A mathematical model for time travel

Welcome for time travellers

On May 2 2017, Newsweek published an article with this title:

Time travel is mathematically possible with mind-boggling model

You may well imagine that, with that title, the article will rather fall into the category of sensationalist papers on seemingly scientific issues. Indeed, in a quick reading of this article I have detected the following inaccuracies:

1. The title does not make clear the difference between a theoretical possibility of traveling in time and building a time machine. That is, the different between theory and practice. What Ben Tippett has developed is a purely theoretical mathematical model.
2. It presents the idea as something new which puts an end to a string of failures and disappointing calculations. Space-time loops, however, are known to be compatible with the general theory of relativity since quite a long time ago. In 1992, for instance, Stephen Hawking came to the conclusion that it would not be possible to use them without negative energy, something that is not known to exist. In 2005, the Israeli Amos Ori proposed a procedure that would not require it, consisting of spinning around an empty toroid region surrounded by a sphere containing enormous amounts of matter (e.g. a black hole). This is not so different from what is being proposed now.
3. Quoting Tippett, the article reads as follows: People think of time travel as something fictional. And we tend to think it’s not possible because we don’t actually do it. But, mathematically, it is possible. This statement is clearly false. If it were true, people would think that interstellar travel or strong artificial intelligence is not possible because we are not currently capable of doing them. The truth is that time travel is excluded, not because just now we don’t know how to do it, but by profound scientific-philosophical reasons. One of the most important arguments is the modified Fermi paradox, which I explained in another post on this blog.
4. The article says this: Traditionally, we think of the universe as being made up of three spatial dimensions, and a fourth dimension representing time. But mathematician Ben Tippett at the University of British Columbia, Canada, says this is wrong. He believes time should not be separated from other three spatial dimensions—instead all four run together, simultaneously. It gives the impression that Tippett has been the first to realize this, when in fact the idea goes back to Minkowsky and Einstein, over one century ago.
5. As I mentioned somewhere else, there are two philosophical theories about time, theory A and theory B. If theory A (the closest to our intuition) is true, then time travel would be impossible by definition. Of course, this is not mentioned in the article, which implicitly assumes that theory B is correct.
6. Finally, the author of these calculations (misnamed design of a time machine) has serious doubts that someone, sometime, will be able to build a machine so that it works in reality. He is quoted thus: Unfortunately, I don’t foresee this as being feasible. And the paper adds: [it would] have to be supported by exotic forms of matter (matter that has never been found in Nature!) So why the sensationalist headline?

 The same post in Spanish
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Manuel Alfonseca