View Full Version : Space-time diagram

hush36

2014-Oct-23, 02:54 PM

On an imaginary spacetime diagram (space on the Y axis, time on the X axis), something moving through space AND time would be represented by a diagonal line at 45°.

As said something’s speed approaches C, it moves through space more than through time therefore the angle between the line and the Y-axis decreases so that, for electromagnetic radation, the line is totally vertical and the angle is 0°.

Heres the question : Is there a “something” that has movement solely in the Time direction so that the angle between the line and the x-axis is 0° ?

NEOWatcher

2014-Oct-23, 03:09 PM

On an imaginary spacetime diagram (space on the Y axis, time on the X axis), something moving through space AND time would be represented by a diagonal line at 45°.

Why 45?

Heres the question : Is there a “something” that has movement solely in the Time direction so that the angle between the line and the x-axis is 0° ?

Wouldn't that simply be something that's not moving?

That would depend on reference frame.

ShinAce

2014-Oct-23, 04:01 PM

If you want to move in the time direction only, then just sit still. Let's say you model the motion of the galaxies in the sky in this manner. Then our milky way isn't moving in space at all, yet it is moving forward in time.

Ken G

2014-Oct-23, 05:04 PM

As said something’s speed approaches C, it moves through space more than through time therefore the angle between the line and the Y-axis decreases so that, for electromagnetic radation, the line is totally vertical and the angle is 0°. The usual way to do a spacetime diagram is set c=1 (so measure distance in light years and time in years, let's say), and light moves along that 45 degree angle. That's because it is we, the observer, who are witnessing the motion of the light-- nothing thinks itself is moving through space! In a spacetime diagram, we are the observer moving only along the x axis, and we regard other things as moving through space and time relative to us, with light moving through an equal measure of both, and everything else moving mostly through time like we are. Objects that are stationary with respect to us are moving only through time, keeping pace in time space with us, while everything else mixes in a little movement through space, essentially at the expense of how we reckon they are moving through time. So for us, light is moving equally through space and time, but we reckon that time is not passing for light, given how we match up our "moments of now" with the "now" seen by that light.

Grey

2014-Oct-23, 05:35 PM

On an imaginary spacetime diagram (space on the Y axis, time on the X axis)...In addition to everyone else's comments, note that it's typical to put space on the x-axis and time on the y-axis. It's just convention; there's no reason you couldn't do it the way you suggest, but since everyone does it the other way, you'll get pretty confused looking at examples of space time diagrams if you're thinking that time is on the x-axis.

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