There is the frame of the cosmic microwave background, which allows you to measure you speed with respect to the average expansion of the universe. It's not special in terms of physics, but it is a reference.

Just thinking out loud here. If the speed of light is absolute (it's not relative right? it doesn't change in any reference frame?), then could the speed of light be considered the base axis, or to continue the metaphor in the parent comment, that the speed of light is the 'outside the car'? Also, doesn't time 'stop' when you are at C, so that the whole idea of space (speed * time) sort of collapses?

> If the speed of light is absolute (it's not relative right? it doesn't change in any reference frame?), then could the speed of light be considered the base axis, or to continue the metaphor in the parent comment, that the speed of light is the 'outside the car'?

The speed of light is the same in any (inertial) reference frame. But you can't use it as a reference to compare other speeds against, because there are more degrees of freedom than you might think: a speed is calculated from both a distance and a time. So you and I could be looking at an asteroid and you say it's stationary and I say it's going half the speed of light, and we'd both be right, even though we both agreed what the speed of light was. Special relativity works like this.

> Also, doesn't time 'stop' when you are at C, so that the whole idea of space (speed * time) sort of collapses?

If you naively plug zeroes and infinities into the equations you won't get good answers, but if you're careful and work with limits then everything works right. What specifically were you asking about when going at C?

You should spend some time doing calculations in the “hypercomplex plane”. That is, something like the complex plane, but where you’ve taken the real number line and added a a new value u such that u² = 1 instead of the value i such that i² = –1.

Try to work out what perpendicular numbers look like in this space, what “circles” look like (hint: like hyperbolae), what the tangents to those circles look like, what kind of metrical relationships arbitrary triangles or other shapes have, and so on.

If you play with this simple 2-dimensional number system for a while, you’ll come to understand the nature of special relativity much better.

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The displacement between two points in spacetime can be either “spacelike” in which case there is some inertial reference frame in which the two points are simultaneous, or it can be “timelike” in which case there is some inertial reference frame where the two points are at separate times, unmoving in space, or the displacement vector can be “lightlike”, which is a boundary between the two – the two points are always in every inertial reference frame separated by some speed-of-light separation.