I Is Time Just a Stubborn Illusion in the Block Universe Theory?

[jbriggs444]

I find this way of picturing things much more sane than to speak of future light cone that exist nowhere in any frame of reference.

What makes you say that the future light cone does not exist in any frame of reference?

 

[tophatphysicist]

As far as the laws of physics, that means that we can't confirm by evidence that the laws of physics apply outside our past light cone.

So, your point is that by not accepting the Block Universe, it is not that this one concept is not accepted as uniquely unconfirmed, rather it is not accepted along with all physical laws and theories not accepted (because nothing can be confirmed that is outside the light cone). For the longest time I was assuming that Block Universe was being uniquely identified by the Physics Forum as a major concept that has been falsely presented as a validated concept. I think it might have a different impact on visitors to the Forum to understand the absence of validation is in the context of the more general principal that there can be no proof of theories (or predictions) regarding events outside of the light cone.

Regarding Block Universe specifically, then, notwithstanding the understanding that Block Universe applies to universe past, present and future, is it accepted that the relativity of simultaneity leads to the concept that space is 4-dimensional and fixed within the past light cone -- and that this conclusion is accepted as valid by the Physics Forum and physics community at large?

 

[PeterDonis]

So, your point is that by not accepting the Block Universe, it is not that this one concept is not accepted as uniquely unconfirmed, rather it is not accepted along with all physical laws and theories not accepted (because nothing can be confirmed that is outside the light cone).

No. I didn't say we couldn't accept that the laws of physics apply outside our past light cone. I just said we couldn't confirm this, since the only way to confirm it would be by collecting evidence, and we can't collect evidence outside our past light cone.

You also seem to be ignoring the fact that "our past light cone" is not a fixed region of spacetime. It grows as we move into the future along our worldline. So even if we can't confirm today that the laws of physics apply in the region of spacetime that is currently outside our past light cone, tomorrow we will be able to confirm it for some portion of that region (the region that is less than one light-day outside our past light cone today).

For the longest time I was assuming that Block Universe was being uniquely identified by the Physics Forum as a major concept that has been falsely presented as a validated concept.

Go back and read my article. I did not say anywhere in that article that the Block Universe was false. I just said that one particular argument that was presented about it was not a valid argument. That's all I said.

is it accepted that the relativity of simultaneity leads to the concept that space is 4-dimensional and fixed within the past light cone

No, because you don't need relativity of simultaneity in order to know that events in your past light cone are fixed and certain. You can observe those events directly without having to know anything about simultaneity at all.

You really need to stop inserting your own preconceptions into what you read.

 

[Ebeb]

... that events in your past light cone are fixed and certain. You can observe those events directly ... .

No, you cannot observe them directly. You observe them after they occurred.

You also seem to be ignoring the fact that "our past light cone" is not a fixed region of spacetime. It grows as we move into the future along our worldline. So even if we can't confirm today that the laws of physics apply in the region of spacetime that is currently outside our past light cone, tomorrow we will be able to confirm it for some portion of that region (the region that is less than one light-day outside our past light cone today).

Does the above confirms that you accept there are events outside the lightcone before they enter into the light cone as observer's time progresses?
Because striclty speaking one doesn't know whether there are events outside the lightcone. Correct? And one -strictly speaking- doesn't know that when you do observe an event, that that event was/existed/occurred outside the lightcone before it is observed.
Does this mean that - and I guess this is what Tophatphysicist is getting at- that special relativity can not necessarily be applied to that zone?
Hence all the simultaneity planes through the apex of lightcone pictures on the internet are wishfull thinking?
Still trying to understand you.

 

[Boing3000]

What makes you say that the future light cone does not exist in any frame of reference?

Projection don't exist, hence the name. The future does not exist, it will eventually. The past ends at the present. That does not preclude us to use projection/prediction,but they are entirely based on the past.

Another way of saying that is I (a particle with rest mass (but no gravitas obviously )) am certainly never going to catch up/intersect with any light cone emitted at any time from my past to "observed" its existence. But other will, mostly mirrors...

I think it is a pretty straightforward definition of the growing block universe

 

[PeterDonis]

You observe them after they occurred.

Yes, which means that when you observe them, they are in your past light cone. That is what I meant by "observe them directly"--you get light signals direct from the events. You don't have to make any calculations or do any extrapolation or prediction.

Does the above confirms that you accept there are events outside the lightcone before they enter into the light cone as observer's time progresses?

No.

Because striclty speaking one doesn't know whether there are events outside the lightcone. Correct?

No, because you haven't observed them yet.

