What is the nature of spacetime?

In summary: I am still having trouble wrapping my head around.In summary, spacetime is a framework that includes geometry and concepts for time. It is not a metaphysical thing. Expansion is just things getting farther apart, and the "big rip" is not in the cards based on our current understanding of cosmology.
  • #71
secur said:
any intuitive picture leads to misunderstandings

Any intuitive picture will involve some level of misunderstanding; but some much more than others.

Also, if the misunderstandings involved in a particuliar intuitive picture are precisely the ones that a given discussion is trying to correct, then that intuitive picture isn't appropriate for that discussion, is it?
 
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  • #72
secur said:
Note that your assertion "spacetime just is" is also an intuitive picture. To say the past "is", or the future "is", is meaningless in the English language. Correct is: "the past was" and "the future will be". This is not just a quibble, and it's not just about grammar.
It is about grammar in the sense that the language doesn't have a construct that allows this view to be expressed at all, but that is a shortcoming of the language, not a problem with what is to be described.
secur said:
In all human experience, only the present "is", not the past or future.
However, in the block universe view, there is no such thing as "the present", while past and future are only relationships to some other event within it. I might "live in the present" as a policy but in spacetime my life has an extent which is fixed, delimited by events that might be called "birth" or "conception" at one end and "death" at the other. My past is the portion of that extent which lies between "birth" and whatever other event along my worldline is being considered.
 
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  • #73
GeorgeDishman said:
... in the far future the current indication is that it may settle to a constant value of 1% in about 175 million years.
tionis said:
What I heard the professor say is that space is currently expanding by that amount every second. I think he was referring to dark energy.
The value of 1% in about 175 million years is what results from dark energy, the much higher rate you mention is what it was during inflation.
 
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  • #74
secur said:
To say the past "is", or the future "is", is meaningless in the English language. ... this statement which is, prima facie, simply a grammatical error.
It might be some other kind of error, but it certainly is not a grammatical error. "The past" is a valid English noun, and "is" is a valid English verb, and a noun followed by a verb is a valid English sentence structure. So "The past is" is not an English language grammatical error.
 
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  • #75
secur said:
To say the past "is", or the future "is", is meaningless in the English language. Correct is: "the past was" and "the future will be". This is not just a quibble ...
You are right about the last part. As has already been pointed out, it is NOT "just a quibble" it is flat wrong.

Just as one example, how would you deal with saying "the past is behind us and the future is ahead of us" ? To say "the past WAS behind us" is true but refers to a state that existed some time ago. "The past is behind us" is a correct statement of current condition.
 
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  • #76
Dale said:
It might be some other kind of error, but it certainly is not a grammatical error.

Colorless green ideas sleep furiously.
 
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  • #77
Vanadium 50 said:
Colorless green ideas sleep furiously.
:oldlaugh: What does this means?
 
  • #78
tionis said:
:oldlaugh: What does this means?
Seems to me like a planned example of a grammatically correct statement that is meaningless to demonstrate that grammar and meaningfulness are not necessarily related.
 
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  • #79
Vanadium 50 said:
Colorless green ideas sleep furiously.
Perfect!
 
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  • #80
phinds said:
Seems to me like a planned example of a grammatically correct statement that is meaningless to demonstrate that grammar and meaningfulness are not necessarily related.
lol that is one funny phrase.
 
  • #81
secur said:
This pedagogical issue, although quite germane to this thread, is tricky. It demands expertise in theory of education, language, philosophy. So it's really not worth pursuing.

Don't say I didn't warn you!

Dale said:
It might be some other kind of error, but it certainly is not a grammatical error. "The past" is a valid English noun, and "is" is a valid English verb, and a noun followed by a verb is a valid English sentence structure. So "The past is" is not an English language grammatical error.

From the first hit google gives for "grammatical error": "Grammatical error is a term used in prescriptive grammar to describe an instance of faulty, unconventional, or controversial usage, such as a misplaced modifier or an inappropriate verb tense."

"The past is" (meaning, it exists right now) uses an inappropriate verb tense.

phinds said:
Just as one example, how would you deal with saying "the past is behind us and the future is ahead of us" ? To say "the past WAS behind us" is true but refers to a state that existed some time ago. "The past is behind us" is a correct statement of current condition.

"The past is behind us" is fine grammatically, a correct metaphorical statement of current condition.

Certainly, the point is disputable. For those of us with a classical education - I learned Latin grammar before English grammar - it's barbaric to use the present tense for past events, much less future. But today the "you are there" style of history is common. "Suddenly Burgoyne throws his redcoats at the Continental Army's left flank. Will the exhausted patriots hold the line? In the driving snow, Washington makes his way to the front, ...") Since you grew up with this sort of thing, your feel for language (such as it is) is very different from mine. Furthermore I admit that "past is" and "future is" can be correct as metaphor, or a statement of mystical belief. But none of this matters, having nothing to do with physics.

"The past is", and "the future is", are strange ideas. That doesn't mean they're not "true". Many brilliant, well-respected mystics have preached that past and future are immediately present to the mind of Nature. Predestinarianism is a Catholic heresy, but IMHO that's a point in its favor. So your deep faith in pre-destination may be justified. Who knows? It's an opinion, an interpretation, an ontology: philosophy. Not science.

