Suppose there was no gravity and space-time was completely flat. This would be like a completely featureless desert. Such a place has two types of symmetry. The first is called translation symmetry. If you moved from one point in the desert to another, you would not notice any change. The second symmetry is rotation symmetry. If you stood somewhere in the desert and started to turn around, you would again not notice any difference in what you saw. These symmetries are also found in “flat” space-time, the space-time one finds in the absence of any matter.

If one put something into this desert, these symmetries would be broken. Suppose there was a mountain, an oasis and some cacti in the desert, it would look different in different places and in different directions. The same is true of space-time. If one puts objects into a space-time, the translational and rotational symmetries get broken. And introducing objects into a space-time is what produces gravity.

A black hole is a region of space-time where gravity is strong, space-time is violently distorted and so one expects its symmetries to be broken. However, as one moves away from the black hole, the curvature of space-time gets less and less. Very far away from the black hole, space-time looks very much like flat space-time.

Back in the 1960s, Hermann Bondi, A. W. Kenneth Metzner, M. G. J. van der Burg and Rainer Sachs made the truly remarkable discovery that space-time far away from any matter has an infinite collection of symmetries known as supertranslations. Each of these symmetries is associated with a conserved quantity, known as the supertranslation charges. A conserved quantity is a quantity that does not change as a system evolves. These are generalisations of more familiar conserved quantities. For example, if space-time does not change in time, then energy is conserved. If space-time looks the same at different points in space, then momentum is conserved.

What was remarkable about the discovery of supertranslations is that there are an infinite number of conserved quantities far from a black hole. It is these conservation laws that have given an extraordinary and unexpected insight into process in gravitational physics.

In 2016, together with my collaborators Malcolm Perry and Andy Strominger, I was working on using these new results with their associated conserved quantities to find a possible resolution to the information paradox. We know that the three discernible properties of black holes are their mass, their charge and their angular momentum. These are the classical charges that have been understood for a long time. However, black holes also carry a supertranslation charge. So perhaps black holes have a lot more to them than we first thought. They are not bald or with only three hairs, but actually have a very large amount of supertranslation hair.

This supertranslation hair might encode some of the information about what is inside the black hole. It is likely that these supertranslation charges do not contain all of the information, but the rest might be accounted for by some additional conserved quantities, superrotation charges, associated with some additional related symmetries called superrotations, which are as yet, not well understood. If this is right, and all the information about a black hole can be understood in terms of its “hairs”, then perhaps there is no loss of information. These ideas have just received confirmation with our most recent calculations. Strominger, Perry and myself, together with a graduate student, Sasha Haco, have discovered that these superrotation charges can account for the entire entropy of any black hole. Quantum mechanics continues to hold, and information is stored on the horizon, the surface of the black hole.

The black holes are still characterised only by their overall mass, electric charge and spin outside the event horizon but the event horizon itself contains the information needed to tell us about what has fallen into the black hole in a way that goes beyond these three characteristics the black hole has. People are still working on these issues and therefore the information paradox remains unresolved. But I am optimistic that we are moving towards a solution. Watch this space.

This article appeared in print under the headline “Do black holes eat information?”

WHY are we here? Where did we come from? According to the Boshongo people of central Africa, before us there was only darkness, water and the great god Bumba. One day Bumba, in pain from a stomach ache, vomited up the sun. The sun evaporated some of the water, leaving land. Still in discomfort, Bumba vomited up the moon, the stars and then the leopard, the crocodile, the turtle, and finally, humans.

This creation myth, like many others, wrestles with the kinds of questions that we all still ask today. Fortunately, as will become clear from this special issue of New ĐÓ°ÉÔ­´´, we now have a tool to provide the answers: science.

When it come to these mysteries of existence the first scientific evidence was discovered about 80 years ago, when Edwin Hubble began to make observations in the 1920s with the 100-inch telescope on Mount Wilson in Los Angeles County.

To his surprise, Hubble found that nearly all the galaxies were moving away from us. Moreover, the more distant the galaxies, the faster they were moving away. The expansion of the universe was one of the most important intellectual discoveries of all time.

This finding transformed the debate about whether the universe had a beginning. If galaxies are moving apart now, they must therefore have been closer together in the past. If their speed had been constant, they would all have been on top of one another billions of years ago. Was this how the universe began? At that time many scientists were unhappy with the universe having a beginning because it seemed to imply that physics had broken down.

One would have to invoke an outside agency, which for convenience one can call God, to determine how the universe began. They therefore advanced theories in which the universe was expanding at the present time, but didn’t have a beginning. Perhaps the best known was proposed in 1948, and called the steady state theory.

According to this theory, the universe would have existed for ever and would have looked the same at all times. This last property had the great virtue of being a prediction that could be tested, a critical ingredient of the scientific method. And it was found lacking.

Observational evidence to confirm the idea that the universe had a very dense beginning came in October 1965, with the discovery of a faint background of microwaves throughout space. The only reasonable interpretation is that this background is radiation left over from an early hot and dense state. As the universe expanded, the radiation would have cooled until it is just the remnant we see today.

Theory backed this idea too. With Roger Penrose I showed that if Einstein’s general theory of relativity is correct, there would be a singularity, a point of infinite density and space-time curvature, where time has a beginning.

“If the early universe had been completely smooth, there would be no stars and life couldn’t have arisen”

The universe started off in the big bang, expanding faster and faster. This is called inflation and it turns out that inflation in the early cosmos was much more rapid: the universe doubled in size many times in a tiny fraction of a second.

Inflation made the universe very large and very smooth and flat. However, it was not completely smooth: there were tiny variations from place to place. These variations caused minute differences in the temperature of the early universe, which we can see in the cosmic microwave background.

The variations mean that some regions will be expanding slightly less fast. The slower regions eventually stop expanding and collapse again to form galaxies and stars. And, in turn, solar systems.

We owe our existence to these variations. If the early universe had been completely smooth, there would be no stars and so life could not have developed. We are the product of primordial quantum fluctuations.

As will become clear (see “Existence special: Cosmic mysteries, human questions”), many huge mysteries remain. Still, we are steadily edging closer to answering the age-old questions. Where did we come from? And are we the only beings in the universe who can ask these questions?

Read more: “Existence special: Cosmic mysteries, human questions“

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