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B C and time

  1. Apr 3, 2017 #1
    The speed of light is constant, in what ever direction, that's not disputed. Now in relativity that as I understand light speed constant C is bound up with time, eg faster you go faster time pass for an outside observer but stays the same in your field of reference. Or to put it another way the faster you go slower time passes for you with reference to an out side observer So is there a constance for time or is time a produce of light speed?
  2. jcsd
  3. Apr 3, 2017 #2
    Each person will observe the other's clock running slow. But each will observe his own clock running at the normal rate. The time that elapses on the clock you carry with you is called proper time and it's a relativistic invariant, as is the speed c.
  4. Apr 3, 2017 #3
    You kind of have that backwards. Basically, in special relativity, clocks moving relative to you tick more slowly. Anyone moving with those clocks won't notice their own time slowed down, it will seem normal to them, and in fact, your time will look slow to them, since you're the one moving from their point of view.
  5. Apr 3, 2017 #4
    There is proper time and proper length. These are the defined units for the temporal dimension and spacial dimension. things move relative to each other throughout, turns out that nothing moves faster than c "across" those two dimensions.

    tl:dr time is a dimension. modeled the same as a line; it's geometric.

    Graphically time is null at c so if doing poetry maybe "time is a product of less than c" :smile:
    everyone always ignores length :H

    btw I didn't read that as you getting the symmetrical situation "backwards", it's just tough to see you not mindful of the use of "faster time" and "slower time".....but I understand you mean comparatively.

    What's key is a particular speed is invariant. (& constant still, i think)
    Last edited: Apr 3, 2017
  6. Apr 7, 2017 #5
    It's hard to understand your statements because you aren't very clear about who the observer is in each case.

    These statements seem to say two different things.
    No, if you are moving relative to another person, you will infer (from careful measurements) that the other person's clock is ticking more slowly.

    Yes, this part seems correct, but it doesn't match the language of the previous statement.

    It's confusing because you say "outside observer" which implies that the outsider is the person who is doing the observing. You also say "faster you go". It only makes sense to the outside observer, because you are never moving according to yourself. Only the outside observer sees you move. But then you say "faster time pass for an outside observer". That only makes sense if YOU are the person doing the observing. The outside observer can't see their own time pass faster.

    "Or to put it another way" implies that you are saying the same thing in two ways, but you said something completely different.
  7. Apr 7, 2017 #6


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    You have to consider what is meant by c being constant. What it means is c is "invariant", in that anyone will measure it as having the the same value relative to themselves, regardless of their motion with respect to anyone else.
    To illustrate, we will use a space-time diagram.
    It is a plot of distance vs time. Time is in the vertical direction and distance is the horizontal direction. lines drawn on the diagram represent objects moving at different speeds. A vertical line represents an object that is "at rest" relative to the diagram (as you move along the line you progress through time but not distance), A tilted line is for something moving relative to the diagram. ( you move through space as you move along the line as well as time.)
    Scale is set so that c is a 45 degree angle.
    Here is such a diagram with three lines representing two velocities.
    The green line is represents motion at 0.5c in one direction, while the blue line represents motion at 0.5c in the other direction. The yellow lines are light. Each line is marked at equal intervals marking off equal points in time. Note that light leaving each line at its time interval of 1 reaches the other line at its time interval of 3.

    We can redraw this as it would appear if the green line were considered "at rest". This is done by rotating the diagram so that the green line is straight up and the blue line leans further to the left. However, just turning the image won't do. As I said, the speed of light is invariant, which means in the new diagram the line representing light must stay at 45 degree angles to vertical. Also, the fact that the light leaves one line at 1 and arrives at the other at three can't be changed either. To accomplish this, we need to draw the new diagram like this:
    We maintained all the requirements, but we had to spread out the time intervals for the Blue line. In the first diagram, When it was 1 on the Green line, looking directly across, we saw that it was 1 on the Blue line. This means that they kept the same time at the same rate. Now, when it is 1 on the green line, it is only 0.6 on the Blue. It takes longer for Blue to go from 0 to one than it take for green to do so. Time ticks slower for Blue than it does from Green.
    If we again redraw the diagram so that Blue is "at rest" we get this.
    Now When blue reads 1, Green is at 0.6 and time ticks slower for Green. This is how time dilation works. Every clock considers itself "at rest" (It is not moving relative to itself) and considers all clocks moving relative to it as ticking slow.
  8. Apr 7, 2017 #7
    How is constant the same as invariant? this has been explored; possibility of a different speed rate which is still invariant.
    are you sure they mean the same thing in this context?
  9. Apr 7, 2017 #8
    Constant meaning the value doesn't change in different reference frames. Invariant meaning the value doesn't vary in different reference frames.
  10. Apr 7, 2017 #9
    Thanks now I think I understand
  11. Apr 7, 2017 #10
    no constant and invariant aren't the same according to the OED 'C' should be called an invariant not a constant. Invariant means does not change at all, constant on the other hand means going constantly eg a car can go constantly at 10 kph or it could go at a constant 100 kph
  12. Apr 8, 2017 #11


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    You can have things which are constant but not invariant, such as my speed if I agree not to accelerate. Every inertial observer will agree my speed is constant, but not what it is. You can also have things that are invariant but not constant, such as the number of balls in a jar. Everyone agrees what it is at a given time (going by a clock next to the jar, for the pedantic), and that it can change.

    The speed of light is both constant and invariant.

    Be wary of trying to apply dictionary definitions to scientific terms. Sometimes they have a specific technical meaning that isn't quite the same.
  13. Apr 8, 2017 #12
    Note that if a car has a constant speed of 10 km/h in some inertial reference frame F, that will not be an invariant quantity. An inertial observer moving relative to frame F will calculate a different but constant speed for the car, it might be, for example, a constant 15 km/h.

    An invariant quantity, on the other hand, can change. For example, the rest energy of an object can change, but all inertial observers will agree on its value both before and after the change. Thus it's not constant.

    The speed of a light beam in a vacuum is both constant and invariant. All inertial observers will agree on the value of that speed, and no inertial observer will ever see that value change.
  14. Apr 9, 2017 #13
    lol good physics these last few posts, the distinction is clear
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