B What is the nature of spacetime?

[PeterDonis]

The state includes this & that particle at this & that time?

Yes.

an initial inflation & *then* the superluminal expansion?

What "superluminal expansion" are you talking about? If you mean the current expansion, it's only "superluminal" if you look at objects far enough apart and use coordinate speed in standard FRW coordinates, which has no physical meaning anyway.

All the while, the fields are there but, with a certain amount of energy given the state of evolution of ... space?

No, the Standard Model fields initially got their energy when it was transferred from the inflaton field at the end of inflation.

When the energy currently in the electrons/quarks/gauge bosons (the mass of these particles) "arrived" there, the assumed quantum harmonic oscillators that make up these fields expanded, ostensibly?

No. The quantum fields don't "expand"; they just have a state at each event in spacetime. The states of the SM fields at events in the region of spacetime just after the end of inflation are much higher energy than the states of the SM fields at events in the region of spacetime before the end of inflation.

I added "time" to "space" just before i posted... important to keep these things straight.

But you didn't. My comment applies to your post after you corrected it, not before. You are using the word "spacetime", but you're still thinking of it as space--as something that can "expand", "change", etc. That is what you need to fix.

 

[GeorgeDishman]

How do we know that spacetime is "there" and not just an "illusion" made by our brain?

If I have a clock ticking on my desk, I can watch the hands move round for a minute. You could sit opposite me with your own clock and watch yours do the same and a screen between us so we can only see our own clocks. In each case, the motion and even the "passing of time" might only be in my imagination. However, if we start together when one says "go" and put our hands after a minute has passed, they will go up together. What stops our hands going up at different times? They say "time flies when you're having fun" so if I enjoy the experiment more than you, my hand should go up before yours. That suggests both clocks are operated by a physical process which is some way depends on a common aspect of the universe in which we live, an objective measurable we call "time" which we share in the experiment by both clocks being at rest in a common inertial frame and at the same gravitational potential.

There are alternative explanations, your clock showing the time I imagine for example, but they rapidly become indistinguishable from pure solipsism in which "I think therefore I am" but my clock and yours, and in fact you and all the rest of the universe are only figments of my imagination.

There is another simpler argument too, if time is only in our imagination, how did the universe evolve from the hydrogen/helium primordial mix to permit life to exist if time is only imagined by lifeforms? All of this is getting very philosophical though.

 

[tim9000]

OK, I see what you mean but that seems to me to be just a particular way of looking at expansion. In other words, you ARE, I think, seeing is as just geometry, which is what it is. The geometry of spacetime is changing with time in a way that carries things away from each other if they are not bound by gravitational or other forces and you are calling the change a "re-defiintion".

I feel like I might finally be getting somewhere with communicating this difficult subject [for me as a layman]

Forget analogies you read in popularizations - they confuse when thought about carefully enough because they are just meant to get across the flavor.

Here is what's going on. Locally - ie in a small region - you can find a region of space-time that is inertial ie particles move with constant velocity without any force. Its like a sphere - pick any small region and its like euclidean geometry. But overall it behaves a lot differently - continue lines they will meet etc etc. The same with GR - locally its very common-sense - overall you get strange behaviors like curved and expanding space-time that in everyday experience looks like a force between objects etc etc.l

I appreciate that analogies are a double edged sword and this is the subject where they are least effective, but...
I understand you're saying that on the huge scale of the universe and galaxies spacetime looks very different do down here on little old Earth, but that doesn't explain that from our reference frame a galaxy is over there one minute, then over there the next. An ignorant caveman such as myself using merely his own two eyes [and a computer] would conclude that space/distance itself has grown, but that is not the case because space is itself merely a framework.

No. The spacetime is curved because of the presence of the masses; the masses don't add any curvature, they are the source of the curvature to begin with.
No. I'm saying that what you are calling different "instants" are just different places in the same spacetime geometry. The spacetime geometry doesn't change; it already contains all the "instants".
Then you're missing a lot of context. I recommend Sean Carroll's online lecture notes:

https://arxiv.org/abs/gr-qc/9712019
No. There is no such thing. You keep on thinking of spacetime as changing. Spacetime doesn't change. The spacetime geometry already contains all the information about "change" through the entire history of the universe; what you are thinking of as "change" is just different places in the same spacetime geometry. The Big Bang is a place in the spacetime geometry of the universe that is very far away from the place in that geometry where we on Earth today are; and the background radiation we are seeing today is in the same place in the geometry as we are, also very far away from the place in the geometry where the Big Bang is.

