Is it still mainstream if we chance the model to 4 dimensional space and put the time as a 5. dimension? Is it possible to chance the equations of the GR to such model, and is it still mainstream or something else? Is it a good or bad possibility?
Is it still mainstream if we chance the model to 4 dimensional space and put the time as a 5. dimension? Is it possible to chance the equations of the GR to such model, and is it still mainstream or something else? Is it a good or bad possibility?
We do sometimes talk about papers on that or similar topics in the Astronomy section. Some String Theory variants propose multiple dimensions, and there is someone at the University of Leiden sometimes pushing for time to have three dimensions. Not much can be said about these unless you have some kind of observation possible that could distinguish. Either way, it is just a model to explain what we see and to predict what we might see soon.
Forming opinions as we speak
Olli, thank you. I haven't had a chance to launch my rant about extra dimensions in over a monyh. OK, here goes, after a quick aside.
(Aside: as was recently suggested for some BB ideas and FAQ's, the extra dimensions idea(s) could use a sticky).
(See following post).
There is a five dimensional theory and several versions of it do have four space-like and one time like dimension. It is called Kaluza Klein theory and was a precursor to String Theory. The primary goal of it was to unify electromagnetic and gravitational theories. I don't think it has ever developed to the point where it can be tested, most of the effort moved on from it to String Theory.
Edit to add: And GR has been generalised to work in it. Although I am not sure how extensively.
Olli, here's a good 5-D reference:
https://en.wikipedia.org/wiki/Five-dimensional_space
Notice that there are no supporting observations and no tests.
Is it mainstream? Eh, I would call it educated mainstream speculation. Maybe we should call it maintrickle.
So, it has been studied in cosmology. Problem with them is that there is no observational support and tests. I did read this Wikipedia article. They considered a 5 dimensional universe. They consider the dimensions as mathematical abstractions without evidence in the reality. But the 4. space dimension can be seen as a reality, it can be the fact that the space of the universe has no outside and everything belongs to it. The 4. space dimension makes exactly that mathematically. The infinite universe is exactly like that, or how this thing is in your mind?
And the string theory, can you tell briefly how it goes, and how it is connected to the 5 dimensional space? Or has it something to do with the multi- universes?
KK Theory is good enough that it predicts the existence of particles, some of which should be very obvious because they are connected to EM fields, and we don't see them. Hence, KK is falsified. That doesn't mean that there can't be something similar that does work, and predicts things we will find, just that KK itself doesn't work.
Forming opinions as we speak
Kaluza Klein theory was based around a single compactified spatial dimension alongside the 3+1 we observe - String theory is based around many compactified spatial dimensions (and some extended ones if you are looking at Brane theories) alongside the 3+1 we observe. String theory has a built in, very desirable, characteristic that if formulated in the critical number of dimensions it is automatically internally consistent.
The idea of 4D space-time is just a mathematical model, like a 5D universe or string theory. That is all physics is: mathematical models (that, hopefully, work).
It has been argued that space-time can only have 4 dimensions: http://space.mit.edu/home/tegmark/dimensions.html
I don't know how realistic that argument is...
I didn't find a good argument against the possibility of 4 dimensional space, where the 4. dimension is the fact that in the space of the universe there is all space and no outside. Then also the time is not necessarily a dimension but something else. We can go only forward in time, and some other things to make it not exactly a dimension, a vector in math as the other four. To not have more than 3 space dimensions, they say in the article only that then there is not atoms or something like that, what is not true.
Does the critic against KK- model also include this kind of model? And is it mainstream or something else? Has this been developed at all? I must look this KK- model in the Internet.
Dimensions are typically orthogonal to each other. "the 4. dimension is the fact that in the space of the universe there is all space and no outside" doesn't appear to meet the criteria to be considered a dimension. So I don't think this kind of model has been considered by the mainstream because it doesn't really appear to be a valid model...
Thanks of all these fine answers! I got the answer. So I have no more need to be in this tread.
The 4. dimension is the fact that there is all and no outside, which is a fact also in reality, all philosophers and astronomers are of this opinion, only it has not been understund in critics of 5 dimensional space that there is the 4. dimension also in the reality this way. One can argue that the 5 dimensional space, 4 + 1 space is mainstream. Mathematically it is so simple as the 4. dimension. I made an article of that [ATM promotional link redacted] It is in finnish language but Google translation comes easily. Always a bad one.
Last edited by PetersCreek; 2017-Mar-10 at 03:45 PM. Reason: Promotional link redacted.