If time stops for an object falling into a black hole, how can LIGO see black holes colliding? - Beyond The World
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If time stops for an object falling into a black hole, how can LIGO see black holes colliding?

A distant observer of someone falling into a black hole would see them slow down, then come to a virtual stop at the event horizon and become “smeared” according to general relativity. So, how does LIGO manage to capture two black holes merging in milliseconds?

Consider the case of GW150914, the first binary black hole merger discovered using gravitational waves. The final black hole formed during the merger has a mass of 60 times that of the Sun, implying a diameter of about 124 miles (200 kilometers). This is the typical size of the remnants from black hole mergers that we observe with the Laser Interferometer Gravitational-wave Observatory using gravitational waves (LIGO).

What distinguishes a light pulse sent by a person falling into a black hole from gravitational waves emitted during a merger? Optical light has a wavelength of about 1,000 nanometers, which is the distance between two light wave crests (a strand of hair is about 90,000 nm wide). The light will be emitted from a location near the event horizon, or point of no return, if someone falls into the black hole and sends out that pulse of light.

Gravitational waves, on the other hand, have no known origin in the space-time region surrounding black holes. When we consider our example signal, this makes more sense. The waves generated by GW150914 have a wavelength of 1,864 miles (3,000 kilometers), which is larger than the system’s size!

In other words, the main difference between gravitational waves and light pulses is that the former can be thought of as being emitted from the entire dynamical, vibrating space-time surrounding the merger of two black holes, whereas the latter can be thought of as being emitted from the entire dynamical, vibrating space-time surrounding the merger of two black holes. They have no relation to any specific location, such as the event horizons of black holes, where gravity is so strong that not even light can escape.

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