Thanks for the link.
Yeah I think I already knew what you were saying about curvature, then again maybe I was saying something wrong and I don't.
Say you pooled all the mass of the universe into some sort of uber-super-massive black hole, would there be the extreme curvature around/in it, then expanding out for ever the curvature of the universe would tend to be euclidian out at infinity?
I'm kind of uncomfortable with this 'spacetime' framework containing all the information of the universe, although I except that you may well be completely correct. But it just sounds like sort of an 'everything is predetermined' sort of thing, that would certainly give fuel to the argument that time is sort of an illusion. But yeah I don't know random quantum fluctuations - God playing dice, the big bang itself doesn't seem to [in my mind at least] fit with that model of existence...also with the heat death of the universe won't all that information be lost anyway...or I suppose the information is just spread infinitely thin until it just exists as a technicality.
Anyway back onto my original point and how you addressed it, taking it to what I infer is its logical conclusion: Space isn't actually getting bigger it kind of is already bigger, just not yet...fair comment?
Like if I took two billiard balls near each other on the table (representing galaxies) then moved them further apart (forgetting that I used energy to do so) and said the time it took to do so made up for the increase in space between them, like they're sort of interchangable sides of the same coin.

If you define the "loaf" this way, then the "loaf" is space, not spacetime.

Okay, the loaf is one axis, the other axis is the oven?
Much like expansion/inflation this thread has gotten away from me again, I'll have to get the the rest hopefully tomorrow.
CHeers!

P.S. Did anyone comment on that 'red shifting' sort of indicating a vector for the planet wrpt the cosmic background radiation? [possibly, I haven't read far ahead in the thread yet to know]

 

[PeterDonis]

that doesn't explain that from our reference frame a galaxy is over there one minute, then over there the next. An ignorant caveman such as myself using merely his own two eyes

You don't see galaxies "jump" from one place to another, discontinuously, with your own eyes. Nor do you see that with any actual measurements. Everything moves smoothly. You seem to be misinterpreting what our cosmological models actually say. They do not say that "space expansion" means things move discontinuously.

Say you pooled all the mass of the universe into some sort of uber-super-massive black hole

That is a different spacetime--a different solution of the Einstein Field Equation. It is not one that can describe the entire universe; it can only describe an isolated object within the universe, excluding the rest of the universe. So this question is not well-defined and doesn't have a meaningful answer.

it just sounds like sort of an 'everything is predetermined' sort of thing

Classical GR is a fully deterministic theory, so yes, "everything is predetermined" as far as GR models are concerned. But GR models don't capture everything about "reality". Our cosmological models based on GR only tell us the large-scale average structure of the universe. They certainly don't tell us every single event at every scale. Cosmology can't predict what you are going to do tomorrow.

I don't know random quantum fluctuations - God playing dice, the big bang itself doesn't seem to [in my mind at least] fit with that model of existence

That's because GR is not a quantum theory. Nor is it a theory of everything. You shouldn't expect it to explain everything.

Space isn't actually getting bigger it kind of is already bigger, just not yet...fair comment?

IMO no. Here is how I would put it: a spacelike slice taken through the observable universe that passes through the event of the Earth "now" has a larger volume than a spacelike slice taken through the same set of worldlines--the same "pieces of matter" that are in the spacelike slice through Earth "now"--at the end of inflation. Notice how I didn't use a single word implying change; I just described two different "slices" taken out of the geometry of spacetime and compared their volumes.

the loaf is one axis, the other axis is the oven?

I don't think this helps any. You need to think of spacetime as a thing, not as a process.

 

[GeorgeDishman]

Like if I took two billiard balls near each other on the table (representing galaxies) then moved them further apart (forgetting that I used energy to do so) and said the time it took to do so made up for the increase in space between them, like they're sort of interchangable sides of the same coin.

It might help to consider the difference between "space" and "spacetime". Objects last for some duration so can you think of two balls that start in contact for a short while then move apart through space as a function of time as being like a forked branch when visualised in spacetime?

 

[Herman Trivilino]

How do we know that spacetime is "there" and not just an "illusion" made by our brain?

I recommend you change that question to the following statement: Spacetime is a model made by our brain.

More generally, this is often a confusion. You're mixing up the model with the thing being modeled. The natural world is what it is. People make models in an attempt to understand it, predict it's behavior, and build stuff.

Once you sort this out in your mind you'll see that your question is based on a misconception. When you study physics you're studying the models, the construction of those models, the use of those models, and the connection between those models and the behavior of Nature.

 

[secur]

Hello tim9000,

The Principle of Relativity (POR) says there is no way to determine, experimentally, an absolute velocity or an absolute reference frame. So any observer can suppose, if he likes, that his own velocity is zero, in his own "rest frame". That led Einstein to dismiss the Newtonian concept of absolute space and time. He didn't actually say it didn't exist, rather it's meaningless scientifically, since we can never determine it. But let's not get into what exactly he did, or didn't, say - that's not important. The point is that in 1905, and still today, the POR holds. Only relative motion has meaning, not absolute.

