Hi Walter

Firstly, it depends what you mean by the A theory and the B theory of time. Presumably you are using these as synonyms for Presentism and Eternalism (respectively), in which case you are talking about the ontology or existence of time(s).

There is also the [McTaggart's] A-series and B-series of time, which refer to a temporal ordering of events, but that's not important right now.

I will use 'A theory' as a synonym for Presentism and 'B theory' as a synonym for Eternalism below.

The B theory holds that all times (and everything at those times) exist...equally. Therefore, those times are available travel destinations, if we are to assume that time travel is possible.

The A theory holds that only the present time (and everything at that time) exists. Therefore any time other than the present time is not an available travel destination. This appears to rule out the possibility of time travel according to the A theory.

However, there is one caveat, which is that the present time is always moving into the future.

The A theory holds that only the present time (and everything at that time) exists. Therefore any time other than the present time is not an available travel destination. This appears to rule out the possibility of time travel according to the A theory

However, there is one caveat, which is that the present time is always moving into the future.. — Luke

Exactly. So, presentism doesn't exclude all time travel. I don't see why presentism as such should be inimical to other kinds of time travel, proceeding at different rates than the normal forward rate.

I don't see why presentism as such should be inimical to other kinds of time travel, proceeding at different rates than the normal forward rate. — SophistiCat

Time travel is not possible according to presentism because those other times (or travel destinations) do not exist (according to presentism). I did not get into it in my previous post, but the caveat I spoke of there (that the present is always moving into the future) can only be viewed in terms of the B theory. According to presentism, no future or past times exist nor any other time except for the present time, and therefore time travel is not possible by presentism's own lights. Any time travel, including time travel at the "normal forward rate", can only be viewed in B theory terms. That is, unless you mix or disregard the tenets of the A and the B theories, because obviously we can and do talk about other times and probably nobody is a true presentist.

According to you, time doesn't pass at all, according to presentism. That can't be right. And I don't mean that in the sense that presentism can't be right, but in the sense that your construal of presentism can't be right.

Time travel isn't possible period. Time isn't a "place you can travel to." Time is simply change ( including motion). Additionally, nominalism is true. You can't literally change from A to be back to an identical A. You'd have A' instead on the second "A" occasion.

You need presentism of course. Travel isn't possible at all in eternalism, given the usual A-definition of 'travel'. I plan to travel to 2024, but it will take me 5 years to do it.

So why did you start the B-theory thread about free will and then totally abandon it? It degenerated into the usualy discussion of what eternalism is rather than its implications.

Suppose that the metaphysics behind the A theory of time is correct, is it possible to travel to the year 2024 (or the "future") or the year 2000 (or the "past") or does time travel require the B theory of time to be correct? — Walter Pound

Travel to 2024 is simple. Just wait 5 years or less and the passage of time will take you there. Of course, it's past travel is where things get tricky. If one assumes presentism, then there is no time that exists apart from the one that we are in in 2019. Because there exists no year 2000 destination to travel to, then it's impossible to really "travel" to 2000 like you would "travel" to France for vacation. But that doesn't mean you can't create your destination. Just think of the universe as if it were a movie playing on a VHS, and hit the rewind button. Give it 19 years and soon you'll find youself in the year 2000. Simple as that. No extra times or 3D universes required and everything is kept in the present.

It degenerated into the usualy discussion of what eternalism is rather than its implications. — noAxioms

Wouldn't it be necessary to have very clear principles of exactly what eternalism is, before we can determine its implications? If we can't agree on those principles, discussing implications is sort of meaningless. One could just change the principles of what eternalism is, to produce favourable implications.

6k
Time travel isn't possible period. — Terrapin Station

There are certain space-time structures that make time-travel possible, if I remember correctly. Didn't Kurt Godel discover one, as did Frank Tipler?

Also there is a theory of time -travel in the multiverse, the catch being that the time you go to will not be in your world.

Presentism is wrong according to these theories.

You need presentism of course. Travel isn't possible at all in eternalism, given the usual A-definition of 'travel'. — noAxioms

What do you mean?


Thinking a bit more about this, if now is an objective fact on presentism, and the Time Traveler is transported some ways into the past or the future, what happens with the now? The situation is different from the normal "time travel" when everyone moves forward into the future in lockstep at 1 second per second, because the Traveler has left everyone else in the future (or the past). The only way this makes sense to me in the presentist framework is if now splits into two nows: one travels with the Traveler (where it entangles everyone else in his present) and one stays with his former contemporaries and continues on at its usual pace.

Note that these considerations are quite apart from the question of existence of the past and the future (which, frankly, make little sense to me).

