Illusion or ignorance?

Every civilization is blind to some things, while others are seen more clearly. This has the consequence that there are problems that a civilization strives to solve, although it is possible to show that they have no solution. This happened, for example, to the Greco-Roman civilization with the problem of squaring the circle with ruler and compass. It fell to the next civilization (ours) to show that it cannot be solved.

On the other hand, we have an evident tendency to deny the existence of what we don’t understand. This is happening to our civilization with two concepts with which we’ve got stuck, that we insist on explaining (away), but don’t have an obvious solution: the flow of time and human self-consciousness. In both cases, many thinkers of the last two centuries have said that both concepts are illusions; that they don’t really exist. Let’s look at it in more detail:

Travelling to the past?

S.Augustin, by Louis Comfort Tiffany
Lightner Museum

In his Confessions (Book XI, chapter 14), St. Augustine wrote these words, still valid today:

What then is time? If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know.

In the current situation of our scientific and philosophical knowledge, we still don’t know what time is.

·         For classical philosophy and Newton’s science, time is a property of the universe. Therefore, time would be absolute.

·         For Kant, time is an a priori form of human sensibility (i.e. a kind of mental container to which our sensory experiences adapt).

·         For Einstein, time is relative to the state of repose or movement of each physical object. There is, therefore, no absolute time.

·         For the standard cosmological theory, there is the possibility to define an absolute cosmic time for every physical object, measuring the time distance since the Big Bang to the present.

·         For the A theory of time (using J. McTaggart’s terminology) the flow of time is part of reality. The past no longer exists. The future does not yet exist. There is only the present. If the A theory is correct, travel to the past is impossible, because you cannot travel to what does not exist.

·         For the B theory of time, the flow of time is an illusion. Past, present and future exist simultaneously, but for each of us the past is no longer directly accessible, and the future is not yet accessible. Einstein adopted the B philosophy of time. In a condolence letter written to someone who had lost a beloved person, he wrote the following:

The distinction between past, present and future is only a stubbornly persistent illusion.

Cyclic Time and Linear Time

Stephen Hawking

In an article published in 1999, in volume 879 of the Annals of the New York Academy of Sciences, Pier Luigi Luisi speaks about the two traditional models of time that have been considered by traditional philosophy and the mythologies of various historical civilizations. They must not be confused with the two philosophical models originated in the twentieth century, the time A and time B of which I spoke in another post of this blog.

• Cyclical time, predominant in Asian civilizations and the Greco-Roman world until the Christian world view took root there. The origin of this model is evident, for many natural phenomena are cyclical: sunrise and sunset; the phases of the moon; the annual movements of the stars, synchronized with the seasons and with many biological phenomena...
• Linear time, prevailing in the three religions who consider themselves descendants of Abraham: Judaism, Christianity and Islam. Linear time can be compared with the course of the life of a living being, which begins at birth, goes on with changes during a certain period, and ends with death.

Irreversible time: illusion, or simplification?

Ilya Prigogine

We know Einstein believed that the passage of time is an illusion. In a letter of condolence he wrote in 1955 he said: ...the distinction between past, present and future is only a stubbornly persistent illusion. To assert this, he relied on the fact that Newton’s equations of gravitation, his own equations of General Relativity, Maxwell’s equations (which apply to electromagnetic waves) and Schrödinger’s equation (which gives the wave function of a particle in quantum mechanics) are all symmetric with respect to time.

How then can we explain the fact that it seems so obvious that time goes from the past to the future? Usually, physicists who believe that time is an illusion explain it by saying that, at the microscopic level, time is actually reversible, but when we move to the macroscopic level, new, emerging phenomena appear, one of which is the irreversibility of time. Let's give an example:

According to the usual theories, the movement of the molecules of a gas is perfectly reversible. If we reverse the direction of time, all the particles behave exactly the same and continue colliding with each other, only they would move in the opposite direction. However, when we consider all the trillions of particles that make up a gas, we see irreversible phenomena arising, such as the fact that the gas always tends to occupy as much space as possible, while its accumulation in a corner of the container is much less likely.

The problem is that our physical theories are based on approximations. Mathematics is a very important tool for physics, but in mathematics there are several kinds of very different problems, which differ in their difficulty to be solved. Let us look at a few:

Is time an illusion?

Albert Einstein

Physicists sometimes deny the reality of irreversible time and consider it an illusion, a psychological phenomenon. In a letter of condolence written in 1955, Einstein said this: ...the distinction between past, present and future is only an illusion, although persistent. A curious way to comfort those who have lost a beloved one. His reasons for saying this were the following:

·         In Newton equations of universal gravitation, if the sign of the variable representing time is changed, the equations don’t change. If we look at the film of a gravitational process, the theory predicts that we won’t be able to detect if the projection was made in the right sense or in reverse.

