By Dr Brian Cox
Presenter, BBC Horizon: What on Earth is wrong with gravity?

Newton's concept of gravity has problems, says Brian Cox

Isaac Newton wrote down his theory of gravity in 1689, and his equations are used to this day to send space probes to the outer edges of our Solar System.
So what could possibly be wrong with our understanding of it?
There are problems with Newton's theory, however. It doesn't quite describe the orbit of Mercury, the closest planet to the Sun, and as Newton knew very well it has nothing to say about what the force of gravity actually is.
It took over 200 years and the genius of Albert Einstein to discover a deeper theory.
Einstein's General Theory of Relativity describes the force we see as gravity as being due to the bending and curving of space and time (or to be more accurate "spacetime") by heavy objects like the Earth and Sun.
Curved spacetime
This is a bizarre concept, but many of us use Einstein's theory every day when we jump in our cars and turn on the satellite navigation system.
Astonishingly, the fact that the Earth bends time has to be taken into account, otherwise our satnavs would drift by 11km per day.
Einstein's theory of curved spacetime beautifully predicts the orbit of Mercury, and much more extreme phenomena out in the Universe.
The Ligo observatories aim to detect gravitational waves

Perhaps the most extreme test of Einstein comes in the form of binary pulsars: two stars as massive as the Sun but squashed to the size of a city, orbiting around each other and spinning thousands of times a second.
Einstein predicts that these exotic stars should spiral inwards towards each other as they release energy in the form of gravitational waves.
Changes in the violent dance of the binary pulsars have been observed at exactly the rate predicted by Einstein, but the gravitational waves themselves have yet to be seen.
This is the goal of the Ligo observatories near Seattle and New Orleans.
Gravitational waves as predicted by Einstein are one of the strangest phenomena in nature.
Broken theory
They are a travelling, stretching and squashing of space and time! If they exist, they will be passing through you right now as you read this, speeding up and slowing down your watch and stretching and squashing your head, fortunately by an amount less than the size of a subatomic particle.
Einstein's laws must break down in the hearts of black holes

So you don't feel them, but, remarkably, Ligo may see their effects. The observation of gravitational waves would be another remarkable triumph for Einstein, but even that will not satisfy physicists like myself.
This is because we know that there are places in the Universe where Einstein must fail. In the heart of black holes, giant suns collapsed to a single infinitely dense point, Einstein breaks down.
And even more crucially, back at the beginning of time, the Big Bang itself, Einstein's picture of space and time is no longer adequate. We physicists are therefore faced with a deep problem.
If we want to truly understand how, and maybe even why, the Universe began, then we must know what space and time looked like right back at the beginning.
Such a theory, if it exists, would be what is known as a quantum theory of gravity  a theory that supersedes Einstein and works not only in the world of planets, stars and galaxies, but also in the subatomic sized world of black holes and the very beginning of the Universe itself.
This quest is the "Holy Grail" of 21st century physics.
Dr Brian Cox presents Horizon: What on Earth is wrong with Gravity? on BBC Two at 2100GMT, Tue 29 Jan or afterwards from BBC iPlayer.

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