum mechanics.

But it is only in relation to the screen that this happens. In relation to another object, the electron and screen are now together in a superposition of configurations,

and it is only at the moment of further interaction with a third object that their shared cloud of probability ‘collapses’ and materializes in a particular configuration–and so on.

Ok. Quantum physics is truly hard and somehow counter intuitive. I promise that this was the harder content of my thread

Next time, we will continue our physical prerequisites with a smaller (and simpler)recap of thermodynamics and how it impacts the concept of time. Stay tuned!

Before we dive into the second part of Carlo Rovelli’s book (the order of time), let’s continue to review some physics prerequisites. We will cover some concepts of thermodynamics this time.

Thermodynamics is ‘a branch of physics that deals with heat, work, and temperature, and their relation to energy, radiation, and physical properties of matter. The behavior of these quantities is governed by the four laws of thermodynamics’ (wikipedia)

The first principle of thermodynamics - also called the principle of conservation of energy - postulates that energy is conserved; it can neither be created (ie from nothing) nor destroyed.

It must already exist in one form, and if the energy "disappears" it must be found in one/more other form.There is also an equivalence between the forms of energy: the amount of energy that has disappeared in one form is equal to the amount of energy that has appeared in another

If a system is not isolated, its energy can vary, but only by exchange with another system (or the outside) whose energy undergoes equal and opposite variation.

BTW, the "renewable energies" (solar, wind) which one speaks more and more in opposition to the non-renewable energies (coal, oil) is a misnomer.

It is indeed the source of these energies which is renewable, not these energies themselves (which would be contradictory with this first principle of thermodynamics).

In thermodynamics, some reactions are said to be spontaneous, they occur spontaneously in one direction only, associated with the flow of time, which cannot be traced back.

If you light a match until its final combustion, there is little chance that you will see it rise from these ashes in the opposite direction.

The same with a waterfall; it is unlikely that the water will return to its source as if by magic..

2 other examples will be more useful for the following:

- the expansion of a gas: if we connect an enclosure containing a gas and another which is empty, the gas will quickly occupy the totality of the overall volume offered. We will never see the gas spontaneously return to the chamber where it was, leaving the other empty.

- The exchange of heat between a hot body and a cold body: heat changes from a hot body to a cold body and they both end up being at the same temperature. You never see two bodies at the same temperature spontaneously exchanging heat so that one heats up and the other cools down

These spontaneous transformations are neither reversible nor invertible under the conditions in which they occur.

The return to the initial state is never possible spontaneously, even by a different path.

So how is this possible? Well, it's the concept of Entropy - introduced in 1854 by Clausius - that will make sense of it all.

The variations in entropy during transformations provide a criterion of their spontaneity.

To be continued (‘time’ is coming 😉)

Entropy is the measure of the "disorder" of a system, the greater the greater the number of these states. Imagine a card game. If you shuffle a brand new, fully ranked game, it gets messy. Shuffle it again: there is virtually no chance it will end up classified as at the start.

And this even if you shuffle him for a very long time. Indeed, among all the "microscopic states" that it can take (52! = 8 E 67), only one is ordered and all the others are disordered. The probability of stumbling upon its orderly state is tiny.

Spontaneous change from order to disorder is possible, but not the other way around.

Well, instead of considering card games, let's instead focus on elementary particles of matter (remember the part on quantum physics).

A rise in temperature increases the entropy of a gas, because the range of values ​​in which their kinetic energy can lie increases (the different levels of energy taken up by the particles).

This notion of "disorder" corresponds to the idea of ​​a loss of information on the microscopic state (position and energy of particles).

Entropy = how microscopically the system is mixed up.

Entropy is therefore a quantity which characterizes the capacity of a physical system to undergo spontaneous transformations: the greater the value of entropy, the lower the capacity of the system to transform.

We can say that it measures the "quality" of the energy available within the system under consideration. During its transformations, energy degrades and becomes less and less usable. Good quality energy is orderly, low entropy energy.

« The arrow of time » means that a closed system loses its ability to evolve as it evolves (its entropy can only increase).

Even though three are 4 laws, thermodynamics is based on 2 concepts: energy and entropy. While the first principle asserts that the first is conserved, the second principle asserts that the entropy can only increase.

These are principles that cannot be demonstrated, except for the consistency and accuracy of the consequences that one draws from them.

On the other hand, it is interesting to note that the equation of the second principle of thermodynamics is the only fundamental physics equation which recognizes a difference between past and future.

The only one that tells us about the passage of time...

All other fundamental physics equations do not talk about time.

Which leads Carlo Rovelli to ask the question: does time exist? Or is it an emerging property in very particular conditions in the universe?

Those pre-requisites (quantum mechanics and thermodynamics) being finished, we will next be able to zoom on the concept of Time.

See ya!

