Time travel doesn't exist because time travel wiped out the timelines where it did

[Bohandas]

Hr. I'm not sure that logic works as well as the article suggests.

It says, "But maybe the biggest paradox of all is also the simplest one: If time travel were possible, wouldn’t we encounter these temporal tourists all the time?"

But, no! That's not a paradox! Paradoxes are things that are logically impossible, not just seemingly improbable or vexing.

Remember the famous Grandfather Paradox - if you travel back in time, and kill your grandfather before your father is conceived, you aren't ever born, and so cannot travel in time to kill him. A time traveler eliminating time travel is logically the same! If time travel removes time travel, then the time travel doesn't exist to eliminate itself!

If you are in a universe that does not allow paradoxes, then either time travel is just not possible, or it is otherwise impossible for you to enter the Grandfather Paradox. If you are in a universe that does allow paradoxes, then you travel in time, eliminate time travel... but you're still there, with your time machine, and haven't eliminated time travel!

Agreed. This is an issue I've put prior thought into. My conclusion was that something like this might happen but that it would take the form of instability in the wormholes used to link time periods rather than the tech not being invented in the first place; it mightn't get off the ground or be usable for anything practical but it could exist; the different time periods would need to interact in the first place before they could destructively interact.

If a hard counter for the instability could be developed, (possibly something based on roman rings?) then interesting things would happen. If we assume self-consistency then events would conspire to prevent people from changing things: People with access to the technology would become lucky - things that they would have both the will and the means to change wouldn't happen in the first place, because that wou,d cause a contradiction. Events (in this case mostly quantum events within the particles of their own brain) would also conspire to make these people apathetic and possibly brain damaged - eroding the will and means to do something that would cause a contradiction. Finally, if they did manage to go back and try to change something they would find themselves stymied by an irresistable run of bad luck. (but if they wanted the good luck they would have to fight apathy and try to change the things that do go wrong even knowing they're going to fail because of bad luck, because the good luck consists essentially of the times where they would have tried this and succeeded.

Also, a form of cleromancy might become possible. For computer operations where it's very obvious whether a result is right or wrong (such as cracking a password for instance) you could use a non-deterministic quantum process as your input and set things so that the system will go reach back in time and interfere with itself if the result is incorrect. This would essentially constrain the non-deterministic process to only occur in the way that corrsesponds to the right answer, becajse any other outcome woukd cause a paradox

EDIT:
Also, with most time travel proposals that are at least semi-rooted in real physics I'm not sure that you could go back prior to the invention of time travel anyway, rendering the question of time traveling tourists moot. I'm pretty sure a pair of temporarily displaced wormholes couldn't take you any earlier than the time that the wormhole pair was created. And as for the alcubierre drive, my understanding is that it doesn't technically move per se, so it's unclear whether the temporal effects of moving faster than light would apply to it; my understanding is that it's more along the lines of how the distance to some very distant galaxies is increasing at a rate greater than 300000000 meters per second due to the expansion of the universe

And then there's Fry's version pf the Grandfather Paradox.

To quote him: "I did do the nasty in the pasty."

to quote futurama again, my (admittedly limited( understanding of the alcubierre metric is that it worls exactly the same as the Planet Express ship: "The engines don't move the ship at all. The ship stays where it is, and the engines move the universe around it"

 

[Bohandas]

Hardly. At the moment, AI doesn't even understand consequences, much less have moral compunction against them.

Is anyone else a little surprised that AI isn't being used more often to replace humans in leadership roles? It doesn't understand consequences, it has no moral compunctions, if you ask it the same thing twice it'll give you three different answers, it can talk persuasively about things it has no understanding of, and it doesn't have a soul.

Those are all the qualities of a successful businessman or politician.

 

[briggart]

lol.

But, what I meant was the answer doesn’t explain very much. I find that things don’t expand to be fine. Just saying that bound systems don’t expand, except to say that that would cause problems, and is not observed, doesn’t provide much, if any, understanding.

If space expands, and light expands as a result, leading to red shift, why wouldn’t a spring experience a similar expansion over time? Even if the spring contracted, there would be a gradual addition of energy. I’ve seen no statement that this occurs, so it must not be happening. Besides, conservation of energy would be broken.

Thanks!
TomB

A gut level proof that bound structures do not continuously expand due to cosmological expansion is that we exist. As we look further back in time, we see the Universe becoming more and more homogeneous. Since expansion should smooth inhomogeneities, the fact that we see them growing with time means that locally the gravitational attraction of matter must be able to overcome the expansion.

Jeans instability provides the basic framework to describe this. You can show that even in an expanding universe, overdensities can actually collapse and form bound structures decoupled from the global expansion, in the sense that overdensities will keep growing forever, although at a slower rate than what would happen in a static background.

This is clearly a simplified model, astrophysical structures like galaxies or cluster of galaxies are not well defined objects with sharp boundaries but they smoothly merge with the surrounding environment and, on the largest scale, the overall expansion. So it is not an "all or nothing" effect, but you smoothly go from the global dynamic to the local motions.

The picture becomes even more complicated when you throw in phantom energy, a type of dark energy with very strong negative pressure. In that case, phantom energy will rip apart bound structures because in an expanding universe the energy density of phantom energy grows with time, exerting more and more pressure on everything else (Phantom Energy and Cosmic Doomsday).