Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question of time

  1. Dec 9, 2007 #1
    Hi, I'm writing a sci-fiction story and have a question I believe is probably somewhat simple. What I am looking for is the smallest measurement of time in the universe that could be considered a constant value. I have no background in physics or anything so while I do consider myself a fan of science, what I would assume might be constants might be completely wrong so I need to ask. Would the the shortest half-life of something be a constant? or the time of an electron orbit around an atom?

    What would take the shortest amount of time, and be constant, no matter where you were in the universe? Or do the laws of physics say that time is different depending on gravity, thereby making it impossible for there to be a constant?

    Thanks for any help.
  2. jcsd
  3. Dec 10, 2007 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I think you are looking for what is called the Planck Time.
  4. Dec 10, 2007 #3
    ok, ty will check that
  5. Dec 10, 2007 #4


    User Avatar

    Staff: Mentor

    To clarify a little though, you are correct that the rate of passage of time is not necessarily constant for two observers.
  6. Dec 10, 2007 #5
    That was sort of an integral part of my question, as it pertains to science fiction. Would a plancks unit be the same on two different planets, 1,000 lightyears apart... let's say, one being a lot closer to the center of the galaxy(maybe effecting the change in gravity more, idk)?

    The answer I'm looking for is, should aliens show up tomorrow, and the clocks on the wall of their spaceships are being measured in Planck time units, would they be using the same value for that figure as we would, or is it a constantly changing figure depending on where you are in the universe/galaxy? Like if you were close to the event horizon of a black hole would it be different?
  7. Dec 10, 2007 #6


    User Avatar

    Planck time is a constant. It's derived from planck's constant, the speed of light and the gravitational constant. If your clock measures in Planck times and the aliens' clock measures in Planck times, and you both share the same frame of reference, then you'll measure the same thing. So if aliens measure Planck's constant, the speed of light in a vacuum and the gravitational constant, and also happen to have square-roots in their maths, then sure, it'll be the same unit of time.

    Anyway, how time is measured by different people depends on relativity, and someone more learned than me can explain that. I don't know how it works at this scale, but generally, when someone accelerates, time slows down for them, meaning that your Planck time will be multiple Planck times of theirs - assuming that rule holds even at this scale. Interesting question, since that's the smallest unit of time measurable - how can you have multiples of a Planck time still = to 1 Planck time (in another person's reference point)?
    Last edited: Dec 10, 2007
  8. Dec 10, 2007 #7
    I guess I follow so far. As far as the acceleration goes, correct my mistakes:

    two ships in space
    each has a clock on the wall using planck units
    ship 1 heads out moving faster than light
    while he is moving the clock on the wall in his ship still appears to him and everyone on his ship to be ticking at the same rate as it was before, but in reality it is not, it's moving.. slower? than the clock back on ship 2 at the original departure point?

    Then if he heads back at the same speed the clocks will both be synchronized again when he gets back?

    also, ignoring any mistakes with any of that, what if on his return trip he comes back faster or slower, are the clocks still synchronized when he gets back?
  9. Dec 12, 2007 #8
    I am bumping this with a second question. In sci-fi, any form of space travel that involves jumping from one place to another in space, instantly, is breaking everything we know about the laws of physics, correct? In the case of this type of space travel, should I even bother trying to deal with the differences in time? Everytime this form of travel is used the vessel is in effect, time traveling, correct?

    Speculative questions like this are difficult to reply to with good information, I understand that, but I am a stickler for making stories as close to what we know to be true as possible because in my opinion, that makes the story itself much more immersive to an intelligent audience. So remember, everytime you physicists watch a movie, tv show, or read a novel, and are baffled by how horribly wrong the scientific concepts are in them, that I was here asking!
  10. Dec 12, 2007 #9

    nothing can move faster than speed of light! ;)
  11. Dec 12, 2007 #10


    User Avatar
    Science Advisor

    That's why the OP indicated that this was a science fiction question.
  12. Dec 12, 2007 #11
    alright then,as i overlooked, the word sci fic
  13. Dec 29, 2007 #12

    Well, when it comes to faster than light travel two things can happen in scifi. One ftl does not affect the clock's speed at all, 2 it affects it for some reason. Faster than light travel also causes people to go back in time in some scifi while in others it does not.

    The slowing of the clock relative to the stationary other, I think, occurs at near light speed in both scifi and real life. That is no matter the direction, going in the opposite direction does not cause acceleration of the clock. So the two clocks would seem to be out of sync at the end. If you assumed the clock would also appear to go slower at ftl speed it should probably behave the same.

    But the question of what would happen to a planck length clocks is interesting in the real world scenario scifish scenario involving relativistic speed. Would the two clocks REALLY be out of sync like regular clocks would be?
  14. Dec 29, 2007 #13
    Nowadays the unit of time 1 sec is defined as a multiple of the period of the radiation emitted between two particular states of a certain atom [tex]^{133} Cs[/tex] at rest. Of course is the atom is moving you get a different result, as everybody else pointed out. Thus this interval of time does not suit your expectations.


    In astrophysics there is a nice problem in the last two decades. From observations we know that the usual matter you see, e.g. electrons, protons, etc is not enough to describe the real world, so we come across to dark matter and dark energy (we call them dark because we don't know what are they :smile: )

    Since you are dealing with science fiction and you have aliens travelling all aroung Cosmos, you can think of the following scenario:

    1. Aliens can understand dark matter whih is made off atoms that do not obey Einstein's special relativity and they transmit radiation at a constant interval of time.
    2. Name one of such atoms, say Wh.is.E. (=Who is Einstein?) :rofl:
    3. Define the new unit of time Wh.is.Se. (=What is second? ) :rofl:

    Thus you don't have to worry about

    because noboby knows what really dark matter is. :smile:
  15. Dec 31, 2007 #14
    cool, this was helpful.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Question of time
  1. A question on time (Replies: 6)

  2. Question on time (Replies: 1)

  3. A question about time (Replies: 3)