Mirrorix

Mirrorix, a hooded watcher of spacetime, stands by a shining crystal tower, a stylised mirror that feels gravitational waves.

The Physics

LIGO measures gravitational waves by comparing the lengths of two 4 km arms with laser light. At the end of each arm hangs a test mass: a 40 kg fused-silica mirror 34 cm across, hung from a four-stage pendulum to isolate it from ground vibration (Advanced LIGO, Class. Quantum Grav. 32, 074001, 2015). On 14 September 2015 a wave from two merging black holes, GW150914, stretched and squeezed the arms with a peak strain of 1.0 × 10⁻²¹ (Phys. Rev. Lett. 116, 061102, 2016).

h ≈ ΔL/L

• h: gravitational-wave strain, dimensionless
• ΔL: change in the difference of the arm lengths (m)
• L: arm length, 4 km

With h ≈ 10⁻²¹ and L = 4 km, ΔL is about 4 × 10⁻¹⁸ m, hundreds of times smaller than a proton.

Honest caveat: h ≈ ΔL/L is the simple order-of-magnitude relation; the exact factor depends on the wave's direction and polarisation. LIGO's light bounces many times in each arm, which is how it reaches this sensitivity.

On the sticker

Mirrorix stands beside a glowing crystal spire with light rings around it; the quad-pendulum suspension is not drawn. Printed text: MIRRORIX with the line h ≈ 10⁻²¹. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.