Gravitational waves (3D)
About this app
Two black holes orbit, merge and ring down: spacetime ripples as a spiral, the chirp signal h(t) runs live, a ring of test masses squeezes. GW150914 as preset, signal to listen to, exaggeration shown.
What gravitational waves are
Accelerating masses emit gravitational waves: ripples in the geometry of space and time that travel at the speed of light. When two black holes orbit each other, the waves carry energy away. The holes move closer, orbit faster, and the signal rises in frequency and amplitude, a pattern called a chirp. The wave oscillates at twice the orbital frequency. It is transverse: it stretches space in one direction and squeezes it across, in two polarisations, + and ×. After the merger the new hole briefly rings before settling (ringdown).
What you see
A grid shows the orbital plane as a rippling sheet on which the wave spreads as a two-armed spiral. Alongside runs the signal h(t) with inspiral, merger and ringdown. A ring of free test masses shows how the wave stretches and squeezes space, and the app gives the length change of a 4 km detector arm as in LIGO, compared with the diameter of a proton. From the masses it computes chirp mass, final mass, radiated energy, final spin and ringdown frequency. Deformations are greatly exaggerated, and the factor is displayed.
Things to try
Start with the GW150914 preset and listen to the signal at its true pitch or two octaves higher. Then change the masses between 5 and 80 solar masses: lighter pairs chirp for longer and end at a higher frequency. Turn the viewing angle ι from 0° (face-on, both polarisations equally strong) to 90° (edge-on, only h₊ and half as strong). Changing the distance changes the measured strain h.
Frequently asked questions
- How were gravitational waves first detected?
- On 14 September 2015 the two LIGO detectors in the USA recorded the signal GW150914, published in February 2016. It came from two black holes of about 36 and 29 solar masses roughly 1.3 billion light-years away. About three solar masses were radiated away as gravitational-wave energy.
- How does a gravitational-wave detector like LIGO work?
- LIGO is a laser interferometer with two 4 km arms at right angles. A passing gravitational wave lengthens one arm and shortens the other by tiny amounts, which shifts the interference of the returning laser beams. The measured strain is around 10⁻²¹, so the arm length changes by a fraction of a proton’s diameter.
- What is the chirp mass?
- The chirp mass is a combination of the two individual masses, 𝓜 = (m₁m₂)^(3/5) / (m₁+m₂)^(1/5). It sets how quickly the frequency of a gravitational-wave signal rises during the inspiral, which is why it can be measured from the signal especially precisely.
- How accurate is the simulation in the app?
- The inspiral follows the Newtonian quadrupole formula and holds up to the innermost stable orbit. Merger and ringdown are stylised rather than numerical relativity, and final mass and spin come from fitting formulas for non-spinning holes. Grid height and ring deformation are greatly exaggerated and the orbits slowed down; the signal panel shows the true time axis.
Subject: Astronomy
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