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
