Gravity Probe B

Plate 34 speaks for itself: “Earth drags spacetime, 37 milliarcseconds a year.”

The Physics

Gravity Probe B carried four superconducting gyroscopes in polar orbit. Their axes slowly turned from the geodetic effect and from frame dragging by Earth's spin.

Ω_fd = G I ω / (2 c² r³)

• Ω_fd: frame-dragging precession rate (orbit-averaged, polar), rad s⁻¹
• G: gravitational constant, m³ kg⁻¹ s⁻²
• I: Earth's moment of inertia, kg m²
• ω: Earth's angular speed, rad s⁻¹
• c: speed of light, m s⁻¹
• r: orbital radius, m

37.2 ± 7.2 mas/yr (polar-orbit average); GR predicts 39.2. Polar orbit at 642 km, launched 2004.

Sources

• C. W. F. Everitt et al., Phys. Rev. Lett. 106, 221101 (2011).

On the poster

Plate 034, “Gravity Probe B”: the pixel-art plate with its slogan “Earth drags spacetime, 37 milliarcseconds a year.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.