Why there is no causality-violating Time Travel
into the past in the OA Universe
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Why there is no causality-violating Time Travelinto the past in the OA Universe |
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.
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 of 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. More details here.
Time Travel – the Options
Once the possibility of time travel into the past is realised, then
this implies that causality can be reversed; events in the universe can
be caused by events in the future. Four hypotheses have been put
forward to describe ways that wormhole travel and the possibility of
reverse causality might be consistent with the observed universe;
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 Close 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 . . . it convinces me.
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 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.