Before we dive into the conclusions of Carlo Rovelli, I suggest those who have missed it to read my former thread about the epistemology of time (and relativity) here :

https://twitter.com/tpierrain/status/1291356717511577602?s=21

Carlo Rovelli is a researcher. Loop quantum gravity (or loop theory) is his area of ​​research.There is no scientific consensus about the quantum properties of time, but this unifying theory of gravity and quantum particle physics(like string theory) is the one that appears to be

In this theoretical framework, the proper time measured by a clock is “quantized”. This means that it takes on certain values ​​and not others. Time is granular instead of continuous.

There is a minimum scale for all phenomena. For the gravitational field, it is called the Planck scale. Planck's time is 10-44 seconds.

At these infinitely small times, quantum effects manifest themselves over time.

Spacetime is a physical object like an electron. It, too, fluctuates. It, too, can be in a ‘superposition’ of different configurations.

The gravitational field not only has a dynamic influenced by masses: it is also a quantum entity that has no determined values ​​except when it interacts with something.

Time only materializes in interaction.

When it does, the durations are granular and determinate only for that something with which it interacts; they remain indeterminate for the rest of the universe.

The absence of time does not mean, therefore, that everything is frozen and unmoving.

It means that the incessant happening that wearies the world is not ordered along a timeline.

Rather, it is a huge and messy web of quantum events.

“The world is more like Naples than Singapore." said Rovelli ;-)

We also don't need to pick a privileged variable and call it "time."

The fundamental world theory need only tell us how the things we see varying in the world vary from one another.

The fundamental equations of quantum gravity are made as follows: they have no time variable (see Wheeler-DeWitt equation in 1967)

The theory does not describe how things change over time. The theory describes how things change relative to each other.

The elementary quantas of the gravitational field are the elementary grains that weave the mobile fabric with which Einstein reinterpreted Newton’s absolute space and time.

It is these, and their interactions, which determine the extension of space and the duration of time.

The relations of spatial adjacency tie the grains of space into webs. We call these ‘spin networks’.

This is the world with no time of elementary physics. Only events and relationships.

The time could be what occurs during a quantum interaction (Alain Connes).

Determining a physical variable is not a harmless operation, it is an interaction.

The effect of these interactions depends on their order (position or speed?) and on the non-commutativity of quantum variables.

This order is a primitive form of temporal order.

Time would therefore be an emerging property.

Moreover, thermal time (related to thermodynamics) and the flow defined by Connes are two aspects of the same phenomenon.

Our fuzzy and indeterminate image of reality determines a variable, thermal time.

Next time we will dwell on this emerging concept at our human level.

We will also see the craziest thing in the theory supported by Rovelli, before concluding.

See you soon! 😉

Ok this is it. Final round 😉.

It’s time now to talk about Carlo Rovelli’s idea that I find mind-blowing.

I won’t put quotes on everything but most of this will continue to be extracted here and there from his book: “the order of time”.

The entire difference between past and future may be attributed solely to the fact that the entropy of the world was low in the past.

Why was entropy low in the past?

Where does the low entropy of the sun come from?

From the fact that, in turn, the sun is born out of an entropic configuration that was even lower: the primordial cloud from which the solar system was formed had even lower entropy.

And so on, back into the past, until we reach the extremely low initial entropy of the universe. It is the growth of this entropy that powers the great story of the cosmos.

But the increase in the entropy of the universe is not rapid, like the sudden expansion of gas in a box: it is gradual, it takes time.

Even with a gigantic ladle it takes time to stir something as big as the universe. Above all, there are obstacles and closed doors to its growth–passages where it occurs only with great difficulty.

A pile of wood, for example, lasts a long time if left alone.

It is not in a state of maximum entropy, because the elements of which it is made, such as carbon and hydrogen, are combined in a very particular manner (‘ ordered’) to give form to the wood.

Entropy grows if these particular combinations are broken down. This is what happens when wood burns.

But the wood does not start to burn on its own. It remains for a long time in a state of low entropy, until something opens a door that allows it to pass to a state of >entropy

Whatever we human beings may be specifically, in detail, we are nevertheless pieces of nature, a part of the great fresco of the cosmos, A SMALL PART AMONG MANY OTHERS.

Between us and the rest of the world there are physical interactions. Obviously, NOT ALL THE VARIABLES OF THE WORLD INTERACT WITH US or with the segment of the world to which we belong.

Most of them do not react with us at all.

Entropy is a relative thing. The entropy of A with regard to B counts the number of configurations of A that the physical interactions between A and B do not distinguish.

The entropy of the world does not depend only on the configuration of the world; it also depends on the way in which we are blurring the world (see quantum mechanics)

and this depends on what the variables OF THE WORLD ARE THAT WE INTERACT WITH. i.e. on the variables with which our part of the world interacts.

This, which is a fact, opens up the possibility that it wasn’t the universe that was in a very particular configuration in the past.

