March 2018 - Stephen Hawking passes away

Geoffrey Compère, Theoretical and Mathematical Physics Unit

Stephen Hawking was a specialist in black holes. He studied fundamental physics, just like you, Geoffrey Compère. Can you explain what these fascinating objects are?

In order to understand black holes, we must go back to Albert Einstein's theory of gravity, as he introduced one of the key concepts in fundamental physics: the theory of general relativity. The idea is that space and time warp in the presence of energy, and especially of mass. Picture a taut sheet. If you place an object on it, the sheet sags. Similarly, space curves around stars, and also a little around the Earth. In the case of a black hole, the curve is such that space itself tears at the centre: this is called a singularity. All around is a sphere of no return known as the event horizon. When one crosses the event horizon, the attraction is too strong and there is no escaping its pull. Stars remain in a state of equilibrium between two forces that counteract each other exactly: gravity, which tends to make the star collapse upon itself, and nuclear fusion, which causes the ‘fire’ that makes the star burn. When a star larger than the Sun has consumed all its fuel —hydrogen—, it collapses and dies. Its mass is so concentrated that it distorts the space around it, forming a black hole, the densest object in the universe.

Can you give us a sense of what these objects are like, while remaining inside the boundaries of theory?

Black holes do not emit light… hence the name! But stars do orbit around them, which lets us detect their presence. This is how we know that at the centre of our galaxy, the Milky Way, is a black hole whose mass is 4 million times that of the Sun: Sagittarius A*. It is the largest black hole known in our vicinity, but it is still much too distant to even consider paying it a visit. This means our understanding of black holes is, first and foremost, mathematical, which is why Stephen Hawking had such a passion for them as a theoretician. In 1970, he discovered that black holes obey the laws of thermodynamics: they have a temperature, and they radiate heat… meaning they are not completely black! This effect is now known as ‘Hawking radiation’.

If black holes emit radiation, it means they have entropy. Entropy is a measure of disorder. For instance, a tidy room could be said to have low entropy, while a messy room would have high entropy. Entropy is proportional to the room's volume. What Stephen Hawking and his contemporary Jacob Bekenstein have demonstrated is that the entropy of black holes is proportional not to their volume, but to the surface area of their event horizon! This was a groundbreaking discovery in fundamental physics.

What will Hawking's legacy be?

His work has opened many avenues of research. Hawking radiation leads us to a paradox that has yet to be solved: since black holes emit radiation, they ‘evaporate’. This means that whatever information has fallen inside them must be sent back out. And yet, according to Stephen Hawking's calculations, information cannot get out because Hawking radiation does not depend on the specifics of what went into the black hole.

So either Einstein's theory of general relativity or the laws of microscopic physics must be changed to account for this. This is known as Hawking’s ‘information paradox’. Much has been speculated on this subject over the past years. Stephen Hawking's major contributions to fundamental physics have made it a more popular topic, as he was able to discuss it with both finesse and poetry while also fighting amyotrophic lateral sclerosis. He has left his imprint in the history of science.