Time: A-series or B-series?

GeorgeDishman

Just checking, I've got a lot to type and I'd be wasting my time if you said yes so leave that aside now.

I want to approach this through a slightly different pair of philosophies, enduratism versus perdurantism, then return to presentism at the end.

Endurantism says that an object is wholely present at one instant of time, i.e. objects are 3-dimensional and change as objects or in relation to other objects as a function of time while perdurantism says that objects as 4-dimensional entities so what we see at any moment is a mere snapshot or a slice intersecting their worldline.

Consider the classic twins scenario. Bob and Alice have their joint 20th birthday party on a space station near Earth in the year 2020 after which Alice is to fly 4 light years to a space station near Alpha Centauri. They agree to meet up for another joint party on the station on her return.

She travels at 0.8c so it takes her 5 years of Earth time and by Pythagoras 3 years of ship (proper) time. Hence she celebrates her 23rd birthday near Alpha Centauri in the year 2025. She then flies back to Earth at the same speed arriving on Earth just in time to have her 26th birthday party on the space station in 2030. Four years later, she will celebrate her 30th birthday on the same station in the year 2034.

For a moment, suppose the endurantist view of existence applies to people but not inanimate objects. Alice exists only at one instant; on a spacetime diagram you can think of a bright point moving along her worldline. The line has no reality, it is merely a representation of the history of her life, she exists only at one point on the line. If there is no aether-like slowing of her metabolic processes, her 30th birthday happens 10 years after her 20th, and she celebrates with a party in the station in the year 2034. Bob also celebrates his 30th in the space station, but since he has been on Earth all the time, he does so in the year 2030. Now since each of them only exists at one instant, they may be in the same place but they are in different years, they cannot meet! (In fact in the endurantist view, the problem would also apply to the space station which could only be present in one of the years.)

The predurantist view doesn't suffer this problem. For example if you think of a person like a "worm" stretching through time, the perdurantist says that all parts of their life have an equal claim to existence. Thus the 26 year old Alice meets up with 30 year old Bob and they can have a joint party in 2030. You could also add other siblings who make similar trips at different speeds and the general result is that the whole of Alice's life must exist equally, not just two moments.

OK, let's relate that back to presentism. Imagine a crowd of people all standing still in a hall. Their worldlines are parallel and in the endurantist there's a bright spot moving along each (this is the moving spotlight idea of course) which identifies their 3D existence at any instant. We can draw a plane through all the dots and that must be "the present" since nobody exists at any other time. You can then construct a vector normal to the plane and that identifies a unique axis for time.

In the perdurantist view however, there are no bright spots on the worldlines of people or objects, all parts of ones lifetime have an equal claim on existence, so you cannot construct a unique plane, all planes are equal, and therefore there is no such thing as a uniquely identifiable physical "present". Equivalently it is impossible to identify a preferred direction for time as a normal to that plane.

In the endurantist model, if you extend the plane of the present to include Alice during her trip or project her clock onto the time axis, you can see that the ship's clock ticks off 3 years while one back on Earth the crowd measures 5 years, hence one or more must be affected in some way by some currently unknown physical process (which seems to imply a Lorentzian approach). For the perdurantist view however, proper time is measured along a worldline no matter how it curves so Alice's clock is keeping accurate time even though it registers fewer years between her departure and return than Bob's because the ticks have the same spacing (invariant interval).

In relativity, the model of the 4-dimensional manifold matches that of the perdurantist view in that there is no unique plane we can call the present and perfect clocks in motion maintain the invariant spacetime sepparation of ticks hence relativity seems to require the perdurantist model if the problem outlined at the start is to be avoided, with Alice being in the year 2034 while Bob has only reached 2030.

IMHO, what all that says is that relativity is incompatible with presentism hence I adopt the perdurantist view and, if I understand the link to A versus B correctly, I have voted 'B' accordingly.

lmoh

Well, that was quite a bit to digest, but your argument was suprisingly clear and easy to read through, even to me, a layperson with little background to SR. The argument does create problems for presentism, but I am already aware of these problems SR creates (so again, I am still confused as to what relevance this has to my #48).

I may have to look over it again, but the only point in which I feel needs addressing is to ask whether or not you are also arguing against the moving spotlight theory of time or just presentism in general. Although you seem to view the moving spotlight view and the growing block view as being equivalent to presentism, I personally prefer to distinguish them (which was why I set them apart in my poll) because they each have some unique differences, differences which may poke a hole in your argument if you are also arguing against something like the moving spotlight view.

On another note, I may have to revise my previous answer (I only answered no because I wanted to see what was on your mind!). As I said, though I am currently committed to the A-series, that is as far as I will go on the issue. I am pretty open when it comes to which A-theory to adopt, which also means I am open to the idea of a hidden present within physics that is compatible with relativity. Now whether or not this hidden present entails an aether theory I don't know (Hopefully you can give some of your input here), but I also want to add that I don't even know whether or not I object completely to the possibility of an aether (though I will follow the mainstream view and just say it does not exist).

GeorgeDishman

Sorry I didn't have time to reply before we went away for the weekend.

