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, 2018 at 16:49
  • Also a possible duplicate of this. Does that thread answer your question?
    – Joachim
    Dec 7, 2018 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. Dec 7, 2018 at 16:54
  • The earlier thread isn't a direct answer to the question asked, thank you nevertheless. Dec 7, 2018 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. Dec 7, 2018 at 18:21

8 Answers 8


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?


There is currently, no technology that allows humankind to travel back in time. If such a technology were to be invented and released to the public at large, then society and civilization would be dramatically transformed. Time travel, particularly, historical time travel, would be one of the greatest and most fascinating adventures ever undertaken by humans. AT LAST, we would FINALLY have a truly, observable historical experience and the days of educated guessing and speculation based upon tenuous evidences, would be "a thing of the past".

However, even if such a perfect or near perfect technology were to come into existence, there are both ontological and even moral questions as to whether or not one should undertake such an awesome journey into the actual, tangible past.

Here's a THEORETICAL example: Let's say this fantastic historical time machine allows you to select any year in the past that you would wish to travel to-(from last year, to the time of Adam and Eve....about 6000 years ago) and let's say, you have always been interested in visiting Ancient Rome. While it would be much easier to plan a vacation to Rome during the summer months and take the expected sightseeing tours of The Forum, the Pantheon and the Coliseum, let's say, that such a summer tour, is just not fulfilling enough for you and that the ONLY WAY to ACTUALLY experience Ancient Rome, is to travel back TO....Ancient Rome-(via this new technology).

Ancient Rome's chronology began around 800 BC/CE and ended at around 500 AD/ CE; so you would have to select one year within a 1300 year timeline when preparing for your time travel visit. Would you want to see the founding of Rome in the year 753 BC/BCE, would you prefer to eyewitness-(via the time machine), the heyday of the Republic-(circa 300 BC/BCE) or if you wanted to visit and actually see the Roman Empire, would you want to visit Rome during the reign of Caligula-(37-41 AD/CE), Nero-(54-68 AD/CE) or visit the opening of the Coliseum in the year 70 AD/CE?

Should you choose to visit the Roman Empire, be prepared to enter an Imperial Dictatorship with FEW citizens, as well as a plurality of slaves and indentured servants with NO human rights. Blasphemy-(that is to say, publicly contradicting the presence of the Roman Deities), would be a crime punishable by death and a seemingly innocuous criticism of the Emperor and his regime, would also be a crime punishable by death. Executions in the Roman Empire, included, crucifixion-(usually along the Apian Way), as well as "being fed to the lions" in the Coliseum.

So the question is, would a contemporary Westerner, who has lived in a Democratic country (and Liberal Age) his or her entire life, be prepared to travel back to a distant, earlier age, time and place? An earlier age, time and place that was anti-democratic, as well as intolerant of religious and intellectual heterodoxies? The likely answer, is, no, you would probably not want to travel back to the Roman Empire, nor would you necessarily want to travel back to the more tolerant and relatively democratic Roman Republic either.

(A historical time machine would redefine the old term, "Culture shock" and its redefinition, would not be so benign or innocuous).

In other words, you wouldn't belong in the Roman Empire or even the Roman Republic. The time travel experience, would probably be too overwhelming and too complicated, as well as too emotionally and too culturally shocking for you to handle. Think about the chronological distance alone; you would be traveling back 1500-2800 years! BEFORE your time. Such a journey would be an incredible and memorable experience, though not necessarily a pleasant or enjoyable experience.

It would be better or wiser for one to plan a trip to Rome IN the year 2021....and NOT visiting Rome TO the year 221!-(either AD or BC).


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.


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, 2019 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.

enter image description here

The past is read-only.


The fundamental laws are all time reversible (except CPT violation). But quantum-states that start out pure, tend to become mixed; information spreads out, the 'thermodynamic arrow of time'. This asymmetry, directionality, is a type of pattern, that is both the mechanism of information transfer and the characteristic of time moving ahead. To move backwards would be to see information do the opposite of spreading out; ie the reverse of perception, ie. we can't percieve it.

Time IS the spreading out of information, so to 'go' backwards would be to withdraw information about you from the world - ie, forgetting.

We can find cases, where it seems like it should be possible to send a signal back in time, but we find it isn't, summarised by Hawking as the chronology protection conjecture.

Anti-particles seem to be directly equivalent to particles going backwards in time, a property essential to Feynman diagrams (Feynman was clear he didn't think that meant they were, but his mentor Wheeler was much more open, as per the 'One electron universe' model he proposed).

Entanglement, which is now widely thought to occur across Einstein-Rosen Bridges (known as saying ER = EPR); this apparently non-local connection somehow manages to be incapable of transmitting useful signals faster than light, summarised as the no-communication theorem.

The final category, theoretical but on solid ground in General Relativity, is closed time-like curves. They require exotic matter we don't understand, but that the accelerating expansion of the universe suggests does exist. A major challenge to them being possible in a quantum-gravity theory is that they can freely create entanglement, whereas quantum theory has very strict rules about that.

A fully isolated quantum system, which may even be of visible size, seems to be able to explore all it's possible iterations at once until it rejoins the universe with a measurement. Possible an abstracted model, say on a qubit assembly, could explore meaningful futures for unusual outcomes, and take steps to make them happen. This is I'd say the closest to reading the future, and it will depend on accurate models, and be limited by sensitivity to initial conditions causing rapid divergences (but a really good model sheds the noise for the core dynamics, which may be tractable).

  • "But quantum-states that start out pure, tend to become mixed" Only if you assume a system in a pure state interacts with an environment that's in a mixed state--in the various versions of the Everett interpretation (including the many-worlds interpretation), the wavefunction of the universe remains in a pure state forever, as discussed in this answer on the physics SE.
    – Hypnosifl
    Jun 4, 2021 at 17:48
  • (also, the black hole information paradox is specifically about the impossibility in QM of an isolated system, in this case a black hole with some matter/energy falling through the event horizon, transforming from a pure state to a mixed state on its own without interacting with anything external)
    – Hypnosifl
    Jun 4, 2021 at 17:53
  • @Hypnosifl: No, it's about whether information can be destroyed. The inside of an existing large blackhole is expected to be isolated into long after the stars have gone out and the CMB cooled below the Hawking temperature. Only then will the information return. A quantum system is isolated, ie behaving 'quantumly', until data leaks out that acts as a measurement. This is much easier to achieve for small systems. As the degrees of freedom rise, it is effectively able to measure itself, reducing possibilities.
    – CriglCragl
    Jun 4, 2021 at 18:04
  • Since a mixed state is just a classical probability distribution on different possible pure states, the transition from a pure state to a mixed state is itself a destruction of information--this is explained in the link I gave, written by a physicist. I don't know exactly what theory you're talking about when you say "only then will the information return" (there are several theories on how the paradox might be resolved in quantum gravity, but all the theories I know suggest the information is in-principle measurable at all times, not that it's lost for a while and then later returns).
    – Hypnosifl
    Jun 4, 2021 at 18:14
  • "A quantum system is isolated, ie behaving 'quantumly', until data leaks out that acts as a measurement." Again, only if its wavefunction is treated as separate from the environment. If you assume that both a subsystem and its environment can be treated as a single system which starts in a pure state, then leaks from the subsystem to environment won't change the fact that the whole system's wavefunction is in a pure state, though the "reduced density matrix" for the subsystem alone will transform into a mixed state (this is what's known as decoherence).
    – Hypnosifl
    Jun 4, 2021 at 18:17

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