Got a letter from GoldBarz asking a question which could be of broad interest.

No reason to restrict discussion to PM. Shd be discussed openly if at all:

Anybody have an opinion?

I say it is possible. Laws have been worked out by physicists for univ. of other dimension.
For example I know the the approach to Quantum Gravity called LQG can be formulated in all dimensions: 3, 4, 5, 6, ...

I guess most physics theories can be formulated in many different dim.
But you can get laws which are ANALOGOUS to the laws we know, but not quite the same and sometimes have some different numbers, like an inverse cube law instead of inverse square. or a 4 instead of a 3, or a pi-squared instead of pi.

Quantum Gravity people sometimes say things like this: when we get an adequate quantum theory of space and time (and the shape of it, which is gravity) then that theory will explain why space has the dimension it does, instead of something else

I dont believe or take seriously stuff that you cant measure or observe or see any detectable consequences of it. So I dont take seriously ideas of "
other universe". for science purpose there is only this.
However one can ask if it could be different.
Could it have different dimension or is this dimension forced by some undiscovered logic. Are other laws possible or is what laws we have forced by some reason. Or is some of what we see just a random accident. Is gravity an accident or did it have to be how it is for some reason?

One can ask that kind question. It makes sense, I think, to ask.
Could be a waste of time, but at least it is not a meaningless thing to ask

To carry any/all of the laws of physics to higher dimensions, you would need to anti-differientate them to that order - just like you derive the 3D volume of a sphere from the 2D area of a circle. The big problem I see is you have no idea what additional constants might emerge. Example, c is a universal constant in 3D+1 space. But would it still be a constant in 4D+1 space - or would it be coupled to a higher order constant? And what about gravity, and the other fundamental constants in our 3D+1 universe? How would you go about measuring any such higher order constants? I would guess it may well be impossible to do so from this universe, hence, meaningless. Does that make any sense?

Consider the first three spatial dimensions. We sometimes talk as if a process of interest were one or two dimensional, but this is really just a convenience. No real object occupies only one or two dimensions, all real objects have extent in all three spatial dimensions. Also with time. There is no real object which has no extent in time.

So, if there are other dimensions in any meaningful sense, then every real object must have extent in every one of them. We only need to count three for our current purposes, plus one of time, but there may be more. Evidence suggests that there may be more. The weakness of gravity in relation to the other forces is one example. The beauty of string theory is another example. I consider the acceleration of the observable universe and the low baryon count as further evidence.

There is no need to speculate that physics might be different in other dimensions. In fact, the idea comes from a common misconception, due mainly to science fiction, in my opinion. "The creature from dimension X" and other such fanciful monster tales use the idea of extra dimensions as a plot gimmick, allowing the writers to bring in scary charachtors and fictional events. That's lots of fun, but is just bedtime stories, not science.

The extra dimensions are not other universes or other worlds. They coexist with us here and now. They have real effects on real objects in our world, just effects that we have not had much cause to notice until now. Marcus made reference to "other universes" which cannot be sensed in any way in our universe, and I agree that such an idea is at best trivial. I don't know, or much care, if other universes exist in which other rules of physics or values of physical constants hold sway.

However, having said that, I will still uphold the idea of branching universes in the many-worlds sense, although I have found it more convenient to think of them in terms of many times rather than many worlds. You may call them other universes if you wish, and say that they are irrevokably separated from our line, but I think that is not entirely accurate. It is only one universe we are talking about, it just has aspects to which we do not ordinarily pay attention. For example, if you persist in scoffing, I might ask you where yesterday is. You do believe in yesterday, don't you? Irrevokably gone, no doubt, but it is still our own universe.

The other times, or other universes if you wish, might show up when we look at events on very short or very long time scales. They are branching out from our every instant, and in every instant are in contact with our own line.

One other supporting point. If you imagine gravity is different in other dimensions, having cubic or quadratic roots or something, then how do you explain that gravity involves square roots even in our three dimensional universe? Vectors, tensors, spinnors and the like all behave themselves very well in three dimensions, even though you may argue that they belong to "other" dimensions, at least by dimensional analysis.

In summary, dimensions are not other places. They are just other ways of looking at this place. Sometimes you only need to look at one dimension, sometimes you might need to look at eight or ten dimensions, in order to understand the physical behavior of real objects. It is just naive to suggest that life cannot exist in higher numbers of dimensions. Life does exist. If it is meaningful to speak of higher dimensions, then life extends itself in all of them, just as every real object must.
Thanks for being here,