And one -strictly speaking- doesn't know that when you do observe an event, that that event was/existed/occurred outside the lightcone before it is observed.

More precisely, you have no way of testing by experiment whether or not the event "existed" before you observed it--because any experiment would involve observation.

Does this mean that - and I guess this is what Tophatphysicist is getting at- that special relativity can not necessarily be applied to that zone?

No, because SR is a model. You can make a model of (hypothetical) events that you haven't observed yet. But the model is not reality. You can't make claims about the events being "real" based on the model. The model is a tool for prediction, not for telling you what's "real".

 

[Dale]

If you start f.ex. with different postulates for explaining phenomena -whether you use different mathematical formulae or not is irrelevant- , then you have a different theory, or not?

That isn't the way I would categorize different theories, but at least it is clear and I can understand what you are saying. You are saying that a theory is defined by its postulates or axioms, I am saying that it is defined by its experimental predictions. I think your usage is non standard, but I don't have an official definition either way.

I would appreciate you get back to me when you do know whether there is a difference between theory and philosophical interpretation

I do know that I (and many of the forum regulars) use the terms as I have described. I do not know if there is an official authoritative source where those terms have been defined. That is why I have made my usage explicit and asked you to do the same.

Suppose one day we develop a set of specific BU mathematical formulae (other than LT), producing same experimental (prediction) results as SR or LET does. Would you call it a different theory than SR?

Based on what I have previously stated defines a theory what do you think I would say?

Hence, when one would say that SR uses 4D objects/units -I agree-, but refute BU, then I'm lost. Completely lost.

Who refuted BU? I certainly did not. I wouldn't even attempt to refute any interpretation. A refusal to accept is not the same as a refutation.

 

[Dale]

So, your point is that by not accepting the Block Universe, it is not that this one concept is not accepted as uniquely unconfirmed, rather it is not accepted along with all physical laws and theories not accepted (because nothing can be confirmed that is outside the light cone).

That is not my point. My point is that the Block Universe is an interpretation, not a theory. As an interpretation it makes no physical predictions that distinguish it from other interpretations.

I accept or reject physical laws and theories on the basis of their experimental predictions. Interpretations (including the Block Universe) make no new predictions, so they do not need to be accepted or rejected.

 

[Ebeb]

Who refuted BU? I certainly did not. I wouldn't even attempt to refute any interpretation. A refusal to accept is not the same as a refutation.

It's not about what my post is about. I'll try again:
You were very clear in that other thread that -your quotes (from this post)- <<Clocks are 4D objects.>><<Measuring sticks are 4D objects.>>
I still don't understand what you mean by 4D objects. Because: 4D objects means: the full worldline. Past present and future in one unit. Block Universe is a 4D object.
Measuring a 4D object is: measuring the 4D block. If SR deals with 4D objects, then SR is about Block Universe. How can SR not be about Block universe if you tell me clocks and measuring sticks are 4D objects? That is what I still don't understand and what I tried to communicate in my post.

Just curious; would you say that in LET the clocks and measuring sticks are also 4D objects?

 

[jbriggs444]

Projection don't exist, hence the name.

The word projection does not appear in the phrase "frame of reference". If you want to restrict a frame of reference to your past light cone that seriously limits your ability to use that frame of reference to make any predictions.

 

[Ebeb]

Yes, which means that when you observe them, they are in your past light cone. That is what I meant by "observe them directly"--you get light signals direct from the events. You don't have to make any calculations or do any extrapolation or prediction.

No.

No, because you haven't observed them yet.

More precisely, you have no way of testing by experiment whether or not the event "existed" before you observed it--because any experiment would involve observation.

No, because SR is a model. You can make a model of (hypothetical) events that you haven't observed yet. But the model is not reality. You can't make claims about the events being "real" based on the model. The model is a tool for prediction, not for telling you what's "real".

This means you don't accept there definitly are events outside the light cone. Hence you only consider your present 'now' as something you are sure of of existing 'now'. Correct?

When Einstein asked whether the moon still exists when he doesn't look at it, you would answer: "I don't know"?
I see the moon. I close my eyes and don't see the moon. I open my eyes, and see the moon again. But you won't agree the moon was definitely still there when I closed my eyes?
Well, no offence, strictly speaking you might be correct, but if this is the kind of "physics" PF sticks to for discussing special relativity...

 

[Dale]

You were very clear in that other thread that -your quotes (from this post)- <<Clocks are 4D objects.>><<Measuring sticks are 4D objects.>>

Yes, experimentally measurable (physical) objects have nonzero extension in the time dimension, so they are 4D.