Thinking a bit more about this, if now is an objective fact on presentism, and the Time Traveler is transported some ways into the past or the future, what happens with the now? — SophistiCat

I presume you ride the 'now' into the future. That's how it worked. To travel to the past, I suppose you'd have to get time to go the other way, and still be able to ride it, but leaving everybody else behind.

In more objective terms, I think time travel to the past would be to cause an instance of 'yourself' to exist at time X, but with memory of time Y, with Y > X. This is pretty easy to do in theory in the forward direction, but not so much backwards, being a violation of the principle of locality.

normal "time travel" when everyone moves forward into the future in lockstep at 1 second per second — SophistiCat

I always wondered what meaning there is being a unit of X per X, which seems to reduce to just unitless '1'. On the other hand, our clocks are dilated mostly due to the gravity well in which we find ourselves, so maybe the rate is still unitless, but still less than 1. How much less is an eye-opening exercise.

There are certain space-time structures that make time-travel possible, i — Inis

Not any spacetime structure that's correct, though. I'm not saying that it's not a popular belief that time travel is possible, but the belief rests on not understanding what time really is.

About the only form of time travel I think is possible, is the form rendered in the twin paradox from relativity (take a trip at close to the speed of light and return), your friends will have aged or even died and much time will have passed on earth whereas you (theoretically) may have aged only a few biological years. Other than that I think it is all science fiction (with an emphasis on the fiction).

Not any spacetime structure that's correct, though. I'm not saying that it's not a popular belief that time travel is possible, but the belief rests on not understanding what time really is. — Terrapin Station

But the Multiverse structure is correct.

According to you, time doesn't pass at all, according to presentism. That can't be right. And I don't mean that in the sense that presentism can't be right, but in the sense that your construal of presentism can't be right. — SophistiCat

You may be right.

To offer some explanation and defence of my view, I consider there to be a perfect symmetry between the motion of Presentism and the motionlessness of Eternalism. Presentism, with its present moment ever flowing into the future is how we experience time, whereas Eternalism with its motionless ontological equality is how we represent or model time. Both are required for our understanding of time. Without the eternalist picture, there is no model or representation of time and we cannot even talk about time. Without the presentist picture, there is no motion and thus no time at all. They need each other. Anyway, that's my probably ill-conceived view in a nutshell. I would consider naming it 'Dumb Presentism', but too many might agree.

In my previous post I argued that only the present time exists according to presentism and that no other times exist. I expect an easy rebuttal to this would be, e.g., that future times don't exist now but that they will exist (or may exist) when they come into existence. Likewise for past times: that they did exist even though they no longer do. Fair enough, but I still like the symmetry of my view. Donning my pure presentist hat for a moment, I might even respond: what other times are you referring to?

One consequence of my view would be that even if the presentist did time travel (to some abnormal temporal destination), then they could be oblivious to having done so because it would always remain the present time for them anyway.

But the Multiverse structure is correct. — Inis

lol

I think it's important to remember that the motivation, the main selling point of presentism is this inescapable subjective perception of being in time - and that includes both the instantaneous now and its temporal progress. However we choose to formalize and articulate presentism, we shouldn't lose track of those basic intuitions, else this turns into a sterile formal exercise.

I presume you ride the 'now' into the future. That's how it worked. To travel to the past, I suppose you'd have to get time to go the other way, and still be able to ride it, but leaving everybody else behind. — noAxioms

This works for the Traveler, but what about the rest of us? What happens to us and our now when the Traveler departs into the future or the past, and now departs with him?

In more objective terms, I think time travel to the past would be to cause an instance of 'yourself' to exist at time X, but with memory of time Y, with Y > X. This is pretty easy to do in theory in the forward direction, but not so much backwards, being a violation of the principle of locality. — noAxioms

We can imagine a world in which, in the year 2019, you stepped into the time machine and suddenly disappeared. Earlier, in the year 1919, an exact copy of you as of 2019 suddenly appeared in a field, fooled around for, say, a week, and then disappeared. Meanwhile, in the year 2019, five minutes after vanishing in the time machine, you reappear, having the memories and other physical changes that your copy had in 1919 at the time of disappearance.

We could tell the same story chronologically, without jumping back and forth between 2019 and 1919. The reason we usually tell these stories achronologically is to emphasize causal connections. But in this telling there are no anomalous causal connections between the past and the future - and that is why it does not count as time travel. Time travel is all about anomalous causality.

