Subtitles section Play video
-
What is polarisation?
-
Light bouncing off objects allows us to see them.
-
Light is an oscillating electromagnetic wave.
-
We see different frequencies of that oscillation as different colours.
-
The electric field component of a light wave can oscillate in any direction perpendicular to the direction of travel. Light from the sun oscillates in all directions, we say it is unpolarised.
-
If the waves have a preferred direction of oscillation, they are polarised.
-
For example, light from your phone is vertically polarised.
-
Your polarised sunglasses are a polarised filter.
-
Here, they let vertical polarisation through.
-
If we rotate them, almost all the polarised light gets blocked. Light produced by a torch is unpolarised.
-
When we place a polarised filter in between, the light becomes polarised.
-
In space, randomly moving hot gas or plasma emits unpolarised light, like the torch.
-
Plasma threaded by a magnetic field is ordered and the emitted light is polarised.
-
The magnetic field acts as a polarised filter.The strong gravity near a black hole bends the magnetic field and twists the polarisation direction of the light from the surrounding plasma.
-
The polarised light rays that manage to escape travel to a distant camera.
-
The intensity of the light rays and their direction are what we observe with the Event Horizon Telescope.
-
Using this knowledge, we can map out the magnetic fields that surround black holes.
