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Do you really understand Einstein’s theory of relativity? – BBC News

5 giugno 2019

Do you really understand Einstein’s theory of relativity? – BBC News

Almost everyone has heard of Albert Einstein, the Nobel prize-winning genius whose theories overturned centuries of scientific thought. But how much do you understand the work that made him really famous – his theory of general relativity? Here, we talk you through the science that changed the way we understand the universe.
“Albert Einstein’s theory of general relativity completely changed the notion of the Universe.

It shed light on the birth of the universe, planetary orbits and black holes.
It also has very practical uses, like in GPS navigation.
But what exactly is this theory and why was it so revolutionary?
Until the early 20th Century, physics was mostly explained in terms of Isaac Newton’s
laws. For Newton, gravity was a force generated
by the mass of an object causing them to attract each other,
heavier objects pulling others more intensely. This is why we stand on the ground on Earth,
said Newton… it attracts us to its centre.
And it’s why planets move around the Sun. But imagine if the Sun disappeared completely.
According to Newton’s theory, the planets of the Solar System would instantly
abandon their orbits, as there would be no gravity attracting them to the Sun.
For Newton, gravity is a force with immediate action
regardless of the distance between the bodies. But according to Einstein’s calculations,
light was the fastest thing in the Universe. Nothing could travel faster than light, not
even gravity. Light takes about eight minutes to cover the
nearly 150 million kilometres that separate the Sun from the Earth.
So, if the Sun disappeared, how could the Earth go off its orbit before us Earthlings
stopped seeing sunlight?
Problems like that suggested to Einstein that gravity could have a different explanation
than Newton thought. Between 1905 and 1915, Einstein developed
the theory of general relativity. He imagined the three dimensions of space
and the dimension of time together as a kind of fabric surrounding us, shaped
by the presence of celestial bodies. He called it space-time.
Imagine the Sun as a heavy bowling ball placed in the middle of a trampoline.
The ball makes the surface of the trampoline dip, right?
This curvature is what we feel as gravity. So for Einstein, the Earth and the other planets
remain in orbit not because the Sun attracts them
but because the Sun is such a massive star that other celestial bodies
follow the curve it generates in the space-time fabric.
Now gravity is no longer considered a force of attraction
between two bodies, as Newton thought. It is an effect of the space-time curvature
on bodies. So according to Einstein, what would happen
if the Sun disappeared? His theory says this disturbance in space-time
would form a gravitational wave that would travel to the planets at exactly
the speed of light. That means we would see the Sun go dark at
the same time as the Earth changes its orbit.
In other words, what Einstein demonstrated is that until then
we had been seeing the Universe in the wrong way.
The general relativity theory turned Einstein into a world celebrity.
Because of him, science (and our imagination) could fly higher and higher.
General relativity not only surprises scientists it fascinates us all.”
Video production: Kako Abraham and Camilla Costa
Editor: Carol Olona
Voice: Claire Press

Source: BCC News – link Video youtube

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