I still don't understand what you mean by 4D objects

I mean an object which has nonzero length, width, height, and duration. Nothing more or less. This is an observable fact, not a theoretical statement.

Because: 4D objects means: the full worldline. Past present and future in one unit.

That is the BU interpretation. It is consistent with the physical observations, but not the same as the physical observations. The physical observations are that objects have some nonzero extension in time, not that they extend into the future.

How can SR not be about Block universe if you tell me clocks and measuring sticks are 4D objects?

In my usage SR is the theory, meaning it is a mathematical framework (Lorentz transforms) together with a mapping from the math to the outcome of experimental measurements (sometimes called the minimal interpretation). The BU uses the same math and mapping to experiment as LET, so both are interpretations of SR in my usage.

The observation that objects are 4D is an experimental fact. It is not "owned" by any specific theory or interpretation. That is something that each interpretation needs to explain, and different interpretations do so differently.

Just curious; would you say that in LET the clocks and measuring sticks are also 4D objects?

Physically, yes. It is an observed fact independent of the interpretation.

What typical LET does is claim that "reality" is not physically detectable and that only an undetectable 3D slice actually "exists". This is precisely why I focus on questions of "physical" rather than questions of "reality" or "existence"

 

[PeterDonis]

4D objects means: the full worldline.

It means a 4D region of spacetime, the "world tube" of the object. It does not necessarily mean that that "world tube", or at least the part of it that is treated as fixed and certain by the model, must extend indefinitely into the future. You can construct models that only have a finite 4D region that is fixed and certain.

 

[PeterDonis]

This means you don't accept there definitly are events outside the light cone.

It means I don't treat events outside the past light cone as fixed and certain, when looking at model. I didn't say anything about whether or not events outside the past light cone "really exist", or are "definite" or any other vague ordinary language term. It's you who keep getting hung up on these words instead of the physics, not me.

Hence you only consider your present 'now' as something you are sure of of existing 'now'.

I haven't said anything about "existing". It's you who keep getting hung up on these words instead of the physics, not me.

When Einstein asked whether the moon still exists when he doesn't look at it, you would answer: "I don't know"?

I would ask him what physical predictions depend on whether the moon still exists when he doesn't look at it, so that we could test the question by experiment.

I see the moon. I close my eyes and don't see the moon. I open my eyes, and see the moon again. But you won't agree the moon was definitely still there when I closed my eyes?

How would you propose to test it experimentally?

if this is the kind of "physics" PF sticks to for discussing special relativity...

You are perfectly free to have beliefs or opinions about things that can't possibly be tested by experiments or evidence. But as far as PF is concerned, they're not "physics".

 

[Khashishi]

It would be helpful if someone stated some alternatives to BU.

[Moderator's note: off topic speculations deleted.]

Another alternative is that space-time is an approximate model which breaks down at some point. Maybe Holographic theory or something with networks or some noncommutative geometry -- who knows?

Another alternative is a many worlds universe. All possible futures exist. All possible pasts and presents also exist, but my consciousness happens to inhabit one subjective present, perhaps with decoherence.. This is consistent with the idea of block universe, but perhaps it should be called the block multiverse.

 

[PeterDonis]

It would be helpful if someone stated some alternatives to BU.

Alternatives for what purpose? Remember that discussion here is focused on physics, i.e., what can be tested by experiment and evidence. If you're just looking for alternatives that make all the same experimental predictions but tell some different story, that's off topic here.

 

[PeterDonis]

Another alternative is that space-time is an approximate model which breaks down at some point.

There is already plenty of theoretical work being done on alternatives of this sort that might make testable predictions. Discussion of that is more appropriate in the Beyond the Standard Model forum (or in some cases we have allowed threads on quantum gravity papers in this or the quantum physics forum). It's off topic in this thread since the model under discussion here is specifically SR.

Another alternative is a many worlds universe.

Discussion of this belongs in the Quantum Physics forum, not here.

 

[Dale]

I haven't said anything about "existing". It's you who keep getting hung up on these words instead of the physics, not me.

I would add that the specific words "exist" and "real" are philosophical (metaphysics) rather than physical.

 

[Boing3000]

The word projection does not appear in the phrase "frame of reference".

I don't see which phrase you are referring to, but that's correct, projection are not real things, neither are frame of reference. They both are mental map.

If you want to restrict a frame of reference to your past light cone

That's exactly what I want to do, and as far as I am aware of, what physics do. All the coordinate input of all equations belong to a past light cone of some FoR, whatever the frame. The output might land in the future, and they are called projection/prediction with regard to that frame.
Unless you are talking about hypothetical prediction based on hypothetical situation, and I don't call that physics, but math.
A lots of misconception about relativity comes from comparing clock spatially separated. That's because frame of reference and especially the future light cone are taken as real thing, while they are not.