PERHAPS instead IT IS US, AND OUR INTERACTIONS WITH THE UNIVERSE THAT ARE PARTICULAR.

We are the ones who determine a particular macroscopic description. The initial low entropy of the universe, and hence the arrow of time, may be more down to us than to the universe itself.

This is the basic idea of Rovelli.

Think of one of the grandest and most obvious phenomena: the diurnal rotation of the skies. It is the most immediate and magnificent characteristic of the universe around us: it turns.

But is this turning really a characteristic of the universe? It is not.

It took us thousands of years, but in the end we managed to understand the revolving of the heavens: we understood that it is we who turn, not the universe.

Something similar might be true for time’s arrow.

The low initial entropy of the universe might be due to the particular way in which we–the physical system that we are part of–interact with it.

We are attuned to a very particular subset of aspects of the universe => orientated in time.

It is hardly surprising that there are ‘special’ subsets in a universe as vast as ours. There is nothing unnatural in imagining that the universe has parts that are ‘special’.

If a subset of the universe is special in this sense, then for this subset the entropy of the universe is low in the past, the second law of thermodynamics obtains; memories exist, traces are left–and there can be evolution, life and thought.

But why should we belong to one of these special systems?

For the same reason that apples grow in northern Europe, where people drink cider, and grapes grow in the south, where people drink wine;

or that I was born where people happen to speak my native language; or that the sun which warms us is at the right distance from us–not too close and not too far away.

In all these cases, the ‘strange’ coincidence arises from confusing the causal relations: it isn’t that apples grow where people drink cider, it is that people drink cider where apples grow.

Put this way, there is no longer anything strange about it.

I don’t know for you, but all this mind blowed me and made me bug a little why thinking about it. 😆

After that, Rovelli ‘s book talk about philosophy and neuro science to explore how time is part of us.

With our memory, we are “coded” to grasp the concept of past, present and future.

A very interesting part too but since my thread is already to long, i’ll let you read his great book to discover all these interesting aspects.

I think it’s time now for a final recap of what we have seen so far in those 2 threads (here I’ll verbatim extracts from Carlo Rovelli):

1. A present that is common throughout the whole universe does not exist

2. The present is a localized rather than a global phenomenon

3. Locally, time passes at different speeds according to where we are and at what speed we ourselves are moving.

The closer we are to a mass, or the faster we move, the more time slows down

4. The difference between past and future does not exist in the elementary equations that govern events in the world (i.e. quantum physics).

In the elementary grammar of the world, there is neither space nor time–only processes that transform physical quantities from one to another, from which it is possible to calculate probabilities and relations

5. Our interaction with the world is partial, which is why we see it in a blurred way. To this blurring is added quantum indeterminacy

6. The variable ‘time’ is one of many variables which describe the world. It is one of the variables of the gravitational field

7. At our scale, we do not register quantum fluctuations. Hence we can think of spacetime as being as rigid as a table.

This table has dimensions: the one that we call space, and the one along which entropy grows, called time.

8. The ignorance that follows from this determines the existence of a particular variable–thermal time- and of an entropy that quantifies our uncertainty

9. We human beings are an effect of this great history of the increase of entropy, held together by our MEMORY that is enabled by these traces

10. Each one of us is a unified being because we reflect the world, because we have formed an image of a unified entity by interacting with our kind, and because it is a perspective on the world unified by memory

11. Perhaps we belong to a particular subset of the world that interacts with the rest of it in such a way that this entropy is lower in one direction of our thermal time (Rovelli’s mind-blowing theory)

12. The directionality of time is therefore real but perspectival: the entropy of the world in relation to us increases with our thermal time

13. We see the occurrence of things ordered in this variable, which we simply call ‘time’,

and the growth of entropy distinguishes the past from the future for us and leads to the unfolding of the cosmos

As disclaimers, one should be aware that:

- The gravitational field has quantum properties is a shared conviction, albeit one currently supported only by theoretical arguments rather than by experimental evidence

- The absence of the time variable from the fundamental equations is plausible–but on the form of these equations debate still rages

- The origin of time pertaining to quantum noncommutativity, of thermal time, and the fact that the increase in entropy which we observe depends on our interaction with the universe are ideas that I find fascinating but are far from being confirmed or widely accepted

- What is entirely credible, in any case, is the general fact that the temporal structure of the world is different from the naïve image that we have of it

This naïve image of time is suitable for our daily life, but it’s not suitable for understanding the world in its minute folds or in its vastness

The mystery of time intersects with the mystery of our personal identity, with the mystery of consciousness...

My (naive?) understanding: we are creatures perfectly matching with our immediate environment (e.g. our memory that allows us to grasp time) where time has probably emerged locally (comparing to the rest of the universe).

This is something that we should continue to think about and to investigate.

Until then, it looks like we are imprisoned in time. 😉

@threadreaderapp can you unroll this please?