- For now Cesium is the standard for counting the second – 9,192,631,770Hz
| Clock | What actually “ticks”? | Transition region | Precision / fractional uncertainty | Gravity impact near Earth |
|---|---|---|---|---|
| Rubidium-87 | electron hyperfine transition | ~6.835 GHz microwave | ~10⁻¹¹ to 10⁻¹² typical | ~90–900 km |
| NIST-F4 Cesium | electron transition | 9.192 GHz microwave | ~2.2 × 10⁻¹⁶ | ~2 meter |
| Strontium-87 | electron transition | ~429 THz optical | ~8.1 × 10⁻¹⁹ | ~7.4 mm |
| Aluminum-27 Al⁺ | electron transition | ~1.12 PHz optical | ~5.5 × 10⁻¹⁹ | ~5 mm |
| Thorium-229 | nucleus transition | ~2.0 PHz VUV | ~1 × 10⁻¹⁹ projected | ~0.9 mm projected |
- moving the clock doesn’t make its precision worse. Its actual frequency changes because time passes at a different rate in the new gravitational potential. That’s a real relativistic effect, not an error in the clock.