You said I was trying to show how the manner in which change is measured (by clocks) seems to support eternalism and be against presentism which the poll then equates to B theory rather than A.

I'm a newbie at philosophy so I may not have grasped the subtleties in the terms but my understanding is that the Moving Spotlight, Growing Block and Endurantism are all different varieties of presentism (and there are probably more).

Lorentz's aether theory (LET) assumed there was absolute time and that motion through the aether resulted in phenomena including length contraction and clock slowing. The result of those is that the "preferred frame" could not be determined thus hiding the existence of a "present". If you consider a hidden present exists, I believe you are forced to revert to something indistinguishable from LET and abandon relativity.

I consider GR to be a valid description of gravity, but I also think that the fact that Hulse and Taylor's evidence strongly supports the existence of gravitational waves. If such ripples in the geometry of spacetime can transport energy and momentum across regions of pure vacuum, it seems to me that spacetime must have some physical existence. That philosophy is called substantivalism in general but you will often see it said that the Hole Argument falsifies that. In fact there are two versions and while the Hole Argument can be used against Manifold Substantivalism, it doesn't cause a problem for Metric Substantivalism which says the gravitational field has a physical existence. Manifold Substantivalism is essentially Newton's "absolute space" so the Hole Argument is a problem for Lorentz's aether.

The Stanford article gives a good description of the argument but doesn't differentiate between the forms, essentially only discussing the manifold variant:

http://plato.stanford.edu/entries/spacetime-holearg/

lmoh

Don't worry about it.

I was more referring to Harrylin's remark about time and change, and how it relates to Mctaggart's arguments. By that, I mean the question of whether time requires change, or change requires time (a "literal flow of time"). As far as I can tell, there isn't much of a connection between both. Your response appears to be going a little beyond what I was saying here, but I won't argue the point further.


They all sound more like varieties of the A-theory, then of presentism. I actually think the differences are quite clear, though. Presentism only says that the present exists, but not the past and the future. The growing block says that the past and present exist, while the moving spotlight view states that there exists a block universe in which all physical times exist, but that there is also a second dynamic time in which moves along this block-time. If you want some more information, you can probably do a search, but I don't have any specific links in mind.

They can all probably be a part of presentism if you essentially define it as a belief in the existence of a present in which time flows (which is what I think you are doing). It doesn't matter what the present is (whether physical or not), just so long as it exists. In that way, then, all A-theories are equivalent to presentism, but I personally don't like this definition, because it is misleading.

Also, as for endurantism being equivalent to presentism, I don't really focus much on the issue to really say whether or not that is correct. But it seems as though you disagree with your earlier quoted phrase,

Ok. I often hear people say that a hidden present is compatible with relativity, but I never got what they meant specifically. I only assumed that this "hidden present" was referring to something more broad.

Fair enough. As I said, I have little knowledge of physics so I probably won't be able to do your argument justice, so the best I can do right now is just take your word for it. I will take some time in order to read it over though when I can.

harrylin

It certainly is more broad than just compatibility with interpretation of a certain theory. Concerning relativity, this is already mentioned in the OP's reference as I cited before:
"it can be plausibly argued that the theory [of relativity ] [..] has no bearing on whether there is such a phenomenon as absolute simultaneity."

In other words: a "hidden present", if I correctly understand what you mean with it, is compatible with a Lorentz ether interpretation of relativity but not with a block universe interpretation of the same. The block universe interpretation is now under discussion here: http://physicsforums.com/showthread.php?t=595021

GeorgeDishman

Well if you take a snapshot, you get a static picture so it can be argued that change requires time (which is what the SEP page mentions) but equally "time" can be considered as emergent based on change. However, it isn't as easy to argue that time requires change to exist but it is plausible to say that, even if it exists indepenently of change, something must change if we are to measure it.

I completely agree with that, what they have in common is a hypersurface (the present, the 'new' boundary of the growing block or the spolight plane) which uniquely identifies a physical simultaneity. That doesn't exist in the block universe model.

Almost, I would say presentism is a belief that a unique "present" moment physically identifies a subset of events in a representational block model and that that subset is subject to change, so if you like it could be "belief in a present in which our perception of a flow of time is emergent from the experience of change whether or not time exists in physical sense". I think that's sufficiently general to cover all the above.

I'm not suggesting they are all equivalent, rather seeing them as different but sharing a common reliance on a meaningful "present".

Personally, I don't think a present can exist if GR is correct, my view is that we each have our own "now" which separates what we remember from the rest ('future part') of our lives, and then communication with others creates the false illusion that it is a shared moment. Every moment of your life is "now" when you are experiencing it so no individual "now" is physically identifiable.

I'm not saying they are equivalent, there are other forms of presentism but AIUI the SEP is saying endurantism could not explain change if neither the past nor the future exists and the present is a static snapshot.

Personally, I don't agree with that but that's another matter. It is of course compatible with the Lorentz Transforms but I think the combination forces an aether-like philisophy rather than non-Euclidean geometry and hence rules out GR.

