ความคิดเห็นที่ 19
Time is a physical frame-dependent concept and quantity in modern physics.
Time was introduced long before Newton in the formulation of many many things, e.g. calendar, terrestrial and celestial motion. Newton was the first, however, to propose a UNIVERSAL time or absolute time which proceeds at constant rate for every observer, regardless of their frame of reference, regardless of how they are moving, regardless of where they are in the universe.
Einstein proposed a relative time in 1905 as fundamental physical property of time in order to resolve the incompatability between Newtonian mechanics and Electrodynamics. Time, according to Einstein, must be relative, depending on how fast you move, in order to achieve the constancy of the speed of light in the vacuum.
Time was then made in unity with space by Poincare and Minkowski, as a mathematical interpretation of the Special Relativity. But this is not the complete story of space-time unity since it made no newpredictions.
In 1916 or so, Einstein completed the space-time marriage starting from his brilliant Thought Experiment that light must bend under gravity, and therefore must the space-time line element representing that path of light. The simplest generalization of curved spacetime geometry is the Riemannian geometry, from which the frame-independent formulation of physics is natural. The theory is dubbed General Relativity Theory. In this theory, time also changes its rate when the gravity changes. Time on the surface of Jupiter runs slower than on the surface of the Earth.
One great prediction of General Relativity is that the spacetime can expand and shrink, i.e. Robertson-Walker metric. This leads to remarkable fact, proved by Hawking and Penrose, that the universe(=spacetime balloon) must have the beginning called singularity. Singularity is where the space and time "begin"(or end, if it shrinks).
There are new ideas and theories on time, e.g. two-time theory, arrow-of-time mechanism. The bottom line is, when you have your theory, you need to make testable predictions and test it against experiments, to find out the correct physical property of time.
จากคุณ :
bj
- [
25 เม.ย. 49 01:48:15
A:129.186.117.172 X: TicketID:038839
]
|
|
|
