Just about two days ago I warned that there will be a lot of nonsense about relativity coming out these days, and promptly there are already three articles here on Science2.0 alone that are exactly what I feared: Physicists with only basic knowledge about relativity claiming that particles cannot possibly go faster than light and portraying anybody who holds otherwise as a moron (not literally, but that seems to be the gist). Wake up guys – it is 2011 – modern physics has come a looooong way since Albert.
In modern physics, it is well understood how particles can travel with superluminal velocity without violating special relativity or causality. I will discuss such a mechanism here and the novel experiments it suggest in case the recent neutrino physics observations do hold up to scrutiny.
One possibility is very intuitive: Our three dimensional universe may well be due to a three (or more) dimensional membrane inside a higher dimensional, so called bulk space. This is called “universe on a membrane” or short “membrane universe” (MU). This is a well known scenario in string theory but not restricted to string theory. In the MU scenario, our light velocity c is the maximum velocity of excitations inside the MU membrane, the latter being by the way the very reason for why the MU universe observes Einstein relativity inside of it. That velocity c may be very small compared to the maximum velocity of particles that are not bound to our MU membrane, those that speed freely through the bulk space with perhaps, as indicated by the OPERA data, velocities thousands of times the speed of light.
If, as was reported, neutrinos actually went faster than light from CERN to OPERA (Oscillation Project with Emulsion-tRacking Apparatus) in Italy’s Gran Sasso, they can do so without any violation of relativity as follows:
(Remark: I wrote this article before I saw Dimension hop may allow neutrinos to cheat light speed, which is saying something very similar to what I point out in the following here and have pointed out before for a number of years now. HOWEVER: My use of extra dimensions here is rather a didactic tool. They are not necessary for the main arguments to go through just the same, i.e. relativity and causality is also not violated without extra dimensions.)
In very high energy collisions, excitations may be shot out into the bulk space. They leave the MU, much like when rogue waves on the ocean hit each other and water splashes far above the surface of the ocean. The splashed water can travel much faster than any of the waves ever could inside the ocean. Our light velocity c, which is the maximum velocity for our universe’s “low energy” excitations in the MU, is much lower than the maximum for those particles that are not attached to the MU. That should not surprise: The maximum velocity for ocean waves is also much lower than the velocity of light c, the latter here being the maximum velocity for the splashed water that is not bound to the ocean. In fact, c is for all practical purposes infinite as far as ocean waves are concerned. In general ‘condensed state physics type’ of emergent gravity scenarios, a large difference between our c and the maximum velocity valid for the bulk space that contains our universe is expected (not in the string theory scenarios I have come across though).
The only sketch of the MU that I found today - a string theoretical example: Notice that the closed strings (gravitons) can reach outside of the membrane while the standard model forces (open strings) are attached to the membrane universe. Neutrinos have only weak interactions; whatever precedes them may come off of the membrane more easily than say muons. It would be thus expected to observe such mechanisms first with neutrinos, as may have happened now.
If this is what has now been observed, then it goes as follows: At CERN, where the high energy collision happens, MU neutral excitations without electro-magnetic or strong interactions are bound weakly enough to the MU that their energy is above the limit below which they would still “feel” the MU. They spread out into all directions, mostly away from the MU into the higher dimensional bulk space. They may travel or jump ‘above’ the MU membrane for perhaps tens of meters along the MU before they re-enter it. How much they, while doing so, on average travel along the orthogonal directions (for example the "height" they reach perpendicular to the MU while doing their ark like trajectory) depends on the specifics of why they re-enter the MU membrane, which I will discuss below.
Traveling only three to 30 meters parallel along our MU means that they re-enter our universe still inside CERN (!). It also means that about ten to 100 nanoseconds seem to be missing; this would account for the about 60 nanoseconds that the neutrinos are reportedly early.
[Warning: MATH! If they jump those first meters with a velocity of maybe ten or hundred times c, they practically spend no significant time jumping, because the rest of the travel time for the 730 km to Gran Sasso is much longer. Without jumping, the neutrinos must spend additional time t traveling along the jump path with at most the speed of light c instead. Those that do not first jump for example 20 meters would need to spend
20 m / c = 20 m /(299792458 m/s) ~ 60 nanoseconds
longer to get to Gran Sasso.]
