Alberto Casas, physicist: ‘Free will is an illusion created by our brain. Everything that is going to happen is already written’

“People like us, who believe in physics, know that the distinction between past, present and future is only a stubbornly persistent illusion.” This is part of the condolence message that the revolutionary physicist Albert Einstein sent to the family of his friend and former colleague at the Swiss Federal Institute of Technology in Zurich (ETH Zurich), Michele Besso, after the latter’s death in 1955.

The letter is included by Alberto Casas, a research professor at the Spanish National Research Council’s Institute for Theoretical Physics (CSIC-UAM), in his latest book, La ilusión del tiempo (or The Illusion of Time). The book serves as the scientist’s vehicle for exploring the branches of physics that have contributed to understanding what he defines as “an essential coordinate for describing the world.” And at each intense and mentally challenging stop along this journey, answers and questions unfold regarding fundamental aspects of everyday existence.

Question. Is time an illusion?

Answer. Time is not an illusion, but the way we perceive it is. It is a necessary coordinate for describing the world: to identify any event, you need three spatial coordinates to define where it occurred and one temporal coordinate to differentiate it from others that occurred in the same place. However, the way we perceive it, and in particular how it passes, can indeed be an illusion.

Q. You state that neither present nor past nor future has an absolute meaning.

A. In our everyday lives, we tend to think of the present of the universe as the sum of all events occurring at any given moment. However, the theory of relativity demonstrates that, for another observer, the events occurring simultaneously are different, and therefore there is no objective present, even though for all observers on Earth the facts that constitute reality are practically the same. But that remains only an approximation. As for the past and the future, the feeling that the past has already happened and is unchangeable, while the future has not yet occurred and is open, may be an illusion due to the fact that we have much more information about past events than about future ones, because of a subtle effect: the increase in entropy (disorder of a system) as time progresses. This causes certain events to leave traces, records, and others not. The former are those we place in the past, and they seem to have already happened, unlike those we place in the future.

The feeling that the past has already happened and is unchangeable, while the future has not yet happened and is open, may be an illusion

Q. Do you mean that the future exists, but we have no data about it?

A. If we set aside quantum physics for a moment, the laws of physics tell us that, starting from a certain state, like the one we are currently experiencing, we can reconstruct both the past and the future. In principle, this would be possible because the laws of physics provide a perfect correlation. It’s like a bouncing ball; at a certain point, you can reconstruct its future, which will be to continue bouncing in whatever direction, and you can also reconstruct its past. When we remember something, what we are doing is reconstructing or deducing how things were at another time based on the data of the present.

Q. So, both past and future are fixed.

A. The only difference is that, due to the increase in entropy, we have much more information about past events than about future ones. But that doesn’t mean they are more real. Quantum physics complicates this perspective because it can make it impossible, with the information available at any given moment, to fully reconstruct either past or future events, but they would still be on the same level.

Q. Is time travel possible?

A. Time travel to the future has been conclusively proven. The Theory of Relativity teaches us that time does not pass at the same rate everywhere. One of the illusions of our perception is that time advances at the same rate everywhere, but that is not the case. For objects that move faster or are in a stronger gravitational field, time passes more slowly. For example, time passes faster at your head than at your feet. This has been experimentally proven. Near a black hole, for you, a year may pass, while far from it, 100 or 1,000 years will have elapsed. This is one way to travel to the future. Another is to travel in a spacecraft capable of reaching speeds close to light speed (something impossible with our current technology). You take a trip through the universe, and upon your return, a century may have passed.

Q. And to the past?

A. It is much more difficult, but the Theory of General Relativity opens the door to that possibility with very peculiar (and unrealistic) configurations of matter and energy that give rise to so-called closed time loops.

Q. But would the past be unchangeable?

A. In these exotic matter scenarios that generate closed time loops, there would be no logical paradoxes because you would return to the same point in spacetime and, therefore, return to exactly the same situation you were in. You wouldn’t perceive anything; you wouldn’t feel that you had made the time journey. So, no logical paradox would occur because you return to the same situation. There would, perhaps, be another way to travel to the past, which is through wormholes: hypothetical, complex, and highly unstable configurations of spacetime, a kind of tunnel. But you could never murder your grandfather because then you wouldn’t have been born in the first place. Nature cannot behave in a contradictory way.

Q. Are there irreversible events?

A. All processes, in reality, are reversible. A classic example is an egg that falls to the ground and breaks. For it to reassemble itself and jump back into our hands is extremely improbable, but not impossible. However, it is so unlikely to happen that, in practice, we call them irreversible processes.

