6 ms·

Physics perfectly well knows what time is. This is an issue with keeping track of time in terms of seconds, minutes, hours and days, and has nothing to do with

by Aqwis 14y ago

Physics perfectly well knows what time is. This is an issue with keeping track of time in terms of seconds, minutes, hours and days, and has nothing to do with not knowing what time is.

chimeracoder 14y ago
What is time, other than a state in which net entropy within a closed system increases (which is already a definition abstract enough that it almost misses the point)?
- andreasvc 14y ago
  That's not really a definition of time, just a way to postulate its direction. Time as a physical unit has been decreed to be the rate of decay of a certain Cesium atom; i.e., a fixed regular interval, whereas the increase of entropy varies from moment to moment. E.g. right now I'm typing and before I wasn't, that doesn't mean time is going ever so slightly faster because of that ...
  - pbhjpbhj 14y ago
    >rate [sic] of decay of a certain Cesium atom // That is time in the same way that leaves moving on a tree is wind. The instance of the radioactive decay indicates the defined period of time has passed within the local frame of reference. That tells us very little useful about time itself.
    - andreasvc 14y ago
      I wasn't making a philosophical statement, I specifically qualified it by saying "as a physical unit." That's still nowhere near precise enough, but I only meant to point out that we typically don't conceive of time as an emergent property of physical processes (i.e., entropy change), but as a local property of a very specific atom, such that time passes at a constant rate, if only because we choose to define it that way. "Telling something about time itself" sounds like a philosophical question which I fear has no definite answer.
  - cnvogel 14y ago
    Decay of nuclei is a completely random process, and you really don't want to use it to define a time-scale. What you had in mind is the SI-definition of the second, which reads (http://www.bipm.org/en/si/si_brochure/chapter2/2-1/second.html http://www.bipm.org/en/si/si_brochure/chapter2/2-1/second.ht...): """ The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. """ The details of this effect are a little more complicated, but it boils down to the fact that you can measure the "angular momentum" of your nuclei when you have them pass through a non-uniform magnetic field. Particles in the one state are deflected differently than particles in the other state. And when you irradiate a particle beam in the F=0 state with the right frequency (9.2 GHz) you can very efficiently swap many particles over to the F=1 state. By adjusting your frequency sufficiently to find the maximum rate of flips, you can can tune for the exact 9'192'631'770 Hz. The cesium particles have not decayed, in principle you could run forever on a certain supply of atoms... even though in practice a Cs-beam is produced on one side, at a hot filament, and dumped to the other side of the clock after passed through the apparatus that performs the steps described above. They will be disposed of when the lifetime of the beam-tube is reached (typically 10 years or so with a few grams of cesium inside).

by Aqwis 14y ago

Physics perfectly well knows what time is. This is an issue with keeping track of time in terms of seconds, minutes, hours and days, and has nothing to do with not knowing what time is.

chimeracoder 14y ago

What is time, other than a state in which net entropy within a closed system increases (which is already a definition abstract enough that it almost misses the point)?

andreasvc 14y ago
That's not really a definition of time, just a way to postulate its direction. Time as a physical unit has been decreed to be the rate of decay of a certain Cesium atom; i.e., a fixed regular interval, whereas the increase of entropy varies from moment to moment. E.g. right now I'm typing and before I wasn't, that doesn't mean time is going ever so slightly faster because of that ...
- pbhjpbhj 14y ago
  >rate [sic] of decay of a certain Cesium atom // That is time in the same way that leaves moving on a tree is wind. The instance of the radioactive decay indicates the defined period of time has passed within the local frame of reference. That tells us very little useful about time itself.
  - andreasvc 14y ago
    I wasn't making a philosophical statement, I specifically qualified it by saying "as a physical unit." That's still nowhere near precise enough, but I only meant to point out that we typically don't conceive of time as an emergent property of physical processes (i.e., entropy change), but as a local property of a very specific atom, such that time passes at a constant rate, if only because we choose to define it that way. "Telling something about time itself" sounds like a philosophical question which I fear has no definite answer.
- cnvogel 14y ago
  Decay of nuclei is a completely random process, and you really don't want to use it to define a time-scale. What you had in mind is the SI-definition of the second, which reads (http://www.bipm.org/en/si/si_brochure/chapter2/2-1/second.html http://www.bipm.org/en/si/si_brochure/chapter2/2-1/second.ht...): """ The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. """ The details of this effect are a little more complicated, but it boils down to the fact that you can measure the "angular momentum" of your nuclei when you have them pass through a non-uniform magnetic field. Particles in the one state are deflected differently than particles in the other state. And when you irradiate a particle beam in the F=0 state with the right frequency (9.2 GHz) you can very efficiently swap many particles over to the F=1 state. By adjusting your frequency sufficiently to find the maximum rate of flips, you can can tune for the exact 9'192'631'770 Hz. The cesium particles have not decayed, in principle you could run forever on a certain supply of atoms... even though in practice a Cs-beam is produced on one side, at a hot filament, and dumped to the other side of the clock after passed through the apparatus that performs the steps described above. They will be disposed of when the lifetime of the beam-tube is reached (typically 10 years or so with a few grams of cesium inside).