First collisions at the LHC with unprecedented Energy! (Ask a particle physisicist)

[Alex K]

@Sappho, which language would you prefer? I can offer German or really really bad Spanish or French

So, those particles we call elementary particles (*) are usually considered to be pointlike (apart from advanced speculative ideas like string theory), because there is no experimental indication that they are not, at the scales we have tested experimentally - this means they are treated as zero-dimensional in space, and one-dimensional in spacetime ("world lines"). In particular, they are treated as point particles in the Standard Model.

Now, these pointlike particles have a quantum uncertainty in their location, which at each point in time is expressed by a wave function

f(x)



Here, x is the putative location, and f(x) squared is the probability to find the particle at position x, roughly speaking. So, the quantum uncertainty is spread out over three dimensions, but we are talking about the uncertainty of one location parameter, and therefore one still talks about it being a point particle. Is that somewhat clear?

An example of a non-pointlike thing: In contrast to the above, the quantum wave function of a string must assign a probability not only to each possible overall location of the string, but to all possible combinations of locations of each piece of the string, i.e. its shape and size. You then get a much more complicated object which is a 1-dimensional thing in space, the shape, size *and* position of which have quantum uncertainty.


(*) It may well turn out that today's elementary particles are not elementary if one looks more closely, i.e. with more energy, i.e. with a bigger collider.