Time is a measure of non-stop, consistent-change in our surroundings, usually from a specific view point.
While the concept of time is self-evident & intuitive, the steady passing of events before our eyes and the orbit of the Moon around our planet, describing its fundamental nature is quite harder.
Even physicists are not sure, what actually happens when time passes. Although they’ve couple of hypotheses.
How does time work?
For centuries, time was considered as a constant, independent force, as if the universe’s progress is governed by single clock.
This description of time changed in 1905, with Albert Einstein’s theory of special relativity.
While the passing of time was already-known to be closely connected to space, this monumental theory was the first to combine space & time into a single field, one with measurements that change depending on the relative motion, or gravitational forces of objects within it.
Basically, it means that time is relative.
Is time real?
Two people moving at the same velocity will each-agree their measures of distance & time match. As one person changes the speed, however, they will see the other’s measure of time & distance change, even as their own stays the same.
Without any reason to prioritize, one perspective of time over another, this means, time is not a constant universal unit at all. It’s a relative measurement that changes as objects move faster or slower or as they are subjected to more or less gravity.
Gravity curves space & time: The stronger the gravity, the more it curves space-time and the more time slows-down.
You can see an example of this in the image following below, which shows Earth’s mass curving space-time.
This is why people on board the International Space Station, which is farther from Earth’s gravity, age very slightly slower than those on Earth.
Is it possible to reverse time?
Of course, for us to-actually see these effects on time, the change in speed or gravitational pull must be huge. But as an observer accelerates towards the speed of light, unique-measures of time become increasingly noticeable.
In theory, as a particle approaches the speed of light, we would see its clock slow down. Once it exceeds the speed of light, its clock would theoretically seem in reverse from our point-of-view. From the particle’s point-of-view, our clock would seem to reverse.
What about time travel?
Similarly, the space contorting volume beyond the horizon of a black hole also distorts perspectives of time.
In our universe, we’ve freedom of space and can move around as we like, but we’re forced to march along time’s arrow in a linear direction.
Calculations show, crossing over a black hole’s horizon would swap those freedoms. So we would no longer have to follow time’s strict arrow of direction, but we would lose the freedom to move around in space, allowing time travel (of sorts).
While these scenarios help us to better understand time’s nature, both speed of light and black hole travel have constraints that prevent us from using them as practical ways to reverse time.
Do not try either at home.
Why is there a future and a past?
Models of space-time can describe measurements of time & space varying from one point to the next, but they do not explain much about time’s stubborn adherence to a sequence of events.
Under these descriptions of time, our universe is a single block of space-time. There is kind of a beginning, the Big Bang, before which our greatest understanding of the laws of physics cannot be applied. There is sort of an end, where change is no longer measured with any significance. But no single slice of time stands-out physically as ‘now’.
“People like-us who believe physics know that the distinction between past, present & future is only a stubbornly persistent illusion,” Einstein once wrote.
There might be a couple of clues to the mystery of time in fields of physics other than cosmology, though. For instance, back in the 1870s the Austrian physicist Ludwig Boltzmann proposed there was a link between time and an increasing level of disorder in the Universe.
By tying thermodynamics principle of entropy to time that only moves in one direction, it hinted at a possible explanation, for why time’s arrow points forward, perhaps our universe moves from a low entropy, highly compact infant universe, to a highly disordered, expansive Universe drifting into the future.
How to slow down time?
Outside of taking a trip into space and away from Earth’s gravity well, there’s a way to slow down time, at least from your own perspective. This has nothing to try do with physics and nature of time itself, but how fast or slow life feels to each of us.
Some researchers say, exposing yourself to new experiences or environments can actually make time seem to pass slower. This might be to-do with the amount of information our brains have to take-in & process, when we are young or learning something new, the world seems to slow down. As we get older and get-into a routine, the days & years seem to speed by.
Unless you’ve a spacecraft, none of this is going to make you age any slower, but knowing that time is a little more bendy than many of us think it’s can be a reminder that we’ve our own ability to change our perception of how fast the days pass, if only a little.
