I've been asked to summarise what MiHsC has to say about the potential for faster than light travel which I discussed towards the end of my recent radio appearance on The Space Show (link). This is of course of great interest scientifically, philosophically and practically for interstellar travel. It is also at the heart of MiHsC, which relies on local dynamics being affected by the far off Hubble edge. I cannot summarise it all in one blog, and the subject is even more on the edge than normal for me, so the tone of this blog will be more like a varied update on work in progress than the surer, didactic blogs I've tended to write recently.

It is well known that special relativity predicts that as an object or spacecraft approaches the speed of light, its inertial mass, as seen by a stationary observer, increases to infinity, forbidding further acceleration and enforcing a constant maximum speed somewhere below light speed (c) dependent on the power of the spacecraft's engine. Now (re: qraals comment below) I realise that faster than light travel is possible for those on the ship due to time dilation, but here I'm looking to resolve larger issues. MiHsC has a tiny correction to make to the usual picture: it says that a constant speed (zero acceleration) cannot be allowed because Unruh waves would then exceed the Hubble scale. So, even as the mass approaches infinity in relativity, and the acceleration reduces towards zero, the inertial mass will start to dissipate due to MiHsC. A tiny relativity-proof acceleration remains: 2c^2/HubbleScale = 6.9x10^-10 m/s^2.

Is there any observational evidence for this prediction? Yes: 2c^2/HubbleScale is close to the cosmic acceleration that was recently found using distant supernovae. Also see galactic jets. This also solves a paradox in that stars just inside the Hubble horizon are moving at just less than c, just beyond it they are moving just above c (relative to us) and so cannot be seen (this is one reason why the night sky is black). In relativity this transition should require infinite energy. Some say this is because space itself is expanding, but I dislike the use of undetectable entities like space in this way. In MiHsC this can be explained naturally by the minimum acceleration.

The huge import of this aspect of MiHsC (a=2c^2/HubbleScale) is that the relativity-proof acceleration is inversely proportional to the size of the Hubble horizon. If we could make a small shell-horizon we might be able to boost this acceleration. I mentioned this in a paper in 2008 and talked about all this at the 100 Year Starship symposium in 2010. I am returning to this subject now, because you can derive the emdrive results from MiHsC by assuming that the emdrive cavity is making such a shell, asymmetrically (see this blog entry) and NASA did detect a change in the speed of light inside the cavity which immediately interested me, but they did not pursue...

One huge problem with all this, also hard to think about, is how might faster than light propagation be reconciled with causality? This is the heart of the problem and so is a good place to start. The diagram below shows space along the x axis, and time (ct) along the y axis for two stationary observers (S1 and S2, light blue lines). It also shows the new skewed spacetime axes (dashed lines) seen by two moving observers (M1 and M2, dark blue lines). With this diagram we can look at the implications of allowing faster than light travel.

One huge problem with all this, also hard to think about, is how might faster than light propagation be reconciled with causality? This is the heart of the problem and so is a good place to start. The diagram below shows space along the x axis, and time (ct) along the y axis for two stationary observers (S1 and S2, light blue lines). It also shows the new skewed spacetime axes (dashed lines) seen by two moving observers (M1 and M2, dark blue lines). With this diagram we can look at the implications of allowing faster than light travel.

Imagine S2 at spacetime point P2 tells a passing M2 that he's just hit his own thumb with a hammer and we allow M2 to send this information to co-moving M1 instantaneously (along their moving x axis: see upper red arrow). M1 just happens to be passing the stationary S1 who he tells about the accident. S1 can now instantaneously tell S2 at the earlier time of P1 about the hammer and S2 can hire handyman Harrison Ford to do it and this changes the future. This paradox arises for instantaneous communication, and also for any communication faster than light speed, but there is something missing from this picture that has forced people like Novikov and Hawking to set up 'Chronology Protection Conjectures' that demand that this sort of thing can't happen. This seems to me a cheat. It should come out of the physics. MiHsC might model this more naturally by bringing in information. Sending information from P2 to P1 destroys a future and there is an energy cost to that, that would preclude this in most macroscopic cases, but maybe not for special quantum cases.

For example, the Einstein-Podolsky-Rosen paper and Bell's tests have shown that spooky action at a distance or future-past interaction probably does occur for quantum systems and I have written a paper (not accepted yet) showing how this can be allowed for quanta since there are only tiny exchanges of information between future & past. I think that time is porous and allows information through in small doses.

Coming back to more practical matters. How might you design a MiHsC-Shell? It would be an array of metamaterials (metal structures) surrounding a spaceship which damp the em-component of Unruh waves likely to be seen by it, at whatever acceleration it is undergoing. If the ship accelerated at 9.8 m/s^2 for example, the Shell would need to deselect em waves of length 7x10^16 m (forgetting relativistic effects for now). If you could do that then the spaceship would have less inertial mass and would be easier to accelerate, and then you'd need to change your tuning to accommodate that change in acceleration. It could also be done by damping more Unruh waves at the front of the ship than the back, for example.

MiHsC offers a tiny chink of light to those wanting interstellar travel, in helping getting to speeds close to c by inertial control (Tau Ceti in a human lifetime), and maybe more, but thinking about space and time is difficult because they are so fundamental. It is rather like trying to rebuild the floor of a tree house while you're standing on it! It's best done with experiments to light the way and for now, I would like to see the great NASA Eagleworks try some more of those emdrive interferometer experiments and publish the results.

References

Bennet, G.L., H.B. Knowles, 1993. Boundary conditions on faster than light transportation systems.

McCulloch, M., 2008. Can the flyby anomalies be explained by a modification of inertia? JBIS, 61, 373-378.