·         The same is true of Maxwell equations, which describe the behavior of electromagnetic waves.

·         The same is true of Einstein equations of General Relativity, which replace Newton equations to describe gravity.

·         The same happens with the Schrödinger equation, the basis of quantum mechanics.

But there is a problem: the equations mentioned do not make all of physics. The second principle of thermodynamics implies the existence of an arrow of time. In 1928, in a book titled The nature of the physical world, the inventor of this term (Arthur Eddington), said the following: if your theory [opposes] the second law of thermodynamics... [it will] collapse in deepest humiliation.

Every physical theory is a simplified abstraction where some parts of reality have been eliminated. If the irreversibility of time is one of those simplifications, it is not surprising that the final result is always reversible. In real events, however, there is no abstraction or simplification. All the physical theories, including the second law of thermodynamics, must be applied together. If this is done, the alleged temporal symmetry goes away.

·        

Newton and his apple

One of the first applications of Newton’s theory describes the fall of an apple. If a film being projected shows several pieces of an apple on the ground, which suddenly set in motion and gather in a single fruit, which then rises upwards until it gets attached to a tree, would we doubt that it has been projected in reverse? The fact that we don’t is a consequence of the second law of thermodynamics.

·         This also applies to the movements of celestial bodies. Imagine a recording of Mercury moving in its orbit, with the sun visible. By studying the solar sunspots we could deduce whether the film is projected correctly or in reverse. Sunspots are a consequence of thermodynamic phenomena.

·         Radioactive decay is another example of a theoretically reversible process that in practice is irreversible. In fact, the proportion of uranium-238 and lead-206 in a rock provides a reliable method to calculate its age. The chain of disintegrations from uranium to lead is far more likely than the reverse chain, although physical theories affirm that both things could happen in theory.

·         Whatever Schrödinger equation says, the Copenhagen interpretation of quantum mechanics requires an irreversible time. If a photon hits an electron with some energy, the electron is left in two overlapping spin states. If the spin is measured, the quantum superposition collapses into a positive or a negative value. This process involves a direction of time: first comes the impact of the photon, then the electron in two superimposed states, finally a measurement and a quantum collapse. The reverse process cannot happen.

In these examples, when all of physics is taken into account without excluding thermodynamics, the supposed reversibility of time disappears. Apparently physicists put their theories above reality, doing the opposite of what the scientific method demands. Not even great men like Einstein were exempt.

El mismo artículo en español
Thematic thread on Time: Next

Manuel Alfonseca

Can time travel paradoxes be avoided?

Cover of Fantastic SciFi with a
Porges story (The Shadowsmith)

In a science fiction story written in 1962 by Arthur Porges, entitled The rescuer, the inventors of a time machine discover that a man has entered the machine to travel back in time. To stop him, they destroy the machine with the man inside. When they are tried for murder and destruction of valuable property, they explain:

This man had taken with him a repeating rifle and five thousand rounds of ammunition. His intention was to arrive at Golgotha in time to rescue Jesus Christ from the Roman soldiers. In short, to prevent the crucifixion. And with a modern rifle, who can say he wouldn’t succeed? And then what?... What of the effect on the future, the entire stream of history, secular as well as religious?

The story is an excellent example of the paradox of predestination mentioned in a previous article, with several more that make us doubt the possibility of time travel. But is there no way to avoid the paradoxes? Is it possible to devise a theory that would remove them making time travel feasible, at least in principle? There have been several attempts to achieve this.

Will time travel be possible?

Hibernation in 2001, a Space Odyssey

The obvious answer to this question, such as it is formulated, is yes, of course it’s possible! We all travel in time at the rate of 24 hours a day.

Naturally, this is not what comes to mind when this question is asked. What is usually meant is this: will we be able someday to make sudden jumps in time, either forward (to the future) or backward (to the past)?

There are several schemes for traveling to the future. If they aren’t possible just now, one day they could become so. For example, perhaps human beings will be frozen and remain in suspended animation, to wake up and resume their ordinary lives a hundred years later. Or they could take passage on a spacecraft, make a trip at relativistic speed, and return to their starting point a century later, whilst for the travelers the elapsed time would have been just one year. In both cases, from the point of view of the persons in question, this would have been a trip forward in time, but in reality no sudden leap would have happened, for time would have kept going on for the hibernated body, though the mind would not be aware, and also for the relativistic traveler, although in this case time would have been accelerated.

But when we speak of time travel, we do not refer to these cases, which are possible in principle. We mean disappearing from the present and appearing in the past or in the future. Will we be able to do this someday?