I have no idea what issue you have with what I say. There was no need for special relativity before some people realize by comparing their past that there was some incompatibilities with their numbers (constant light speed) when relating them galilean'ily. None of those numbers come from the future of any frame of reference.

that seriously limits your ability to use that frame of reference to make any predictions.

I actually think it is the exact opposite: my ability to do prediction is rooted in past observable/fact, not hypotheticals.

The BU is not a physical construct but a vague philosophical object where nobody even bother to specify how to assign the 4 numbers at all point.

The growing block universe is the product of special relativity, and all FoR have only access to a slice of it which is the called the past.

 

[Ebeb]

It means I don't treat events outside the past light cone as fixed and certain, when looking at model. I didn't say anything about whether or not events outside the past light cone "really exist", or are "definite" or any other vague ordinary language term. It's you who keep getting hung up on these words instead of the physics, not me.

I haven't said anything about "existing". It's you who keep getting hung up on these words instead of the physics, not me.

Let's then use events that occur or don't occur. Would that suit you?

I would ask him what physical predictions depend on whether the moon still exists when he doesn't look at it, so that we could test the question by experiment.

How would you propose to test it experimentally?

You are perfectly free to have beliefs or opinions about things that can't possibly be tested by experiments or evidence. But as far as PF is concerned, they're not "physics".

Let's then cencentrate on your "fixed and certain" of your BU article.
Unfortunately after nearly 4 decades dealing with SR I still don't seem to have the intellectual capabilities to grasp the rationale of your article at once, so I have to take it one a step at a time. I would appreciate if you treat them one at a time.

When does an event "car hits tree" get your label "fixed and certain"?
If I understand you correctly, it gets labeled at the moment the event is observed, i.e. at your present event.
Correct?

But the above doesn't mean that you know that the event occurred before you observed it.
Correct?

Hence all events you put in that past lightcone, are events you observed. Otherwise they cannot be in the past lightcone.
Correct so far?

If all the above is correct, then strictly speaking it is only safe (physically safe) to use (apply) the SR model for( to) the past lightcone (= observed events). Because for all the other events (because we don't even know whether there are any other events) it's wishfull thinking. A full model of spacetime including simultaneity lines thorugh the apex is in fact only based upon suppositions and uncertain predictions based on past observations.
Correct?

(3) All events in the past light cone of a given event are real (i.e., fixed and certain) for an observer at that event.
Reference https://www.physicsforums.com/insights/block-universe-refuting-common-argument/

They are fixed and certain, but you don't know whether those events actually occurred before observation that event of observation.
Correct?

In that case I would consider "fixed and certain" a label you put on the image you "receive" in your eye or consciousness, but not on the event itself that occurred, because we don't know there was an event that occurred before observation. (Strictly speaking we don't even know whether we have eyes. Only our consciousness tells us something like that).
Yes, I have to get to the bottom of this, because if you insist on the importance of 'observing an event', then I want to know exactly what you mean by that to be able understanding what you try to convey in your posts and article.

Because we don't know whether events occurred or not before being observed, strictly speaking that means that such a past light cone with observed events is only a mental model representing what your consciousness came up with, because we don't even know that 'seeing' a car smashing the tree means that it occurred before it is seen. (I now start doubting what you mean by observing/seeing an object. You are not sure whether there was an object before observation, hence observing/seeing is just the conscious act of an "observed image" popping up in your mind...?)
If all the above is correct, then I think you should consider 'solipsism' one of the options summed up in your article. But you don't: your article states:

Proposition (2-1) is false,
Reference https://www.physicsforums.com/insights/block-universe-refuting-common-argument/

Hence somewhere along the line I must have lost you.

 

[Boing3000]

In that case I would consider "fixed and certain" a label you put on the image you "receive" in your eye or consciousness,

Or more reliably a sheet of paper.

but not on the event itself that occurred,

real event have no "label". You may describe them like "a photon emission, or the cat died", but only YOU can give it some 4D label with your FoR.

because we don't know there was an event that occurred before observation.

That physicts 101, we don't use seer detector in physicts.

(Strictly speaking we don't even know whether we have eyes. Only our consciousness tells us something like that).

If you get someday in a dramatic car crash, you may be in a position where you don't have eyes anymore no matter what your consciousness tell you.

strictly speaking that means that such a past light cone with observed events is only a mental model representing what your consciousness came up with,

Observation labelled in meter an second are not 'invented', they are observed/measured. Consciousness have nothing to do with that.

because we don't even know that 'seeing' a car smashing the tree means that it occurred before it is seen.

