Little thought experiment. #GeDANKen

A man wishes to travel through time to save the love of his life.

Only, he can't.

While he can't revisit the past, he discovers a way he might be able to change it. How could this work if time travel is impossible? He has access to great amounts of energy and SciFy gizmos.

Time travel is impossible. He does not want to recreate the past, but rather, replace it. He wants to erase the loss, perhaps even such that it never happened, even from the viewpoint of a higher dimensional observer.

Parallel worlds are considered coping strategies, he thinks he can do better than just seek out a universe where he's died and she remains.

Maybe the impossibility of total reversal caused the universe where she died and he had the ability to truly erase it to never exist in the first place? Thus preventing her loss...


closed as unclear what you're asking by Mołot, Separatrix, Frostfyre, L.Dutch, dot_Sp0T Jun 17 '17 at 13:19

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    $\begingroup$ I will do this in the form of a comment since I really think you should do this: Please specify what he wants to do, this is rather vague as it stands. Does he want his love back? He can't simply change one aspect of the past, that would create a huge paradox. How did he lose her? I think more information is needed. My list is far from complete, this needs a lot of work I feel. I also added new options $\endgroup$ – Raditz_35 Jun 17 '17 at 9:15
  • $\begingroup$ I believe this would require predestination, the universe would have to have never truly decided on the matter, such that neither loss nor survival could have truly existed. Something mind boggling like that, likely causing an infinite loop $\endgroup$ – user39453 Jun 17 '17 at 9:31
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    $\begingroup$ Time travel or its alternatives demands clarity of thought. Your conceptualization is not coherent enough. Travel to a parallel where his love is alive is an easy alternative -- except he also might be alive there. Taking grievance counselling is possible. Not as interesting, but realistic. The universe is big enough to absorb petty tinkering like time travel. Be concise about your objective & look at what is needed to achieve it. $\endgroup$ – a4android Jun 17 '17 at 11:51
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    $\begingroup$ You added the part where you disallow bringing a copy of the wife from a parallel world after an answer suggested that solution. It is generally frowned upon to edit the question in ways which would disqualify already existing answers. Best way to ask is if you alteady ahead know the limits you want; it can be tricky to think of all aspects before asking, but there is a sandbox for questions where you can get help before asking the question "for real". $\endgroup$ – Mrkvička Jun 17 '17 at 12:27

Here's a fun idea. Not exactly the answer the OP asked for; but serviceable for the character described.

Premise: Time travel IS impossible, he cannot go back, and he cannot do anything in the past that affects the future. Period.

That said, what he learns while researching time travel is a way to recover lost information from the past, in the present. We do this all the time, in the form of archeology and the study of fossils; we learn of things that actually happened, perhaps five or ten thousand years ago.

This is analogous but a million times more precise: Our hero can actually read, down to the atom, the structure of any object in the past. He can't read the quantum states of all that, but which atom is where: He can read that. He can't change it, but he can make a copy (think of a Star Trek transporter).

That applies to the brain and body of his great love. So, using a great amount of energy, he picks a moment in time when she was last healthy, and makes a copy of her at that moment materialize in his lab. A little electrical shock to reboot her quantum states, and bingo: There she is, in the flesh, with all of her memories and personality, waking up in what seems to her a hospital. She has questions, like "WTF happened to me", etc.

In the past the original continues to her death, as must be the case. But he has his love back, unchanged from the last moment he saw her.

That said, HE is not unchanged, he is (say) 30 years older, an obsessive compulsive reclusive billionaire, emotionally devastated and withdrawn after suffering through 30 years of her loss. (Heck, let's say he caused her death through some careless act, and rightly blames himself.)

He is nothing like the man she loved. Further, let's talk about everything else she loved: Both her parents are dead; her sister died of cancer. All of her friends have aged 30 years, from her point of view she time travels thirty years into the future with no way back. Everything she knew, her routines and friends and acquaintances, her work, her politics, her understanding of the world, even the man she loved, is all irretrievably amputated.

In a way, her life was still lost: She had a network and it is gone, dead and broken and moved on from her for 30 years; the people alive in it can hardly remember her. Say they were in their mid twenties and met in college: She was in love with her college sweetheart (our scientist) a young and newly minted PhD that loved his life, was excited by his quantum research, full of ideas and vitality!

