Alberto Casas' fundamental guide to understanding the Nobel Prize in Physics: "We are alive thanks to the uncertainty principle"

Alberto Casas, physicist, researcher and author of 'The Quantum Revolution'.BBVA

Alberto Casas, born in Zaragoza, at 63, has considered opening the doors of quantum physics to the general public, even knowing that it is "counterintuitive" and that Albert Einstein himself questioned it.

However, he defends that it is the best explanation of "the hidden mechanisms of reality".

Casas, doctor and research professor at the CSIC at the Institute of Theoretical Physics (CSIC-UAM), has published

(Ediciones B, 2022), a fundamental, basic guide that is as understandable as it can be, without losing rigor, about the science called to unravel the deep and inadvertent mechanisms of nature, the origin of the universe and the future of a practically unimaginable computation whose pioneers have just been distinguished with the Nobel Prize in Physics.

Ask.

What is quantum physics?

Response.

It is the most powerful theory we have and, so far, without a single failure to describe nature.

Perhaps one day a flaw will be discovered, theories are not sacred and do not have to be.

Q.

By parts.

Liliana, the intern who is the protagonist of

, before the entourage of the businessman boasting of quantum knowledge, replies with an example: a person changes when they feel observed.

Can love explain the wave function of a particle, which evolves depending on whether we observe it or not?

A.

[Smiles] Love can be a clever metaphor for quantum physics.

There is something of that.

The wave function is one of the most mysterious aspects of quantum mechanics, and possibly with elements that are still a bit ambiguous.

According to the orthodox postulates of the theory, in the state of a particle, as long as it is not observed, many possibilities can coexist.

For example, you can be in many places at once.

The value of the wave function at a point is related to the probability that, when looking at it, we find it at that point.

This materialization of the particle in a specific place is what is called the collapse of the state or of the wave function.

As can be seen, in quantum mechanics, observers have a leading role.

Love can be a clever metaphor for quantum physics.

There is something of that

Q.

Quantum mechanics is inherently probabilistic, but I don't know of any stone thrown into the air that doesn't fall.

R.

We are going to give an example of this, because here the microscopic world works in a very different way from the macroscopic one: if I throw a ball against a wall, we all know from experience that the ball will bounce and that it will not go to the other side, which is called tunnel effect.

However, according to the theoretical prediction of quantum mechanics, the ball has a chance of actually passing through the wall as a ghost, but it is so infinitesimal that we will never see a ball perform this feat.

When we do the same experiment with elementary particles and with microscopic barriers, it turns out that the particles, although they do not have the energy to cross the barrier, some do, and in the proportion predicted by quantum mechanics.

According to quantum mechanics, a ball has a chance to go through the wall like a ghost, but it is so infinitesimal that we will never see it perform this feat

Q.

“We are alive thanks to the uncertainty principle”, you affirm.

R.

It is called the uncertainty principle, although it is really a theorem, a consequence of the postulates of quantum mechanics, but it is called a principle for historical reasons.

And what it says is that there are physical magnitudes that we cannot know simultaneously.

A typical example is the position and velocity of a particle: we cannot accurately know both magnitudes simultaneously.

Sometimes it is interpreted as that, when we measure the position, we perturb the speed and it is impossible to know both at the same time, but the thing is deeper: what the principle says is that the particle cannot have the position and velocity defined at the same time. speed.

It is not that we are unable to measure them simultaneously, but that they cannot be well defined simultaneously.

If one is well defined, the other cannot be.

Now,

we are alive thanks to the uncertainty principle because, if this were not the case, the electrons of the atoms would fall into the nucleus: everything would collapse and elements and molecules would not be able to exist and, therefore, there would be no life.

But if they fell into the core, they would acquire a "too well defined" position and speed, which is prohibited by the uncertainty principle.

More information

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Q.

What is state overlay?

A.

In our common life, we are used to objects occupying one position and only one.

However, quantum physics allows an electron to be in a superposition of states, associated with different positions, for example A and B. As long as we do not measure the position, two realities will coexist: the electron located in position A and in position B This is experimentally proven over and over again.

Again, in the macroscopic world we don't see it, but in the microscopic world, it is more than proven.

Q.

And the theory of the many worlds?

R.

It has a lot to do with that “magical phenomenon”, in quotes, that takes place, for example, when an observer measures the position of a particle and the state of its collapse, forcing the particle to materialize in a specific position.

In the orthodox interpretation of quantum mechanics it is not clear which beings are qualified as observers capable of producing this collapse: only human beings?

maybe any system of a certain size, like an animal or a detector?

The many worlds theory is the radical assumption that collapse never really occurs.

