Many philosophers argue that time travel is possible without causally affecting the past?

Is it even possible to time travel without causally affecting the past.

The grandfather paradox is an extreme case which limits a time traveller from killing their grandfather to ensure their own existence. However, the mere existence of a time traveller and very minimal interference with the grandfather (or anyone for that matter) would cause another version of the past; thereby violated the consistency principle? Is it possible without causally affecting the past at all since even the existence of a time traveller adds additional matter to our universe compared to the past without the time traveller?

  • Not if the past has always been that way - there is no reason to assume travelling back in time changes anything. I cannot relate this to any philosophical writings, however. – Joachim Dec 7 '18 at 16:49
  • Also a possible duplicate of this. Does that thread answer your question? – Joachim Dec 7 '18 at 16:52
  • Thanks. How has the past always been that way. Let's say John was born in 1950. It's now 2018. Currently, the past does not have a history which includes John. If now, he finds a time machine, and goes back to 1950. History now includes John. Hence, his existence has causally changed past. – PhysicsEnthu Dec 7 '18 at 16:54
  • The earlier thread isn't a direct answer to the question asked, thank you nevertheless. – PhysicsEnthu Dec 7 '18 at 16:56
  • How many philosophers have argued that time travel is possible? That looks to me more of a physics question, and I have seen arguments for stable closed timelike curves (time travel without changing the past) in physics contexts. – David Thornley Dec 7 '18 at 18:21

In order to consider the issues of time travel, it is important do make the distinction between the phenomena (what we perceive) and the noumena (what actually happens in nature, but we don't have access to). In general, time travel implies several logical flaws, which few people seem to take care of, and most conclusions are essentially affected by such flaws.

Things, interactions and causality are facts at a phenomenal level. We perceive them, but that doesn't mean that they are physical facts. Causality is the mental-rational relationship between two events, created only by our understanding. Causality and time generate a chain of events in our understanding, and we create relations between them. For example, a queue of dominoes that fall down in order. If the cause is the Domino-1 fall down, the consequence is the domino-2 fall down, and so on.

When we speak of time travel, at the phenomena level, we are idealizing a change in the causal chain. For example, killing my grandparent in the past is equivalent to the assumption that domino-9 (me) falls down without domino-1 (my grandparent) falling down ever. Is that possible? Not within the rules of causality of this universe, as we've learned them. Remember that the rules of causality govern science (Newton's laws, relativity or Darwin's natural selection are examples).

In consequence, time travel, at the level of our understanding, is just a fallacious idea, and any further conclusion (what happens with causality if I kill my grandparent in the past) is impregnated with such fallacy. In order for time travel to be possible at this level, causality should prove wrong. At this level, time traveling is a nice popular fantasy, equivalent to religious beliefs.

At the noumenal level, causality, things and relationships don't exist as we experience them in our daily lives. Wheeler's experiment proves that the past can be changed. But we don't have the capability to understand the observations of reality at this level. Quantum Mechanics, QM, show that our understanding (that we receive through our senses as phenomenal features) of reality (noumena) is essentially flawed, and the universe behaves in strange ways, which cannot be grasped by our understanding (yes, we have complex formulas, but that doesn't mean that they have a rational meaning). QM is so complex to understand, that we've been forced to assume the possibility of an infinite number of universes (which is not a scientific fact!) to solve the issues of our understanding.

So, it is also a fallacy to conclude that time travel would be possible by amplifying small QM facts, and granting them properties of the macroscopic world (e.g. the whole body of a person would be transported to 1918, with its clothes, memories, the air of his lungs, and the atoms that surround his belly in a radius of 2m).

Then, since time traveling is a fallacious consideration, it is pointless to discuss the consequences of such event.

An equivalent question would be: if Zeus made a miracle in the past, can he undo it without changing my present?


No doubt some will consider this too fantastical, but premonition could be considered a form of time-travel. Borrowing an anecdote from Jonathan Bricklin's The Illusion of Will, Self, and Time: William James's Reluctant Guide to Enlightenment, page 94

The lady had dreamed that she saw her old coachman falling from the top of the carriage to the road, landing on his head. Later that day after a long drive during which she had forgotten completely ahout her dream . . . she saw the coachman leaning back in his seat, as if he were not well. She called to him to stop the carriage, jumped out, and motioned to a nearby policeman. Just then the coachman swayed and fell off the hox, The policeman was by then near enough to catch him and keep him from landing on his head and being severely injured, as the dream had foretold.

You could consider that the lady had 'experienced' the possible future and sent a message back through time, altering the past. It may be just a story, but it illustrates a 'workable' time-travel scenario that avoids the problem of anachronous grandchildren and weapons from the future.

  • 1
    Premonition is real. The instances of it are too numerous to be contradicted. It is not necessary that the person have experienced the future event and have relayed it to the past; all that is necessary is for God to perceive what will happen and alert the mortal of it. – pygosceles Nov 14 '19 at 19:36

Is it even possible to time travel without causally affecting the past?

Yes, in only one direction: forward.

Backwards time travel is impossible. The proof for this is simple: Backwards travel unrolls to forward travel. This is necessarily true in order to preserve causality. If one were to reset all things (except oneself as the time-traveler) to the state they were in at an earlier point in time, and then play forward from there, modifying the so-called "past" state and the future trajectory, the whole history will still have been compounded into a single linear progression respecting causality (see Git rebase). The only way to avoid this dilemma is by revisiting or recalling the past in the present without any ability to change past events.

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The past is read-only.


Well I wanted to share my point of view of time travel I like physics so this is how I look at it:

All of this is from my original thougths, so just my opinion.

Is time traveling possible? Yes, but not in the way most people think.

What do I mean by that? When you travel in time the only reality that exists is what the traveler experiences as present, Imagine like a youtube video in which you can go forwards or backwards, but when you go backwards you destroy everything that is the future for your current "present" what i say is that in reality you destroy everything that hasn't happened, What i also mean is that time traveling (to the past) is actually destroying(modifying in the correct way) the current state of a system.

And you can only go to the future by building it.


If one were to accept time as a record book of the occurrences in reality, and if reality is determined as operating within the confines of mathematics/logic, then affecting a past event in the already established timeline would have some effect in the future.

Thought experiment: hitting a cue ball in pool, or throwing a ball of paper to some area of a room; if one were to go back into the past and somehow influence the trajectory of said cue ball or ball of paper, the objects will be placed in a different position prior them being affected. And to state that the objects will still land in predestined positions, regardless of going back in time to try influencing the course action, would pronounce fate. But I seem to understand fate as being unreasonable as it violates logical causes to empirical observations of our world.

The question is how much change of the past do you consider as changing the past?


No such thing as a physical time travel. Time is an invented construct to measure differences in change.

There is no grandfather paradox because that is based on a false premise.

As for causality - an effect can alter the cause, it's called overlapping and because the effect affects the cause you cannot sometimes determine the true cause of something. This has nothing to do with time directly though.

What you can do:

You can consider time traveling to the past by digging through your memories or by studying things of the past.

You can consider time traveling to the future your imagination and planning.

You can alter the past and control the future by re-writing history in the way some if not all governments do: rewrite things of the past history and enforce them by 'law' to favor a certain development of the future.

That's about where it stops.

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