Quite the opposite: only after seeing/recording the event, you can know for certain where and when it occurred in your past. This is basic causality.

 

[Ebeb]

Yes, experimentally measurable (physical) objects have nonzero extension in the time dimension, so they are 4D.

I mean an object which has nonzero length, width, height, and duration. Nothing more or less. This is an observable fact, not a theoretical statement.

I.o.w. considering an object that continues to exits (withiut past/present/future 'existing' out there) as time goes by is sufficient to label that whole proces a 4D object?
I start seeing what you mean: in fact you use a part of spacetime model, and call it 4D, but it doesn't mean that past/present/future "co-exist" in that 4D object.
I find this a bit sloppy. I would only use 4D object for something that somehow exists in its globality. But because I get the impression that "objects before they are observed" are not part of physics language, my interpretion of 4D object actually doesn't make sense. Is that it?

That is the BU interpretation. It is consistent with the physical observations, but not the same as the physical observations. The physical observations are that objects have some nonzero extension in time, not that they extend into the future.

In my usage SR is the theory, meaning it is a mathematical framework (Lorentz transforms) together with a mapping from the math to the outcome of experimental measurements (sometimes called the minimal interpretation). The BU uses the same math and mapping to experiment as LET, so both are interpretations of SR in my usage.

The observation that objects are 4D is an experimental fact. It is not "owned" by any specific theory or interpretation. That is something that each interpretation needs to explain, and different interpretations do so differently.

Physically, yes. It is an observed fact independent of the interpretation.

I personaly would certainly not use the word 4D object in a LET context. In LET there are 3D objects. The fact -in LET framework- time goes by doesn't make them 4D objects. But in SR there are only 4D objects where past/present/future are part of the object. In LET the future and past are not part of the object, because in LET it exists only 'now'. In SR that's a different story.
That's why I find the communication about 4D still so confusing. I think it's because we don't agree SR is only about Block Universe. And my discussion with PeterDonis starts bringing to light that he refutes relativity of simultaneity requiring BlockUniverse because he only wants to take for 100% true the photon hitting his eye, hence not being 100% sure there are events outside his lightcone.

What typical LET does is claim that "reality" is not physically detectable and that only an undetectable 3D slice actually "exists". This is precisely why I focus on questions of "physical" rather than questions of "reality" or "existence"

But by stating that in LET there is -quote- <<an undetectable 3D slice actually "exists">> (actually in LEt only one such a 3D slice does exist) means in LET we do accept and believe there are events simultaneously occurring 'now' with my present now event. But for SR all of a sudden we are not allowed making a statement about 3D slices "existing"?
Seems LET concentrates more on 'reality' and 'existing' than SR does? Really?

 

[jbriggs444]

That's exactly what I want to do, and as far as I am aware of, what physics do. All the coordinate input of all equations belong to a past light cone of some FoR, whatever the frame. The output might land in the future, and they are called projection/prediction with regard to that frame.
Unless you are talking about hypothetical prediction based on hypothetical situation, and I don't call that physics, but math.
A lots of misconception about relativity comes from comparing clock spatially separated. That's because frame of reference and especially the future light cone are taken as real thing, while they are not.

A frame of reference has neither a future light cone nor a past light cone. An event has a future light cone and a past light cone. A frame of reference is part of the mental model by which we analyze a physical situation. Yes, that means that a frame of reference is not physical.

Your claim was that the future light cone in a frame of reference does not exist. But that does not follow.

Frames of reference are not physical. Their existence or not is not (whatever you decide "existence" means) is not a matter of physical fact. Every portion of a frame of reference has just as much claim to "existence" as any other. One may quibble about the "existence" of events within a past light cone or a future light cone. But not with the existence of portions of a coordinate system within which these events are spoken of.

 

[Ebeb]

I would add that the specific words "exist" and "real" are philosophical (metaphysics) rather than physical.

I.o.w. even labeling your own present event with 'real' or 'exist' is not physical?
This is getting even worse than anticipated.
I'm really lost here.

 

[jbriggs444]

I.o.w. even labeling your own present event with 'real' or 'exist' is not physical?
This is getting even worse than anticipated.
I'm really lost here.

Recall that "physical" means measurable. Dale is not questioning whether you your present event is "real" or whether it "exists". It is pretty clear that whatever those terms are taken to mean, you "really" "physically" "exist" are your present event. At least from your own point of view. But the meaning of "real" and "exist" is considerably fuzzier when applied to other events. If those terms do not mean "measurable", what do they mean?

