# Timetraveller killing himself in the past

Say timetravel was invented, with wormholes for example. And a 27 year old guy decided to go back in time to when he was 12. He would shoot and kill the 12 year old self.
Or in another case, he goes back to when he was not even born yet, and kills his mother.

What would happen to the traveller? Would he just continue on existing, and his death at younger age would just happen in "a parallel universe", or would he cease to exist?
If its the ladder, which seems odd to me, then HOW would he cease to exist? You cant just vanish, right?
Also, when you go back in time, are you not adding mass (the atoms you consist of) to the "past universe"?

fawk3s

ghwellsjr
Gold Member
I guess those who claim to believe in the possiblity of timetravel are pulling the wool over your eyes. I wouldn't pay them any more money if I were you.

PAllen
I guess those who claim to believe in the possiblity of timetravel are pulling the wool over your eyes. I wouldn't pay them any more money if I were you.

Yeah, I am perfectly willing to explore the math of the singularities and closed timelike curves that GR allows, as well tachyons that can be made mathematically consistent with SR. Yet I would happily bet a large sum that none of these exist in the real universe.

atyy
Or in another case, he goes back to when he was not even born yet, and kills his mother.

Macbeth. Or more accurately, Macduff.

PAllen
If find the following paradox much more amusing and challenging than shooting yourself/parent in the past. One reason is that things like Novikov consistency preclude many paradoxes but not the following (which Greene argues you just must accept, but I don't):

I go back to Shakespeare's time and discover he has some idea about the play Macbeth, but has writer's block and can't get it going. You give him a copy of Macbeth, he loves it and produces it (no plagiarism, since he wrote it). So who really wrote Macbeth?

None of the main consistency or 'censorship' hypotheses prevent plays, symphonies, etc. that have no causal creation. This is enough for me to conclude these ideas are simply garbage and don't occur in the real world.

Imo, the whole grandfather theory is wrong. The moment you travel back in time, the physics and matter would have all changed at that point. You are now a new living thing at that time.

it's a continuum. There's no going back. this is it. no worm hole. in a smbh all matter squashes to subatomic, then pure energy to an exit - so no traveling through time. tomorrow is a new day, forget the past. trekies.

fawk3s, for a very good popular book by a physicist that discusses these "bootstrap" and also the potent "no choice" paradoxes, see:

"Black Holes, Wormholes & Time Machines" by Jim Al-Khalili

Yeah, I am perfectly willing to explore the math of the singularities and closed timelike curves that GR allows, as well tachyons that can be made mathematically consistent with SR. Yet I would happily bet a large sum that none of these exist in the real universe.

Yeah, if backwards time travel were possible, that begs the question, "Where are the time travelers from the future?"

On the other hand, time travel to the future is easy and doesn't violate conservation of mass/energy.

You just have to travel at a velocity far greater than anything else in the Universe.

Matterwave
Gold Member
Just because GR admits solutions with closed time-like curves, doesn't mean that they exist. At the end of the day, GR is still a mathematical model, and with all mathematical models we must interpret them correctly.

Just because the math says something, doesn't mean it exists. Experiment should be the ultimate judge.

DaveC426913
Gold Member
This is entirely speculation and so does not belong her on PF.

It just occurred to me how time travel could occur without creating any grandfather paradoxes.

By traveling through time you end up outside your own light cone. That means that after the trip through time you end up back in the same time as before, but you can't have a cause-effect relationship with anything in your own past. i.e. you cannot go back an kill yourself because you will arrive too far away that you couldn't get there at any speed less than c.

It also just occurred to me that I'm not the first person to think of this.

Say timetravel was invented, with wormholes for example. And a 27 year old guy decided to go back in time to when he was 12. He would shoot and kill the 12 year old self.
Or in another case, he goes back to when he was not even born yet, and kills his mother.

What would happen to the traveller? Would he just continue on existing, and his death at younger age would just happen in "a parallel universe", or would he cease to exist?
If its the ladder, which seems odd to me, then HOW would he cease to exist? You cant just vanish, right?
Also, when you go back in time, are you not adding mass (the atoms you consist of) to the "past universe"?

fawk3s

Well, it has never happened so we just plain don't know. The adding mass to the past universe seems odd to me too, but if it were done then that would be that.

