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Black waves, soft hair and infrared triangles - Graham72 - 30-10-2026
Memories are made of this. Research by theoretical physicists Sabrina Pasterski and Andrew Strominger examines how gravitational waves leave permanent imprints on spacetime.
When cataclysmic cosmic events occur, such as colliding black holes or exploding stars, they radiate gravitational waves that carry away energy and angular momentum. These waves don’t just pass by: they leave space permanently changed. A ring of free-floating masses stays stretched after the wave has gone (the displacement memory effect). Pasterski, Strominger and Alexander Zhiboedov discovered a second effect, spin memory: a passing wave causes a permanent relative time delay for light travelling in opposite directions around a loop.
These memory effects form one corner of the “infrared triangle”, which shows that three separate ideas are the same physics seen three ways: the memory effects, hidden symmetries of spacetime far from any source (supertranslations and superrotations), and “soft theorems” describing very low-energy gravitons and photons. Strominger, Stephen Hawking and Malcolm Perry used these ideas to propose that black holes carry “soft hair”: information recorded on the event horizon when zero-energy particles fall in. They suggested it as a possible route toward resolving the famous black hole information paradox, though whether it can do so is still debated.
The attached program for cp/m microbees shows the first part: the displacement memory produced by supertranslations, and the spin memory produced by superrotations. It doesn’t show soft hair. Soft hair isn’t light or motion that could be drawn, even exaggerated, but a change in the black hole horizon’s properties, so there’s nothing to animate. The display is hugely exaggerated: a real displacement memory is far smaller than an atom over the size of the Earth, and a real spin memory delay is far too small to measure with today’s detectors.