TWIL #008 - Light Bends and Spacetime is the Reason
Light always travels in a straight line - but 'straight' in curved spacetime looks like a curve to us, which is why massive objects bend light.
- #physics
- #relativity
- #spacetime
Light appears to bend around massive objects like stars and galaxies. This is called gravitational lensing, and it's been observed, measured, and used in astronomy for decades. But why does it happen if light is massless?
The answer is that light isn't actually bending. It's taking the straightest possible path through space - the problem is that space itself is curved.
Einstein's general relativity reframes gravity not as a force between masses, but as massive objects curving the geometry of spacetime around them. The more massive an object, the more it warps the spacetime fabric nearby. Light (and everything else) then travels in what's called a geodesic - the shortest path in that curved geometry.
From our perspective, watching from flat spacetime far away, that path looks curved. From the photon's perspective, it went straight the whole time.
A useful analogy: draw two "straight" lines on a flat piece of paper and they never meet. Draw the same lines on the surface of a sphere (like longitude lines on Earth) and they converge at the poles. The lines are still locally straight - the surface they're drawn on just isn't flat.
This prediction was first confirmed during the 1919 solar eclipse, when Eddington photographed stars near the sun and found their apparent positions shifted exactly as Einstein calculated. It made Einstein famous overnight.