**Physicist**: A “dimension” is a direction. So, we’re in “3 dimensions” because there are only 3 directions, and every other direction is just a combination of those 3 (and their negatives). For example, by telling someone how far north, east, and up to go you can direct them anywhere on Earth. Time is also considered to be a dimension, but there are some subtleties involved with that.

When string theorists talk about space having 10 dimensions they’re talking about all but four of those being “tiny loop dimensions”. We’re used to thinking of a direction/dimension as extending outward forever, but these tiny directions just make a short loop and bring you back to where you started.

For example, if the universe had two normal dimensions, and the third dimension was a “1-meter loop dimension” then you could move, say, north/south and up/down no problem, but if you looked east or west you’d see the back of your head a meter away. But unlike a mirror, you could physically poke the back of your head. Or give yourself a haircut, because why not.

The smaller dimensions are like that, but loop on a distance that is much, much smaller than the smallest particle. As a result, it doesn’t really make sense to talk about movement in these tiny dimensions, basically because there’s no place to go.

So, if you want to avoid being hit by something, you just step to the side (in any of our 3 dimensions). You have the option because you have the room. In the loop dimensions there isn’t enough room, so as far as we (and even particles) are concerned, they may as well not be there.

String theory (which is where all this “10 dimension” talk is coming from) is the only theory of everything on the table today, but unfortunately it hasn’t made any verifiable predictions (yet). The extra, but undetectable, dimensions are just one example of that.

The portal picture is from here, and the alleyway photo is from here.

Short Answer: For the same reason flatlanders on a piece of paper don’t notice that the piece of paper thy live on exists in 3 dimensions (or 10).

next question: if these extra dimensions have no room for anything, why does string theory have them?

good question but the fourth dimension is not understood yet so we cannot get to fifth unless we understand time or what ever it is

When quantum physicists see quantized values, they immediately begin to suspect that there are some hidden restrictions somewhere in the system. For example, sound waves in air can have (practically) any wavelength, but sound waves on a guitar string can only show up in a tiny, discrete, set of wavelengths that divide the length of the string (these are called the harmonics).

Particles themselves are discrete, and have properties like charge, lepton flavor, baryon number, etc. that are all discrete integer numbers. One of the reasons behind coming up with these extra tiny dimensions is that they introduce a restriction (the size of the dimension) which makes standing waves show up in integer multiples of some set frequency, similar to the way the fixed length of a guitar string forces a particular set.

So…if there are 10 dimensions, can there existence be proved by experiment? And would one be able observe them in some way.

They probably can’t be observed directly, but there may be some very slick way to detect them through some secondary effects and clever math.

so if. then if i travel at the speed of light and if could see mirror behind while travelling at the speed of light what kind of dimension would i see at mirror.

How bout the ‘brane theory’ saying we live in a 3 dimensional ‘brane’ but our ‘brane’ is right up against other ‘branes’ that can be 4, 5, 10 or any number of dimensions there are.

I could not resist commenting on this one: Those who espouse the the Brane Theory have little or no “brains” (please smile)

Yes my moderation is: it was just a pun..a joke not meant to do any harm. Come on fellow scientists lighten up. The second law of thermodynamics says we are not going to live forever (at least on this planet) so enjoy your work. I promise to be a better fellow human being and not be such a wise guy. amen

Can’t waves of small amplitutes nd longer wave lengths move through this dimensions?

There is one clue which helps to understand dimensions. For example to explain time as a dimension in 4D- 3-space and one time dimension, we can assume the body at rest and think of time passing. i.e a body at rest will have (0,0,0) space dimension and time dimension as “T”. So the time passed between two events for a body at rest is t2-t1 and the initial and final coordinates are given as (ct,,0,0,0) and final coordinates are (ct2,0,0,o). Similarly to understand 5th dimension we must assume body at rest in time dimension too and think of dimension that is moving.

I read another article about 10 dimensions where dimensions 5 through 10 were described as “possibilities”. The following video also describes this concept (http://m.youtube.com/watch?v=p4Gotl9vRGs)

I just wanted to ask whether this concept has scientific merit and is this in any way related to the way 10 dimensions are described in your article.

The reason for why we can’t see the fourth dimensions is the reason for why flat landers can’t percieve the depth .

Some phycisist say that the 5th dimension could be the orthogonal of that of the fourth.Time moves in one direction further more it is a line in a representation of the 4d.The 5thcould be the possibility of every casualty or the parallel worlds to be simplified.

Just out of curiosity, are mathematics themselves largely restricted to the dimension we currently perceive them in, or are they a universal language that acts in different ways depending on what dimension they are acted upon. Perhaps, for example, you may have English but depending on the location it could be spoken with an accent and have its own grammar, or would it simply not even be relevant, and be replaced by something unfathomable to us?

@Jared

The notion of a dimension is well understood (defined) mathematically. It’s as easy to do math in three dimension as it is in 40 (it’s just that in 40 there are typically more numbers to keep track of).

Question: Why are the dimensions there in the theory, if there isn’t enough room for anything to move in them?