Everyone knows what the time is right?

What is time?

Could you explain it to someone else?

Take a few minutes to think about how you can explain what time is to a friend.

The most important thing about time is that

THINGS WEAR OUT

This is called the Second Law of Thermodynamics. It is the most important law in the Universe.

Your clothes wear out; your trainers get smelly and wear out, what happens to a strawberry if you leave it in a bowl for four weeks?

As things wear out they get messy too. There is a scientific name for a state of mess. The amount of mess in the Universe always increases. A number called Entropy measures how messy the Universe is. Entropy always gets bigger. As your bedroom gets messier, entropy grows.

But how does your room get tidied up? It doesn't happen by magic. Somebody has to work at it!

When somebody tidies up and makes a messy room look neat, it looks as if the Second Law of Thermodynamics has been broken. Tidying up has made Entropy smaller in the room. BUT outside in the Universe, energy has been used up to make the room tidy.

Work uses up energy, and using up energy always makes Entropy increase.

Everything we do uses up energy from the Universe.

THIS IS THE BIG IDEA

If Entropy did not increase, there would be no difference between the past and the future. Therefore, as Entropy increases time passes.

You can see Entropy increasing and time passing just by popping ice cubes into warm water.

Before the ice melts, there is a pattern of ice in the water. After the ice melts, there is no pattern. All the water is the same. Entropy has increased and time has passed, because energy has been used from the Universe.

If you are really bored you can try and watch time run backwards.

Get a glass of cold water and watch it until some of the water turns into ice and the rest gets warm. WARNING before you try this you must know that it can not happen. Heat never flows from a cold thing into a hot thing. It always flows from the hot to the cold one…

Another way to understand time.

All of the energy used on Earth comes from the Sun (except for a very tiny amount created by atomic fusion)

Even the coal and oil we use in power stations were made by plants that needed sunlight to help them grow. It takes a huge amount of energy to make even the simplest of things, like glass.

 You could make time stand still just by taking away the Sun and the stars. If you got rid of the Sun and the stars, there would be no energy for living things to draw on anywhere in the Universe. There would be no way to make order out of disorder. The temperature everywhere would be the same. Time would not flow. Nothing would happen, anywhere.

If everything really could always stay the same, there would be no time at all. It just wouldn't exist. It is because things wear out that time passes. The everlasting flow of time is like an endless river that carries everything and everyone along with it.

But it doesn't stop there being a past and a future. Time always moves forwards, so you have to make something before it can exist for you to break it.

If someone has made a glass in the past you can use it in the present and it can be broken in the future.

The arrow of time points from the past to the future. But that doesn't explain why we seem to be moving steadily from one to the other. Nobody knows why time flows along like a river instead of like a set of still photographs.

The most important thing to understand about time is that, no matter what we do, we just can't stop it passing.

Time passes at a rate of 60 seconds every minute, 60 minutes every hour, 24 hours every day, every day of your life. And it always goes in the same direction. Time is always passing and it can't be held back or altered. Things that have happened can't be "unhappened". What's done is done.

 THE BIG BANG

There are many theories about the origins of the Universe and time. Stephen Hawkin has said, "all theories about origins break down at the Big Bang." Can there be definitive answers? Some say yes and others choose to think about other things. Generally the ideas are divided into two schools of thought; religious and scientific. Those with a faith in God or gods would often say that God or gods created everything. Since God or gods can do almost anything then this might be possible. There are myths, Scriptures and many legends that try to understand creation and origins. Science would say that as human knowledge increases we will one day find the answer and today's theories might lead to tomorrow's answers.

Imagine a balloon dotted with blobs of paint. If you blew up the balloon to twice the original size, all the paint blobs will be further apart.

If you measure from any blob, you'll see that the other blobs have moved away from it. Imagine that each blob is a galaxy. Whichever galaxy you were in, it would look as if all the other galaxies were moving away from you, and that you were the centre of the Universe.

If the Universe is getting larger then it must have been smaller in the past. Imagine winding time backwards. There must have been a time when all galaxies touched. Before that, a time when all stars touched. And, before that, a time when all the atoms in the stars were squashed together. One of sciences explanations is that everything in the Universe came from a tiny fireball. This is known as the big bang.

BUT… WHAT HAPPENED BEFORE THE BIG BANG?

Some thoughts … The Big Bang was not like a lump of compressed matter exploding into empty space. There was nowhere for the Big Bang to expand into, because space was not made by the Big Bang.

There was no "before" the Big Bang, because time was made in the Big Bang.

The Big Bang is the edge of time, the moment when time began.

Time has only existed since the Big Bang.

Let's keep it simple… Space and time seem to be very different things. You can move about anywhere you like in space. But you can only move one way in time, from the past into the future.

In 1905, the great scientist Albert Einstein realized that space and time are connected. And he found that the way space and time look to you depends on how fast you are moving.

In 1907, Hermann Minkowski used geometry to explain what Einstein had already discovered in terms of geometry. Minkowski said that time is a dimension like space, and that space and time together make up the four dimensions of spacetime, that humans experience. Time is a 'direction' like space, but the time direction is at right angles to all of the space directions. So as things move though space they can move in any direction and at all sorts of speeds. But nothing can move through space fast than light.