Why do bulk excitations re-enter our universe? There are several possibilities that could be perhaps distinguished in the data, two of which are:
1) While electro-magnetic, strong, and so called weak interactions are bound to the MU, gravity is universal and reaches out into the bulk space. Gravity may pull them back into the MU (this is a common string theoretical assumption). Gravity is very weak, but since CERN may just be at the threshold energy for this effect, the 'splashed stuff' may stay very close to the MU, being pulled back in quickly on the order of tens of nano seconds.
2) The other possibility is that the bulk space is either ‘compactified’ (circular, periodic) or perhaps our universe is actually made up from multiple such membranes packed at some distance apart. These scenarios count as one here, because in these cases there are practically, as seen from the “splashed”excitations’ point of view, parallel membranes that they will collide with. They leave the MU but soon hit a parallel membrane [which is the same MU they came from in the so called compactification scenario] and get stuck again. Once they are stuck again, these excitations become usual neutrinos that travel further with a velocity just below c.
The MU scenario is just one – there is a shock wave scenario that is not so intuitive to laypeople. All such scenarios do not touch the relativity that is valid inside the MU: Relativity theory is not violated! The superluminal velocities here can also not violate causality, although relative to carefully selected reference frames inside the MU, the particles actually go somewhat into those reference systems' past (however, no excitations ever enter a past light cone!). Causality is here trivially guarantied by the mere fact of that the MU lives through the more fundamental time of the bulk space-time around it (or in other words: the past 'simply does not exist anymore' according to the more fundamental bulk time).
If you do not believe me and like to read literature about these scenarios, there are many sources. References to for example the MU and the fact that superluminal speed does not necessarily violate causality you can find in my up to date discussion of these topics “Supporting abstract relational space-time as fundamental without doctrinism against emergence”. Sorry for promoting my own work here, but if you want a non-pseudo science and up-to date discussion of above light speed phenomena in modern physics, this is the only one I know that openly discusses precisely these issues, especially in Section 4. There is even a whole sub section “4.1. Causality preserving superluminal velocity but no time-travel” where you will find references to other scientists’ peer reviewed work also.
What if the Effect is Real?
I have already clearly enough pointed out that I personally expect that the superluminal neutrinos will go away as nothing but a systematic error. However, if the superluminal neutrinos stay with us, a scenario like I discussed is most likely to account for it. If so, I will be very glad to have published already in December 2009, two years before this discovery.
If the effect stays, the following considerations will become important:
1) Given the same particles and collision energies, the average bulk space-time jump distance and therefore the time of flight difference should be independent of the overall travel distance. I.e., building an OPERA detector not at 730 km distance but say only 200 km or say as far as 2000 km away should result in the same tens of nanoseconds early arrival time.
2) If re-entry is due to gravity, the average jump distance (I will not discuss shock waves here), depends much on the collision energy. More energy means longer jump distance implies in turn an earlier arrival time.
3) If re-entry is due to a compactified bulk space instead, the jump distance depends more on the size of the involved compactified dimensions (compactification radius and so called warping) and the capture probability of the bulk-excitation collision with the MU membrane.
Interested people should model these assumptions and look for their signature in the available data.
4) If the MU scenario is valid, it is likely that the MU rest frame and the cosmic microwave background (CMB), a rather special reference system left over from the big bang, coincide. This is because the Theory of General Relativity insists on that such a unique (!) background exists in any cosmos (even in some without a big bang). Relativity theory thus not only allows us to use any reference system (Earth, space ship, …), but it moreover suggests that the CMB may be the one to pick, say in case you desire to make contact with an perhaps underlying physics from which relativity emerges, for example an MU.
In principle, one should expect some bias in the arrival time due to the direction of the CERN-to-OPERA line of sight relative to the flight direction of the earth inside the CMB. Earth is bound to the sun which in turn is bound to the Milky Way galaxy, which moves with about 600 km per second through the cosmic reference system, that is 0.2% of the speed of light. This is only a tiny fraction of the other velocities involved here, namely the speed of light and even faster than light speed, but nevertheless – such neutrino experiments would perhaps be the only ever doable ‘ether drift experiment’ (many have been suggested, I even myself suggested one that employs tunnel junctions in the linked essay, but the practicality of those working out is far less than building a better OPERA detector as far as I can see).