Are space and time the same thing?
If so, how can that be?
Space is a vacuum.
More correctly, an imperfect vacuum.
A near-total emptiness, but not completely empty.
A vacuum is a space devoid of all matter, but no vacuum is truly devoid of all matter.
No vacuum is truly perfect.
Intergalactic space, for example, contains a few hydrogen atoms per cubic meter.
A perfect vacuum isn't possible because quantum theory dictates that energy fluctuations known
as virtual particles are constantly popping in and out of existence in space.
But what's the relation between space and time?
Everything must occupy an area of space, and relative to every area of space is a measure
of time which can be described as the rate of any movement through that space.
At the beginning of the last century, Albert Einstein published his genius work of General
Theory of Relativity which explains how what we perceive as gravity actually comes from
the curvature of space and time.
This theory showed the world that space itself isn't a flat unchanging, absolute entity.
Rather it's woven together, along with time, into a single fabric, like an outstretched
blanket, called spacetime.
The spacetime fabric is continuous and can get distorted by mass and energy.
The sun, which is a massive object, bends spacetime blanket in such a way that light
no longer travels in a straight line as it passes by the Sun.
A black hole, which is an even more massive object, can force light to start traveling
around it if it gets too close.
Because a black hole completely twists and bends space around it, any passing light will
eventually be dragged in.
And for everything in the universe to happen, it takes time.
Time is the fourth dimension.
If you want to schedule a meeting with someone, you need to describe not only the three-dimensional
location which is length, width, and height, but also time.
So if you want to explain the behavior of our universe you need to think in terms of
spacetime.
The reason you see stars is because light travels from them to our eyes.
But it takes time for light to reach our eyes.
Sirius, for example, the brightest star in the night sky, is about 9 light-years away
from us.
The star's light that we can see tonight began its journey 9 years ago.
When we look at Sirius, we are observing what the star looked like 9 years ago which means
we are technically looking back in time.
The distance between us and Sirius is equal to that taken by light to cover over a period
of 9 years.
And that's why we can never know what the universe is like at this very instant.
The universe clearly doesn't extend just in space, but in spacetime.
Space and time are the same thing, or at least two different sides of the same coin.
And this becomes obvious, especially at relativistic speeds approaching the speed of light.
Light always travels at the same speed.
Einstein's special relativity suggests that the speed of light is constant.
Thus, space and time had to be combined into a single entity in order to keep light's speed
the same for all observers.
And then, time slows down as things speed up, and can be shifted by gravity, such as
starlight passing by a black hole as we explained earlier.
If you traveled at the speed of light, time would actually stop.
That's because you would be moving WITH time.
But what would happen if you moved faster than light?
In that case, you would move backward in time.
The speed of light remains unchanged even if the observer moves relative to it.
Time, for humans, is just imaginary.
It's imaginary in the sense that when we talk about time, in our mind that consists of the
past, the future, and the present.
But in physics, everything is a big infinite now.
Past, present, and future are all the same.
Time doesn't pass or flow.
Time is there.
Together with space, they exist as spacetime.
Physicist John Wheeler perfectly sums up Einstein's general relativity in 12 words "Spacetime
tells matter how to move, matter tells spacetime how to curve".
That means matter is doing the moving rather than space itself.
And it is not just moving through space.
It is moving through spacetime.
The idea of space expansion could be wrong.
Space is probably not expanding.
Instead, the objects are moving farther from one another.
In the absence of matter, you can not measure space.
And matter was born in a state of flying apart.
Things have been flying away ever since the big bang occurred, and it doesn't mean that
more space has been created.
Space doesn't get created.
Space simply is.
The past, the present, and the future are simply there and are part of spacetime.
Though there will certainly be better models and theories in the future, especially due
to the fact that relativity is not fully reconciled with quantum mechanics, Einstein's general
relativity remains our best understanding of the cosmos on the largest scale.