Embedded physics · Comment questions in progress
Why does the arrow of time move forward?
It is our familiar experience of the direction of time that we remember the past but not the future.
Basic framework
The tendency for entropy to increase gives a distinct direction to macroscopic processes such as the melting of ice.
crux of the debate
Why this aspect is strong ties back to the question of why the early universe was low entropy.
Today's knowledge frontier
Memory leaves physical traces; These traces produce records about the past.
Why are space and time unified?
Special relativity shows that measurements of length and duration may vary between observers moving at different speeds; However, it shows that the speed of light remains the same. Therefore, space and time are not independent scenes, but four-dimensional space-time in which events take place together. Even simultaneity may depend on the movement of the observer.
Gravity is more than a force
In general relativity, what keeps planets in orbit is not thought of as an invisible string pulling them from afar. Matter and energy change the geometry of space-time; Objects follow the most natural paths possible within this curved geometry. The deflection of starlight near the Sun, details of Mercury's orbit, atomic clocks, and gravitational waves are tests of this framework.
open problems
General relativity is extraordinarily successful at large scales, quantum mechanics at subatomic scales; However, in cases such as the interior of black holes and the earliest moments of the universe, it is necessary to use the two theories together. The search for quantum gravity is the main reason for the emergence of different programs such as loop quantum gravity and string theory.
How is it tested?
Atomic clocks measure different times depending on altitude and speed differences. The GPS system requires both special and general relativity corrections. The bending of light around large masses and the gravitational waves measured by observatories like LIGO are experimental traces of space-time geometry.
Where is the border?
General relativity points to its limit at singularities; We do not have an experimental theory of gravity that is fully integrated with quantum mechanics. Wormholes may be possible solutions to equations, but there are no observations showing that they are stable and traversable.
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Albert Einstein — explained gravity as space-time geometry.
Kip Thorne — developed the physics of black holes, gravitational waves, and wormholes.
Source: Einstein Online