 

[puzzled fish]

Wait a minute...

Is there a notion of Gaussian curvature and differentiation without a manifold and an already existing temporal dimension? Put SR aside, how can you do GR without a BU?

 

[jbriggs444]

Is there a notion of Gaussian curvature and differentiation without a manifold and an already existing temporal dimension? Put SR aside, how can you do GR without a BU?

You can do differentiation just as well with something that does not "exist" as with something that does. i.e. "Shut up and calculate" applies.

 

[Boing3000]

Your claim was that the future light cone in a frame of reference does not exist. But that does not follow.

What I cannot be follow is how this is my "claim" and how it differs from "yours"

A frame of reference has neither a future light cone nor a past light cone

Frames of reference are not physical. Their existence or not is not (whatever you decide "existence" means) is not a matter of physical fact.

I see no interest to quibble over the word "existence", nor that you repeat what I just said. What is not physical is not physical. None of those non-physical 4D numbers are part of the BU.
There are as many BU as there is FoR, most of them unconnected, and those BU are growing.

Every portion of a frame of reference has just as much claim to "existence" as any other.

I didn't even know that FoR could claim anything, but I certainly don't claim they are useless. FoR have precise boundaries (and domain of application)

One may quibble about the "existence" of events within a past light cone or a future light cone.

Have you ever heard of someone contesting the existence of event in the past of a light cone ? I have never heard of such thing.
Have you ever heard of someone observing the existence of event in the future of a light cone ? I have never heard of such thing.
So I have never heard of any quibbling...

But not with the existence of portions of a coordinate system within which these events are spoken of.

The portions of a coordinate system ends exactly a those events. There is nothing beyond to be spoken of because FoR are not physical.

 

[jbriggs444]

What I cannot be follow is how this is my "claim" and how it differs from "yours"

The distinction is between the map and the territory. The map can exist regardless of whether the territory does.

A coordinate system does not end at the edge of a future or past light cone.

 

[puzzled fish]

You can do differentiation just as well with something that does not "exist" as with something that does. i.e. "Shut up and calculate" applies.

No, I cannot because I do not have a manifold to begin with. No overall Gaussian curvature at a point, sorry.

So, in GR you tacitly assume that there is a BU. If it's really there or it exists is irrelevant. The fact is that GR as a theory has a block universe at its heart.

 

[tophatphysicist]

No, I cannot because I do not have a manifold to begin with. No overall Gaussian curvature at a point, sorry.

So, in GR you tacitly assume that there is a BU. If it's really there or it exists is irrelevant. The fact is that GR as a theory has a block universe at its heart.

Excellent way to state it, puzzled fish. One of the arguments used to avoid accepting the Block Universe is that there is an alternate theory: LET. But, how is LET an alternate theory to General Relativity? I have the same feeling that you've expressed here. However, I do feel that it is relevant whether the universe exists as a 4-D physical object with actual 4-D physical structure. At the same time I acknowledge that there are physicists who feel that consideration is philosophy--not physics.

Our society at large looks to science to help in forming their world view. They don't look to science to find out how to calculate. The world view of physicists has helped keep away ghosts and goblins and other beliefs that would distort our approach to life. Einstein is still my standard bearer for how to approach physics and what physics is all about.

 

[jbriggs444]

No, I cannot because I do not have a manifold to begin with. No overall Gaussian curvature at a point, sorry.

I can take the derivative of f(x)=10t-9.8t^2 at t=1. If I decide not to toss the baseball, that derivative is still correct.

 

[Boing3000]

The distinction is between the map and the territory. The map can exist regardless of whether the territory does.

We are only discussing maps about existing territory on PF.

A coordinate system does not end at the edge of a future or past light cone.

This makes no sense. Coordinate system have no light cone, as I explained to you in my first response.

Coordinate system ends at the last event recorded in them. Unless you are aware of an event with 2117 AD as time coordinate. There is no event with coordinate 1 trillions light years south of Manhattan either.

Coordinate system exist when applicable => to record/label event, they have precise extent like every map... here be dragons

 

[Dale]

I get the impression that "objects before they are observed" are not part of physics language, my interpretion of 4D object actually doesn't make sense.

It makes perfect sense, it just isn't physical.

In LET there are 3D objects.

Yes, but they are not physical (experimentally measurable).

That's why I find the communication about 4D still so confusing.

I think the part that you are confused about is not the "4D" but the "physical". You are having a hard time recognizing that the philosophical statements of an interpretation are not "physical".

But for SR all of a sudden we are not allowed making a statement about 3D slices "existing"?