But today there's a new wrinkle: the Cosmic Microwave Background, CMB. Its existence is not a "law of physics", but it happens - accidentally, as it were - to define a practical "rest frame" for the universe, against which we can compare our motion. It turns out, as you know, that our galaxy is moving through it at a moderate speed, much less than c.

Now, consider those distant galaxies which, naively, are receding from us at speeds approaching c. Standard cosmology says that there are more distant galaxies, receding even faster than light. How can this be, when c is a speed limit that nothing can surpass?

Here's the key fact. According to everything we know, at the location of those distant galaxies, they're moving through the CMB at speeds comparable to ours. So from their point of view, they're nowhere near the speed of light. From their point of view, we're the ones receding at an enormous speed.

The way to make sense of this is to accept that the CMB does, in practice, define an "absolute rest frame" of the universe. All galaxies are moving slowly by that standard. But spacetime itself is expanding. That's why very distant galaxies can recede faster than c, even though relative to their local space they're approximately stationary, just like us.

The speed-of-light limit applies only to our motion relative to local space. It has nothing to say about the expansion of spacetime. That expansion can be (much) faster than light speed.

This is just an intuitive picture, of course, but as such it's valid I believe. Study the FLRW solution of Einstein's field equations, and the physics related to the surface of last scattering (the origin of the CMB) for the exact situation.

 

[PeterDonis]

spacetime itself is expanding

No, space is expanding (at least, on the interpretation you are describing). Spacetime does not expand; it doesn't change. It just is.

 

[secur]

No, space is expanding (at least, on the interpretation you are describing). Spacetime does not expand; it doesn't change. It just is.

True. For something to expand it has to be changing in time, which "spacetime" can't do. In the "Block Universe" all times exist at once - although even the term "at once", or "simultaneously", implies an instant of time, which is not right.

But to say it "just is" implies it must be taken on faith, no further explanation possible - that also is not right. And it brings up Bill Clinton's objection: it depends what you mean by "is".

We don't really have the language to describe the situation; there's no substitute for the math.

Apart from that, don't you agree this is a sensible intuitive picture, getting across the basic idea?

 

[tionis]

I heard that space is doubling itself every some to the minus some every second. Is this true, and if it is, is there a difference between expanding and doubling? To me, expanding space is sort of like stretching and doubling is like exponential. Let me see if I can explain what I'm asking: suppose I take a region of space and I put it inside a geometrical shape, if space is doubling, then I should see the number of geometrical figures increase exponentially, whereas with expansion, I only observe one geometrical shape grow. Which is it?

 

[GeorgeDishman]

I heard that space is doubling itself every some to the minus some every second.

What you saw was talking about the cosmological effect called "inflation" which, if it is correct, finished around 10^-32s after some fiducial "start".

To me, expanding space is sort of like stretching and doubling is like exponential

If it expands by 100%, it has doubled, and if it expands by 100% in every period T, then the result is exponential so there isn't really a difference. If inflation is the correct model, then it doubled very rapidly during that period, perhaps doubling every 10^-34s.

Since inflation stopped, expansion has been much slower and at a variable rate so not exactly exponential. Currently distances are increasing on average by 1% roughly every 140 million years, 10.8 billion years ago it was 1% every 44 million years and in the far future the current indication is that it may settle to a constant value of 1% in about 175 million years.

 

[tionis]

Thanks, George. 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 difference in my mind is that two objects in the same cube of space I referred to earlier, separated by a distance, but without going outside the imaginary boundaries of said cube, would not grow apart if space was doubling itself because it would just duplicate itself around that cube as oppose to the cube growing in size which would then separate the two objects within. My understanding is that every second there is more space than before, so is this because space cannot occupy the same space of another piece of space and so it duplicate itself? Is all very confusing lol.

 

[PeterDonis]

don't you agree this is a sensible intuitive picture, getting across the basic idea?

Not really, since it leads to a number of common misunderstandings, some of which appear in this thread.

Which is it?

Neither, really. When people talk about "expansion of space", they are talking about a feature of a particular set of coordinates. The direct physical observable is redshift--we observe light from distant galaxies to be redshifted, and the redshift gets larger as the galaxies get further away from us (though the distance itself is not observed directly, we observe brightness and angular size and infer distance from that--the actual direct correlation is between redshift and brightness/angular size). The "expansion" is what we get when we put all this data into a model using FRW coordinates and look at coordinate distances as a function of coordinate time. None of this implies the things you are saying.

 

[tionis]

Neither, really.

Hi Peter. I found the video. The statement starts @ 31:15. Is the professor wrong?

 

[PeterDonis]

I found the video.

We're not talking about pop science interpretations. Yes, many pop science sources will talk about "space expanding". So will professors when they are trying to give analogies to lay people. But the actual physics is what I described.

