Before answering the question, I will define the different meanings of the word “possible”. In philosophy, there are different senses of the word “possible”. Besides, it’s good to know the different senses in how a thing made be said to be possible.
First of all, there is a sense in saying that a thing is possible in terms of its logical coherency. For example, a person might say a philosophical zombie or “p-zombie” in David Chalmers’ thought is “logically possible” or “logically conceivable.” This simply means that the idea of a living body with similar brain states and behaviors but lacks consciousness is a logically consistent idea in itself and that it doesn’t present a contradiction of terms or meaning. The concept of a dragon may also be something that’s considered to be logically possible or conceivable in that seems to be a consistent idea in itself and doesn’t present a contradiction. A “circle-square” or a “married bachelor” would be something that’s a logical impossibility by contrast. A “married bachelor,” for instance, presents a violation of the Principle of Non-Contradiction because it presents a contradiction in meaning. A person cannot be married and unmarried at the same time. Likewise an object cannot be both a square and not a square but a circle at the same time.
Moreover, there is the notion of “metaphysical possibility” or “metaphysical conceivability”. If something is “metaphysically possible” then it is said that it could technically exist or happen in reality. So if someone says that p-zombies or dragons are metaphysically possible then that person is holding that a p-zombie or a dragon can actually exist. Of course, if one thinks for instance that there is an omnipotent Deity out there then quite plausibly p-zombies and dragons would be metaphysically possible provided that God decides to create such entities with his power. To say something is “metaphysically impossible” by contrast would be to hold that something absolutely cannot exist or cannot occur in reality. “Circle-squares” are metaphysically impossible since they imply a logical contradiction and logical contradictions cannot exist.
In addition to logical possibility and metaphysical possibility, there is also the notion of “physical possibility”. To say that something is “physically possible” is to say that it can happen within the known laws of nature and states of affairs in the world. It may be physically possible for instance that a person can jump three feet up in the air but not fifty feet in midair. Of course, what is “physically possible” doesn’t necessarily exhaust all what may be metaphysically possible. It may be, for instance, impossible physically for a person to jump fifty feet in the air on this planet with the normal laws of nature in place. But if God decided to alter or suspend the laws of physics for a moment it may be possible in reality for a person to jump fifty feet in the air on earth.
In considering logical possibility, it may be a good indication that a thing or event may be metaphysically possible or something that can technically exist. Some philosophers consider logical possibility to always be an indication that something is metaphysically possible. Others argue that while logical possibility may generally be a good indication that something is metaphysically possible, it’s not something that necessarily entails that a thing or event can actually exist. For instance, philosopher William Lane Craig may grant that an infinite number of objects are logically possible or exempt from a contradiction in terms. However, he would make the case that an infinite number of entities are metaphysically impossible.
I tend to agree with the idea that logical conceivability is only something that generally indicates that something is metaphysically possible but it may not always guarantee a metaphysical possibility. There may be other reasons besides a thing being incoherent that may indicate that something is absolutely impossible. For instance, there might not be any detectable contradiction in the notion of an entity being intrinsically evil by its existence. But if an omnibenevolent Deity exists as the source of all things, then it would be metaphysically impossible for an entity to have an intrinsic evil existence. This is because it would seem that an omnibenevolent Deity cannot create a thing with an intrinsic evil existence as many theologians like Augustine would argue as such.
Nonetheless, logical possibility is important in philosophy because it not only helps establish what can be actualized in reality but it also helps in discerning identity or distinction among things. If one wants to know whether consciousness is something identical or distinct from material objects, then all one has to do is produce imaginary scenarios or “thought experiments” in which either living bodies or conscious states exist independently of each other. If there’s nothing contradictory in the notion of all possible material objects existing without having consciousness including a living human body with all the proper functions then consciousness and material entities cannot be the same thing. The notion of a living body, like in the thought experiment of a p-zombie, is not diminished if we suppose there’s no subjective awareness going on with it. By contrast, supposing a triangle can exist with only one side would be a contradiction in terms or would not be logically possible because a triangle by definition has to have three sides in order to exist.