Please don't take my word for it, just see it as an argument I have advanced, I'm very much a beginner in this discipline too and may well be wrong.

lmoh

I'm osrry if this response sounds a little off, since I am just trying to catch up with our earlier discussion. It seems as though you are responding on a weekly basis (which kind of throws me off), even though you come on every other day.

Understood. But applying this definition of "present" to the following:

I don't completely agree here. As I suggested before, arguments from SR or GR cannot affect something like the moving spotlight model, so it is not the knockdown argument that you seem to be suggesting it is. IMHO these arguments have more to say about the physical block time then about the time in which the spotlight/s move (of course, what the "present" refers to in this model would probably be completely distinct from that suggested by presentism and the growing block model).

Your argument would make more sense here if you are arguing that there is no physical present (as in absolute simulataneity of events within the block universe). If you are arguing against presentism as I defined it, then I would be more inclined to agree.

Well, I don't always agree with you, but when it comes to physics, I am not really confident in what I know. Until I take my own time to consider these issues myself, I am willing to concede to what you are saying here. At least you have a physics PhD unlike me .

GeorgeDishman

Sorry, that wasn't intended. I have limited time available and I don't like to rush off a glib reply to topics where some deeper thought is called for. This is an area where I am feeling my way rather than repeating something I've already gone into in detail.

I've had a look down the page and I saw you defined time as a measure of change. That may be reasonable but doesn't in itself lead to a definition of the present. Sorry if you've done it before but can you repeat that definition, just give me a post number if I've missed it.

No, I got a BSc with Hons in 1974 and then went into the electronics industry, I'm here to learn as much as to pass on what I know. :-)

lmoh

No problem. Its best to take a slow and steady approach. That is not to say that I am willing to take a break (I usually don't ), but I often find that taking some time off from certain issues allows me to get a better understanding of the issue, when I am committed to it. We will just see how this discussion goes.

I have tried to read this over, but I am not sure what you are trying to say. I don't think I have said that time being a measure of change would lead to a definition of the present, but I think I have given some ideas of what the present can mean back in #55, where I have defined the present of the A-theory as being the moment in which time flows (this is a crude definition, though). But what that moment is however, can vary among the different theories (presentism, to me, would incorporate a physical present, while the moving-spotlight view would have it placed within a time separate from the block-universe). AFAIK, this does not necessarily say anything about reductionism or platonism with respect to time.

(I never really got what you meant when you responded to my initial remark about time and change a couple of weeks back, I don't know if that was what you are referring to.)


Oh, I thought that degree you referred to was a doctorate. But at least it is still better than having no background at all .

GeorgeDishman

I have read the last few posts and I think we are talking past each other partly.

You poll equates A-series with presentism etc. and B-series with eternalism. I am certainly in favour of some form of eternalism to avoid discarding GR due to the existence of a present but I am less sure about A versus B:

http://en.wikipedia.org/wiki/A-series_and_B-series

http://www.iep.utm.edu/time/#H7

I can see there is a connection but I could also see that it could be related to whether the Big Bang was the start of time, thus every event occurs at some specific co-moving cosmic age and relationships like "before" and "after" are secondary, versus a view that the Big Bang was an event that happened in time while time itself is infinite into the past, thus all event times can only be relative and the Big Bang is merely a convenient origin for our temporal scale. I think I still go with the B-series for my answer but I suspect they may be different decisions.

Darken-Sol

sweet.

Darken-Sol

has any one ever made a mobius strip? when u put a line in the center it ends up back where it started. i have no evidence to back it up but i get a hunch time is kinda like that.

GeorgeDishman

The way we see time behave in experiments leads us to see it slightly differently. One analogy is that it is more like a skater going from one end to the other of a frozen lake, they generally progress towards the destination but with a slight zig-zag to either side. What we call time is the length of the zig-zag path, not the length of the lake.

The Earth's orbit around the Sun means you travel from birth to death in a spiral like a spring with hopefully about 70 turns on the coil. Your age is measured along the helix, not directly between the ends, it is the length of wire needed to wind the coil.

Perdurantism says your life is a coiled spring and equally real at all points, presentism says you are a skater, only real at one spot on the track you leave.

lmoh

I am not sure what you mean here by "A versus B" or what about it you are so unsure about. What is the distinction between "A-series or B-series" and "A versus B"?

They are both certaintly issues within the philosophy of time, but that is as far as I will go. Whether or not time is finite or infinite is not important to whether or not time flows. We can say that time never began flowing or did at the beginning of the universe for the A-series, or just say that there is a block of either infinite or finite size in the case of the B-series (Though the moving spotlight theory kind of incorporates both options).

(I appreciate the discussion, but for now, I will just bow out. I have since lost interest and I am not sure if this conversation is going anywhere. You can respond if you want, maybe I will reply, but I am going to take a break.)

GeorgeDishman

I am unsure whether a choice between "A-series" versus "B-series" is the same as a choice between "presentism" versus "eternalism". The way you asked the poll suggested you see them as being the same.

I got bored typing "series".

It's been interesting but I think we have gone as far as we can so I'll leave it at that too.

Best regards

Evo

Good place to stop.