She was looking forward to their wedding and their life together, the children they would have and raise, the careers they would work through together. Not some geezer older than her father at the far end of his career, somehow mysteriously rich, a recluse in some futuristic compound professing his love for her.

She is a smart girl; she understands everything that happened. but what does she do now? How does she acclimate? Eventually, after the stages of grief, she will break down emotionally and cries for all she has lost, accepting the impossibility of changing it.

How does she move on? Does she fall in love with a different young scientist?

Or here is a fun twist: She convinces our hero she does not return his love, she doesn't know him. She loved the man of 30 years ago; but clearly (to her) that man died when she died.

Our scientist realizes she is right, and what he must now do: As a selfless act, he runs his machine one more time; to retrieve somebody else from the past: Himself, from 30 years ago. And when his younger self wakes up, he tells him: "We need to talk."

  • $\begingroup$ Fantastic answer! Noticed the OP choked down on the question after this. Really, the only solution s/he is pushing towards is a universe where both characters do not exist, so no problem exists. $\endgroup$ – rebusB Sep 29 '17 at 16:07

First thing is Special Relativity suggests that the Universe is a 4-dimensional solid with each point being the four coordinates of position and time - the past has no more reality than the future, but also no less; all of time exists at once. In such a Universe, we call each point an "event". The change of any one event, it is known, can only be described statistically. The probability of something happening in a certain way is not 1 or 0. Quantum mechanics allows for a range of outcomes, up to "1 or 0" or down to "random".

Let's say he has a remote space-time manipulator device capable of using tachyons to change the QM probability of an event actually happening; that is to change the fields at event x in a desired direction. There are a couple of things that have to be considered: first is focus. That is, if I focus the tachyons to hit event x, how accurate can I be, and is there any 'spill-over' to near-by points?

Next is energy, how much energy does it take to change an event? Next is propagation. Does a change in a past event propagate instantaneously thru the time-line? It seems obvious to me that the fastest it can propagate is the speed of light. It also seems obvious that any change must propagate at least faster than 1 second per second since otherwise it will never catch up to the present. What is more or less arbitrary is whether this propagation occurs at a constant rate, or whether it depends on space-time distance from "now" or perhaps even the energy used (or the focus size).

You could have any sort of law, afaik. Energy decreases with time, increases, stays constant, varies higher and lower. Same with rate of propagation. If I make two changes to the past, one on 1/1/2016 and another on 1/1/2000 then the one in 2000 has to propagate thru 1/1/16 for the 1/1/16 change to "work" (or to stick). Any change in 2000 might change the fields in the light-come, and void any changes after 2000. Seems to me that it gets more difficult to change the deep past than the near past, so that energy should increase with distance from 'now'.

The other problem is if the propagation speed is not infinite, then you have to wait for any change to move up the time-line to your present before you can (meaningfully) change something else.

Given that it would be impossible to predict all of the consequences of any change (harder as the change gets older), then changing a moment ago will be much easier than changing a century ago. But smaller changes a century ago will have much larger effects than a similar change of a moment ago. So, which do I do: send a couple of cosmic rays down to flip a traffic light right before her car crash or add an atomic flaw in a processor in the integrated circuit in the traffic light 10 years ago, which causes it to be out-of-service on the date the crash happened? Or what do I do if she died due to a high genetic risk of cancer? Do I mutate her DNA (and then she won't be the same person) or do I figure out how to get her cured? Can I do the latter? How can I be certain that what I do will be effective? Do I care that to save her, I must kill another (or ten or thousand others)? How far back can I work? Am I the only one with the gizmo? Or are there millions of people changing the past?

If I didn't already mention it, there's probably two ways to change the past:

  1. targeted. This atom absorbed this photon and went into this higher energy state which lead to ....


  1. random→ I changed a probability of something in 1099 and the Allies lost WW2.

Who knew? Well, maybe I figured it out. Maybe I know that to change events on a personal but not global level, I have to restrict my tinkering to below certain energies and no farther back than 10 or 20 years, maybe.

Decades ago (I forget when) IBM wrote its initials with atoms on a surface. The easiest thing to affect in the past is information. Maybe you could change a couple of atoms and write yourself a message. Of course, if you had no reason to be looking for such a message, then it's unlikely that you'd notice it.

The one thing about time-travel that really bugs me (Doctor Who knows better) is that we know we are traveling in both space and time. Opening a portal to yesterday will result in you being thousands of miles out in space. Any device which works on the past, must be able to work at variable distances as we spin and orbit and travel thru our Universe.

