Sorry, says the barman, we don't serve neutrinos. A neutrino walks into a bar.
The announcement last week that neutrinos emitted from CERN were tipping up at Gran Sasso a tiny fraction of a second early has led to doubts in some quarters over the accuracy of Einstein's special theory of relativity - and spawned jokes like the one above.
With so many potential causes for inaccuracy in experimental instruments, though, it's unlikely that Einstein's going to be proved wrong.
And now observations of the effects of gravity on light over large distances have indicated that Einstein was bang on the money with his theory of general relativity.
Astrophysicists at the Dark Cosmology Centre at the Niels Bohr Institute have managed to measure how light is affected by gravity on its way out of galaxy clusters. It's the first time that the gravitational influence on light has been measured on a cosmological scale.
The observations confirm the theoretical predictions that redshift - whereby the wavelength of the light from distant galaxies is shifted more and more towards the red with greater distance - is affected by the gravity from large masses like galaxy clusters.
Radek Wojtak and his team analysed measurements of light from galaxies in around 8,000 galaxy clusters, looking at the wavelengths of the light coming from galaxies lying in the middle of the galaxy clusters and those on the periphery.
"We could measure small differences in the redshift of the galaxies and see that the light from ga laxies in the middle of a cluster had to 'crawl' out through the gravitational field, while it was easier for the light from the outlying galaxies to emerge," says Wojtak.
Then he measured the entire galaxy cluster's total mass and with that got the gravitational potential. By using the general theory of relativity he could now calculate the gravitational redshift for the different locations of the galaxies.
"It turned out that the theoretical calculations of the gravitational redshift based on the general theory of relativity was in complete agreement with the astronomical observations," he says.
"Our analysis of observations of galaxy clusters show that the redshift of the light is proportionally offset in relation to the gravitational influence from the galaxy cluster's gravity. In that way our observations confirm the theory of relativity," explains Radek Wojtak.
The discovery also aids research into dark matter and dark energy. While no one knows what dark matter is, they know what its mass and thus its gravity must be - and the new results for gravitational redshift back up existing models for dark matter.