Even it it DID happen maybe there is some reason we don't know it happened, or we know it happened but we don't know how, or... who knows?

Saw
Gold Member
I go back to Shakespeare's time and discover he has some idea about the play Macbeth, but has writer's block and can't get it going. You give him a copy of Macbeth, he loves it and produces it (no plagiarism, since he wrote it). So who really wrote Macbeth?

None of the main consistency or 'censorship' hypotheses prevent plays, symphonies, etc. that have no causal creation. This is enough for me to conclude these ideas are simply garbage and don't occur in the real world.

Hmm... Imagine that the book sent from the future is... 20 tomes of a detailed universal history. Imagine that it comes in electronic format and it is uploaded in the internet in a public server, so everybody can read it. So far, so good, still history has not changed. But what a hard job would the censors have in preventing people from changing it...!

This is entirely speculation and so does not belong her on PF.

It just occurred to me how time travel could occur without creating any grandfather paradoxes.

By traveling through time you end up outside your own light cone. That means that after the trip through time you end up back in the same time as before, but you can't have a cause-effect relationship with anything in your own past. i.e. you cannot go back an kill yourself because you will arrive too far away that you couldn't get there at any speed less than c.

It also just occurred to me that I'm not the first person to think of this.

This is nothing more than the "multiverse" theory, IMO.
You go back in time, but from then on, you're in a different universe.

Still doesn't answer the question, "Where are all the time travelers from the future?"
You'd think one or two would've shown up by now, considering the fact that accelerating expansion of space implies no end to the future other than heat death.

Who exactly gets to determine what is or is not "entirely speculation" that "does not belong her (sic) on the PF"?
I don't envy that person(s), him or her. LOL

Dale
Mentor
2021 Award
Still doesn't answer the question, "Where are all the time travelers from the future?"
You'd think one or two would've shown up by now, considering the fact that accelerating expansion of space implies no end to the future other than heat death.
The types of time travel machines that are permitted by GR do not allow time travel to points before the construction of the device. So the answer to the question "Where are all the time travelers from the future?" is simply that the time travelling device has not yet been built so the time travelers cannot travel to now.

Say timetravel was invented,

Say perpetual machine was invented... but not in this universe.

And a 27 year old guy decided to go back in time to when he was 12. fawk3s

Think about what you are suggesting. If a 27 year old guy went back in time he would get younger and unlearn what he had learned in the intervening 15 years. He wouldn't be a 27 year old guy looking at a 12 year old kid. He'd BE that 12 year old kid.

What you are really suggesting is that the 27 year old guy continues moving forward in time, but the rest of the universe goes back in time 15 years.

no this is not correct you are talking in the same time frame

The distortion unit reaches its target destination by using very sensitive gravity sensors and atomic clocks. The basic unit of calculation is the second. So yes, in a sense you do “dial in” in a date and the computer system controls the distortion field. At maximum power, the unit I have is capable of traveling about 10 years an hour.

Unfortunately, time travel is not an exact science. There is inherent error and chaos in the computers ability to make accurate calculations. Based on the current technology of the clocks and sensors, distortion units are only accurate to about 60 years or so. So no, . The divergence between the worldline of origin and the target worldline would be too great. If one were to try and travel back that far, history would look nothing like what you would expect.

The types of time travel machines that are permitted by GR do not allow time travel to points before the construction of the device. So the answer to the question "Where are all the time travelers from the future?" is simply that the time travelling device has not yet been built so the time travelers cannot travel to now.

Your answer is that a 100 year old theory doesn't allow it?
Not very convincing.

DrGreg
Gold Member
Your answer is that a 100 year old theory doesn't allow it?
Not very convincing.
This is the "Special & General Relativity" forum, so any answers you get here will be in terms of those theories. If you want an answer in terms of a theory that doesn't yet exist, you'll have to travel into the future to find it.

DaveC426913
Gold Member
Your answer is that a 100 year old theory doesn't allow it?
Not very convincing.