Hope this helps.


The multiverse theory can be usefull in that case. Let me explain, if this theory is right, an infinity of alternative univers could existe. He just ne to find the right one.

Here the right universe is a one were the love of his life is not dead, and they have never meet before/the man never existed in that universe. In another univers where the man kill his copy in that universe and just live like nothing happen.

I suggest you to watch the Rick and Morty ep 6 saison 1, this episode talk about that.

  • $\begingroup$ Oh 2 people one thought. Would you like me to delete my part of the answer saying the same thing since I have more stuff? $\endgroup$ – Raditz_35 Jun 17 '17 at 9:01
  • $\begingroup$ He has the specific desire to alter the past, not cope by finding a parallel world. $\endgroup$ – user39453 Jun 17 '17 at 9:02
  • $\begingroup$ @user39453 Living in an universe where x hasn't happened is completely equivalent to altering the past actually $\endgroup$ – Raditz_35 Jun 17 '17 at 9:05
  • $\begingroup$ I don't think that is true. An instance of his love died and he isn't content to just find another copy of her. Seems pretty human to me. He wants to get as close as he can to reviving her. $\endgroup$ – user39453 Jun 17 '17 at 9:13

You asked for science-based and reality-check, so You can't really travel through time and parallel universes are unlikely at best.

If You want your character to get his wife back IMHO You have two options:

  1. Rebuild a clone, if done down to molecular level (i.e.: brain structures replicated) you'll get even memories back.
  2. Postulate we are in a simulation and convince the simulators to load a previous "save game".

My answer does not really bring back the love of his live (except for the last one) since you did not specify what you meant by "replacing the past".

The past is what you tell people what the past is. Even today, free democracies are mostly writing history as they please. Rewritting the past is easy. He could simply tell everyone that something happened differently and just like those entire countries, at one point he believes it himself.

A connection to the past is made via the impact of a certain event. If he tries to to change the things that are specifically impacted - for example his loved one would've wanted a house with a graden so he buys that house.

If we go sci-fi again, he could alter the memory of everyone on earth even, undoing the effects with arbitrary sci-fi equipment is easy. The only one who will remember is he until he also forgets as those countries did.

Another options are particles that move faster than light, called tachyons. You said time travel is impossible, but those particles just go backwards in time, I think this is a major difference. If the woman had a tumor for example, he could treat it with tachyon radiation in the future.

Check out the film "Vertigo". A woman is murdered and the protagonist replaces her with another woman (ignoring the actual story of the movie).

One might even do the total recall. He plugs himself to a virtual reality machine/drug/whatever and thinks he remembers things that did not happen. One could also alter his memories in a similar way.

There of course is the parallel universe thing, let's call it the Rick and Morty approach. He travels to an universe where she still exists and brings her back or lives there. This is pretty sci fi.

  • $\begingroup$ I don't think these suggestions solve the problem. One core question is whether a static high dimensional multiverse existence could plausibly be replaced by something within it given what we know. $\endgroup$ – user39453 Jun 17 '17 at 9:06
  • $\begingroup$ The consequences of this are to the effect that the universe never decided between the two cases in the first place. $\endgroup$ – user39453 Jun 17 '17 at 9:08
  • $\begingroup$ @user39453 It might be nice if you wrote in your question what the problem was that needs solving then. Maybe you think it is implied, but it is not written $\endgroup$ – Raditz_35 Jun 17 '17 at 9:08
  • $\begingroup$ So in reality, neither exist. $\endgroup$ – user39453 Jun 17 '17 at 9:08
  • $\begingroup$ Sorry, this is just my theory, I didn't want to load the question $\endgroup$ – user39453 Jun 17 '17 at 9:10

/he wants to erase the loss/

The loss is not a thing like a brick. It is his subjective experience of the event and how he feels about it. He can change that. He can alter his perception of the past.

He could do it by science fiction methods, like transcranial magnetic stimulation, radiosurgery or even surgery, obliterating the area in the brain where those memories are kept. Or altering the part of his brain where his feelings are kept. People with right brain damage can become cold and unfeeling. One such would remember she died but not feel any particular way about it.

Or he can erase it incompletely, with time. The sharp edges wear away little by little as life washes over them. If this is for a story, after he tries all the other methods, he should wind up realizing he is doing this method. The way people do.


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