How can you photograph a dark hole in a dark wall at night? You can’t.

Credit: NASA/JPL-Caltech/IPAC If light can't escape a black hole, how can you photograph it? Galaxy M87, imaged by the Spitzer Space Telescope, 25 April 2019. It was a logical consequence in the end that space and time is relative, but it took a long time for us to realise it. Without that principle, that the speed of light is constant, much of physics just wouldn’t work. So you can never separate space and time, if you accept the fact that light is really what defines everything and that the speed of light is constant. On the fundamental level, the ticking of a clock is really the oscillation of the light waves or similar types of waves, and that changes as you go through curved space. Or at least what you measure as time changes – because time is a derived quantity: it’s something that you always measure by a clock, but how quickly is a clock ticking? Light has to travel a longer path, but then also time needs to change to have to keep the velocity the same. So, when you think of gravity as a change in space – of the gravity of a body leading to a deformation in space – then suddenly the length scale changes. The ratio between the two is fixed, but individually the two can change.Įverything seems so strange: time slows down, almost actually comes to a standstill, light just disappears, nothing comes out. That was a big realisation for Albert Einstein, that light always moves with the same speed, and it’s deeply ingrained in our theories of physics today.Īnd what is speed? Speed is length divided by time miles per hour, kilometres per hour. And light is the only thing that’s constant in the Universe. Sometimes that’s yourself, and we know that time is relative: a few seconds can feel like ages or go past way too fast.īut on a basic level, you’re always measuring space and time with light. If you think about it, what is space? What is time? Have you ever touched space? Have you ever touched time? How do you experience it? It all boils down to the properties of light. That, at least, is the theory that Einstein proposed to explain the mysterious nature of gravity.

Falcke, Radboud University What is 'space-time'? Can a black hole warp time itself? No light can escape a black hole, but it can be bent around it, which means a black hole's 'shadow' is wreathed in bright light. So nothing that goes in is able to come out, at least according to the theory of gravity. Then surrounding this is a perimeter, so to speak - not a line, but a virtual sphere - where if you cross that border, you’ll never be able to come back: the event horizon.Īnd that’s true for yourself if you were to walk into it, but it’s also true for light. If you have a big star exploding and collapsing into a black hole, there is no force that can stop that collapse, because as more matter piles up and clings together, the force of gravity just becomes stronger and stronger and wants to pull it together even more. There is something in this Universe, around this black hole, that we cannot see and measure with current technology – and maybe never will – and that’s why it’s a dark and mysterious region, shielded by an event horizon where nothing escapes. It’s not a real hole, but a hole in space and time because it punched a hole into our observable Universe. Gravity and spacetime curvature – Einstein’s great idea about explaining gravity – becomes so extreme that anything that comes too close will never be able to escape. Credit: EHT Collaboration What is a black hole?Ī black hole is an enormous amount of matter compressed into a tiny amount of space. The supermassive black hole in galaxy M87 was observed by the Event Horizon Telescope and announced to the world in April 2019.

Very first image of a black hole