Institute of Astronomy

What happens to light falling into a black hole?

Published on 15/01/2014 

I can't find an explanation that my brain likes for my query.....
My logic thinks that light should 'pause' at the event horizon of a black hole, to an observer away from the outside of the event horizon.  I'm imagining firing a flare away from me into a black hole from a distance.  As the flare moves further away from me it becomes smaller, but once it reaches the event horizon it's size will remain the same from the distance where the observer remains stationary/fired the flare.  If observer moves closer to the light it will become larger as if the flare was coming back to me.
Now, going back to the initial query... being light has the fastest speed, at the point of the event horizon would the speed of light not be countered with the 'speed' of the gravitational pull of the black hole and just pause?

One of the things about relativity that can be a bit difficult to get your head around is that light always seems to move at the same speed, no matter how fast you are travelling, or in what direction. What happens instead is that the Doppler effect changes the wavelength of the light, similar to the way that the pitch of a siren on an ambulance or a police car seems higher as it is coming toward you, and then lower as it is moving away.

With light, if you are moving toward the light source then the light will seem bluer, whereas if you are moving away from it the light will seem redder.  Gravity has the same effect, so what happens to the flare as it approaches the event horizon is that it seems redder and redder, until at the moment that it actually crosses the event horizon the wavelength has been stretched so much that is undetectable.

Page last updated: 15 January 2014 at 12:04