An interesting implication. Apparently, theories go stale simply because a certain amount of time has passed, despite being tested every single day in countless labs around the world, as well as finding its way into consumer applications - devices which can only work if the theory is accurate.

I agree with DrGreg. You'll have to enlighten us about your theories from the future which are newer and less stale.

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Dale
Mentor
2021 Award
Your answer is that a 100 year old theory doesn't allow it?
Not very convincing.
Sorry, I missed this response. In addition to DrGreg and DaveC426913's comments, I would point out that the evidence supports my comment. Since, as you yourself mentioned there is a clear lack of time travelers from the future, so either it is impossible or it is possible but limited by GR-like restrictions.

DaveC426913
Gold Member
...there is a clear lack of time travelers from the future, so either it is impossible or it is possible but limited by GR-like restrictions.

Welllll, this argument is so weak no one takes it (or says it) seriously. We could use the same argument to prove it is "impossible" that there is any life in the Milky Way. And it would be an argument just as full of holes.

Dale
Mentor
2021 Award
How so?

DaveC426913
Gold Member
How so?

The X-doesn't-exist-because-if-it-did-why-don't-we-see-it argument.

Apologies. I stand corrected. It is not a weak argument; it is a fallacious argument.

The fact that we don't see time-travelers walking around (I guess with silver-jumpsuits and jet-boots?) is not evidence that time travel doesn't exist.

If the logic held, we could say aliens do not exist since otherwise we would be up to our knees in them.

Dale
Mentor
2021 Award
The fact that we don't see time-travelers walking around (I guess with silver-jumpsuits and jet-boots?) is not evidence that time travel doesn't exist.
But it is evidence that, even if time travel does exist, for some reason time travelers can't travel here. GR provides a good reason why not.

Btw, from an empirical standpoint the fact that we don't see X is evidence against X.

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DaveC426913
Gold Member
But it is evidence that, even if time travel does exist, for some reason time travelers can't travel here.
No it isn't.

My utter and continuing absence from China is not evidence that General Relativity is at work preventing me from going there. In fact, you can't prove I haven't been to China and that I'm not visiting it regularly (in disguise of course).

Dale
Mentor
2021 Award
My utter and continuing absence from China is not evidence that General Relativity is at work preventing me from going there.
Obviously not. Your presence in China is equally likely under GR as under alternative theories of gravity.

In fact, you can't prove I haven't been to China and that I'm not visiting it regularly (in disguise of course).
There is a difference between evidence and proof. For something to be evidence for a hypothesis it merely needs to be more likely under the hypothesis than under alternative hypotheses. For it to be proof it must follow with logical certainty. That is the difference between deductive and inductive reasoning.

DaveC426913
Gold Member
Obviously not. Your presence in China is equally likely under GR as under alternative theories of gravity.

No, by your logic, you would deduce that, since you've never actually seen me in china, there is something preventing me from going there. You would suggest my absence is evidence of the impossibility of intercontinental flight (or at the very least, that I am on a no-fly list).

The only thing preventing me from going to China is that I have chosen not to. This says nothing about the physics.

There is a difference between evidence and proof. For something to be evidence for a hypothesis it merely needs to be more likely under the hypothesis than under alternative hypotheses. For it to be proof it must follow with logical certainty. That is the difference between deductive and inductive reasoning.

Thing is, we are not talking about physics here, we are talking about human behaviour, policies, historical interests and slim chances of actually being ibn the place at the right time. And you're deducing the motivations of a people centuries - perhaps millennia - removed from ours.

In order for the lack of extant time travellers to be on the news, all the following would have to be true:

- it would have to be *feasible* (not requiring a toroidal BH or light years long cylinder)
and economical (not requiring a country's output of energy)
- they would have to want to pursue as a regular thing
- they would have to come to our time (as opposed to, say prehistoria)
- they would have to come to our place (as opposed to, say the ocean depths)
- they would have to have a reason to expose themselves to us in a way that we can recognize.
- etc.

The fact that we do not see time travelers is only evidence that not every one of the above conditions have been met. You cannot deduce the behaviour of our descendants, and then constrain their actions to a given course based on your deductions.