Why No Time Travel in OA
Why no Time Travel
Image from Steve Bowers
Time travel into the future is not only possible but commonplace in the OA Universe (relativistic travel, nanostasis, etc.). Unfortunately, this is a one-way journey. Here is one answer why we don't have time travel into the past in the OA universe.

Why Time Travel Would Destroy the Universe

Time travel is very dangerous and uncomfortable to contemplate, and even if possible, will no doubt never be useful.

It is possible to imagine ways breaking the chain of causality using wormholes as currently described in OA. The wormhole mouths can be displaced in time as well as in space; if one wormhole mouth is carried off on a relativistic spacecraft then brought back, it becomes displaced in time by time dilation. In effect it is the equivalent of the younger twin in the famous Twin Paradox; but it is possible to travel through the mouth of the hole to connect the younger mouth with the older mouth and vice versa. This would allow a traveller to step back into the past.

Time Travel: the Options

1. The Boring Physics conjecture - all kinds of time travel are impossible, and wormhole travel of any kind is also impossible in every case. This may the case in the real universe, but in the Orion's Arm universe the existence of traversable wormholes is a basic precept, so that hypothesis cannot apply within the context of OA.

2. The Chronological Protection conjecture — wormholes are possible, but some sort of physical mechanism or set of mechanisms prevents them from becoming time machines. Matt Visser described one such mechanism whereby a flux of virtual particles would be generated every time a time machine created a time loop, known as a Closed Timelike Curve (CTC). A related possibility is the existence or formation of an event horizon between areas of space where time travel is possible (certain wormholes) and areas where it is not possible (the rest of the universe). This is the situation that exists in the Orion's Arm scenario; it may, or may not be the situation in the real world too.

3. The Self-Consistency principle - in this scenario you can go back in time, but you can't change history. This principle was developed by Igor Novikov in the 1980's (Old Earth calendar). If there is only one timeline in the universe, then going back in time forces you to act only in ways that are consistent with the known history of events; that means you cannot kill your grandfather before your father is conceived, no matter how hard you try. This limitation of action might lead to absurd developments; if it is possible to send tourists back in time to watch historical events, they might become so numerous that they make up the majority of people present at such events. Dealey Plaza might be full of time tourists in November 1963 but they cannot act to prevent the assassination of Kennedy, or act in any way inconsistent with the known historical evidence.

However if there is more than one timeline in the universe, as described in the Many Worlds Hypothesis, then time travel can be consistent — going back in time simply creates a new, consistent timeline, one of countless others created by quantum events. These timelines are each self-consistent, but as every act of time-travel changes the timeline and creates a new history, it is effectively impossible to travel to one's own past; only to the past of a different timeline, created by your own presence and potentially very different to the one you started in.

4. The Radical Rewrite conjecture - in this scenario you can change your own history, which means the whole of time is mutable and can be shaped to one's own whim. If there is only one universe, history becomes fluid. You might go back and change history, then another time traveller will change it again almost immediately, and the present will be an ever-changing kaleidoscope, with a different President or Prime Minister every day; you will go to sleep in a mansion, and wake up in a mobile home; your car will change from a Buick to a Volksvagen while you are driving it. What is worse, you will not be aware of such changes happening — you will suddenly have a whole new past and a new set of memories created by a meddling time traveller long ago. In some versions of this scenario, each time traveller which goes back in time effectively destroys his or her own future existence, and becomes orphaned in time.

Such a lack of continuity will invoke the Chronological Protection Conjecture: the universe will morph and mutate until it reaches a history in which time travel has never been invented, ever, at any time in the past or future. This is the only stable state for the universe, the one we are in now. The CPC has been proposed by many people, including Hawking, Asimov and Larry Niven.

Chronological Collapse

If the universe does allow reverse time travel, usable by sentient/sophont entities, it won't stop at one or two little historical research trips . . .

If there is no effective chronological protection mechanism, the universe of today will be overrun with travellers from the future. Even if there is no 'Big Rip' where the Universe tears itself apart through accelerating expansion, hundreds of trillions of years from now the cosmos will be a slowly dying place. Even red dwarf stars will eventually burn out, leaving the inhabitants of the far future only their dying embers to gather energy from, although the creation and merger of black holes could perhaps keep civilisation going for an (admittedly very long) while. Eventually the entities of the far future will be limited to reversible computation to save energy. This means confining themselves to a very limited set of mental processes. This prospect would surely not appeal to the heirs of once-mighty advanced civilisations. If time travel were possible then refugees from the far future would flood back, sometimes in multiple instances.

The future sophonts would come back in an exponentiating wave to constantly change the present and the past, and whole galaxies of material particles will begin to exist in space time reference that did not have them before - some? many? most? matter and events may turn out to be acausal, going round and round in closed timelike loops and increasing the total mass of the universe, which may begin to collapse in the distant future, sending chronistic refugees in massive tardises back to our time thus accelerating the collapse; increasing the mass of the present day universe until it collapses. The collapse will get closer to the present day, until it eventually happened yesterday and we will cease to exist . . . believe me, you don't want to go there.

Text content by Steve Bowers
Page uploaded 4 December 2003 last updated 11 November 2008