He explained this phenomenon with

it was almost useless for our ancestors who looked up to the sky to understand what's really there.

and then

came Einstein.

he explained the same phenomenon with another theory:

**the general relativity**

**(including the**

special relativity)

special relativity)

**the law of universal**

**g**

**n**

**o**

**t**

**a**

**t**

**i**

**v**

**a**

**r**

**i**

Everybody noticed at least once that apples fall from the apple tree.

though this theory seemed perfect to write down physical circumstences on

Earth

of space and time.

that gravity between masses results from their warping

that gravity also effects time.

this is called gravitional time dilation.

this means that time flies with different 'speed' in regions of different gravitional potential.

so time is an independent dimension, and with the other 3 spatial dimensions, they make up spacetime, the 4-dimensional world we live in.

and the only thing that remains the same for every observer in the universe is the speed of light

the theory explains...

Light travels 300000 kilometres/ 186000 miles in only 1 second. Nothing can move faster.

physicists discovered cosmological redshift of light. (which is an optical Doppler-effect.)

they realized that stars out of our galaxy all suffer redshift, therefore they get further and further from us.

light also travels in time. so if we look at the stars, we can only see the past.

which states that at the very beginning, the whole universe was a single point, with extreme density and temperature,

then it started to expand rapidly (like an explosion), and the expansion is still going on for at least 13 billion years.

so they could model the

Big Bang theory

this was the state of singularity, where the number of dimensions are infinite. There was no space and no time.

with Einstein's theory,

scientists also discovered

black holes

the very heavy radio stars, whose gravitational field is so strong that even light can't escape from them. that's why they are 'invisible'

The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole.

but what is relativity?

imagine that you're in an elevator with your friend

what you see is that the world around you is moving

but from an other point of view, you're the ones who actually move

Following this logic we can easily admit that everything is relative.

except the speed of light.

The speed of light is a constant. Light consists of photons, the particles whose mass is 0.

this is something that never changes.

Nothing can reach the speed of light that is heavier than photons.

Therefore the only things which can exceed the speed of light are things with negative mass.

Einstein realized that matter and energy are interrelated and made the famous formula that expresses this relationship mathematically:

E = m x c

2

speed of light

mass

energy

interesting consequences

DO YOU NOW SEE THE LIGHT?

imagine your friend now flying with his spaceship

and yourself watching him

with your binoculars

exactly when he's half-way between two twin stars,

suddenly the stars explode in exactly the same time

as you know, the light beams from both exploding stars travel with the same speed,

and you see that they reach you at exactly the same time

but what he sees from the inside,

is quite different.

as he travels towards the 2nd star, he actually reaches the beam from the 2nd star first.

1st

2nd

From different reference frames, there can never be agreement on the simultaneity of events.

so he will think that the 2nd star exploded first.

**who's right?**

Einstein tells us that both are correct, within their own frame of reference.

This is a fundamental result of special relativity:

**so these are the miraculous rules of our universe**

we have all experienced the Doppler-effect: it's the phenomenon when an ambulance passes by you with its sirens turned on. The sound you hear when it approaches is different from the one when it departs.