I’ll answer in my next blog post, why time travel should not be possible even though the mathematics say it should be possible. And can we test this?

Although many physicists would love to agree with you, they can’t. Despite a tremendous amount of effort to prove the contrary, the laws of physics as they stand today do not forbid time travel. [It’s only extremely impractical — like reversing the rotation of the galaxy.]

The case that you’ve considered can be dealt with by imposing the principles classical consistency or quantum consistency to eliminate paradoxes.

Moreover, there are reasonable theories of gravity (of which Einstein’s General Relativity is a special case) that permit wormholes (i.e. time machines) in the absence of (fantastically impractical) “exotic” matter (e.g. Gauss-Bonnet and Lovelock gravity).

Source: The Physics of Stargates — Parallel Universes, Time Travel and the Enigma of Wormhole Physics by Enrico Rodrigo.

Time dilation and the “light speed limit” are both things that, if one were to anthropomorphize the Universe, suggest that it is quite a friend to the _individual_, but not to large groups / societies.

Time dilation means that, given enough energy and means to survive awesome G-forces from rapid acceleration, the crew of a spaceship can go star and even galaxy hopping, seeing so very much of the Universe, with much of the (subjective) time spent enjoying / colonizing / strip mining / studying the destinations and very little spent actually travelling.

The “light speed limit”, of course, means that those embarking on such a trip are effectively saying good bye to everything they leave behind. For some, this will be seen as a downside. I see it as an incredible positive. Small communities will be able to go in peace, untroubled by the rather horrific prospect of the spread of some ever-expanding interstellar society, a cancerous social plague of arbitrary laws and homogenity. As it is, on Earth our nation states are inefficiently and cripplingly large, and are the single greatest threat to individual freedom. In a Universe where superluminal travel was impossible, the sorts of atrocities and restrictions on individual liberties that large societies cause would, gratefully, at most be limited to individual planetary systems. This seems merciful.

All of that also only really applies if we remain human. If we’re talking about superluminal interstellar travel, then we may as well also talk about indefinite life extension and eliminating aging, and mind upload technology. Both of these have a solid theoretical foundation, which no one can say about superluminal travel; the only issues with them are practical, not theoretical. If we didn’t age, then barring accidents we could take that time dilated space voyage, come back years / centuries / millenia later and catch up with those left behind. With mind uploading and mind emulation, the whole species could travel together living in a virtual reality, all on a massive server farm. Alternatively, it’d allow us to leave a copy of ourselves behind so that no one has to miss us and we can pursue goals at home, and take copies of those we’d miss or need; when we come home, both copies of us integrate our data and personal experiences into a single person. Admittedly this might sound very “sci-fi”, but again, if we accept a position of scientific physicalism when it comes to consciousness and mind (and I see no reason not to do so), these are things that we absolutely _will_ eventually be able to do (possibly as early as 50 years from now or sooner)… and again, no one knows for sure if we will ever be able to effectively travel faster than light.

Or, bringing it back to the original topic of the post, how _any_ ways around the cosmic “light speed limit” and effective superluminal speed wouldn’t be a form of backwards-in-time time travel.

The presence of time dilation shows that Lorentz-Fitzgerald transformations are present. Therefore we are limited to subluminal velocities. Therefore, galactic and intergalactic space exploration will take millions of years.

Imagine if we could figure out how to ‘bypass’ Lorentz-Fitzgerald transformations. That is why I suggested that there is the need for new ‘tunneling’ theories.

I would say the opposite — it could the major solution to interstellar travel. A ship being piloted at near luminal speed to a star system 1,000 light years away might only require a few months time **for the crew involved*. They would hardly age at all…

For the pilots, it might seem like just a few weeks to get to Alpha Centauri in this case, even if to their colleagues back on Earth it would appear to take them 4.5 years, and another 4.5 years to return. When they return from their apparently short trip, a decade would have passed back on Earth, but to the pilots it would have been a short trip and they would have hardly aged at all. Assuming G-forces could be overcome that is…

On a more serious note — it may be technically feasible to send *information* through time — for example Hawking Theories predict particles can flow backward in space-time on the event horizon of black holes — and there are claims out there that such future-scanning technology is already being piloted — although these claims mostly amid… loopers.

Having said that there was an apparent NDA breach at BNL back in 1999/2000 on such developments which caused a bit of a stir in some internet forums — and more recently I acquired a document from LBNL apparently on same said clandestine research at BNL.

Imagine this. Someone, let’s call him Ben, invents a really important breakthrough in technology. Then others, say Albert and Hal, concur and invite Bill to fund this technology. And the world progresses, and the technology becomes a major economic driver and Ben and his colleagues make tons of money off this technology.

Then some unrelated third person, say Gary, gets very upset that Ben and his colleagues are successful, so he recruits Martin to pay for the cost of travelling through time to steal the invention from Ben. It happens. The world lines change, and Gary and Martin are now wealthy, but of course not as much as Ben was because they don’t fully understand this technology. Wrong time line.

When Ben realizes what happened, he talks to Albert and Bill about what should have happened. Bill funds a team to go back and change the past to what it should be. Back to the normal timeline.

Imagine if this went on, back and forth, for every small and big issue in the world. What do you think would happen?

But of course all names are fictional and not in any way related to any know or unknown persons, even if they are from a different time or planet.

Humans like to think of themselves as rational but they are not.

I won’t disagree with you that causality is not never violated in Nature. We haven’t observed it and at the present time we don’t know how to prove or disprove it. The tricky part is that even if one could observe causality violated, we could not be sure that this is ‘an apparent’ violation due to how we measure and observe this causality violation, or due to undiscovered properties of the Universe.

Regarding quantum gravity, Robert Nemiroff and his colleagues (http://www.space.com/17399-gamma-ray-photons-quantum-spacetime.html) looked at gamma-ray burst and found that parts or all of quantum gravity may be invalidated.

So we have to wait and see if the mathematics of quantum gravity could become the real world physics of quantum gravity. This requires more astrophysicists searching through the data to find some more occurrences of these events.

I’m less confident of string theories. Lee Smolin in his book The Trouble with Physics (http://www.thetroublewithphysics.com/), explains that string theory is dependent on particles expanding as their energy increases. But Lorentz-Fitzgerald transformations (of Special Relativity and earlier) require particles to contract.

In my book on gravity modification I have proposed an experiment that could confirm or disprove this. Again we have to wait until someone conducts this experiment.

And at the end of the day you may be right, that causality violations do occur in Nature, and would that keep us awake at night, at the thought of all the new inventions we could come up with!