Setterfield: This quote can be found in my second response to Mark Kluge, here.
Thank you for this question and bringing my quote back to my attention. Until I had checked on the data a few minutes ago, I was under the impression that tachyons had been experimentally verified. They were first discussed by Gerald Feinberg in 1967 in an article entitled "Possibility of Faster-Than-Light Particles" in Physical Review 159: 1089–1105. I recalled watching some cloud-chamber experiments at Adelaide University in the very late 1960's or early 1970's where the doctoral student pointed out to me some strange particle behavior. The student commented that they thought they might be tachyons, since the tracks had features in time and location which were consistent with that.
Your question has forced me to go back to the verified historical data, and you are right. Despite what I saw and was told, tachyons have not been experimentally observed, and therefore only remain a theoretical possibility. If they do exist, they would travel faster than light, since the speed of light is their minimum speed, not their maximum. In briefly reviewing the literature on tachyons, most thinking about them seems to have been confined by the theory of relativity. It might be wise to consider other options in case they actually do exist with some characteristics that are different from what relativity predicts. The basis of relativity is called into question by several facts. First, the Cosmic Microwave background Radiation (CMBR) presents an absolute universal rest frame against which all velocities can be compared. As Martin Harwit points out, this means that relativity can only be valid on very small scales, and not for macroscopic phenomena. Secondly, since the speed of light is inversely dependent upon the strength of the Zero Point Energy (ZPE) and the ZPE has increased with time, the current speed of light may not be the limiting velocity in the cosmos.
This second point was discussed in a New Scientist article for 1st November 2008, p.28-31 where it was pointed out that "there is a definite but unspecified maximum speed [in the universe] that the sum of individual relative speeds cannot exceed ... Light's special position in relativity is a historical accident ... Not only is light not necessary in relativity - there's no room in the theory for it." So the speed of light as we have it now need not be the maximum speed. Indeed, the ZPE research suggests that the maximum speed which attained at the origin of the cosmos was about 10 billion to 100 billion times higher. This leads to the speed of gravity issue.
Research by the late Tom Van Flandern on double star systems indicates that the speed of gravity may be of the order of 10 billion times the current speed of light or more. If this is so, the propogation of gravitational phenomena may reflect the original upper maximum speed. Since the speed of light is dependent upon the density of virtual particles, gravitational waves may be so long that virtual particles present no hinderance to them. If so, they might then travel at what has always been the upper maximum speed in the cosmos. It will be interesting to see how this aspect of the research develops.
With thanks again for helping me to get this straight,
July 16, 2009