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Paradox-free time travel is theoretically possible

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Re: Paradox-free time travel is theoretically possible

#111

Earlier quoted context omitted.

Would you mind expanding on that?

If you send an apple back in time the conservation of energy is broken. The energy sum of everything increases in the past.

1. Not necessarily: nobody said the time-travel couldn't pump energy at your destination to compensate for the energy the future is sending.

2. What does it have to do with the third law of thermodynamics (which is about the impossibility to reach a temperature of 0K)

Re: Paradox-free time travel is theoretically possible

#112

I watch a lot of both American-made movies and TV shows as well as a lot of Japanese anime. One thing I find curious is how different the models for time travel usually are. In American media, it seems more often than not, time travel is modelled as if the past and the future are simultaneously existing parallel worlds, where the future is affected by the past in "real-time", e.g. Back to the Future's Marty gradually…

I think it depends on the show in anime. SPOILERS FOLLOW. Erased is not a parallel world, but someone trying to change their past. Madoka finds Homura repeatedly doing whatever is possible to save Madoka. Steins:Gate is timelines and parallel worlds (think of how many times Okabe again and again tries to save Mayuri). Girl who Leapt Through Time is the Groundhog Day story, same day over and again; bit like Tatami Galaxy. No idea where something like Haruhi Suzumiya might fall where time is played with in many different ways.

Meanwhile Evangelion (the later films) and Attack on Titan both seem to be veering towards some kind of inability to stop the same grand narrative cycle repeating again and again...

Re: Paradox-free time travel is theoretically possible

#113

I've never seen the past or future. Only physical things still or moving, combining with other things or not ... What we call 'the past' is a mental picture of where physical things used to be positioned and their condition. But everything in 'that now' has changed. Just as when we 'travel' in physical space, when we get 'back home' it has changed. If I were to 'time travel' I'd be gone from now. But everyone else wo…

> So where was the 'instant of time' I travelled to stored so I could go there? Not where, when. [1] The block universe ontology is pretty commonly offered as an interpretation of general relativity. People consider GR as pretty strong evidence for such an ontology. Personally I don't but I'm just saying your criticism isn't very good. 1. https://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time...

That article doesn't mention general theory or relativity, quoted discussions indicate that eternalism mostly relies on geometric expression of (plain) Minkowsky spacetime, which is interpreted literally as indication of existence of global time dimension. General theory or relativity is bad news for eternalism, because curved space is more obviously local and thus less compatible with eternalism which favors globality.

Re: Paradox-free time travel is theoretically possible

#114
I’m not an expert, but why would the paradox they are talking about be a problem for the concept of time travel?

To me, a believer in the multiverse, it seems that the obvious answer is that in our particular instance of the universe no one in our future has figured out time travel, or have not gone back in such a way to let us know about it. For whatever reason.

We pay too much attention to what we perceive to be the “arrow of time”, the direction. Forward, backward? What’s the difference really? Is it our illusion of “free will”, that somehow there is more to the atoms of our body than can be explained by the same laws of nature that governs the rest of the universe, that simply won’t accept that we might just be like billiard balls moving along predetermined paths (admittedly unbelievably complex ones) and if we accept that, then the direction of time doesn’t really matter at all.

Re: Paradox-free time travel is theoretically possible

#115

Time travel violates the law of conservation of energy. By travelling back in time, you yourself are injecting more mass/energy into a system. So it's great as a science fiction trope, but if you're going to talk physics, that needs to be addressed.

Just consider the entire thing (spacetime) as one system.

Re: Paradox-free time travel is theoretically possible

#116
> In other words, a time traveler could make changes, but the original outcome would still find a way to happen — maybe not the same way it happened in the first timeline but close enough so that the time traveler would still exist and would still be motivated to go back in time.

> "No matter what you did, the salient events would just recalibrate around you," Tobar said.

I'd love to see how this works when you go back in time and kill your toddler self.

> "The universe is deterministic and attempts to alter Past Event X are destined to be the forces which bring Past Event X into being,"

But WHICH events? If things change around you when you change the past, then by definition some events will have happened, and others not (otherwise NOTHING would change, including your arrival in the past, and therefore time travel cannot occur). After all, a single breath you took at a specific point in time is an event. Waking up 2 seconds later is an event. Using your left foot first as you walked out of the house that day instead of the right is an event. So who decides which events are "important" enough that "forces bring Past Event X into being"? You'd need some kind of omniscient, omnipotent intelligence in order to accomplish that.

The very logic behind this idea is a contradiction.

Re: Paradox-free time travel is theoretically possible

#117
post #74
post #57

Earlier quoted context omitted.

I could see how a universe with such rules could exist. But is it our universe? In the universe you describe, wouldn't we already have seen time travellers from the future? The only explanation I can think of for why we haven't seen any, if we live in such a universe, is that we need to build an "arrivals hall" first, and that as soon we manage to do that, people from all over start popping in for a visit.

I can’t see how a universe like that could exist. Perhaps if some time-traveler was unlucky, their gun might jam, but I can’t figure out how you could successfully thwart a “well-prepared time traveler” who knows all the details of the situation they’re jumping into and has double-checked their gun after making the jump.

The logical extension of this would be, given how local interactions might affect events across the globe over time, that the entire past becomes immutable. That would reduce time travel to a read-only experience, so it wouldn't have any utility beyond time tourism.

Re: Paradox-free time travel is theoretically possible

#119

Earlier quoted context omitted.

If you send an apple back in time the conservation of energy is broken. The energy sum of everything increases in the past.

1. Not necessarily: nobody said the time-travel couldn't pump energy at your destination to compensate for the energy the future is sending. 2. What does it have to do with the third law of thermodynamics (which is about the impossibility to reach a temperature of 0K)

> pump energy at your destination

What does that even mean? This is the kind of hand-waving the OP was pointing out.

He probably meant the first law.

Re: Paradox-free time travel is theoretically possible

#120

Earlier quoted context omitted.

If you send an apple back in time the conservation of energy is broken. The energy sum of everything increases in the past.

1. Not necessarily: nobody said the time-travel couldn't pump energy at your destination to compensate for the energy the future is sending. 2. What does it have to do with the third law of thermodynamics (which is about the impossibility to reach a temperature of 0K)

I think I messed up the number.
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