Thursday, June 27, 2019

Travelling to the past?

S.Agustín, por 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.

On the other hand, we have the concept of an arrow of time, proposed in 1927 by Arthur Eddington, of which there are several variants:
  1. The thermodynamic arrow, related to the second principle of Thermodynamics, which states that in a thermodynamically isolated system, entropy (which measures lack of energy organization) always increases.
  2. The cosmological arrow, for the universe had its origin in the Big Bang and since then it evolves continuously.
  3. The psychological arrow, which makes it possible for us to distinguish the past from the future.
A few weeks ago, some media outlet claimed that a Russian team has managed to reverse the flow of time in the laboratory, sending an electron to the past. A few days later, the news had fizzled out. As usual, the media have goofed on a scientific news, although the fault is not always exclusively theirs, for scientists use sometimes communication facilities to exaggerate their results and increase the media impact of their research.
The first thing to note is that the article in question, describing the experiment, has been available in arXiv for over a year, so it isn’t easy to see why it suddenly became food for journalistic exaggerations.
In the second place, what this article describes is not an experiment carried out in a laboratory, as some media have suggested, but a computer simulation. There wasn’t, therefore, a travel to the past, but a simulation of something that may not be a travel to the past, as we’ll see below.
Third, what the article describes is the passage of an electron from a more probable to a less probable state. In other words, a supposed transgression of the second principle of thermodynamics. Yes, there are people who identify (incorrectly, in my opinion) the thermodynamic arrow of time with the flow of time. This identification is far from proven.
Alan Turing
Fourth, the article and its recent popularization in the media insist on pointing out that the simulation was carried out in an IBM quantum computer. This is irrelevant, for a quantum computer, as Turing proved more than 80 years ago, can solve just the same problems as an ordinary computer. Yes, it may be able to solve some problems much more quickly (although this has not yet been proven), but it does not have the ability to solve any new problems.
Let’s hope this example is a lesson for the media, so they won’t try to count their eggs before they are hatched when they see the possibility of giving an impact headline to some news in the scientific field. That headline seldom corresponds to reality.

The same post in Spanish
Thematic thread on Time: Preceding Next
Manuel Alfonseca

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