Einstein said that anybody, anywhere in the Universe, is entitled to say that they are at rest, and to measure all movement relative to themselves. This is called the SPECIAL THEORY OF RELATIVITY

Einstein's theory says that when an object moves past you, it shrinks a little bit. And when a clock moves relative to you, time runs more slowly for the moving clocks. We don't notice this in everyday life, because the effects are only large enough to notice if something is traveling at close to the speed of light - 300,000 kilometres per second (186,000 miles per second).

THE BIG IDEAmoving objects shrink, moving clocks run slow. Time itself runs more slowly for moving objects.

If you travelled in a space ship moving at close to the speed of light on a journey that lasted a year, when you came back to earth you would find that, although you were only a year older, centuries would have passed on earth. This is a kind of one-way time travel.

The way space shrink and time stretches for moving things is because space and time are connected in spacetime.

Think of a square block of rubber, like an ordinary pencil eraser. One side of the eraser represents time the other side (at right angles) represents space.

If you stretch the eraser in the 'time' direction it gets thinner in the 'space' direction.

The Special Theory of Relativity describes how time stretches when space shrinks, and vice versa. But when you want to work out accurately what happens when spacetime is bent, you need a more comprehensive theory, since the Universe is a sphere things don't travel in straight lines and gravity has an influence on everything that moves.

What you need is the GENERAL THEORY OF RELATIVITY

The general theory explains everything that the special theory explains but makes allowances for the effects of gravity.

So what is… GRAVITY?

Imagine a stretched rubber sheet, like a trampoline. Now put a heavy weight, like a bowling ball, on the sheet. It bends.

The gravity of a star, like the Sun, makes a similar dent in spacetime. Now try to roll a marble across the rubber trampoline with the weight on it, the marble's track would be bent sideways by the dent in the rubber sheet. If a planet (or anything) moves past the Sun (or anything), the planet's track is bent sideways by the dent in spacetime. That's what gravity is.

If you put a very, very heavy weight on the rubber sheet it would stretch the sheet so much that it would form a hole, like a very deep well with narrow sides. If you put a very heavy lump of matter at a point in space, it would make a 'hole' in spacetime - a bottomless pit of infinitely stretched spacetime. Anything could fall into it, but nothing could ever get out. Not even light could escape. So it is called a BLACK HOLE

You don't need very much matter to make a black hole. When a star has used up all its nuclear fuel, it cools down and shrinks into a small ball. If the ball of the dead star has as much matter in it as three stars like our Sun put together, it will make a black hole.

There are lots of stars around that are at least ten times the size of our Sun they will all form black holes as they burn out.

So at the centre of a black hole there might be a star with the mass of the Sun but as small as a golf ball. Its gravitational pull on spacetime will attract everything to it, including the light that passes by.

Anything that falls into a black hole is crushed by gravity. It is squeezed into a mathematical point, with no volume at all, at the centre of the black hole. . According to the General Theory of Relativity, stuff falling into a black hole could go right through it and be turned into the beginning of a completely new universe, in a new Big Bang. This wouldn't mean that the stuff crushed into a black hole bounces back out into our Universe. It would be in a different universe- another bubble of spacetime, connected by a tiny tunnel, called a… wormhole

If this happens every time a black hole forms, our Universe may be just another bubble in the froth of universes, like bubbles in a glass of coke. Our Universe might even have been born of a black hole in another universe. Where does the other end of a black hole go? Some scientists think that black holes are the gateways to tunnels to other universes. But why couldn't the tunnel come out in another part of our own Universe?

If wormholes like this existed, it would be possible to jump into a wormhole and come out of the other end, far away, near a distant star. It would take hardly any time at all to cross through a wormhole into another part of our Universe, like a short cut through space.

Scientists at first did not think that this was possible, but Einstein's General Theory of Relativity does not forbid the idea of wormholes to exist. Tunnels through space are completely, scientifically reasonable according to the laws of physics.

And a hole trough space is really a hole through spacetime. That means that the other end of a wormhole could come out not just in a different place but most certainly in a different time. Or it could come out in the same place but in a different time.

So the means are available to travel trough time. If you jumped through a wormhole, you could come out of the other end not just anywhere in the Universe, but anywhen in the Universe.

Just one thing to think about… the diameter of a wormhole might only be the size of an atom.

There might be a way to travel through time, but what about paradoxes…a paradox is a seemingly absurd or contradictory statement, even if actually well founded. Suppose you travelled back in time and you landed on your granny and killed her when she was just a little girl. That means your mother would never have been born, so you would never have been born either. So you could not have travelled back in time in the first place to land on your granny. This must mean that there must be a law of physics that forbids time travel. Some scientist believes that there could be just such a law that prevents such paradoxes from happening.

What is the future of time? Scientists are not sure. Some say that there is an everlasting expansion like a balloon that never bursts but just gets larger and larger. Other scientists say that the Universe will expand to a certain size and then contract to the size it was at the Big Bang and then increase in size again. This fluctuation will continue forever.

But religion has its say too. Many religions have said that God or gods have formed the Universe and it will also end as God or gods command. Some Christians say that Jesus the Christ will return, rule for 1,000 years and then finally do away with all sadness and wickedness and make a new Universe for people to live in, in everlasting peace and harmony with God.

Today we understand some things… tomorrow we will understand more… someday we might understand much more.