Furthermore, what is considered to be metaphysically possible or impossible will greatly depend on a person’s views on the nature of reality and the nature of God. For instance, Benedict Spinoza would consider free will to be metaphysically impossible because the nature of God according to him cannot have free will. By contrast, a thinker like George Berkeley would hold that it’s metaphysically possible (and actual) that individuals have free will because that trait comes from God and God also has this power of free will within himself. Of course, what’s considered to be metaphysically possible or impossible is often thought to be determined by the nature, powers, and limitations of God or the eternal self-existent Being. In a similar manner, whether one thinks that time travel is metaphysically possible will greatly depend on one’s views of the nature of time and other factors.
First of all, is time travel logically possible or exempt from contradiction? It seems that time travel is logically conceivable and doesn’t necessarily imply a contradiction. A contradiction in time travel would only arise if we suppose that person goes into the past or future and erases an event and brings about a new one. The so-called grandfather paradox would be an illustration of supposing that time travel changes the past and future. The grandfather paradox calls us to imagine a person traveling into the past and killing his own grandfather and thus the time traveler erases his own existence in the future. But the grandfather paradox fails to demonstrate that time travel necessarily implies a contradiction as David Lewis and Nicholas J.J. Smith point out. This is because it’s possible to conceive of time travel happening without erasing any past or future events. It’s just that if time travel into the past were to happen then no one would permanently kill their grandfather because it’s already been established that the time traveler exists partly due to his grandfather. Any time travel into the past or the future would only be part of the original events of the past or future without altering the events in time. At any rate, it seems that time travel can theoretically occur without changing any events.
Of course, people can posit parallel universes or a similar “many worlds hypothesis” time travel where an alternative timeline is created when one travels into the past to resolve the grandfather paradox. But even in the scenarios of a person traveling into the past and killing their grandfather (or someone that looks like his grandfather) on a parallel universe or on a new timeline that’s branching off the old sequence of events, there’s no actual changing of the original past or original timeline. There is only affecting a specific chain of events even when we suppose a time traveler lands on a parallel timeline similar to our own and kills his grandfather or someone that’s genetically identical to his grandfather. Regardless, whether the time traveler lands within the past within our timeline or in another similar parallel timeline, events are only affected at most by the time traveler, events are not in any form erased and replaced with new events.
I don’t know if there are parallel universes or timelines out there. But there’s absolutely no need to postulate them to explain the logical possibility of time travel. We can assume that there exists only one unified sequence of events or a single timeline in which the time traveler moves from the present to the past or future. Given what we know, that there is at least one cosmic timeline or history of events, if the time traveler were to land into the relative past and meet his grandfather then he will not kill his grandfather since it’s already been established within the past that the grandfather will live on, and contribute to the traveler’s existence. Or, if the traveler manages to shoot down his grandfather then perhaps the grandfather might miraculously recover from the shots and then live on to contribute to his grandson’s future existence. Either way, the time traveler can only affect the past but he cannot undo any event that has already occurred. And if the time traveler where to meet his grandfather within the distant past then it would be always the case that the time traveler has met his grandfather within the past; it would not be a case where the time traveler was absent in meeting his grandfather within the past and then later that changes and now it’s part of the past that the traveler meets his grandfather. Once something happens within the past then it’s something that’s always been part of the past (or future).
Hence time travel seems to be logically possible and it can be done in theory without implying any changes in history. Is time travel metaphysically possible? The answer to that question would greatly depend on how one defines the nature of time. If one accepts the traditional view of time—the presentist theory—then time travel would be metaphysically impossible. This is because with a presentist model of time both past and future events don’t exist and so there would be no future or past moments to travel to as such. However, if one accepts an eternalist theory of time like block universe where all events of the past, present and future co-exist then time travel would be metaphysically possible or at least possible in theory. In order for time travel to occur, the events of the past or future have to exist in order for one to arrive at those different moments. A block universe model would allow the events of the past and future to exist for the time traveler to arrive at as such.
Now a B-theory of time combined with the idea that an all-powerful Deity exists would seem to entail that time travel is metaphysically possible. If God wanted to remove a person from the perceived present onto a relative past or future period of time it seems that He could do so with his omnipotent power. The divinely caused time travel here would work like teleportation in the Star Trek series. It would appear, for instance, that God could teleport an individual from 2020 AD to 7000 BC. And that God could teleport a person into the past or future in a logically consistent manner without changing the past or future. So if the future person is transferred from 2020 AD into 7000 BC then it would always have to be the case that man was originally located at 7000 BC without changing any events in 7000 BC. The same would be true if the man were teleported from 2020 AD to 2200 AD. If the person arrives at 2200 AD through some time travel procedure then it would always be the case that the man arrived at 2200 AD without changing any events.