You can certainly make such a statement, but it is a philosophical statement not a physical one.

I.o.w. even labeling your own present event with 'real' or 'exist' is not physical?

Yes, it is philosophical, not physical. Do you have a realimeter or an existometer that you can use on yourself to experimentally measure your present realness?

 

[jbriggs444]

We are only discussing maps about existing territory on PF.

Any time you utter the phrase "future light cone" you are talking about a region extending into the future.

Coordinate system have no light cone, as I explained to you in my first response.

You are the one saying that coordinate systems do not extend to the future light cone. I say they do.

Coordinate system ends at the last event recorded in them.

Nonsense.

Coordinate systems cover regions with and without significant events, recorded or otherwise. They are a framework against which events can be recorded. Or predicted.

Coordinate system exist when applicable => to record/label event, they have precise extent like every map... here be dragons

And they cover regions which may exist or not or not yet exist.

 

[Boing3000]

Any time you utter the phrase "future light cone" you are talking about a region extending into the future.

... and none of those regions a is part of a BU. Some will, eventually.

You are the one saying that coordinate systems do not extend to the future light cone. I say they do.

For once: you contradict yourself there.

A frame of reference has neither a future light cone nor a past light cone

Secondly I don't say coordinate systems don't "extent to future light cone". I say coordinate system extent stop to the farther event(observed or predicted) plotted from the center, in any dimension.
You keep bringing up "light cone", and I don't know why.

Coordinate systems cover regions with and without significant events, recorded or otherwise. They are a framework against which events can be recorded.

The input/past

Or predicted

The output/future

And they cover regions which may exist or not or not yet exist.

And the future section is not part of any global vague/mythical/philosophical BU, not even part some a local FoR based one.

Whatever you think non-physical frame coordinate system covers, the territory it represents stop at recorded events.
And every event you plotted beyond this limits are just figment of your imagination...
The future isn't plotted anywhere in any coordinate system, only some best guess of it, in some limited range

 

[puzzled fish]

I can take the derivative of f(x)=10t-9.8t^2 at t=1. If I decide not to toss the baseball, that derivative is still correct.

And what is x = 10t-9.8t^2 pray, if not a block universe? The fact that you can change your mind ( initial conditions ) and choose a different solution is irrelevant.

A solution to the EFE is a solution. Once you have picked it, you cannot change it. You can transform it covariantly, but this is as much or little you can do.

 

[jbriggs444]

x = 10t-9.8t^2 pray, if not a block universe

It's a function definition. Shut up and calculate.

 

[PeterDonis]

Let's then use events that occur or don't occur.

What does "occur" mean? How do I test, experimentally, whether an event has "occurred"? The obvious answer is to observe it, but if I observe an event, it must be in my past light cone.

When does an event "car hits tree" get your label "fixed and certain"?

Meaningless question. Any event in your past light cone is fixed and certain. What events are in your past light cone depends on what event on your worldline you are treating as your "present" event.

If I understand you correctly, it gets labeled at the moment the event is observed, i.e. at your present event.

There is no "labeling" involved. You are treating "fixed and certain" as something that has to happen to an event, physically. It's not. It's just a property in the model.

But the above doesn't mean that you know that the event occurred before you observed it.

I haven't said anything about "know". Once again: you have a model, and the model treats events in the past light cone of some chosen event, the one you are calling the "present" event on your worldline, as fixed and certain.

all events you put in that past lightcone, are events you observed.

Strictly speaking, they are events you could have observed, at some event on your worldline prior to the event you are calling your "present" event, just looking at the causal structure of spacetime. Whether you actually observed them depends on things that are irrelevant to this discussion, like whether you were paying attention.

If all the above is correct, then strictly speaking it is only safe (physically safe) to use (apply) the SR model for( to) the past lightcone (= observed events).

Nope. You still don't understand what a model is. A model is a tool for making predictions. You don't have to predict what happened in your past light cone, at some chosen event you are calling your "present" event, because those events are fixed and certain in the model. You only have to predict events outside your past light cone. That's what the model is for.

A full model of spacetime including simultaneity lines thorugh the apex is in fact only based upon suppositions and uncertain predictions based on past observations.

Any event in the model that is not in the past light cone of whatever event you are calling your "present" event is not fixed and certain in the model; it's predicted, and the prediction is not 100% guaranteed to be correct. That is true. Any model will have the same property--there will be some things that are fixed and certain, and some things that are predicted and might be wrong.

They are fixed and certain, but you don't know whether those events actually occurred before observation that event of observation.

I haven't said anything about "know". See above. You need to get rid of your preconceptions and stop reading things into my posts that I didn't put there.

