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I am not a theorist, but if anything I think the 26 free parameters is a pretty significant understatement in terms of predictive physics. Experimentalists have

I am not a theorist, but if anything I think the 26 free parameters is a pretty significant understatement in terms of predictive physics. Experimentalists have measured orders of magnitude more than 26 measurements which are incorporated into our understanding of physics.

pdonis 10y ago
The number of free parameters is not the same as the number of experimental measurements; you are correct that there are orders of magnitude more than 26 of those. The point is that the equations of the theory can correctly model all of those experimental measurements only if 26 parameters are included whose values cannot be predicted by the theory; their values can only be obtained by analyzing all of those experimental results and finding the combination of 26 values that best matches the data, given the equations of the theory. It is hoped that a more fundamental theory, to which our current SM would be an approximation, would reduce the number of free parameters required, i.e., would allow the equations of the theory to match all of the data with fewer numbers included whose values the theory cannot predict.
- selectron 10y ago
  Like you said the SM is an approximation, so even with 26 free parameters there is a lot of physics that is left unexplained. In my experience it seems like far more than 26 free parameters which go into MC simulations, for instance with something like Madgraph. Perturbative QCD predictions also seem to rely heavily on experimental measurements. I'm not sure things like half-lifes can be predicted accurately from 1st principals either. So I don't think it is correct to say that from 26 parameters and first principals we can predict all experimental results. Even if we could do this in theory, in practice we do not have the computational power to do it, so we rely on approximations which are implicitly guided by experimental results. I agree that the number of parameters needed is less than the number of experimental measurements though, I confused that.
  - pdonis 10y ago
    > Even if we could do this in theory, in practice we do not have the computational power to do it, so we rely on approximations which are implicitly guided by experimental results. This is true, in practice we can't do all of the derivations from first principles. But AFAIK this is an issue of, as you say, computational power; it is not an issue of theoretical insufficiency (at least within the domain that the SM covers, i.e., the non-gravitational interactions in flat spacetime with gravity being negligible). We know what equations we would have to solve; we just can't solve them analytically and we don't have the computing power to solve them numerically in all cases.

pdonis 10y ago

The number of free parameters is not the same as the number of experimental measurements; you are correct that there are orders of magnitude more than 26 of those. The point is that the equations of the theory can correctly model all of those experimental measurements only if 26 parameters are included whose values cannot be predicted by the theory; their values can only be obtained by analyzing all of those experimental results and finding the combination of 26 values that best matches the data, given the equations of the theory. It is hoped that a more fundamental theory, to which our current SM would be an approximation, would reduce the number of free parameters required, i.e., would allow the equations of the theory to match all of the data with fewer numbers included whose values the theory cannot predict.

selectron 10y ago
Like you said the SM is an approximation, so even with 26 free parameters there is a lot of physics that is left unexplained. In my experience it seems like far more than 26 free parameters which go into MC simulations, for instance with something like Madgraph. Perturbative QCD predictions also seem to rely heavily on experimental measurements. I'm not sure things like half-lifes can be predicted accurately from 1st principals either. So I don't think it is correct to say that from 26 parameters and first principals we can predict all experimental results. Even if we could do this in theory, in practice we do not have the computational power to do it, so we rely on approximations which are implicitly guided by experimental results. I agree that the number of parameters needed is less than the number of experimental measurements though, I confused that.
- pdonis 10y ago
  > Even if we could do this in theory, in practice we do not have the computational power to do it, so we rely on approximations which are implicitly guided by experimental results. This is true, in practice we can't do all of the derivations from first principles. But AFAIK this is an issue of, as you say, computational power; it is not an issue of theoretical insufficiency (at least within the domain that the SM covers, i.e., the non-gravitational interactions in flat spacetime with gravity being negligible). We know what equations we would have to solve; we just can't solve them analytically and we don't have the computing power to solve them numerically in all cases.