The idea of time reversal has for the most part always been restricted to tales of science fiction and fantasy, and the belief that time consistently moves forward, away from the past and toward the future, is generally accepted without question.
Yet this common belief is not based in scientific theory or concluded from the fundamental laws of physics, so says Nobel Laureate Anthony Leggett, who gave a talk entitled ‘Why Can’t Time Run Backwards?” last Thursday in a packed Hoyt Auditorium. Rather, it is assumed to be true because that is all that we have ever been able to perceive.
‘We automatically assume that we can remember the past and affect the future,” Leggett said. He called this perception the ‘arrow of time” and continued on to say, ‘The problem is that at the most basic level that we understand… the fundamental laws that we believe that nature obeys appear to make no distinction between the forward and backward directions of time.”
For centuries, this paradox has perturbed physicists, who have strived to find some scientific explanation of either why time is restricted to one direction or why we can only perceive one direction. Leggett spent the next several minutes of his lecture describing some of the fundamental laws of physics and why they do not restrict time to one direction.
He provided the most detail on Isaac Newton’s three laws of motion: objects will remain either at rest or in uniform motion unless acted on by some external force; every action is counteracted by an equal but opposite reaction; and force is equal to mass times acceleration.
To explain why these laws are independent on time, Leggett threw an object into the air and then caught it. If you were to film him doing that, he said, and then played the film backward, it would look the same as when you played it forward.
Going along with this is the question of causality. Leggett asked the audience to picture a cannon firing and then asked if the cannon’s initial position and velocity caused its final position and velocity. If this causality exists and is observable, then in theory we should be able to use the laws of physics to predict the future.
‘All of the future [would be] laid out in front of you,” Leggett said. ‘You can, at least in principle, calculate exactly the whole future of the universe… [but] in the framework of Newtonian physics, there’s really no basis for the idea that the past causes the future.”
Leggett then described another example of a physical phenomenon that physicists began to use in their attempts to explain the apparent irreversibility of time. If you had a closed box that was divided into two halves by a screen so that one half was filled with a gas and one with empty space, and then if you took the screen away, the gas would spread out to fill the entire box. This phenomenon is due to the second law of thermodynamics, which defines the forward direction of time as the direction in which many naturally occurring physical phenomena, such as the expansion of gas in the box, must take place.
When gas is confined to a certain volume and the boundaries of this volume are taken away suddenly, the gas will expand. However, the reverse case the gas contracting in order to allow the partition to be put in place will not occur.
‘It will never be the case that the gas spontaneously goes back to this earlier state,” Leggett said.
Physicists eventually came to the conclusion that though the fundamental laws of physics are independent of time, these laws are valid on a microscopic scale, and the physical phenomena that we experience every day occur on a macroscopic (larger) scale.
Thus the arrow of time that we perceive only necessarily pertains to that particular macroscopic level.
So while the conclusions made explain the apparent paradox of the arrow of time, they provide no definitive conclusion as to whether or not time actually can run backward the answer is, as of now, unanswerable.
Because he was presenting these complex ideas to an audience consisting of people with mixed degrees of previous knowledge, Leggett attempted to make his lecture easy to understand by describing examples from everyday life.
Leggett also used his own apparent fascination with the subject to keep the audience’s attention.
Lombardo is a member of the class of 2010.