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

One clock runs slower than the other. Neither of them tracks the pace of the advancement of the present. If there was a device that could do that, you'd have your empirical evidence for the view.

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

Yes they do, all other factors being equal.

Edit: I read this last comment wrong. Negate this reply....

What do you mean "they rewind"? — SophistiCat

Well, I buy into neither presentism nor time travel, so I'm trying to imagine how a presentist would envision travel to a time that is no longer existent. Somehow you need to find yourself in a world with the state being some prior state except for you being in it, which sort of seems to require a physical rewind of all state (except the part where 'you' appears in it), thus dragging the present back to that prior time. It really makes no sense to me, but it makes no sense in the block view either, so go figure.

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.

Well relativity gets you that, and it even looks normal to the 'travelling' person.

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.

I don't think it makes sense for a presentist to propose a divergence of time. A divergence of worlds, sure. That avoids some paradoxes, but time going forward for Fred but backwards for me in my machine, no. If my machine does that, then it just creates a new world now that looks like my old world did X years ago, without actually alter the course of 'the present'.

As I said, it is actually quite easy to achieve time travel in the forward direction (clock moving at other than the usual subjective rate). You can do it with anesthesia or cryonics. The former doesn't halt aging like the latter does. They can also just manufacture a new 'you' a thousand years from now, which is possible at a biological level but probably not a molecular level, and certainly not subatomic.
The subjective experience is fairly similar for all three. You look at a clock, there is a discontinuity, and suddenly the date on the wall is different. It may or may not involve a feeling of 'waking up'. This differs from sleep where there is a definite subjective sense of how long you've been out.

None of these methods work with a negative time rate. Travel to the past is just nice fiction, despite quantum experiments that can be interpreted as cause well after effect.

Yes they do, all other factors being equal. — noAxioms

Really? How does GMT+1 manage to remain +1 to GMT if the time zones run at different rates?

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

I don't see how the reading on the clock has anything to do with whether or not the present exists.

My bad. I read your comment as suggesting that they didn't run at the same rate.

I don't see how the reading on the clock has anything to do with whether or not the present exists. — Metaphysician Undercover

Hey, we actually agree on a point...

I have noticed that we do agree at a few points, but then we go in different ways.

I don't see how the reading on the clock has anything to do with whether or not the present exists. — Metaphysician Undercover

Perhaps you don't, but you cannot explain, given an objective present, why the clocks diverge.

Perhaps you don't, but you cannot explain, given an objective present, why the clocks diverge. — Inis

Even if there were no clocks, the present still "exists" and change still happens and therefore time (a derivative concept of change) passes. If there were no change there would no time.

Perhaps you don't, but you cannot explain, given an objective present, why the clocks diverge. — Inis

Both clocks traveled the same amount of spacetime. However since one clock traveled further in space, it therefore traveled less in time. Which is to say, it ticked at a slower rate than the clock on the ground.

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

I am aware of at least some of the problems of presentism (including this one), but it seems to me that presentism needs to be assumed in order to address the topic of this discussion.

In response to your question, the clocks are reunited and measured from a preferred frame of reference, so can't we also talk about presentism and eternalism from a preferred (e.g. Earthly) frame of reference, at least for the purpose of this discussion?

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? — TheMadFool

Time travel may not be possible, or we may not have discovered how to do it yet, but I think we can entertain the possibility for this discussion at least.

One clock runs slower than the other. Neither of them tracks the pace of the advancement of the present. If there was a device that could do that, you'd have your empirical evidence for the view. — noAxioms

I'm not sure how this relates to what I've said. Perhaps you're just noting another problem with presentism in relation to the rate at which time flows? Again, I think we need to assume presentism (or that "the A theory of time is correct") for the purposes of the topic of this discussion. However, if you all just want to discuss the failings of presentism then have at it.

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. — SophistiCat

If that is the case, then the standard conception of backwards time travel of simply going back to the past is strictly impossible, because your actions from there on out will inevitably affect the present, and everything from the time you have left will not go on "as if nothing happened".

However, I don't think there is any specific definition of what time travel is, apart it being some mechanism that can get you to a world which is in "2000 AD". There are many different ways which one can conceivably do just that in fiction, some of which involve divergence (like multiverse theory) and others which don't (like simply rewinding the universe).

Even if there were no clocks, the present still "exists" and change still happens and therefore time (a derivative concept of change) passes. If there were no change there would no time. — prothero

We know that an objective present cannot exist because the clocks disagree. All there can be are relative presents.

And presentism cannot explain why this happens, whereas eternalism has a fully worked out scientific theory called General Relativity, which explains it.

One clock runs slower than the other. — noAxioms

I'm not sure how this relates to what I've said. — Luke

Apparently I was responding to a different quote, the one just below the one quoted in that response. It is about how time dilation doesn't invalidate presentism.
That's two posting mistakes I've made now.

