What is time, and why does it advance?

What is time?

By Thomas Kitching, UCL

Imagine time flowing backwards. People would rejuvenate instead of growing old, and after a long life of progressive rejuvenation – unlearning everything they know – they would end up like a sparkle in their parents’ eyes. It is time as it is depicted in a novel by science fiction writer Philip K. Dick but, surprisingly, the direction of time is also an issue cosmologists grapple with.

While we take for granted that time has a given direction, physicists do not: most natural laws are “time-reversible,” meaning that they would work just as well if time were defined as a decline. So why is time always moving on? And will he always do it?

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Does time have a beginning?

Any universal concept of time must ultimately be based on the evolution of the cosmos itself. When you look at the universe, you see events that have happened in the past – it takes a light time to reach us. In fact, even the simplest observation can help us understand cosmological time: for example, the fact that the night sky is dark. If the universe had an infinite past and an infinite expanse, the night sky would be completely bright – filled with the light of an infinite number of stars in a cosmos that has always existed.

For a long time, scientists, including Albert Einstein, thought the universe was static and infinite. Observations have since shown that it is in fact expanding, and at an accelerating rate. This means that it has to come from a more compact state which we call the Big Bang, which implies that time has a beginning. In fact, if we look for a light that is old enough, we can even see the relic radiation from the Big Bang – the cosmic diffuse background. Realizing this was a first step in determining the age of the universe (see below).

Relativity

But there is a catch, Einstein’s special theory of relativity shows that time is… relative: the faster you move relative to me, the slower time will pass for you relative to my perception of time. So, in our universe of expanding galaxies, rotating stars, and whirling planets, the experiences of time vary: everything is past, present and future is relative.

It turns out that because the universe is on average the same everywhere, and on average looks the same in all directions, there is a cosmic time. To measure it, it suffices to measure the properties of the cosmic diffuse background. Cosmologists have used it to determine the age of the universe: its cosmic age. It turns out that the universe is 13.799 billion years old.

The arrow of time

So we know that the weather probably started during the Big Bang. But there remains a nagging question: what exactly is time?

To unpack this question, we need to examine the fundamental properties of space and time. In the dimension of space, you can move forward and backward; commuters experience it every day. But time is different, it has a direction, you always go forward, never backward. So why is the dimension of time irreversible? This is one of the major unsolved problems in physics.

To explain why time itself is irreversible, we have to find processes in nature that are also irreversible. One of the few such concepts in physics (and in life!) Is that things tend to get less “orderly” over time. We describe this using a physical property called entropy which encodes how ordered something is.

Imagine a gas box in which all the particles were initially placed in a corner (an orderly state). Over time, they would naturally seek to fill the whole box (a messy state) – and getting the particles back to an orderly state would require energy. It is irreversible. It’s like breaking an egg to make an omelet. Once it spreads out and fills the pan, it will never turn ovoid again. It’s the same with the universe: as it evolves, the overall entropy increases.

A growing disorder

It turns out that entropy is a really good way to explain the arrow of time. And while it may appear that the universe is becoming more orderly rather than less – moving from a wild sea of ​​relatively evenly distributed hot gases in its early days to stars, planets, humans, and weather articles – it it is nevertheless possible that it increases in disorder. This is because the gravity associated with large masses can drag matter into seemingly ordered states – the increase in disorder that we believe must have occurred being somehow hidden in gravitational fields. So the mess could increase even if we don’t see it.

But given nature’s tendency to prefer disorder, why did the universe start in such an orderly state? This is still considered a mystery. Some researchers argue that the Big Bang may not have even been the start, there may in fact be parallel universes where time flows in different directions.

Will time end?

Time had a beginning, but whether it will end depends on the nature of the dark energy that causes it to expand at an accelerating rate. The rate of this expansion can eventually tear the universe apart, forcing it to end in a great tear; alternatively, dark energy can disintegrate, overthrowing the Big Bang and ending the universe in a Big Crunch; or the universe may simply expand forever.

But would any of these future scenarios end time? Well, according to the weird rules of quantum mechanics, tiny random particles can momentarily arise out of a vacuum, which is constantly seen in particle physics experiments. Some have argued that dark energy could cause such “quantum fluctuations” to give rise to a new Big Bang, ending our timeline and starting a new one. While this is extremely speculative and highly unlikely, what we do know is that it is only when we understand dark energy that we will know the fate of the universe.

So what is the most likely outcome? Only time will tell.

Thomas Kitching, lecturer in astrophysics, UCL

This article originally appeared in The Conversation. Read the original article.

Conclusion: what is time and why does it advance? Cosmologist Thomas Kitching of University College London explains how the arrow of time points to the future.