It appears that an all-powerful Deity can teleport an individual into the past or future and allow that person to affect the past or future within certain limits. Again, none of this seems to imply a contradiction like a “circle-square” and it doesn’t to seem imply anything metaphysically impossible given God’s omnipotence and the premise of eternalism. It also seems possible that the Deity could just send a person to a past or future event without having that time traveler affect anything in the event. The time traveler could just be a passive observer of a historic event without interacting with anyone or anything going on in the event. At any rate, I would say that time travel is metaphysically possible given the premises of a timeless Deity and a static theory of time. It appears that God could cause a person to time travel just as he could choose to create a dragon or a unicorn. As some would argue and rightly so, if God is beyond time and he views of all time at once then it seems that the Deity has the ability to cause a person to time travel.
Now I am not saying that time travel to the extent that happens in movies and science fiction stories has occurred or will occur in the future. It’s extremely improbable that we will be able to invent a time machine to travel significantly into the future or past. Nonetheless, it seems that time travel is metaphysically conceivable given that there is a timeless omnipotent Deity who can technically make present individuals at different moments on the block universe.
Is time travel physically possible? According to many scientists, it’s physically possible to travel into the future in an infinitesimal manner but time travel into the past hasn’t been clearly established. It’s apparently possible to travel into the future as Michio Kaku explains in Physics of The Impossible (P. 219):
[T]ime travel to the future is possible, and has been experimentally verified…The journey of the hero of The Time Machine into the future is actually physically possible. If an astronaut were to travel near the speed of light, it might take him, say, one minute to reach the nearest stars. Four years would have elapsed on the Earth, but for him only one minute would have passed, because time would have slowed down inside the rocket ship. Hence he would have traveled four years into the future, as experienced here on Earth. (Our astronauts actually take a short trip into the future every time they go into outer space. As they travel at 18,000 miles per hour above the Earth, their clocks beat a tiny bit slower than clocks on the Earth. Hence, after a yearlong mission on the space station, they have actually journeyed a fraction of a second into the future by the time they land back on Earth. The world record for traveling into the future is currently held by Russian cosmonaut Sergei Avdeyev, who orbited for 748 days and was hence hurled .02 seconds into the future.)
As far as I know, the Russian cosmonaut Sergei Krikalev now holds the world record on traveling slightly into the future. So time travel into the future seems to be physically possible according to many scientists. If what scientists say is correct and time (or the measure of it) slows down as a person moves faster as Einstein’s special theory of relativity would indicate and they’ve been able to measure how astronauts travel into the future at very tiny amounts then this is good evidence that time travel (at least towards the future) is something that can technically happen. There are also many scientists that believe that special relativity supports eternalism which would make time travel possible at least in principle. This isn’t just because of the so-called “time dilation” when an object moves at high speeds “time slows down” for that object in comparison to other things that are at rest or moving at lower rates, but also the relativity of simultaneity in Einstein’s theories.
According to Einstein’s relativity of simultaneity notion, two events can be happening at the same time from one frame of reference or perspective, while the same two events can be perceived as happening at different times from another frame of reference. Now an eternalist theory of time like the B-theory or the other similar idea, the moving spotlight model, can accommodate the differing perspectives of the two events because all events and times are equally real given the premise of eternalism. However, a presentist would have a greater difficulty trying to accommodate the relativity of simultaneity because if only the present moment is real then this time theory would seem to entail that one of the frames of reference is correct and the other perspective is wrong. It would seem that the presentist would be committed to saying that the two events either did objectively happen at the same time or at different successive times. But there seems to be no way of discerning which frame of reference is correct over the other one. For this reason, many scientists including Einstein have taken special relativity as supporting eternalism.
At any rate, if what many scientists are contending is correct then time travel into the future seems to be physically possible. Overall, to answer the lead question of this essay, “Is time travel metaphysically possible?” I would say “yes.” Given the premises of an all-powerful Deity, the B-theory of time, and some of the scientific evidence with the time dilation of moving objects, it seems that time travel can technically happen. Do you think time travel is metaphysically possible?