Again, I think we need to assume presentism (or that "the A theory of time is correct") for the purposes of the topic of this discussion. However, if you all just want to discuss the failings of presentism then have at it.

Presentism and Eternalism are two different metaphyscial interpretations of the same empirical data. Since time travel would be an empirical experience, it should in principle make zero difference whether presentism or eternalism is assumed. Under current empirical physics, both metaphysical views forbid time travel to the past, and neither forbids forward travel. Hence I see little point in needing to assume one metaphysical stance when discussing if a physical act is possibility or not.

Presentism and Eternalism are two different metaphyscial interpretations of the same empirical data. Since time travel would be an empirical experience, it should in principle make zero difference whether presentism or eternalism is assumed. Under current empirical physics, both metaphysical views forbid time travel to the past, and neither forbids forward travel. Hence I see little point in needing to assume one metaphysical stance when discussing if a physical act is possibility or not. — noAxioms

Eternalism isn't metaphysical if it's part of our best physical theories. Both general relativity and quantum mechanics tell us that the universe as a whole is at rest. This was realised early on in GR but took a while to be understood in QM.

This means that presentism isn't metaphysical either, it's just wrong.

What is metaphysical, however, is the claim that an objective observer-independent Reality exists. If you take the view that reality is observer-dependent, then presentism may be rehabilitated, but at what cost? Alternatively, you could take the view that our philosophy of time need not be compatible with our scientific theories. That seems even worse!

We know that an objective present cannot exist because the clocks disagree. All there can be are relative presents. — Inis

No clock or other device measures objective time, so this doesn't follow. All clocks run slow, and some slower than others, so it is to be expected that they don't always agree.

Eternalism isn't metaphysical if it's part of our best physical theories. — Inis

I don't think any of those theories assert it, despite the typical interpretation of relativity. I know Einstein held eternalist views due to the implications of the theory, but that's mostly because it is the simpler view, without a needless addition that adds nothing to the theory.

Anyway, relativity doesn't make any references to 'the present', but doesn't deny it either. It actually (reluctantly) acknowledges a preferred foliation, which can be interpreted as relativity supporting at least one aspect of presentism. Still, an objective ordering of events is not an assertion of the existence (or lack of it) of a present, which makes no empirical predictions.

Both general relativity and quantum mechanics tell us that the universe as a whole is at rest. This was realised early on in GR but took a while to be understood in QM.

This means that presentism isn't metaphysical either, it's just wrong.

I actually don't follow what you're trying to say here. I don't know what it would mean to say that the universe is not at rest (has a nonzero velocity???), so there doesn't seem to be any meaning to saying it is at rest.
Then you say this makes presentism wrong, and I don't see the connection.

I like good arguments against presentism, and I even have a unique one of my own, but I think they're all faulty.

What is metaphysical, however, is the claim that an objective observer-independent Reality exists.

Yes, that would be a metaphysical claim, and one independent of the eteralism/presentism debate. The latter concerns the nature of the universe (is it 3D or 4D?), but the former is something deeper, and seems to rest on a sort of undefined meaning of 'exists'. Being a relativist, I don't make sense of something being said to just 'exist'. It exists in relation to something (which need not be an observer), and I'm not sure in relation to what the universe might be said to exist or not exist. So such statements need to be defined by those that makes such statements.

If you take the view that reality is observer-dependent, then presentism may be rehabilitated, but at what cost?

If 'I' am an observation, that observation is taken at some event, and that means 'I' am an event, in relation which 'the present' very much has meaning. If 'I' constitutes a defined series of specific events, then those events are not simultaneous and none of the events is more special than any of the others, except perhaps the two endpoints. 'The present' could still be defined as the last event of the series of events that defines 'I'. Presentism is not wrong there, and the cost doesn't seem too high.
Notice that it is sort of an idealistic view. 'I' defines the present, not the other way around. That's a bit of cost I think.

A lot of what was just posted can be illustrated with the balloon analogy.

The universe is like the balloon, with the galaxies painted on it, moving around a bit, but generally moving away from each other as the balloon expands. oom in and there is you and I on it.

Presentism says the the balloon is like, well a balloon, without an interior. Only the present surface is the actual part of the balloon. Eternalism says that all of the space, both interior and exterior points are part of the object. There is no surface to the thing at all, defining points that are interior or exterior. Growing block says it is a solid sphere, with an outside surface, but points outside of that are not part of the object.
I find these all to be metaphysical differences, but others do not. My claim for my stance there is that there are no empirical distinctions between these three scenarios.

The metaphysical claim that the object exists independent of observation is a different claim, and can be claimed or denied by all three scenarios. The object, whatever its nature, exists or not. Again, this is metaphysical since again, there are no empirical distinctions between these two scenarios.

