There are days when I despair at the state of scientific education and knowledge in the world. This is one of them.

In my wildest imaginings I never thought that I would have to say this, but the particle physics community has not opened a portal to hell.

For those who have not read about this odd theory yet, let me explain. As many of you are aware, on July 5 the Large Hadron Collider restarted after a break, and set a record for the highest energy particle collisions in a manmade experiment. (Nature itself creates far more energetic reactions constantly, including in our own atmosphere, which will be important to note later in this article). This will likely result in a lot of interesting new information on subatomic particle physics and the properties of the fundamental particles of nature.

And now there is a conspiracy theory circulating the internet that claims that the LHC also opened a portal to another dimension, and has sent the entire planet into a parallel universe.

I wasn't going to comment on this lunacy, but it does give me a chance to comment on a misunderstanding that I have heard from a few more sane but still scientifically less literate people. And that is, what do we mean when we say that the LHC is searching for extra dimensions.

There are multiple unproven theories in modern physics that posit the existence of additional dimensions, and I suspect that they are the kernel of information that resulted in these latest bizarre claims. From the century old Kaluza-Klein and Nordstrom theories that explored adding a single extra spatial dimension to the Universe, to the more modern superstring theories that can only exist in eleven dimensional spacetime, and the brane multiverse models that suggest we are living on a membrane that is floating through some even higher dimensional space, there are many valid scientific reasons to believe that we actually exist in something more than the four dimensional spacetime that we already are aware of.

But these are not parallel Universes of the sort seen in science fiction movies.

It is impossible for the human mind to visualize four dimensions, so let me work with something smaller here. Imagine looking at a single line in space. It has exactly one dimension - specifically its length. But as you look closer, you would find that it is not actually a line, but a tube. In addition to its length, it also has another dimension that is perpendicular to that one large dimension, in the form of a circular cross section. And so this line is actually two dimensional, but with one dimension much smaller than the other. 

In our Universe, the equivalent would be to look very closely at a single point in space and discover that it is actually a very tiny sphere or a donut, or some other shape. And this is not something that exists in our Universe, but somehow is perpendicular to every direction we can see, as the circular cross section of the line is perpendicular to its length.

And these higher dimensions are tiny. Over a decade ago I did some work in this field as part of my doctoral research, and even then we knew that these hidden dimensions had to be on the atomic scale in size. There are some models that allow slightly larger dimensions, but they are not accessible to experiments such as the LHC.

The reason that these models are discussed in connection with the LHC is due to a phenomenon known as Kaluza-Klein modes. In essence, any particle that exists in our Universe could travel into these higher dimensions, and effectively appear to have a much higher mass. (The process is very interesting, but beyond the scope of this article). The result is that at high energies, such as those reached by the LHC, we could find a second copy of the Standard Model of particle physics, but with all of the masses of fundamental particles shifted upwards by a certain amount. This shift would prove the existence of at least one extra dimension, and its size would tell us the scale of the hidden dimension.

My own belief is that existing constraints on extra dimensions from astrophysics and cosmology have already excluded anything that the LHC could measure, but I would be quite happy to be proven wrong on this point.

As an aside, I would add that there are theories in which parallel Universes do exist and contain other worlds and other life forms - but they are completely disconnected from our own. There is no possible way that the LHC or any other experiment in this century will ever be able to connect the two Universes. And they are very speculative with no evidence to support their existence. (However if there is sufficient interest, I will write an article about such theories at some point in the future)

So that is my not-so-brief explanation of why the LHC has not and will not be creating portals to other dimensions. That is simply science fiction, with no basis in reality.

As so much on the internet is.