What is time? That is a question that no one knows the answer to. Before we can approach it, we must ask another question: what time do we mean? Measuring the time clocks? That is not time, but a representation of time by a machine. Clock time is a human construct, something we devised to understand what we feel deep down about the nature of time: that time is a measure of change, that things change, and that if we have some degree of control over this change , we can better learn to quantify these. However, it is the sense of the passage of time that is so puzzling. We know it’s there, we feel it, we see it in the mirror as we see ourselves aging. Yet we can’t get a grip on it.
The essence of time dominates our existence and yet escapes our comprehension.
The universal story of time
Time is not like space. We are free to move in space, left, right, backward, forward. But time is a kind of prison. We cannot control it and we cannot move around it. It’s moving forward and that’s it.
Yes, the theory of relativity has changed our perception of the constant flow of time. But time still flows, even if it works differently for observers moving relative to each other. Einstein showed that a moving clock ticks slower than one at rest. But they both tick, and time moves forward for each. One of the cornerstones of the theory of relativity is that light is the fastest in the universe. Traveling at the speed of light is impossible for anything that has mass. Only light itself can reach the speed of light. (So are gravitons, hypothetical particles that carry gravity and are believed to be massless.) So, perhaps for light, time doesn’t exist. But since light is not aware of existence, it probably doesn’t care about the passage of time.
But we do care. And what modern science has shown us in spectacular ways is that everything that exists has a history. From living things to rocks to the universe itself, there is always an embedded time story that tells the story of that thing. When it comes to the master of all time, the story that contains all the stories, we have to go to the universe itself.
What we do know is that time began about 13.8 billion years ago with the Big Bang. That event marked the beginning of time. We understand time as a measure of change. What changed in the dawn of time is that the universe as we know it came about in ways we still don’t (or may not) understand. What changed scientifically, however, was that the extremely high density of matter and energy began to thin out due to the expansion of space itself — an expansion that unfolded over time and is still ongoing.
The details of this story depend on the kinds of things the universe contains. The cosmic recipe determines how the universe changes over time and what future it will have. There are essentially two possibilities. In one, the universe will continue to expand forever. Since stars have finite lives, they will be extinct at some point far in the distant future. Stellar corpses will blanket the universe, from slowly smoldering white dwarfs to black holes of various sizes.
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But drama can be added to this, depending on what kind of matter the universe contains. If the current recipe remains viable, there will be three main ingredients: dark matter, dark energy, and the material we’re made of — the normal matter of protons (actually quarks) and electrons.
A cold death, or destruction by fire
Assuming normal matter and dark matter are stable in the long run, dark energy determines the future of the universe. If dark energy, this ethereal substance of unknown composition, is a constant – that is, if its density does not change and it maintains a fixed volume regardless of cosmic expansion – then the expansion of the universe will continue to accelerate. In extreme scenarios, it can have so much negative pressure that it tears everything apart, breaking matter back down to its basic ingredients. Instead of ‘from dust to dust’ the cosmic epitaph would be ‘from particles to particles’.
But it’s also possible that dark energy isn’t like that and that its negative pressure will diminish over time, no longer fueling the rapid expansion of the universe. Acceleration will decrease and the universe will retain the faded stars and their corpses, all resting far from each other – the ultimate cosmic solitude. The sad thing is that in a cosmos with no room for entropy to grow, matter cannot reorganize itself into something interesting. This is the cold cosmic death scenario. If nothing changes, time itself loses its function and ends.
Another possibility is that the expansion will slow down. If there is enough matter, it can reverse itself and push the universe from expansion into contraction. Eventually, matter that has been dispersed for billions of years will reconsolidate to a small volume, heat up, reach crushing densities and… well, it depends. It can go into a Big Crunch, the reverse of the Big Bang; or it can reach a point of maximum contraction and then bounce out into a new phase of expansion. This is the bounce universe model, where the universe trades periods of expansion and contraction, and never the point of infinite density or initial singularity† Time is ticking, although each cycle needs new clocks. Each end of time marks the beginning of a new age, a new cycle of existence. This is a somewhat more reassuring picture than a cold death, even if each cycle involves destruction by fire.
What we learn from our current cosmological models is how lucky we are to exist exactly when it is possible to exist. Of course, no luck is involved here. We exist now because this is the moment when matter can agglomerate into thinking blobs like us. In other ages there would be no stars that last a lifetime enough to guess his fate. So if time will end, it will be because beings like us will end too. In a universe without conscious beings aware of the passage of time, without the awareness of past and future, the very concept of existence is meaningless. That should give us pause when we consider how small we are in the vastness of space. Small yes, but as far as we know, we are the ones who keep all of cosmic history in our minds.