Can Time Go Backward? Caltech Physicist Says No

Time travel, in which a character will go backward into the past and embark on an adventure, is a popular theme in science fiction. What is the reality, though, that someone could go back in time? As reported on January 5 in the Times of San Diego, Caltech Research Professor Sean M. Carroll discussed this question during a presentation earlier this month at the Reuben H. Fleet Science Center in San Diego.

Many people have lounged on a beach during an all-too-short vacation and thought briefly about the nature of time simply because they don’t want it to pass, and wondered if there was anything that could change things. Time, however, only moves forward in one direction in day-to-day life. This forward movement is referred to as the “arrow of time,” and is something over which we have no apparent control.

Time’s movement helps to bring order to the universe. As Ray Cummings wrote in his 1922 novel, The Girl in the Golden Atom, “Time is what keeps everything from happening at once.” A question that needs to be answered before you can address the ability to go back in time is what pushes time forward in the first place. If you know that, perhaps you can determine if time can be pushed the other way.

Since time can’t be seen, it is not usually treated like other things that make up the universe. However, time is a property that is as real as anything else that can be measured in the cosmos. Albert Einstein realized this and coined the term “space-time” when talking about the totality of the universe. Much of his analysis considered this duality as he tried to work out other problems.

The key to the movement of time, and the thing that drives it forward, is something called entropy. This term is not in most people’s everyday vocabulary and so it quickly makes eyes glaze over. However, entropy is merely the tendency for conditions to go from ordered to disordered. In fact, it’s easier to think of it as a synonym for order. Entropy is the amount of disorder. Low entropy has more order than high entropy.

The scent from a vase of flowers placed at one side of a room will travel across the room as the molecules spread. Dropping an egg will cause it to smash into many pieces and form a sticky mess. The ordered conditions of the scent or the egg move toward disorder. No one expects the scent of the flowers to re-enter the plant, or for the egg to suddenly re-assemble itself.

The universe's expansion and the "arrow of time" move forward in one direction as entropy increases.

[Image Credit: NASA/GSFC]

Time was produced at the Big Bang at the same instant that every other property of the universe came into being. The universe began in an infinitely small point and expanded to its current size. All of the matter and energy that is presently contained in the universe was present at the Big Bang. As the universe expanded quickly and violently, the cosmos moved from ordered to disordered, carrying all of the properties of the universe with it, including time. With the forces at work in the universe, the increase in entropy pushes time forward as they expand the universe.

Professor Carroll’s take on this was pretty straightforward when he dismissed the possibility of time travel. He said, “Time is a one-way street. We will always just move into the future.”

The force that makes time go forward is a part of the universe. The prospect that someone can go backward in time to help a relative, assassinate a dictator, or start a vacation over seems, unfortunately, pretty dim.