Timed tests, bedtimes, timed miles, the world seems to revolve around time but, according to physicist Albert Einstein, “the distinction between past, present, and future is only a stubbornly persistent illusion.”

      Physicists define time as the progression of events from the past into the future. It is not something that can be seen, touched, smelt, or tasted but, humans still measure it. While many physicists argue over the many theories of time there is one concept that appears to be agreed upon: the arrow of time. The concept was first explored by astronomer and physicist, Sir Arthur Eddington. According to Eddington, time appears to be inherently directional. The past lies behind us and is “fixed and immutable” the future, on the other hand, lies ahead and is not necessarily fixed.  Andreas Albrecht, a physicist at the University of California gives perspective to the arrow of time providing the example that “eggs can be scrambled but not unscrambled and shattered teacups do not spontaneously reassemble.”

     The original theory of time as stated in 1905 by scientist Albert Einstein. While most people think of time as a constant, physicist Albert Einstein showed that time is an illusion; it is relative. According to Einstein, it can vary for different observers depending on your speed through space. time slows down or speeds up depending on how fast you move relative to something else.  Einstein gives an example that a person “approaching the speed of light, a person inside a spaceship would age much slower than his twin at home.”

     General relativity also provides scenarios that could allow travelers to go back in time, according to NASA. One possibility could be to go faster than light, which travels at 186,282 miles per second (299,792 kilometers per second) in a vacuum. Einstein’s equations, though, show that an object at the speed of light would have both infinite mass and a length of zero. This appears to be physically impossible, although some scientists have extended his equations and said it might be done. A linked possibility, NASA stated, would be to create “wormholes” between points in space-time. While Einstein’s equations provide for them, they would collapse very quickly and would only be suitable for very small particles.

      While Einstein’s theory provides a base for the understanding of time, physicist Carlo Rovelli believes it oversimplifies the concept. Rovelli is one of the creators and champions of loop quantum gravity theory, one of several ongoing attempts to marry quantum mechanics with general relativity. In contrast to the better-known string theory, Rovelli states that loop quantum gravity does not attempt to be a “theory of everything” out of which we can generate all of particle physics and gravitation. Rovelli wrote down his theory in a book.  In the first, “The Crumbling of Time”, Rovelli attempts to show how established physics theories deconstruct our common-sense ideas. In part two, “The World without Time”, Rovelli puts forward the idea that events (just a word for a given time and location at which something might happen), rather than particles or fields, are the basic constituents of the world. In the final section, “The Sources of Time”, Rovelli reconstructs how our illusions have arisen, from aspects of thermodynamics and quantum mechanics. He argues that our perception of time’s flow depends entirely on our inability to see the world in all its detail.