Further to my post on 31 December 2019, I have now finished *Time Travel and Warp Drives: A Scientific Guide to Shortcuts through Time and Space* by Allen Everett and Thomas Roman. I learnt a lot. Which is good.

In a Newtonian universe, there is no fundamental speed limit. (In modern terms, we could say that the speed of light was infinite, although, of course, we already knew in the C17th that it wasn't). But a ship accelerationing at 1*g *will still only have reached a speed of about 300,000 km/s after a year. So, it is still going to take you a long time to get between the stars unless you are able to tolerate very high accelerations.

Special relativity though offers us a tantalising possibility: time dilation. If we can get close to the speed of light, time slow down. So, in a 1*g* torchship, we can get to arbitrary distances in a few years or decades of ship time. This is better than a Newtonian universe, at least to those on the ship. But, frustratingly, there's a catch. Relativistic effects don't really start hitting in until we are close to the speed of light. The amount of energy required to get to 0.9c, much less 0.99c or, particularly, for a continuing accelerating ship is pretty large. Where would you get this energy from? The vacuum energy would be my guess, but until we work out how do that, relying on time dilation to get us around the universe on semi-reasonable timescales seems dubious. But surely nature wouldn't be so cruel to hold up this possibility only ti snatch it away.

But, of course, we don't just have special relativity. What of general relatvity. Here the prospect are even more tantalising. The notion of the wormhole has been around for decades. The *SFE* claims it was coined by John A. Wheeler in 1957. There's obviously a link here to the concept of hyperspace. Wheeler was drawing on work by Weyl in 1928, who called his workmholes one-dimensional tubes. The term "hyperspace" was used by John W. Campbell in *Islands of Space* (1931), but had been used earlier by Kirk Meadowcroft in "The Invisible Bubble" (1928). Campbell's story uses hyperspace as we understand it today. By 1935, John Buchan could have a character who knew "...less about women than he knew about the physics of hyperspace." Where did Buchan get the term? Of course, popular descriptions of the fourth dimension had been around for decades, but Buchan's use of "physics" rather than "mathematics" is suggestive. Did he get the world from a young relative or an article or book on higher dimensions or general relativity. Ot had he read Campbell?

We have had some notions of using black holes as ways to get around the universe, but, famously though, it is not until Carl Sagan talks to Kip S. Thorne about *Contact* in the laye 1980s that the modern notion of FTL wormholes, grounded in general relativity, really gets going. What wonders what precursors there were to Thorne's work.

An FTL drive is a time machine - or at least can be. I wish I could see that. *Time Travel and Warp Drives *is reasonably technical, but despite having a physics degree and, at one time understanding, to some degree, why time dilation had to occur, I just don't have the time, patience and desk these days to be able to work through the reasoning with pen and paper to understand fully why FTL = time travel.

The immediate problem with wormholes is that, in order to bend spacetime in the appropriate ways, they require exotic matter. Now, there are lots of different types of exotic matter, but here is meant a specific form of very exotic matter: matter with negative energy. Now, we have plenty of lumps of positive energy just lying around, but not so much with negative energy. Perhaps we could make some somehow, perhaps with the Cassimir Effect. That was the thinking back in the late 1980s/early 1990s.

Now, the Einstein field equation of general relativity also tantalisingly includes solutions that involve closed timelike curves, which are time machines. This has been known since the 1930s and Goedel in 1949 was explicitly looking for time travel in his rotating universe (hence why he kept asking Dyson if they had found it - evidence that the universe is rotating - yet). It was Tipler in the 1970s who popularised the notion that CTCs could be created by rotating very long pieces of matter at high speed. This seemed like a plausible route to a time machine.

A problem with Tipler's scheme is that it involves an infinitely long cyclinder of matter. Now if I have a cylinder centimetres or nanometres or femtometres wide and many AU or parsecs long, I might think it is near enough to be being infinitely long that I can ignore the end effects, but, sadly, no. It really does have to be infinitely long.

But we don't live in an entirely Einsteinian uinverse. At some point, quantum mechanics will rear its (complicated|ugly) head. And I will talk more about the consequences of that in part II of this article tomorrow.

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