GeorgeDishman said:
It is about grammar in the sense that the language doesn't have a construct that allows this view to be expressed at all, but that is a shortcoming of the language, not a problem with what is to be described.

Fine, but I think there's also a problem with what is to be described.

GeorgeDishman said:
However, in the block universe view, there is no such thing as "the present", while past and future are only relationships to some other event within it. I might "live in the present" as a policy but in spacetime my life has an extent which is fixed, delimited by events that might be called "birth" or "conception" at one end and "death" at the other. My past is the portion of that extent which lies between "birth" and whatever other event along my worldline is being considered.

This is (unlike grammar) the heart of the matter. One can argue that Minkowski space "proves" the Block Universe. The theory doesn't allow us to define a universal present, past and future along some absolute time line. There are infinite such divisions, one for every observer. Time must be treated just like a space axis, with every point in past or future "existing now".

The theory certainly allows Block Universe interpretation, But SR and GR math doesn't require it. Minkowksi space is also compatible (more or less) with the normal ontology: the past was; only the present is; the future is not yet, but will be. If anyone cares I can explain why that's so, but of course it's off topic.
 
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  • #82
secur said:
"The past is" (meaning, it exists right now) uses an inappropriate verb tense.
Why? What specific grammatical rule does it violate which makes it inappropriate? Nouns, even a noun like "the past" don't have a tense, so using them with present tense is appropriate grammatically.
 
  • #83
secur said:
For those of us with a classical education - I learned Latin grammar before English grammar - it's barbaric to use the present tense for past events, much less future.

You do know that the "historical present" was used in Latin, right?

secur said:
"The past is", and "the future is", are strange ideas.

Not in relativity. "The past" and "the future" are just names for particular regions of spacetime, and when considering physical models in relativity, one is considering models in which spacetime is a geometry, an object that does not "change" or "evolve" but just "is". What we experience as temporal relationships and "change", in the spacetime model are geometric relationships that do not "change".

secur said:
Minkowksi space is also compatible (more or less) with the normal ontology: the past was; only the present is; the future is not yet, but will be.

Only on appropriate interpretations of "past", "present", and "future". And on such interpretations, there is another region, which you haven't mentioned and which does not even have a name in ordinary language: the spacelike separated region, which Roger Penrose calls "elsewhere" in The Emperor's New Mind. This region is not "past", because it can't causally affect us in the "present" (here and now); it's not "future" because we can't causally affect it; and it's not "present" because it's not here and now and there's no invariant way to pick out events that are happening "now" but not here. So the "normal ontology" leaves something crucial out.

Basically, you're trying to use your own ideas about grammar to constrain how we can use language to describe physics. That's problematic in two ways. First, since we're describing physics, the structure of the physics, not the structure of the language we use to describe it, must take precedence. Second, physical theories are not formulated in ordinary language; they're formulated in math. So any ordinary language description is going to be at best an approximation, not a precise description, and quibbling about fine points of grammar when the whole description is already known to be imprecise seems pointless.
 
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  • #84
secur said:
This is (unlike grammar) the heart of the matter.
Thank you.
secur said:
One can argue that Minkowski space "proves" the Block Universe.
That's a bit general, I would say the Twins Paradox is indicative but I suspect a dozen people would give you a dozen reasons, philosophy is like that ;-)
secur said:
The theory doesn't allow us to define a universal present, past and future along some absolute time line.
We have to be careful there, I know what you mean but the regions bounded by the past and future light cones of any individual event are invariant.
secur said:
There are infinite such divisions, one for every observer.
One for every event, not every observer.
secur said:
Time must be treated just like a space axis, with every point in past or future "existing now".
Yes and no, we can treat it like a spatial axis though directed by the arrow of time, but you cannot say the past and future are "existing now" since "now" is a location on that axis which denotes the boundary between "past" and "future", it makes no sense in talking of a line to say "P is the set of all points to the left of point N, and all of those points also exit at point N.". The closest I could suggest would be "Time must be treated just like a space axis, with every instant in past or future "existing always" but that is also wrong since it implies events stretch for the full length of the axis. That's what I meant by the language being inadequate.
secur said:
The theory certainly allows Block Universe interpretation, But SR and GR math doesn't require it. Minkowksi space is also compatible (more or less) with the normal ontology: the past was; only the present is; the future is not yet, but will be. If anyone cares I can explain why that's so, but of course it's off topic.
I'd like to see how you would apply that to the Twins Paradox, I don't think you can do it without invoking a preferred frame (think of the Moving Spotlight philosophy as an alternative), but I believe the FAQ for this forum says discussion of the Block Universe is forbidden and I guess alternative philosophies are too so please send me a P.M. if you want to take it farther.
 
  • #85
Ok, historical present tense is legitimate grammatically, so I was wrong. However, Virgil and Caesar never meant the past event actually "exists now" (or, "exists always", whatever); it was just a rhetorical device. There's a huge difference between such poetic license, and actually believing in an "existing" past, or a pre-destined "existing" future. Such faith is given only to holy mystics and theoretical physicists.

