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

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.

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