The passage explores the concept of time and gravity, based on Einstein's theory of relativity. According to this theory, time flows at different rates in different gravitational fields. The author explains this phenomenon by comparing time to a stream, where massive objects like planets create a "drag" that slows down the flow of time near them. This results in a gradient of time flow, with time passing more slowly near the massive object.
The author also explains the concept of 4-velocity, which is the combination of an object's velocity through space and time. In a gravitational field, an object's 4-velocity is rotated, causing it to accelerate towards the massive object. This acceleration is paid for by a deceleration in the time direction, resulting in time passing more slowly near the massive object.
The passage also touches on the concept of gravity and how it affects the path of light, including the phenomenon of gravitational lensing. The author notes that gravity can cause light to bend around massive objects, and that this effect is a result of the curvature of spacetime.
Finally, the passage ends with a discussion of the potential for gravitational waves to be used to test ideas in quantum gravity, including the detection of primordial gravitational waves from the inflationary epoch.
Here are the key facts extracted from the text:
1. Clocks run slow in gravitational fields.
2. GPS satellites tick faster by a factor of 1-in-a-billion, which can throw off their position accuracy by 11km per day.
3. The speed of light is constant for all observers.
4. The weight induced by acceleration is fundamentally the same as that induced by gravity, according to the equivalence principle.
5. Objects with mass warp spacetime, causing time dilation.
6. The presence of mass and energy warps spacetime, with the most intense part of that warping being in time.
7. In a gravitational field, objects move through time more slowly.
8. Velocity through time increases away from a massive object like the Earth.
9. The 4-velocity of an object is pointed almost entirely in the time direction.
10. Light travels at the speed of light through space only, and not at all through time.
11. Photons do not have a 4-velocity, which is defined according to the ticking of one's own clock.
12. Gravitational lensing is the bending of light in a gravitational field.
13. The flow of time determines the path of even timeless particles like photons.
14. The universe may have undergone an eternal inflation, with our universe being a bubble that formed when inflation ceased.
15. Gravitational waves can be used to test ideas in quantum gravity.
16. Primordial gravitational waves from the inflationary epoch may be detectable in the gravitational wave background or indirectly through their effect on the cosmic microwave background.