The nature of time: an unsolved puzzle

A new approach tries to answer one of the oldest question of human history: “what is time?”

– By Claudio Bogazzi, PhD, Science Writer –

The passage of time is probably one of the simplest aspects of human perception.  Time has always been associated with the passing of seasons and the cycles of celestial objects. And yet, modern physics does not have any special rule regarding the passage of time. Time does not flow. Time simply is.

A new Perspective article published in Frontiers in Physics, by Dr. James M. Chappell and colleagues focuses on the nature of time.

The researchers give a view of time as an intrinsic geometric property of a three-dimensional space, without the need for the specific addition of a fourth dimension. A new approach that tries to answer one of the oldest questions of human history: “what is time?”

The view of time has deeply changed through history. Isaac Newton, one of the founders of classical mechanics, thought of time as something “…absolute, true […], flows equably without regard to anything external.” Newton believed that time was like a theatre stage and that the physical phenomena were its actors. Time can be measured and it is the same for all observers.

This concept of time as something absolute survived for more than three centuries. Then, in 1905, Albert Einstein, with his theory of Special Relativity, changed the way we think about time. Space and time were combined into a single continuum, the spacetime.

There is no absolute time, according to Einstein. My time and your time are different depending on how fast we are moving from each other. The faster I am moving, the greater the magnitude of the time dilation I am experiencing. In our normal daily life, we do not notice any time dilation effect. Usually for a commercial airplane flying from London to New York this difference is a few microseconds. However, this effect can be quite dramatic for speeds close to the speed of light (300000 km per second).

Einstein did not stop. Almost ten years later, he published what is now considered his masterpiece: the theory of General Relativity. Gravity is not just a force of attraction between masses, it is a property of the spacetime itself. The more massive a body is, the more distorted the spacetime around it. This distortion is what we experience as gravity. Massive objects such as stars and planets warp the fabric of spacetime and thus “attract” other bodies. Consequently, not only the motion of a body but also its gravity can slow time.

General Relativity changed how physicists perceive time. Both past and future are fixed. Time should be thought as something laid out in its entirety, with all past and future events there. As the physicist Paul Davies wrote in his book About Time: Einstein’s Unfinished Revolution: “Posing the simple question “How fast does time pass?” exposes the absurdity of the very idea. The trivial answer “One second per second” tells us nothing at all.”

General Relativity, however, fails to explain why we experience an arrow of time. There’s more: the laws of physics show no preference towards any direction of time. At the microscopic level, the realm of atoms and particles, the physical processes are mostly time-symmetric. This means that if we would reverse them in time, the physical laws would still be true. The American physicist John Wheeler once said: “If you ask an atom about the arrow of time, it will laugh in your face.”

Yet, at macroscopic level there is an obvious direction of time that creates a distinction between past and future: an egg that drops from a kitchen table will smash into pieces and its final state will be completely different from its initial state. This is related to the Second Law of Thermodynamics, which states that in an isolated system the entropy, the quantity the measures the state of disorder, always increases with time. The Second Law implies that the world is asymmetric in time with respect to the amount of order. As time goes by, an isolated system will become more disordered. Is that all? If so, why do we perceive the direction of time? Is it something uniquely related to the human brain? Scientists do not know the final answer.

Much progress has been made to understand the nature of time but the main question, “what is time?” is still unanswered. Scientists do not know whether time is simply a fourth dimension of the spacetime continuum or whether there is more to it. Chappell’s article provides new insights into the nature of time, and it does that by challenging conventional views of spacetime.

Read the full article: Chappell JM, Hartnett JG, Iannella N, Iqbal A and Abbott D (2016) Time As a Geometric Property of Space. Front. Phys. 4:44. doi: 10.3389/fphy.2016.00044

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