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The first detection of gravitational waves in 2015 created huge excitement because it confirmed a long-standing prediction of Albert Einstein’s general theory of relativity and opened up a completely new way of observing the universe. Physicists have also been scrutinizing data from the growing number of gravitational-wave detections for “echoes” – the existence of which could mean that our understanding of relativity is incomplete. Physicists in Canada and Iran have found tentative evidence for such echoes gravitational waves from colliding black holes, and now say a stronger signal exists in data from colliding neutron stars.

Many physicists believe that general relativity is incomplete because it is at odds with quantum mechanics, leading to the information paradox when considering the extreme gravitational fields generated by black holes. Relativity tells us that whenever anything, including light, crosses a black hole’s event horizon the information it contains is lost to the rest of the universe forever. But quantum mechanics requires that information can neither be created nor destroyed. This is a problem given the existence of Hawking radiation, which implies that black holes can evaporate away to nothing and in the process erase all of the information that flowed into them.

If gravitational-wave echoes exist, it would suggest that black holes are not bounded by a classical event horizon but instead by a quantum-mechanical Planck-scale structure. One such structure put forward by theorists is the “firewall”, which would destroy any object passing through it but retain that object’s information and so keep it outside the black hole. Firewalls, however, are controversial. While physicists generally agree that quantum mechanics comes into play deep inside black holes – even though it is impossible to see its effects – they are largely sceptical about its role outside the event horizon.

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Category: Science