Eternalism isn't metaphysical if it's part of our best physical theories. Both general relativity and quantum mechanics tell us that the universe as a whole is at rest. This was realised early on in GR but took a while to be understood in QM.

This means that presentism isn't metaphysical either, it's just wrong.

What is metaphysical, however, is the claim that an objective observer-independent Reality exists. If you take the view that reality is observer-dependent, then presentism may be rehabilitated, but at what cost? — Inis

A presentist need not deny observer-independent reality. Instead they are describing reality from a preferred reference frame - their own (as Luke discusses here).

That is compatible with the universe as a whole being static and unchanging (and thus a kind of eternal present independent of time).

This is similar to Alice measuring an electron in a spin up state. That is true in her reference frame. Yet that is compatible with the electron being in superposition in a different reference frame.

A presentist need not deny observer-independent reality. Instead they are describing reality from a preferred reference frame - their own — Andrew M

And someone else can describe reality from a different reference frame and they both disagree. They don't just disagree on the time, but disagree on the factual state of affairs in their relative presents.

If there exists an observer-independent presentist reality, how can "descriptions" of it be different?

Can you give an example of a factual disagreement?

In the time dilation example, one clock traveled a different spacetime path to the other. Once each clock's reference frame is factored in, there is no factual disagreement - time simply elapsed at a different rate for each clock.

- time simply elapsed at a different rate for each clock. — Andrew M

What do you think "time elapsed at a different rate" means? Suppose you and I meet. It is the present when we meet. Then we go our separate ways, and meet again later. It is still the present when we meet the second time, and it was the present for each of us during the entire intermediate period. But for each of us, there is a different amount of time passed between the two meetings, if we take differing spacetime paths. Doesn't this just mean that there is not a fixed quantity of time between any two distinct points of the present? So we can say that for any two points in time, there is not a fixed amount of time between those two points, because the quantity of time between them varies according to the spacetime path that a person or thing takes to get from one to the next.

What do you think "time elapsed at a different rate" means? Suppose you and I meet. It is the present when we meet. Then we go our separate ways, and meet again later. It is still the present when we meet the second time, and it was the present for each of us during the entire intermediate period. But for each of us, there is a different amount of time passed between the two meetings, if we take differing spacetime paths. — Metaphysician Undercover

Yes.

Doesn't this just mean that there is not a fixed quantity of time between any two distinct points of the present? So we can say that for any two points in time, there is not a fixed amount of time between those two points, because the quantity of time between them varies according to the spacetime path that a person or thing takes to get from one to the next. — Metaphysician Undercover

It just means there is no absolute (or frame independent) time. Here's an example in terms of the twin paradox.

Suppose Alice and Bob are twins. On the day they both turn 20 years old, Bob travels into space at high speed and returns on the day that Alice turns 30 years old (according to Alice's clock on Earth). But Bob is 26 years old (according to the clock on his spaceship) and has only aged 6 years. Less time has elapsed for Bob than for Alice. (Example here.)

Can you give an example of a factual disagreement? — Andrew M

Literally everything in relative motion inhabits a different present. These presents become more strikingly in disagreement as relative speeds increase and with distance. A classic example of this is Penrose's Andromeda Paradox, inappropriately named, because it is not a paradox.

Suppose Alice and Bob are twins. On the day they both turn 20 years old, Bob travels into space at high speed and returns on the day that Alice turns 30 years old (according to Alice's clock on Earth). But Bob is 26 years old (according to the clock on his spaceship) and has only aged 6 years. Less time has elapsed for Bob than for Alice. (Example here.) — Andrew M

So I may conclude that from the point in time when Bob left, to the point when Bob returned, the amount of time which passes is dependent on one's frame of reference. Can I make the further, more generalized conclusion, that the amount of time between any two points in time, is indeterminate?

Can I make the further, more generalized conclusion, that the amount of time between any two points in time, is indeterminate? — Metaphysician Undercover

Just curious what you're thinking there. Why would it be indeterminate?

It's indeterminate because from every different frame of reference there is a different amount of time between the two points. Therefore there is no fixed value for that time period.

It's indeterminate because from every different frame of reference there is a different amount of time between the two points. Therefore there is no fixed value for that time period. — Metaphysician Undercover

It seems like you're thinking something like:

* There's a "Master Time" that overarches all other time,
* Those other times pass differently relative to each other,
therefore
* From the perspective of the "Master Time," there's an indeterminate amount of time between any two points.

Is that right?

If I assumed a "Master Time", then I couldn't conclude that time is indeterminate. The Master Time would necessitate a determinate time. So, remove your premise of Master Time. Make your perspective the human perspective. From the human perspective, there is a variable amount of time between any two moments in time depending on the frame of reference. Therefore time is indeterminate.