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TENET, Christopher Nolan's most recent blockbuster
everyone is talking about
All I have for you is a word: TENET
The movie is great
If you haven't watched it,  we recommend you to do it
but after you watch this video
where we explain 3 key ideas of the Physics behind TENET
The video is essentially spoiler-free
And if you have watched it, you also probably need some help
so we advice you to watch the video
to understand almost everything
The first key idea is the reversibility of time
The movie is based on the idea that it is possible
to reverse the direction of time for certain objects or people
The action is full of inverted phenomena
Cars speeding backwards
bullets which start from the impact point
and move backwards to the gun which shot it
or will shoot it
They are inverted versions of familiar objects and phenomena,
but time-reversed.
The underlying idea is that all laws of physics are invariant under time-reversal
If I take any phenomenon on an isolated system
make a video of its evolution
and I play it backwards to see how it looks with time reversed
I can see that it is perfectly compatible with the laws of Physics
It's indeed true that essentialyl all interactions in nature
are invariant under time reversal
Gravity, electromagnetism,
even phenomena at the level of the atom or the atomic nucleus
are invariant under reversal of time.
Well, actually not ALL physical phenomena
are invariant under time reversal
There exist some phenomena in particle physics
which are not exactly identical with time running forward or backward
There are some phenomena which do not respect this symmetry
This is related with the so-called violation of the CP symmetry
which takes place at the level of particle physics
for instance in the decay of some particles, like kaons
This is a tiny effect, with no consequence in our daily life,
it rather occurs in some radioactive processes,
and in particle physics collider experiments, like the LHC
In fact, the Large Hadron Collider in Geneva
has a detector, named LHCb,
dedicated to this kind of phenomena
and which studies CP violation
CP violation is one of the key ingredients
in the generation of the matter-antimatter asymmetry in the universe
which explains why we are made of matter, rather than of antimatter
But that's a bit out of the point now, so we recommend you to watch this video for further info
The fact that some particle physics phenomena
do not respect time-reversal invariance
is very likely the reason why radioactivity
and radioactive elements are so present in TENET
But, since this is not explicitly mentioned in the movie,
let's move one, although particle physics will be back later on in this video
The second key idea is what they call inverted entropy
We have just said that the laws of physics
are invariant under time reversal
However, we don't see inverted processed in our daily life
We see eggs falling and smashing
but we don't see smashed eggs regenerating and jumping up
We also see icecubes melting and evaporating
We don't see water puddles spontaneously turning into icecubes
This is because we are talking about objects
made out of millions of millions of millions of millions of particles
macroscopic objects
The fact that macroscopic phenomena
evolve in a naively not time-reversal invariant way
is known as the second law of thermodynamics
which states that the entropy of systems always increases
If you want to learn more about entropy, we recommend you to watch this video
A sketchy explanation is that
the entropy of a systems is a measure of its degree of disorder
Smashed eggs are more disordered than unbroken eggs
Similarly, an icecube is an ordered crystalline structure
whereas a water puddle, or evaporated water
is far more disordered, because it's distributed over the whole room
Entropy increase with time defines the thermodynamic arrow of time
which introduces a notion of forwardness in time
An important property of the second law of thermodynamics
is that it is probabilistic
For instance, if we consider a set of water molecules as vapour in a room
it is far more likely that they are disordered in the whole room
than cornered up together all in 1 cubic centimeter
Conversely, if we start off with all molecules cornered up in 1 cubic centimeter
and we let the system evolve in time
the most likely outcome is that the molecules end up distributed in the whole room
Out of pure probability, there is a preferred direction of time
which corresponds to this thermodynamic arrow of time
But there is absolutely nothing in the laws of physics
against having millions of millions of millions of millions
of molecules in the air of the room
with perfectly adjusted positions and velocities
such that they start colliding, slowing down, and concentrating
ending up with all simultaneously in a cubic centimeter
in a corner of the room
or forming a water puddle, or eventually an icecube
It is extremely unlikely, even to ensure
that this has never ever happened once in the whole history of the universe
So one of the key ideas in TENET is that they have discovered
some technology allowing them to invert the entropy of objects
We'd rather say, to invert the direction of entropy increase of objects
We thus see smashed cars that regenerate and start running
or shattered glass which regenerates and spits off a bullet
flying back to the gun which (un)shot it
The third key idea is...
