If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

## Physics library

### Course: Physics library>Unit 16

Lesson 2: Minkowski spacetime

# Starting to set up a Newtonian path–time diagram

We introduce a graph that's a lot like your good old position-vs-time graph—but with a twist!

## Want to join the conversation?

• At , Sal says that "as we get more into physics, we'll see that maybe we shouldn't necessarily think of time as driving; maybe position, in some ways, is driving time." What is Sal referring to? Is there a relevant Khan Academy video or Wikipedia page? • Is it that if at , the photon at "c" velocity travels the same distance in less time than the spaceship, making it feel as if less time has passed? Or not? I get a little confused with relativity. • hey sal, what is the meaning of word space-time? • at when sal draws the graph of position of light he doesn't take into consideration the fact that the person is moving with constant velocity . In the future videos you will learn how that the speed of light (3 x 10^8 m/s) is absolute - it is always constant no matter from what frame of reference we look at it (even if we are moving).

The Newtonium way of looking at things thought of space and time like this;
1) The passage of time is the same for all frame of references, meaning time is absolute.
2) Measured space is the same for all frames of references.

But as you will learn, this can't be.

Instead, Einstein's Theory of Special Relativity talks about how light is absolute, but space and time are not.

Now we know space and time are not absolute, they are really just different directions in this continuum spacetime. That is why people say space/the universe has 4 dimensions, the last one being time. Space = 3D and time = 1D, when you put these together you get Spacetime which is is representing that time isn't a separate absolute, and space a separate absolute, they are just different directions of spacetime.

This switch from Newtonian/Galilean thinking may take a while to sink in - just keep learning!

Hope this helps,
- Convenient Colleague
• I see that at huge speeds, time can depend on speed, but if you can put it either way, why switch the axes we are used to? Does this demonstration work with time as x and position as y? • what is the equation for time dilation due to speed • Given that, "Inertial frames obey Newton's first law, while non-inertial frames don't. That happens because non-inertial frames are accelerating or spinning somehow, which creates pseudo forces within them, denying the first law." Why do accelerating frames create non-inertial forces actin on the accelerating masses (but they do not come from the environment they are intrinsic to the mass that is accelerating. When a body orbits the earth the earth gravity supplies the force responsible for the acceleration in the orbit,but if this is taken to be a bucket full of water the water is pushed against the bottom of the pail so it doesn't fall out becayus their is a centripetal force on the water outward and this is the so called non-inertial forse (we can feel this force when we accelerate around9are carried around relative to the ground) on a merry-go-round.
(1 vote) • There is no force pushing the water against the bottom of the bucket but there is a force from the bucket pushing on the water to make it accelerate in a circle. There only seems to be a force pushing the water against the bottom of the bucket from a non-inertial frame rotating with the bucket.

When you look at the bucket from a non-spinning frame of reference all of the forces make sense but from the spinning frame of reference there is this mysterious force pulling on the bucket and water.
• Why , if you change position very quickly (travel near the speed of light, for example), you move through time more slowly? I can see that the path length of the light relative to the frame the light clock is moving relative to is longer than the path length of the light relative to the frame the clock is "at rest" in, so that the time interval will be longer for the frame the clock moves relative to than the interval relative to the frame the clock is at rest in BUT why does this mean that the time flows slower relative to the frame the clock moves in , than the time flow in the clock where it is relatively at rest with respect to the frame its light path is refered to. Why couldn't we say it flowed faster than that of the clock when it was "at rest"?
(1 vote) • In special relativity the restriction that the speed of light in a vacuum is measured to be the same by all inertial observers regardless of the relative motion of the observer and the source of light requires that the time and space axes of observers in different inertial frames to have different orientations. This means that distance in space and the distance in time between two events are not necessarily the same for different inertial observers. What is the same is the total space-time distance between events. For the math to work out requires that the space-time interval to be s = (x^2)+(y^2)+(z^2)-(t^2). Because of this there is a contraction of space and time.

Here are a few good videos expanding on this. The first three are TedED videos on space-time and the 4th is from the PBS Spacetime YouTube channel:  