How many dimensions are there?
The word gets used two ways, and people rarely separate them.
One is a measure of a thing or a place.
How long, how wide, how far along.
The other is another place a thing could be.
Somewhere you can't reach by walking.
The first meaning is the plainest. A dimension is a measure.
A number you read off a ruler.
Start with a point.
It has no measure. It is somewhere, but there is nothing to say where it is against.
One axis, one number.
Slide along a line and a single measurement tells you exactly where you are.
Two axes, two numbers.
Now you can place anything on a surface. The coloured path is the measuring: this far across, then this far up.
Three axes, three numbers.
Across, up, and toward you. Every position anything can hold in space is an answer to those three questions.
Three, because that is everything where can ask.
Whatever we can conceivably measure as location is already covered. A fourth spatial number would have nothing left to say.
Time is called the fourth dimension, but it does a different job.
The other three say where. Time says what happens next.
Take one set of three numbers.
Where something is at one instant. A snapshot, complete on its own.
Take another set. Then another.
Each one is a full answer to where. The thing has moved between them.
Time is the progression between sets of locations.
The action that carries one set of measurements into the next.
Put the sets in order and you get a path.
Where, plus when. The fourth measure is the order the locations come in.
The second thing people mean is parallel.
Not a position on a ruler. A whole causality running beside this one.
Follow one timeline.
At some moment, something goes the other way.
Slightly different, or very different.
Each branch keeps its own full chain of cause and effect, running at the same time as all the others.
Parallel does not have to mean different.
Take the branches away. Keep one timeline, one copy of it.
Parallel can also be positional.
Different locations on the same copy of the same timeline. Nothing differs except where along it you are.
Another kind of elsewhere. Measured in which, not how far.
The third place is scale.
Size is not only a property of things. It is somewhere you cannot walk to.
Everything you have ever seen sits within a few steps of you.
Each tick on this ruler is ten times the one before.
Go down. Cells, atoms, nuclei.
The smallest scale we have probed directly is about 10−19 metres.
Below that is a long dark.
The Planck length, 1.6×10−35 m, is where known physics stops making predictions.
Now zoom out. Count the steps from us to each end.
35 orders of magnitude down to the Planck length. 27 up to the edge of the observable universe.
We are closer in scale to the biggest thing than to the smallest.
Sixteen orders of magnitude no instrument has touched.
Room for whole sets of causality to share our space and our time, separated from us only by size.
The fourth place is frequency.
Not where, not when, not how big. How it vibrates.
Think of a radio.
Every station is in the room right now, in the same space. None of them interfere. Each is sealed off by its frequency.
Now suppose matter works the same way.
Sets of particles that oscillate only with their own kind. They cannot touch, bond with or signal anything on another frequency.
Same universe. Same timeline. The exact space you are in now.
Whole planets could be here, built from particles we can never interact with.
Never, except maybe through gravity.
Mass bends the same space for every layer. It may be the only thing that crosses.
A cousin of dark matter.
Physicists do discuss hidden sectors of particles that share our space and barely touch us. Still speculative, but not idle.
So, how many dimensions are there?
It depends which where you mean.
Three numbers to place a thing, one to order its changes.
Then three more questions: which branch, which size, which frequency.
Five ways to be elsewhere.
axial · time · parallel · scalar · frequential
None of them is contained in the others.
Where are you?