Good old Wikipedia. Cesium is no longer the scientific standard of the second... it is not precise enough. Eric Goldstein -- On Mon, Dec 20, 2010 at 10:49 AM, Laurence Cuffe <cuffe@xxxxxxx> wrote: > > > On Dec 19, 2010, at 10:07 PM, Don Williams <dwilli10@xxxxxxx> wrote: > > > Today, the fundamental unit of time suggested by the International System of > Units is the second, since 1967 defined as the second of International > Atomic Time, based on the radiation emitted by a Caesium-133 atom in the > ground state. Its definition is still so calibrated that 86,400 seconds > correspond to a solar day. 31,557,600 (86,400 × 365.25) seconds are a Julian > year, exceeding the true length of a solar year by about 21 ppm. > > At 06:58 AM 12/17/2010, Carlos wrote: > > Don: > I could copy definitions, every viewpoints, etcetera from > different sources, but I think it would be more easy to read this > Wikipedia article: > http://en.wikipedia.org/wiki/Time > > Carlos > > Yes, I have read that entry and in the first paragraph it says: "Time has > been a major subject of religion, philosophy, and science, but defining it > in a non-controversial manner applicable to all fields of study has > consistently eluded the greatest scholars." > ________________________________ > I just noticed the above incomplete response on the subject. I had been > holding this message in order to find a statement by Einstein, that he > couldn't define time, but it's buried somewhere in a 400 page bio I have > been reading and I can't easily find that sentence Just trust me that he > did say that. > > Einstein even used time as a fourth dimension, and also attempted to prove > that the universe is curved back on itself. Current evidence now points to > a flat universe. He even won a Nobel Prize in Physics, but not for his work > on relativity. > > Interesting situation, Hawking wrote a Brief History of Time, but even there > he didn't define the term. > > It's clear that even though there seems to be no scientific definition of > time, we all use it, mutually understand what the word means, and are not > hampered in our daily lives by the lack of such definition. > > DAW > > > Scientific definition of a unit of time. Wikipedia, Units of Time. > Today, the fundamental unit of time suggested by the International System of > Units is the second, since 1967 defined as the second of International > Atomic Time, based on the radiation emitted by a Caesium-133 atom in the > ground state. Its definition is still so calibrated that 86,400 seconds > correspond to a solar day. 31,557,600 (86,400 × 365.25) seconds are a Julian > year, exceeding the true length of a solar year by about 21 ppm. > We can, as scientists, define time with about the same level of certainty > with which we can define distance, and mass. All of these entities when we > look closely become quite slippy as concepts. For instance, the length of a > meter stick, which seems a very concrete thing, turns out to depend in a > very real way on which way and how fast you are traveling when you measure > it. > Similarly duration depends on the relative velocities of the observers > discussing it, and also depends on how deep into a gravity well you have > descended, This effect is real, a height difference of 12 meters can put two > Iron atoms out of sinc with each other so that they can no longer have the > same energy levels. (http://en.wikipedia.org/wiki/Pound-Rebka_experiment) > Even before and after is up for grabs. So long as two events are too far > apart for a light beam to get from one event to the other before the other > event happens in some rest frame, than its possible to find another rest > frame, where the second event will happen before the first. > This is all fascinating stuff, and although I touched on it in college, I > find MIT open courseware, and even ITunes U (both free!) a great source of > world class lectures on this material. > All the best > Larry Cuffe > --- Rollei List - Post to rollei_list@xxxxxxxxxxxxx - Subscribe at rollei_list-request@xxxxxxxxxxxxx with 'subscribe' in the subject field OR by logging into www.freelists.org - Unsubscribe at rollei_list-request@xxxxxxxxxxxxx with 'unsubscribe' in the subject field OR by logging into www.freelists.org - Online, searchable archives are available at //www.freelists.org/archives/rollei_list