-Time travel... -No
The third key idea is time inversion
There are quite a few subtle points here
If you take a thermodynamics system
and you make its entropy decrease
this still does not mean that time is running backwards for this system
This is in fact a problem we scientist find in some newspapers headings
In some physics experiments, it has been possible to achieve that some microscopic systems
evolve such that their entropy decreases
their thermodynamic arrow of time is effectively inverted
When this gets to headlines, journalists mostly talk about time travel etc
What can we say...?
If you want to learn more about time travel,  for real, go check out this video!
So, coming back to TENET, the burning question,
leaving aside the practical problem of inverting entropy for large systems like cars, bullets, or people,
is to explain, or make plausible, that their time is running backwards.
This is very important in TENET
It's really necessary for the plot that inverted objects run backwards in time
For instace, in some operations
there are two teams, the time-forward red team
and the inverted blue team moving backwards in time
and gives information to the red team
about the future of the red team
which is in the past of the blue team
There are two ways in which Nolan
expresses in TENET that inverted objects run backwards in time
The first is the psychological experience of the main characters when they're inverted
We really experience with them that they are moving backwards in time
because we see events more and more towards the past.
This has a lot to do with the so-called psychological arrow of time
the way our brain feels, our conscience feels
the passage of time.
In other words, we remember the past, but not the future
This a fascinating topic,
not fully understood from the viewpoint of Neuroscience
but which we can roughly understand
from  thermodynamic point of view
using the earlier idea of entropy
The question is, when one inverts the thermodynamical arrow of time,
is the psychological arrow of time also inverted?
The argument to make this plausible
was already explained by (drum role) Stephen Hawking
in 1988 in his first popular science book "A brief history of time"
where in chapter 9 he writes about the arrows of time.
This chapter explains the different arrows of time
such as the cosmological arrow of time
which indicates the direction of expansion of the universe,
the thermodynamical arrow of time
and the psychological arrow of time.
And explains that the psychological arrow of time
is aligned in the same direction as the thermordinamical arrow of time
basically because the process of forming memories of events
is a process that generates entropy.
It takes energy to have our brain create new memories
and the process releases energy in form of heat
which has an enormous entropy.
Hence the direction of formation of new memories
is the direction of entropy increase,
the thermodynamic arrow of time
And now that we're talking about heat
let's mention one issue that TENET if kind of skipping over
namely, the interplay between inverted objects and their environment.
For instance, a human body, just because it is at 37ºC,
is continuously emitting a black body radiation
What happens with this radiation? It's not clear if it's inverted or not
Similarly, a bullet hitting a wall or a glass
releases its energy as heat among the atoms of the wall or glass
What happens with all those atomic motions? Are they inverted or not?
Because walls are in general not inverted objects
So the time inversion in the movie could work in its full form
only if all the particles participating in the event are inverted as well
including particles in the environment, which have not experienced the time inversion process
So...
The only reference to the issue of the environment
is a vague explanation that inverted persons have trouble with gas exchange in breathing
and must wear a mask
It is not clear why, beyond the fact that is is a good way
to distinguish inverted or ordinary people
depending on whether they wear mask or not
Nice trick, Nolan!
It's also equally unclear all about the inverted explosion
in which the fire freezes....
Let's not  make spoilers!
To finish with the psychological arrow of time,
let me mention that it is not explained in the movie,
but the whole theme of nature and creation of memories
is very present in Nolan's movies
For instance, "Memento", or "Inception"
Also, the exchange of information from the future to the past
is a key ingredient in Intestellar
where at the end of the movie... Spoiler Alert!
there is information exchange from the future to the past in Murph's library
But, back to the point,
the second way in which Nolan tries to convince us
that inverted people move backwards in time
is by invoking the Wheeler-Feynman theory
in which positrons are like electrons, but moving backwards in time
By the way, Richard Feynman is one of the most charismatic physicists of 20th century
and a genius in scientific outreach for the general public
We recommend you to check out his talks, many of them online,
and also his books, in particular the Feynman Lectures on Physics
They are available free online, see the link in the description of the video
Enjoy!
