Soft hair on black holes, making concrete on Mars and exploring the cosmos in 2016

By Hamish Johnston

This week’s Red Folder looks to the cosmos, starting with a spiffy new video from the European Space Agency. The slick presentation is a preview of some of the extra-terrestrial exploits that the agency has planned for 2016. This includes the landing of the Schiaparelli probe on the surface of Mars. This stationery lander will survey its Martian environs to find a suitable location to drop the ExoMars rover in 2018. The mission’s namesake is the Italian astronomer Giovanni Schiaparelli, who mapped the surface of Mars and was the first to use the term canali to describe the straight lines that were thought to exist on the surface of the planet.

It’s possible that someday humans will colonize Mars and this will involve building dwellings and other structures on the Red Planet. In preparation, Lin Wan, Roman Wendner and Gianluca Cusatis at Northwestern University in the US have come up with a recipe for making concrete on Mars. The trio reckon that any successful colonization of the Red Planet will have to rely on local building materials because shipping stuff from Earth would be horrendously expensive.

Their concrete makes use of the large amounts of sulphur on Mars, which they say will form chemical bonds with Martian soil to create a material that has comparable strength to concrete here on Earth. You read more about this out-of-this-world material on the “Emerging technology from the arXiv” blog.

From the practical to the sublime, 40 years ago Stephen Hawking argued that information is destroyed when it plunges into a black hole and cannot be recovered as the black hole evaporates by giving off Hawking radiation. This argument is well grounded in physics but it leads directly to the famous “black-hole-information paradox”. This points out that if Hawking is right, a complete description of a physical system at one point in time (before the information plunges into the black hole) cannot be used to predict the properties of that same system at some time in the future. This failure to predict violates several fundamental tenets of quantum mechanics and physicists have been arguing about the paradox ever since.

While the physics surrounding this and other related black-hole paradoxes is enough to make your eyes water, some of the names that physicists have dreamt up will have you crying with laughter. My favourite is the no-hair theorem, which states that a black hole only has three attributes: its mass; electric charge; and angular momentum. This means that information about matter and energy falling into the black hole is subsumed into these three values and can never be recovered. In other words, a black hole appears “bald” to an observer with no structure (or “hair”) to provide information of what has previously fallen into the abyss.

Now, Hawking has joined forces with his University of Cambridge colleague Malcolm Perry and Andrew Strominger of Harvard University to write a paper with the fanciful title “Soft hair on black holes”. As the title suggests, the paper offers a way of recovering information from a black hole in the form of quantum pixels that exist at the black hole’s event horizon. They speculate that these pixels could be “soft photons” with zero energy and also look at whether these pixels could actually soak up all the information going into the black hole, preserving it for the future. If you want to know the answer, you will have to read the paper or check out Sabine Hossenfelder’s summary on her Backreaction blog.