# Q & A: our dimensions are numbered?

Q:
I have been reading about string theory, and how they propose multiple dimensions above our perception. Everyone seems to assume that the three that we percieve are at the bottom(dimensions 1,2,and 3). Has anyone thought that they could in fact be, for example, 5,6,and 7. We percieve light and sound in the middle of the spectrums, and it makes sense that we could be in the middle dimensionaly as well. Any thoughts or research in this area? I would be very interested.
- Michael French (age 32)
Veneta, OR
A:
When we talk about the spectra of light or sound, there's a natural variable (frequency) which we use to put the different components in order in a meaningful way.

Think about those dimensions for a minute. Do they come with numbered tags from the Dimension Outlet?

Mike W.

(published on 01/07/2011)

## Follow-Up #1: Ordering our spatial dimensions

Q:
What I mean is, they always talk about "higher" dimensions. How do we know that we are not in the middle of the spectrum instead of at the ends? Could there be "lower" dimensions?
- michael french
A:
Michael- I apologize for being too brief and cute in the last answer. Here's a fuller explanation. Let's say we call our three spatial dimensions 1-3. Let's suppose, for the sake of argument, that in a more complete M-theory there are 7 more spatial dimensions. Let's call them 4-10. Then our theoretical colleagues try, with some marginal bits of success, to calculate predictions for observables using this M-theory.

What would happen if we had chosen to call our dimensions 8-10, or 3-5? The calculations of observables would be absolutely unchanged because the names we call the dimensions never enter into the calculations.  So we view the choice of names of the dimensions as purely an aesthetic question, with no scientific meaning.

As Galileo said, it wouldn't matter if we called them polenta.

Mike W.

(published on 02/03/2011)

## Follow-Up #2: physics in higher dimensions

Q:
Putting "the names (of dimensions)" aside, do all objects/energy in these "extra" dimensions obey all the known fundamental physics laws as we know? such as uncertainty principle, conservation of energy/momentum/probability/charge etc.? These "proposed" extra dimensions have no special properties whatsoever but each and every one of them is (or acts) just like another spacial dimension we see everyday in our 3D world? How are spacial dimensions connected with fundamental physics laws? Which fundamental physics laws(properties/mechanisms) explain/necessitate the existence of spacial dimensions as we observe? For example, is "uncertainty principle" required for spacial dimensions to exist?
- Anonymous
A:
Wow, that's quite a set of questions. I'll try to handle some of them.

Conservation of probability should certainly hold in any type of physics. It's hard to see what it would even mean for it not to hold.

The main motive for thinking of higher dimensional physics is to find a way to work gravity consistently into the framework of quantum mechanics. So in that program, all the basic quantum properties would survive. That includes the momentum-position uncertainty relations, which aren't tied to any specific number of spatial dimensions. Also, conservation of energy and momentum follow by Noether's theorem from the translational symmetries of time and space. So the time part would still apply.

The proposed other dimensions do not currently behave like the 3 with which we're familiar. In most pictures, they're all currently curled up on a tiny scale, not extended. Some versions, I think, have other extended dimensions but not ones that just mix with our 3 by rotations. So they're different.  I'm not quite sure what becomes of conservation of momentum in any little curled up spatial dimensions. You should find somebody more knowledgeable.

The common suspicion is that our current laws of physics reflect a particular way that the extra dimensions curled up. There may be a huge number of different ways (some estimates are about 10500) allowing many different laws of physics at the level we deal with, all based on some as yet unknown deeper law.

You'd also need somebody more expert to find out whether conservation of charge necessarily works in some deeper physics in which the electroweak force is integrated with the strong force and gravity, regardless of the number of dimensions involved,

Mike W.

(published on 03/12/2013)

## Follow-Up #3: number of dimension for uncertainty relation

Q:
You said "the momentum-position uncertainty relations, which aren't tied to any specific number of spatial dimensions." but for uncertainty relations to hold true the dimension must be higher than a point or point-like because it seems impossible for anything to move at a point. Is this the case or not? So, the dimension must be at least a line-like or string-like for uncertainty relations to hold true?? So uncertainty relations are tied to or applicable to the dimension higher than a point?
- Anonymous
A:

Yes, you need at least one dimension.

Mike W.

(published on 11/16/2013)