Why does time pass
It is a question so profound that few people would even think to ask it yet its effects are all around
Human beings live in a perpetual present inexorably sealed off from the past, but moving relentlessly into the future
For most people time seems to be something that is just out there a thing ticking away in the background fixed immutable
Time seems to go in one direction in one direction only, but physicists see it much differently
John Archibald wheeler said time is what stops everything from happening at once I
Wish I could explain what it would be like
For there not to be time
But I can't we experience time
We are psychological creatures that have time and that's kind of one of our defining characteristics, so if time weren't around
We don't really have even a good way to
Describe what happens because happens, right?
That's a verb right there that has to do with time
So I think once you start wondering about this you get much further into the realms of philosophy than physics
I actually don't think it's that mysterious. We live in a world
That's full of stuff and this world happens over and over again with the stuff in slightly different positions
Time is just the label on all those different moments that constitute the history of the universe
It's very much like the page numbers in a book
One of the great minds who changed the way science thinks about time was Albert Einstein in
1905 he published his special theory of relativity
Which described the motion of objects moving near the speed of light?
In it he demonstrated that time passes differently in different places depending on how those places are moving with respect to one another
So first I'm Stein studied the effect of
Fast motion, and he found that space and time got mixed up if you moved more quickly and that's his special theory of relativity
the theory of relativity is so called because
Even the difference between time and space is relative to what observer is doing the measuring in
Particular to how fast you're moving
This is something that we never notice in our everyday lives, but once you start moving close to the speed of light
relativity becomes crucial
Einstein showed that the faster one travels the slower time goes for the traveler at the speeds at which humans move this is
imperceptible
But for someone traveling on a spaceship at speeds close to that of light
Time would slow down compared with its passage for people on earth
One of Einstein's great insights was to understand
That different people moving at different speeds through the universe
see
Different events as being simultaneous there was another important aspect of Einstein's theory which he didn't even realize when he published it
The time was woven into the very fabric of space itself
It was actually not line Stein but another German mathematician named Hermann Minkowski
Who pointed out that Einstein's theory could be thought of as a theory of a single?
Four-dimensional space-time and I signed when he first heard this idea said well, that's just a mathematician talking
That's not really something that is very useful to physicists
But once he tried to fit gravity into his theory of relativity. He realized the space-time concept was crucial
Einstein used this insight to help develop his general theory of relativity which incorporated gravity
He published it in 1915 ten years after his special theory of relativity which was focused solely on motion
Einstein tried very hard he worked for ten years after inventing special relativity
To fit gravity into the framework and eventually he realized he needed a new framework, so he invented what we call the general
theory of relativity
Where Einstein says that this space-time stuff this four-dimensional world in which we live has a life of its own?
Space-time itself can move it has dynamics it has curvature
and it's that curvature that geometry of space-time that you and I
Experienced as the force of gravity and then he found the time was also
Changed by your proximity to a very massive object so like a black
Hole is the best example
But also just how close we are to the earth or to the Sun or anything else that has a gravitational field
changes the passage of time
With the general theory of relativity he demonstrated that massive objects warped the fabric of space-time
It is this curvature that causes time to slow down near them
Accordian general agility time
slows down near massive bodies
It slows down any place that gravity pulls you tour
Time slows down in proportion to the gravitational pull of a nearby object so the effect would be strong near a black hole
But milder near the earth, but even here it can be detected
Einstein's theories had to be taken into account when the GPS system was set up, otherwise it would have been inaccurate
These ideas about space and time being relative sound very
Wild and far out and removed for everyday experience, but to physicist. They are very very important. There's something we observe every day
One of the interesting things about Einstein's equations is that none of them suggest time goes in only one direction
They work equally well if time goes forwards or backwards
This is in contrast to everyday experience
One of the conundrums is that?
The laws of physics for the most part do not distinguish between time going forward and time going backwards and yet the actual
World that we experience certainly is different
forward to backwards one scientist who puzzled over the directionality of time was Arthur Eddington a 20th century
astronomer who defined the concept of the arrow of time based on observations made by the 19th century physicist Ludwig Boltzmann
In space there's no preferred direction. There's no difference in the laws of physics
If you look left or look right, but there seems to be an enormous difference between the past and the future
That's what we call the arrow of time, and it's a little bit mysterious
Why is the past so different from the future? We think it's not a difference in the deep down fundamental laws of physics
It's actually just a difference because of the universe in which we live
The arrow of time is just time as we experience one second following one other second following another second
It's just the x psychological experience of time and the fact that
Things change as we experience them, but it seemed to happen in the world forward so buildings get built
But they decay slowly, and it's that decay, there's at least partially responsible for the arrow of time
It's the increase of entropy so in thermodynamics. We know that things get more random over time and
That appears to be very fundamental to our experience of time and what's happening in the universe?
The arrow of time is based on the second law of thermodynamics, which says the disorder known as entropy increases with time
For example a building left untouched will slowly decay into its surroundings it will disintegrate into a more chaotic state
But it is highly unlikely that the building will become more orderly over time
This is because there are many more ways for a system to be disorderly than orderly
There can be many ways for something to break for instance
but only one way for it to be put back together again a
System will be less disordered in the past and more disordered in the future. This is the arrow of time
So how can the arrow of time be reconciled with Einstein's equations if time can go forwards and backwards according to relativity?
Does that mean it's possible to go backwards in time?
We actually don't know whether it's possible to travel backwards in time. We do know it's possible to travel forwards yesterday
I travel forwards 24 hours and Here I am right now
You can't help but travel forward in time our best guess is that you just can't travel backward in time
The theory of relativity does allow time travel to the future if I go down near a black hole
And stay there for one hour you may go forward by seven years by the time
I come back, so you've moved forward in time seven years while I move forward in time just one hour
Einstein's theories do allow for the formation of wormholes in space these are shortcuts that link otherwise distant places in the space-time continuum
So a wormhole is
a hole in
Space and time that comes out somewhere else
And it does it much faster than the speed of light potentially so it would be a way of
violating what we think of otherwise as a fundamental speed limit in Einstein's theories and
It does it in a way that doesn't obviously violate
Einstein's theories, but we're not sure that there's any actual way you can build a wormhole
They might be shut off from the rest of space and time and so could never actually use it to do anything
Although wormholes are theoretically possible, they're a highly implausible proposition
That's because the equations suggest enormous masses and energies would be required to create and manipulate one
There are two problems of the wormholes one is if you don't have one how you create it
And the other is if you do have one. How do you hold it open?
So it doesn't collapse and prevent you from going through and in both cases
What we understand about these questions
points toward a
Conclusion that wormholes are probably forbidden by the laws of nature
What remains then is a mystery?
Theory fails to forbid traveling backwards in time, but practice suggests it might just as well be forbidden
For now it would appear the arrow of time cannot be reversed
No, one knows. Why time passes, but it seems that no matter how people look at it
It goes in one direction in one direction only
You