PeterDonis said:
Basically, you're trying to use your own ideas about grammar to constrain how we can use language to describe physics.

No, although perhaps it sounded that way. I'm just trying to get across how radical and unsensical "Spacetime just is" is. So radical it can't even be stated correctly with language, or comprehended by a human mind. For us mortals trapped in time, "exist" means - can only mean - exists now. As we all know, we must turn to the math for a precise statement. "Spacetime just is" is (at best) only an intuitive picture.

Pre-destination is philosophy not science. You don't seem to realize how questionable it is, and how solid the evidence must be if you expect others to go along. Telling physics students they must take it on faith is guaranteed to drive away those who think. That's good for them - they can make more money elsewhere - but bad for physics. And there are other, equally dubious, philosophical stances they must sign up to if they want that "A".

But here's the real problem: the math doesn't actually support the statement. First let's look at SR, and the common misconception that it forces Predestinarianism.

The POR says that all inertial frames are "equally good". We have no way of identifying Nature's "preferred frame". Therefore - and here's the mistake -, there isn't one. But in fact, POR allows us to arbitrarily identify any frame as "preferred" and the math will still work. It's true that, by Occam's razor if nothing else, this is unjustifiable. But for the sake of argument let's suppose that at every spacetime point the velocity which makes the CMB isotropic defines the "preferred" frame. Then obviously Nature can use this to distinguish past, present and future without contradiction.

Identify a "sheet" of "preferred" points, for every "instant of time", such that the following holds. The union of their past light cones (the "past") is disjoint from the union of their future light cones, and each disjoint from the "present" sheet. The union of the three is all of spacetime. Furthermore all such present-sheets are disjoint and their union is all of spacetime.

Then Nature can arrange it so that at every "present" point, the past has been but is no more, the present is, and the future is not yet, but will be.

It's very important to note, this "preferred" frame doesn't actually have to be real. Suppose we have some way to know for certain - independent of the actual SR math - that no preferred frame exists. For instance, Einstein (blessed be his name!) said so: i.e., it's Revealed Truth. My proof still demonstrates that pre-destination is not entailed by the actual SR math! It's a pretty subtle point, but I'll hope you can understand it without further explanation. If not let me know. There are, BTW, other ways to demonstrate this obvious fact, but this is the simplest.

What about GR? The same proof works fine, except for extreme circumstances like singularities and wormholes, where it becomes impossible to partition spacetime in this manner. But the following General Rule applies (and also to similar circumstances that might arise in String Theory, etc). If it seems to prove pre-destination, it has no experimental support.

"Preferred frame" is verboten, but that applies only to the claim that it's "true". I'm using it as a hypothetical device to show that pre-destination is not entailed by SR. Let no man accuse me of actually believing this blasphemy! I've got enough troubles already without the Inquisition after me.

Indeed, the fact that these topics are verboten is the best proof that pre-destination is unprovable. If it were real science, you wouldn't need to prohibit questions, because you'd have answers. Doubters must be burned at the stake, instead of convinced by reason, only when the doubt is justified.
 
  • #86
secur said:
There's a huge difference between such poetic license, and actually believing in an "existing" past, or a pre-destined "existing" future.

Using the analog of the "historical present" in relativity does not mean you have to believe in the "block universe" view. It can just as well be a "rhetorical device", intended to briefly describe the physics in ordinary language while realizing that the description is not completely accurate. No ordinary language description of physics will be completely accurate; ordinary language is too vague and imprecise. If you really want a precise description, you use math, and then all of the issues you're discussing simply don't exist.

secur said:
As we all know, we must turn to the math for a precise statement.

Yes. So basically you are saying "the picture we get using ordinary language is imprecise". We all agree on that. So what more is there to discuss?

secur said:
the math doesn't actually support the statement.

The math does not require the block universe interpretation. That's true. I wrote a whole Insights article about it:

https://www.physicsforums.com/insights/block-universe-refuting-common-argument/

Nobody in this thread has claimed otherwise, as far as I can see.

Basically you seem to me to be arguing against positions that nobody in this thread is taking.
 
  • #87
secur said:
I'm just trying to get across how radical and unsensical "Spacetime just is" is. So radical it can't even be stated correctly with language, or comprehended by a human mind.
This is clearly wrong. Part of the appeal of the block universe idea is how easily comprehended it is.

I find your repeated religious references completely inappropriate, particularly the comparison with the Inquisition. We issue warnings and delete content, we don't torture and kill.
 
  • #88
PeterDonis said:
The math does not require the block universe interpretation. That's true. I wrote a whole Insights article about it:

https://www.physicsforums.com/insights/block-universe-refuting-common-argument/

Nobody in this thread has claimed otherwise, as far as I can see.
I haven't said anything in the thread but just for the record, I do feel relativity is problematic for presentist philosophies. However, I think your Insight article is absolutely correct, the Andromeda Paradox is not a sound basis for the argument.
 
  • #89
This seems to be a good point at which to close the thread.
 
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