I always wondered what meaning there is being a unit of X per X, which seems to reduce to just unitless '1'. On the other hand, our clocks are dilated mostly due to the gravity well in which we find ourselves, so maybe the rate is still unitless, but still less than 1. How much less is an eye-opening exercise. — noAxioms

Yeah, I only invoked that formula in order to represent the difference between normal time flow and time travel as a difference in the rate of time flow (as seen from the presentist standpoint). Note that in non-inertial reference frames your local time still passes at the usual rate.

"I presume you ride the 'now' into the future. That's how it worked. To travel to the past, I suppose you'd have to get time to go the other way, and still be able to ride it, but leaving everybody else behind."
— noAxioms

This works for the Traveler, but what about the rest of us? What happens to us and our now when the Traveler departs into the future or the past, and now departs with him? — SophistiCat

Well, they 'rewind' along with the rest of 'history', which isn't even a violation of physics. Only what you (the 'traveler') are doing is a violation.

We can imagine a world in which, in the year 2019, you stepped into the time machine and suddenly disappeared. Earlier, in the year 1919, an exact copy of you as of 2019 suddenly appeared in a field, fooled around for, say, a week, and then disappeared. Meanwhile, in the year 2019, five minutes after vanishing in the time machine, you reappear, having the memories and other physical changes that your copy had in 1919 at the time of disappearance.

Pretty much the standard depiction, yes. I think Back to the Future did almost exactly this.
The story typically involves a vehicle, emphasizing that it takes to 'the past' like that is a place. Nobody does it with a say a pill or scan-and-teleport with reverse causality. Tron and Star Trek sort of do the scan depiction, but to a different place, not to the past.
Some depictions just have a device like a wristwatch that does the job for you. Some depict a portal that requires a receiving portal on the other side.

We could tell the same story chronologically, without jumping back and forth between 2019 and 1919. The reason we usually tell these stories achronologically is to emphasize causal connections. But in this telling there are no anomalous causal connections between the past and the future - and that is why it does not count as time travel. Time travel is all about anomalous causality.

I think it very much counts if there is a guy in 1919 with memory of 2019. The way you tell the story puts emphasis where the storyteller wants it, but there would be little dispute of time travel to somebody with such memories in 1919, however little he might be able to convince the locals there.

I think it's important to remember that the motivation, the main selling point of presentism is this inescapable subjective perception of being in time - and that includes both the instantaneous now and its temporal progress. However we choose to formalize and articulate presentism, we shouldn't lose track of those basic intuitions, else this turns into a sterile formal exercise. — SophistiCat

I agree, and I've never said otherwise. What I've said is that, according to my view of presentism, no other times but the present time exist, and time travel can only be viewed from an eternalist or B theory perspective of time.

I agree, and I've never said otherwise. What I've said is that, according to my view of presentism, no other times but the present time exist, and time travel can only be viewed from an eternalist or B theory perspective of time. — Luke

How about doing a simple time dilation experiment? Synchronise atomic clocks, and take one on a flight around the world. When the clocks are reunited, they no longer agree on the time. How is that possible under presentism?

I just read the wiki article on the theory. It seems science has a preference for the B theory of time. It says it kinda fits with the other theories like relativity, etc. Sadly, that's the limit of my understanding.

If time is a fourth dimension (relativity) then it becomes natural to expect an extension in it. We can move to and away from a particular location in the 3D of space, why would the 4th dimension of space be excluded from such a possibile property?

The question that bothers me is why are there no instances of time travel? Why is it difficult? We see travel in 3D space - it's so commonplace that no one even notices it. What is so special about the 4th dimension?

How about doing a simple time dilation experiment? Synchronise atomic clocks, and take one on a flight around the world. When the clocks are reunited, they no longer agree on the time. How is that possible under presentism? — Inis

Are both clocks at the present when they are reunited? The time displayed on the clocks is irrelevant to presentism, so long as the clocks always remain at the present.

Are both clocks at the present when they are reunited? The time displayed on the clocks is irrelevant to presentism, so long as the clocks always remain at the present. — Metaphysician Undercover

If the "Present" existed, then the clocks would read the same.

How about doing a simple time dilation experiment? Synchronise atomic clocks, and take one on a flight around the world. When the clocks are reunited, they no longer agree on the time. How is that possible under presentism? — Inis

That has been done many different times and many different ways and the result is the same, clocks run at different rates under the influence of gravity and acceleration. The time reading on a clock however has nothing to do with presentism. Time is not fundamental, what is fundamental is change and process, and the rate at which a clock runs, or humans age, varies with gravity and acceleration. There is a fundamental misunderstanding about what time is (a derived concept from change) and what clocks do (they are processes that run at different rates under different conditions).