Is the professor wrong?

He isn't "right" or "wrong". He's talking about an interpretation, not about the actual physics. He's not trying to give you an actual physical model that you can use to draw physical conclusions. He's just making an analogy for people who don't know the physical model and don't want one.

 

[tionis]

He isn't "right" or "wrong".

But Peter, he gave a very specific number for the doubling of space every 10^-35/s . Where did he pull this one from lol?

 

[PeterDonis]

he gave a very specific number for the doubling of space every 10^-35/s

First, he's talking about the inflationary epoch there. It has nothing to do with what's happening today.

Second, that number still describes a coordinate-dependent quantity, at least if you're going to use the phrase "doubling of space" to describe it. You have an equation in the model where the spatial scale factor is an exponential function of time. The number he gave is just the time constant in the exponential. But if you're looking for an actual physical invariant described by that number, it isn't "doubling of space"; the invariant is the rate at which comoving observers see other nearby comoving observers accelerating away from them (the technical definition is the expansion scalar of the congruence of comoving worldlines).

So the professor is right that the number he gave occurs in the model and has a specific technical meaning there. But he's neither right nor wrong, physically speaking, when he says that the number describes "doubling of space"; that's an interpretation, not the actual physics.

 

[tionis]

Awesome explanation. Thank you!

 

[secur]

Apart from that, don't you agree this is a sensible intuitive picture, getting across the basic idea?

Not really, since it leads to a number of common misunderstandings, some of which appear in this thread..

But any intuitive picture leads to misunderstandings. If it were exactly correct it wouldn't be an "intuitive picture", rather it would be "real physics". As you say, in this thread, there are a number of common misunderstandings due to pop-sci "intuitive pictures". That's unavoidable when teaching any complex subject. Leonard Susskind mentions somewhere that what you tell beginners must be a bit wrong; you can't nail it all down in the first hour. The goal is: by the end of the semester, make sure all those necessarily imprecise statements are corrected in the student's mind.

You can't reject an intuitive picture on the grounds that it isn't exactly right: none are. You could, on those grounds, refuse to give a beginner any intuitive idea at all, just throw them into the most difficult details, sink or swim. But that goes against basic and universal pedagogical practice.

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. In all human experience, only the present "is", not the past or future. When the student hears that statement, they get an intuitive picture which has some validity ("Block Universe") but strictly speaking makes no sense. It "leads to a number of common misunderstandings", to say the least. Does that mean it's no good? No. By the end of the semester they'll understand what you mean by this statement which is, prima facie, simply a grammatical error.

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.

 

[cube137]

Not really, since it leads to a number of common misunderstandings, some of which appear in this thread.
Neither, really. When people talk about "expansion of space", they are talking about a feature of a particular set of coordinates. The direct physical observable is redshift--we observe light from distant galaxies to be redshifted, and the redshift gets larger as the galaxies get further away from us (though the distance itself is not observed directly, we observe brightness and angular size and infer distance from that--the actual direct correlation is between redshift and brightness/angular size). The "expansion" is what we get when we put all this data into a model using FRW coordinates and look at coordinate distances as a function of coordinate time. None of this implies the things you are saying.

The above is interesting description. So one can say space is really an illusion as we are only inferring from the brightness/angular sizes and redshifts and inferring "distance" from them as well as any expansion. This may be why quantum entanglement works and why space can be united with time because space is not what we think it is.. only an illusion. But then when we see a car moving in front of us. Can we say we only observe the other car sizes, colors and ours and infer spaces from that. Or is space only an illusion when referring only to redshift or Big Bang? How do you apply it to everyday spaces?

 

[PeterDonis]

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?

 

[GeorgeDishman]

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.

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.

 

[GeorgeDishman]

... in the far future the current indication is that it may settle to a constant value of 1% in about 175 million years.

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.

 

[Dale]

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.

 

[phinds]

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.

 

[Vanadium 50]

It might be some other kind of error, but it certainly is not a grammatical error.

Colorless green ideas sleep furiously.

 

[tionis]

Colorless green ideas sleep furiously.

What does this means?

 

[phinds]

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.

 

[Dale]

Colorless green ideas sleep furiously.

Perfect!

 

[tionis]

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.

 

[secur]

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!

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.

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.

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.

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.

 

[Dale]

"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.

 

[PeterDonis]

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?

"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".

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.

 

[GeorgeDishman]

This is (unlike grammar) the heart of the matter.

Thank you.

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 ;-)

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.

There are infinite such divisions, one for every observer.

One for every event, not every observer.

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.

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.

 

[secur]

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.

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.

 

[PeterDonis]

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.

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?

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.

 

[Dale]

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.

 

[GeorgeDishman]

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.

 

[PeterDonis]

This seems to be a good point at which to close the thread.