Feynman did his PhD with John Archibald Wheeler,
another enormous physicist of the 20th century
especially in topics of General Relativity
He actually coined the term "Black Hole"
Well, these ideas of Wheeler and Feynman
about positrons as electrons moving back in time
are actually the seed of basic principles
in the Feynman diagram formalism
in Quantum Field Theory,
in which antiparticles are like particles, but moving back in time
Well, actually, the symmetry to be applied is CPT
C is charge conjugation, reversal of the sign of the particle charge
P is parity, that is left-right reversal
in particular, reversal of the spin state of the particle
and T, which is the very operation of time reversal
So, in Quantum Field Theory,
a process in which an electron and a positron meet and annihilate
is equivalent to an electron moving in time
suddenly turns around and starts moving back in time
This is very similar to what happens in TENET
when ordinary and inverted people
enter the inversion maching, moving forward or backwards respectively
and disappear.
It's equivalent to a person entering the machine
and coming out of it as inverted moving back in time
This also fits with the moving, when the main character is warned
that if his inverted and ordinary selfs meet
he may end up in annihilation
But that's something we already know since Back to the Future
This all sounds great, and makes very cool movie effects
but let's check out a few things that don't quite fit
-  Did you come up with anything? -You're not going to like it
For instance, when an electron and a positron meet and annihilate
the energy stored in their masses is released as photons
This does not take place in the inversion machines in TENET
We'll have to trust this is part of the time inversion technology
On the other hand, if we grant that inverted objects
are made of particles travelling back in time, i.e. antiparticles
then any particle in the inverted object
could annihilate with the particles in the non-inverted environment
it need not be the inverted and ordinary selves that meet and annihilate
it could be any inverted particle and the particles of the environment
This is in a sense part of the so-and-so treatment of environment in TENET
Finally, in the Feynman diagram formulation of Quantum Field Theory,
although there are formally particles moving back in time,
all physical processes are such that there is never information transfer from the future to the past
This would violate causality, and the rules of Special Relativity.
In the movie they brush over this when they discuss the Grandfather paradox
The way in which this is solved in Quantum Field Theory
is something very analogous to the double slit experiment
The idea is that to compute any physical effect
you have to sum over the different ways in which this can happen
This is the so-called path integral
This is briefly mentioned in the movie by Robert Pattinson's character
when he recalls parallel universes and superposition of realities etc
and when he mentions he has a Master degree in Physics
If you don't, you can check out this video where we explain path integrals and antiparticles
And if you do, you should check it out as well!
Coming back to the path integral, to the sum over ways in which a quantum phenomenon can take place,
the point is that some of these ways can involve antiparticles,
particles moving back in time,
but you have to sum over all possible ways, and when you do the sum,
there are cancellations and there only remain contributions that respect Relativity.
Causality is not violated.
There is no information transfer at speed larger than that of light,
and equivalently, there is no information transfer from future events to past events
So this prevents any information transfer from the inverted blue team to the red team
I let you think about it
Ok, I hope you found these explanations interesting
and useful to understand more and more layers of the many in TENET.
In any event, nothing of what I explained serves to understand
why an inverted bullet is far more deadly than an ordinary one
Or why the algorithm would lead to the destruction of...
Let's not give spoilers!
But that's all part of the freedom of creation of the director,
who already warns us in the moving, when one of the scientists says
- Don't try to understand it, feel it
So, since it has been very entertaining
and has given us an excuse to talk about Physics and the nature of time
you are forgiven, Christopher
That's all. Thanks for watching, and see you at the beginning
This reversing the flow of time, doesn’t us being here now, mean it never happens?