That has been done many different times and many different ways and the result is the same, clocks run at different rates under the influence of gravity and acceleration. The time reading on a clock however has nothing to do with presentism. Time is not fundamental, what is fundamental is change and process, and the rate at which a clock runs, or humans age, varies with gravity and acceleration. There is a fundamental misunderstanding about what time is (a derived concept from change) and what clocks do (they are processes that run at different rates under different conditions). — prothero

How does gravity alter the rate of clocks under presentism?

How can the present be at different times under presentism?

Clocks around the world have 24 different times representing the different time zones. Before agreement clocks in different towns had different times, rail travel made synchronizing clocks necessary, etc. The time reading on any particular clock has nothing to do with presentism or with the philosophical notion of time.

Clocks around the world have 24 different times representing the different time zones. Before agreement clocks in different towns had different times, rail travel made synchronizing clocks necessary, etc. The time reading on any particular clock has nothing to do with presentism or with the philosophical notion of time. — prothero

Why do clocks on the earth and in orbit run at different rates?

Clocks in different time zones don't run at different rates.

From Wikipedia : Time Dilation

According to the theory of relativity, time dilation is a difference in the elapsed time measured by two observers, either due to a velocity difference relative to each other, or by being differently situated relative to a gravitational field. As a result of the nature of spacetime,[2] a clock that is moving relative to an observer will be measured to tick slower than a clock that is at rest in the observer's own frame of reference. A clock that is under the influence of a stronger gravitational field than an observer's will also be measured to tick slower than the observer's own clock.

Such time dilation has been repeatedly demonstrated, for instance by small disparities in a pair of atomic clocks after one of them is sent on a space trip, or by clocks on the Space Shuttle running slightly slower than reference clocks on Earth, or clocks on GPS and Galileo satellites running slightly faster.[1][2][3] Time dilation has also been the subject of science fiction works, as it technically provides the means for forward time travel.[Experimental testing[edit]

Hafele and Keating, in 1971, flew caesium atomic clocks east and west around the earth in commercial airliners, to compare the elapsed time against that of a clock that remained at the U.S. Naval Observatory. Two opposite effects came into play. The clocks were expected to age more quickly (show a larger elapsed time) than the reference clock, since they were in a higher (weaker) gravitational potential for most of the trip (c.f. Pound–Rebka experiment). But also, contrastingly, the moving clocks were expected to age more slowly because of the speed of their travel. From the actual flight paths of each trip, the theory predicted that the flying clocks, compared with reference clocks at the U.S. Naval Observatory, should have lost 40±23 nanoseconds during the eastward trip and should have gained 275±21 nanoseconds during the westward trip. Relative to the atomic time scale of the U.S. Naval Observatory, the flying clocks lost 59±10 nanoseconds during the eastward trip and gained 273±7 nanoseconds during the westward trip (where the error bars represent standard deviation).[39] In 2005, the National Physical Laboratory in the United Kingdom reported their limited replication of this experiment.[40] The NPL experiment differed from the original in that the caesium clocks were sent on a shorter trip (London–Washington, D.C. return), but the clocks were more accurate. The reported results are within 4% of the predictions of relativity, within the uncertainty of the measurements.
The Global Positioning System can be considered a continuously operating experiment in both special and general relativity. The in-orbit clocks are corrected for both special and general relativistic time dilation effects as described above, so that (as observed from the earth's surface) they run at the same rate as clocks on the surface of the Earth.[41]

The rate at which a process occurs, mechanical clock ticking, biologic or chemical is affected by the gravitational field in which it occurs. Again nothing much to do with presentism and very little to do with the nature of time itself except to show that keeping time is just measuring the relative rate of change. Time itself as we typically talk about or conceive of it does not exist. There is no absolute universal fixed time (Newton), passing at a uniform rate.

Well, they 'rewind' along with the rest of 'history', which isn't even a violation of physics. Only what you (the 'traveler') are doing is a violation. — noAxioms

What do you mean "they rewind"? The idea of time travel is that someone (or something) is moving in time (at a different than normal rate), while everyone and everything else goes on as if nothing happened. But how this divergence is possible if there is only one now is something I can't wrap my head around. It would make sense if now diverged as well.

What I've said is that, according to my view of presentism, no other times but the present time exist, and time travel can only be viewed from an eternalist or B theory perspective of time. — Luke

No times but the present exist, and wherever the present is, that is what exists. Yesterday the present was thataway, and now the present is thisaway. Nothing about presentism says that the present has to stay in one place.

No times but the present exist, and wherever the present is, that is what exists. Yesterday the present was thataway, and now the present is thisaway. Nothing about presentism says that the present has to stay in one place — SophistiCat

Right, I've never denied that either.