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What is Time?

  1. Nov 28, 2008 #1
    What is Time according to Physics?

    And here's a hypothatical situation and question:

    If the rotation of planets around Sun stops, there won't be any more days and nights, it would stop where it is, but would that mean that time has stopped advancing?
  2. jcsd
  3. Nov 28, 2008 #2
    This topic by pure coincidence covers your question (however it is not resolved there), from page two onwards.

    To answer your question, the rotation of the planets stopping wouldn't really be physical, they can't just stop. You'd be robbing them of their angular momentum, and all the planets would be subject to gravity (and therefore not invariant under time, time isn't simply discardable). Time isn't defined by night or day (neither is it defined by vibrations of a Cesium atom). Anyway, read the topic, maybe it will help you. :)
  4. Nov 29, 2008 #3
    Time is a "duration of passing" where progress of some sort can be measured therefore, determined to have occurred. For instance, holding your breath for 1 minute demonstrates a passing of time, as you'll feel your body requiring air, surely an indication of the progression of your body's metabolism and a "passing of time".

    Assuming we could actually stop the planets from revolving around the Sun for a short period of time, would your need to breath air stop? Would clocks stop? Would it prevent your independent movement on the Earth's surface? Of course not, therefore, time continues even though one of the normal methods for determining the day and night is no longer occurring. Naturally, we wouldn’t want to stop the orbits even if we could. :wink:
  5. Nov 29, 2008 #4


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    No. If you manage to make this happen, all you have done is to break one of your clocks (and kill billions of people :smile:).

    From that other thread:
  6. Nov 29, 2008 #5
    Great answer around this comment...but unlikely to be understood by the poster.

    Time is really an elusive subject. The only thing I can think that we really know is that it varies with velocity like space according to your reference frame. And it's "direction" may be linked to increasing entropy characteristic of our universe. It seems to have emerged from the initial big bang or big bouce or whatever started our universe along with space, energy, gravity and other forces, but which is "fundamental" is unknown. Time permits change; without it everything would be static, unchanging, and the universe could not evolve. So its conceivable that in another random quantum fluctuation and another universe time did not emerge..so that universe would be "stuck" in some static condition. Quantum theory typically conjectures time, like space, is discrete (has a minimum interval) and this is suggested but I don't think experimentally proven yet...

    It takes a while to understand how subtle time is. I found I had a better feel for it after reading perspectives from people like Michio Kaku, Lee Smolin, Brian Greene, and Roger Penrose in THE ROAD TO REALITY even has an understandable discussion with his really sophisticated mathematics.

    Does string theory have any insights regarding time? I can't think of any...
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