# What is relative and what is not

1. Apr 21, 2014

### Whitefire

As all of you, I like to think about why and how Things happen; and as all of you I often feel frustrated by just how basic are the things we really don't know.

What would help me at least a bit is a meaningful list of things, values, concepts that are relative (in physics), and another comparable set of concepts that are not relative. For example:

Relative: time flow ("forward"?), space (distance, size), movement in space (?)

Not relative: speed of light, Planck constant, quantities (number of stars, particles...)

I would like the lists to be as accurate as possible, but the more general items on them, the better. I am really after some generalizations. Can you please help me add things to the lists? Can the things I wrote be questioned? Please do not get too specific, like listing different particles; and also please do not go outside of physics into things like emotions or psychological concepts (although they cen obviously be relative as well).

Thoughts, anyone?

2. Apr 21, 2014

### Staff: Mentor

Relative: components of tensors, coordinates

Invariant: tensors, events

I am not certain, but that may be a complete list.

3. Apr 21, 2014

### Fredrik

Staff Emeritus
To me, "relative" means coordinate dependent and "not relative" means coordinate independent. So I have to consider most of your examples wrong.

Time flow: Not sure what time "flow" is, but different coordinate systems can disagree about which event happened first.

Space: The distance between two objects is certainly relative. Think about length contraction in special relativity. Different coordinate systems also disagree about which slice of spacetime to call "space".

Movement in space: Velocities are assigned by coordinate systems, and different coordinate systems assign different velocities. The concept of "non-accelerating" is however coordinate independent.

Speed of light: OK in SR, as long as we stick to inertial coordinate systems.
The speed of light is the same in all inertial coordinate systems in SR, and at the origin of all local inertial coordinate systems in GR. But there are other coordinate systems.

Planck constant: OK

Number of stars: The number of stars in a region of space right now depends on what slice of spacetime we call space, and that's determined by the coordinate system. The number of stars in spacetime is however coordinate independent.

Number of particles: Check out the Unruh effect