Q. Is free will an illusion?

A. If the laws of physics are deterministic, then it’s clear we don’t have free will because everything that’s going to happen is already written. But neither does quantum physics, which is inherently probabilistic, facilitate free will. Events may not be predetermined, but their probabilities are, and we can’t affect them in any way without violating the laws of physics; we have no control over them. That’s why I believe free will is an illusion created by our brain, the feeling that we control our actions. When we do something, it seems to be the result of a free choice, but in reality, the decision had already been made. Many neurological experiments indicate that decisions are made a fraction of a second before we are aware of them. From the point of view of physics, it can’t be otherwise: there is no physical law that, in my opinion, allows for the free choice of a conscious being like ourselves. It’s not a very pleasant conclusion, I admit, but it seems to be what physics says.

Q. So we are not responsible for our actions?

A. The only reasonable way to behave in practice is as if we were free to choose, because otherwise, it would be absolute chaos. If we want to protect ourselves, for example, from someone who commits a crime, we have to hold them accountable for their actions. On the other hand, even if we are not truly free, a decision made consciously and consistently with all your moral values is not the same as a decision made by a sick person who is unaware of what they are doing. In practice, we cannot attribute the same degree of responsibility to them, but that doesn’t mean that, at a very deep level, we lack control over what we do.

Q. Is aging also due to entropy?

A. Biological processes are processes in which entropy increases, implying natural degradation. To sustain them, you need a supply of energy with very low entropy, which is the useful kind. For example, the Earth’s heat alone is useless to us because it is almost in thermal equilibrium. It is completely useless for biological processes, which occur thanks to the continuous supply of high-quality energy with very low entropy, provided by the Sun.

Q. Physicist Sabine Hossenfelder says that we die from the increase in entropy.

A. It is true that the process of degradation and death of a living being can be seen as a process of increased entropy and a return to thermal equilibrium. But given that we have a permanent supply of low-entropy energy from the Sun, in principle, it would be possible to reverse that degradation process. I see no physical contradiction in being able to reverse the aging process by harnessing useful energy from the Sun or another very low-entropy energy source.

Q. So we don’t die, but we are transferred to a macroscopic state with different entropy.

A. It depends on how you define death. If it means your brain stops functioning, you lose consciousness, and the information you possess is dispersed into the universe, then yes, we die. If you define death as the complete destruction of information, then, in that sense, we can say that we don’t die because the information is still there, dispersed, in a totally irretrievable way, but it’s still there. It’s like a burned book: the information it contained survives in the ashes and the combustion gases, even though it’s practically impossible to recover.

Q. If someone were able to revert that new state to the initial state, could they recreate existence?

A. Ultimately, we are physical systems. It’s like a wrecked car. From the remains, you could eventually rebuild it. A living being is a highly organized, complex system, but, in principle, from the remains of a deceased person, it would be possible to reconstruct the person as they were initially (at the cost of increasing overall entropy). It would be incredibly difficult, of course, but I see no impossibility of doing so.

We can say that we don’t die because the information is still there, scattered, in a completely unrecoverable way, but it’s still there

Q. The many-worlds theory, which also challenges our perception of time, means that we live in parallel realities, but are only aware of one.

A. It’s important to clarify that this is a hypothesis, specifically about what happens during the observation process. According to the orthodox interpretation, called the Copenhagen interpretation, when a physical system is in a superposition of states and you observe it, only one of those possibilities materializes, and the rest disappear. This is the interpretation we all use in practice, but it has significant conceptual problems. Another, more conceptually satisfactory interpretation, in my opinion, is the one defended by Hugh Everett, according to which the state doesn’t collapse: the system doesn’t materialize into one of the possibilities, but rather all of them continue to coexist in superposition. From your point of view, you are only aware of one of the quantum states, but there are other versions of yourself, a vast number, that are aware of the other alternatives. You only perceive one of the possible evolutions of the world, but others are happening simultaneously in the same place, even if they are invisible.

Q. Do we live many lives simultaneously but are only aware of one? Can I warn my other self not to sign the loan?

A. [Laughs] I insist that it’s a hypothesis. An example I use is that if you buy a lottery ticket, when the draw takes place, one of your “selves” will see that number come up. That is, all possibilities end up being realized in some branch of your quantum state. But it’s just a hypothesis, albeit a perfectly serious one, for interpreting quantum mechanics. It must be added, and this is important, that interaction between “the worlds” is impossible. That is, once they have been created, there is no longer any possibility of communication or of them affecting each other. Communicating with other parallel worlds would be excluded in this hypothesis.

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