So mathematics tells us that the joint system, formed by the observer and the particle, globally enters a state of superposition and we have two contradictory realities coexisting:

Q.

What is entanglement and how does it occur?

R.

It is a consequence of the postulates of quantum mechanics and, surely, the most surprising physical phenomenon or the one that is furthest from the predictions of classical physics.

Entanglement is that, if we have two physical systems, what happens in system 1 can be correlated with what happens in system 2, that is, affect it instantaneously.

We can have two electrons millions of kilometers away from each other, at opposite ends of the solar system.

If they are in an intertwined state, what is done to one of them will instantly affect what happens to the other, just like telepathy.

Q.

Is there such telepathy?

R.

The phenomenon is experimentally proven and is fascinating.

It is very curious, however, that this kind of instantaneous communication between states cannot be used to transmit useful information faster than light;

which would have violated the theory of relativity.

Q.

And teleportation?

R.

It is also another very surprising phenomenon that has been experimentally verified in elementary particles.

In principle, there would be no theoretical difficulty in doing it also with macroscopic objects, even with human beings, although the technology is absolutely light years away from being able to achieve such a thing.

It's science fiction right now.

But with elementary particles, even with millions of elementary particles, it has been achieved.

When we talk about teleportation, it is not that we take a particle at a point, destroy it and it reappears thousands of kilometers away.

It's not like in

There is no transmission of matter.

What happens is that the state of the initial particle is recreated in another particle located at another point.

In principle, to achieve this, an enormous amount of information would have to be sent, since the particle can be in infinite states.

The most surprising thing about quantum teleportation is that much of this information is not transmitted by conventional means, but is done precisely thanks to quantum entanglement: that mysterious communication that is not useful on its own, but that, combined with a little of conventional transmission of information, it is capable of recreating the complex state of a particle.

Most of the transmission of information is done through this kind of telepathy and, furthermore, instantly,

In principle, there would be no theoretical difficulty in also teleporting macroscopic objects, including human beings, although the technology is absolutely light years away from being able to achieve such a thing.

Q.

You also state that we have neither experimental evidence nor a guaranteed theory about the origin of the universe.

R.

We have experimental evidence of what happened in the universe one second after the Big Bang and it matches perfectly with what the theory predicts.

Now, if we go back to even more primitive moments, things get more and more difficult because temperatures and densities are higher and higher and there comes a time when we do not have a proven theory that describes nature at those scales.

Furthermore, the Big Bang leaves open the question of why there is a universe and where the matter and energy that populates it came from.

There is a very interesting speculation that the universe could have arisen as a quantum fluctuation out of nothing.

This is more of a suggestive idea than a perfectly mathematically formulated idea, because nothing, by definition, is something that does not exist.

Q.

Absolute stillness does not exist?

A.

As we said before, a particle cannot have its position and speed well defined at the same time.

Let's take the example of a pendulum: we can imagine it perfectly still, but it really isn't possible: if we were to look at it with a formidable microscope, we would see that it really has a certain movement because it cannot be perfectly still (exact zero speed) and in a certain position.

Q.

And avoiding noise in quantum computing is impossible?

R.

It is one of the great problems of quantum computing.

A quantum computer is made of a material made up of atoms and these have a certain speed, they cannot be perfectly still, which induces undesirable disturbances.

This noise can be reduced by cooling the systems to a temperature close to absolute zero, -273 ºC.

But you can't stop atoms completely, because of the uncertainty principle.

And this is not the only source of noise.

The most important is decoherence.

The universe is full of dark matter that we do not know what it is and cannot be described with current theories of particle physics.

it has to be something else

Q.

What is decoherence?

A.

A classical computer works by manipulating bits, that is, sequences of zeros and ones.

In contrast, the building block of quantum computing is the qubit, which is essentially an arbitrary superposition of the zero and one states of a bit.

The huge advantage of a quantum computer is that, using qubits, it can compute trillions of different combinations of bits at once, instead of having to perform the computation trillions of times.

But these quantum superpositions of states are very sensitive to interaction with the environment, which tends to degrade them until they are useless for that operation.

This effect is what is called decoherence.

Q.

Quantum physics still leaves fundamental questions unanswered.

Which?

R.

Perhaps the most important is to make quantum physics and the theory of relativity compatible, two formidable theories but that, in their ins and outs, are inconsistent with each other.

In addition, there are many things for which we do not know the explanation.

For example, the universe is full of dark matter that we do not know what it is and cannot be described with current theories of particle physics.

It has to be something else.

We know it's there, but we don't know what it is.

With the known particles there are also many mysteries.

For example, no one knows why elementary particles have the mass they do.

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