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B Flow of time

  1. Dec 29, 2017 #1


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    I know that under some conditions time runs slower or faster depending on the speed we have relative to an inertial frame of reference. However if I understand correctly for an observer that is moving with say 90%c though time runs much faster, for him the time runs in a normal way (I mean he doesn't experience the reality around him like in fast forward motion in a video play, he experiences like time is ticking in a normal way).
    Is it possible to create a situation where one observer experiences the reality around him like it is in slow motion or like it is in fast forward? Does this have to do with relativity or it is something that has to do with substances that affect the function of our brain and our senses?

    And if we reach 100%c does time completely stops?
    Last edited: Dec 29, 2017
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  3. Dec 29, 2017 #2
    Its important to remember that, relative to yourself, you are always stationary. So, for you, time always flows normally. Only other things can be moving relative to you, and when they do, you will observe their clocks ticking more slowly (negligibly until they reach significant fractions of the speed of light, of course). So, if I fly by you in a spaceship going 80% of c, you'll see my clock ticking at 60% the normal rate. However, from my perspective, you are the one moving at 80% so my clock will be ticking just fine from my perspective, but yours will be ticking at 60% the normal rate.

    Nothing can move at or above the speed of light relative to you. The Lorentz factor does approach infinity as v approaches c (and its reciprocal approaches 0), so it might feel natural to conclude that when something reaches light speed its clock will appear to stop entirely. However, that is an impossible condition to reach and when v=c the Lorentz factor is undefined, providing no actual answer, which is fine because its impossible to actually do that.
  4. Dec 29, 2017 #3


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    If you mean "can I sit here and see my watch ticking fast or slow", then not really. You are essentially comparing your own internal experience of time to the watch - that is, you are using yourself as a crude clock. A key feature of relativity is that all types of clock are affected the same way, so the watch will always look normal to you (barring drugs or fever or anything like that which might mess with your sense of time).

    However, clock tick rates are affected by gravitational potential. Clocks higher up tick faster. So in principle you can see this now. Lie on the floor and look up at the wall clock, or put a clock on the floor and climb a ladder. I gather that there are clocks stable enough to detect the effect with a height difference as low as a meter. Of course, your own sense of time is too crude to actually see this unless you were in a gravitational field strong enough to turn you into strawberry jam.

    Note that the equivalence principle applies, so being in an accelerating rocket works just as well as being in a gravitational field. Including the bit about strawberry jam.
    Relativity says that all clocks tick the same way under the same circumstances. You can, of course, analyse a clock in terms of its component parts and how they are affected and show how this comes together to produce the effect on the clock.
    "Elapsed time" doesn't have a coherent definition for things moving at lightspeed. But only massless particles can do it, and they don't wear watches, so it doesn't matter.
  5. Dec 29, 2017 #4
    Time isn't running, things (and persons :-) are.
  6. Dec 29, 2017 #5


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    Well not sure, to be honest I think time is flowing somehow though it is certainly NOT a fluid...
  7. Dec 29, 2017 #6
    I guess most people do, but I don't. Time, imho, doesn't possess a property 'velocity' and it cannot, therefore, 'run' slow(er) or fast(er). The only thing stated by SR about time, is that time differences may differ from frame to frame (as measured in one). That does not necessarily include the existence of a velocity of time, you just have more or less of it.

    By the way, I even do not think of time as an independent existing quantity (let alone that it be the underlying 'motor' or 'carrier' of events), I would rather say it is the result of events, or a similar concept (so the other way around!). But that is philosophy, which doesn't change any equation or law of physics.
  8. Dec 29, 2017 #7


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    Far and away the esiest way to understand special relativity is the block universe. In this model, time is just a direction in a 4d whole. Nothing actually changes or moves and time doesn't flow in any sense. We only have the impression of change because we see the present and compare it to our memory of the past, and we naturally model the present as somehow distinct from the past.

    I recommend looking up Minkowski diagrams (they're just displacement-time graphs with the time axis vertical and a clever choice of units). They are effectively 2d slices through the block universe and are an excellent way of developing intuition about relativity.
  9. Dec 29, 2017 #8


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    Time, as experienced by a person, is a measure of the "distance" (more precisely, the interval) along that person's worldline. That is all it is in relativity. Coordinate time is simply picking a family of observers and using the nearest of their wristwatches for remote time marking.

    Different observers may experience different times between events because they follow different routes of different "lengths" between those events.
  10. Dec 29, 2017 #9


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    If you are aboard a spacecraft zooming around at relativistic speeds, you will measure things going slowly on planets you pass, and they will measure things going slowly on your spacecraft. But to you on the spacecraft, things in the spacecraft are normal, and to them on the planet things on the planet are normal. That is required by the postulate of relativity.
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