Strictly speaking we don't even know whether we have eyes. Only our consciousness tells us something like that

We are talking about SR here, not about theories of consciousness. You are making this way too difficult.

if you insist on the importance of 'observing an event', then I want to know exactly what you mean by that

Again, we are talking about SR here, not about theories of consciousness or metaphysical questions about how we can observe or know anything at all. You are making this way too difficult.

somewhere along the line I must have lost you

Because you keep on wandering off into the weeds of theories of consciousness and metaphysics, instead of just looking at the simple model I am describing. In the model, there is some event that you call your "present" event. Events in the past light cone of that event, in the model, are treated as fixed and certain. Events not in the past light cone of that event, in the model, are not treated as fixed and certain. That's it. That's all there is to it.

Your approach, taken to its logical conclusion, would say that we cannot do physics at all unless we first understand how consciousness works. That's absurd. People have been doing physics for centuries without knowing how consciousness works, except for the basic rough and ready pragmatic knowledge that everybody has just by being conscious and going through their daily lives. That is enough for what we're discussing here. If it isn't enough for you, then I'm afraid nothing anyone could possibly say in this discussion is going to satisfy you. Sorry.

 

[PeterDonis]

Our society at large looks to science to help in forming their world view. They don't look to science to find out how to calculate.

But one of the key things science tells people about forming their world view is...to shut up and calculate; stop worrying about what "exists" or what's "real" and focus on what can be tested by experiment and evidence.

The world view of physicists has helped keep away ghosts and goblins and other beliefs that would distort our approach to life.

Yes, and it did so not by saying they weren't "real", but by saying there was no evidence for them.

Einstein is still my standard bearer for how to approach physics and what physics is all about.

Then you've missed another key lesson of science, which is: there are no "standard bearers". Science is not an authority that tells you what to believe. It's a tool that you need to learn how to use to form your own beliefs for yourself.

 

[Denis]

I would add that the specific words "exist" and "real" are philosophical (metaphysics) rather than physical.

Once using realism as an assumption to prove observable predictions (Bell's inequality) which one cannot prove without making this assumption, realism it is physical hypothesis.

No, because SR is a model. You can make a model of (hypothetical) events that you haven't observed yet. But the model is not reality. You can't make claims about the events being "real" based on the model. The model is a tool for prediction, not for telling you what's "real".

Of course, there is a difference between the model and reality. But if it is aimed to be a model for reality (and not simply fantasy) it also tells us what is real now. And hopes to predict successfully what happens in the future.

But, he didn't decide just by the force of pure reason. He decided based on measurements and observations culminating in the relative nature of simultaneity.

No. Once we have two interpretations, one with and one without a 4D block universe, and above agree with all measurements and observations, the preference for the block universe is pure reason.

If LET is a philosophical interpretation but not a theory, then why calling it a theory?

It is simply following some tradition, naming convention. You could also name it "preferred frame hypothesis" or so.

The convention to name things which make the same observable predictions different interpretations of the same theory is reasonable. But it should not force us to rename traditional names.

 

[PeterDonis]

if it is aimed to be a model for reality (and not simply fantasy) it also tells us what is real now

If you want to define "real" as "whatever is fixed and certain in the model", that's fine with me.

 

[PeterDonis]

Once using realism as an assumption to prove observable predictions (Bell's inequality) which one cannot prove without making this assumption, realism it is physical hypothesis.

"Realism" meaning that specific assumption in that specific argument, yes. But that doesn't seem to be what the posters in this thread who are making an issue of "realism" mean by that word.

 

[puzzled fish]

It's a function definition. Shut up and calculate.

Fine!
But that's the way automata think, not humans. It would be very easy for a sophisticated computer to come up with various random functions and diff. equations as solutions to various problems.
But Gauss and Riemann began with tangible objects and geometry not "functions". And GR is a geometric theory. If you want to regard it as an approximation of a futuristic functional theory is ok with me, but it would not be GR.

 

[jbriggs444]

Fine!
But that's the way automata think, not humans. It would be very easy for a sophisticated computer to come up with various random functions and diff. equations as solutions to various problems.
But Gauss and Riemann began with tangible objects and geometry not "functions". And GR is a geometric theory. If you want to regard it as an approximation of a futuristic functional theory is ok with me, but it would not be GR.

Meh. If you can't measure it, its a bit pointless to argue about it.

 

[tophatphysicist]

Meh. If you can't measure it, its a bit pointless to argue about it.

What about quarks? What about strings? What about the relativity of simultaneity and existing 4-D physical universe structure of 12 billion years ago? (PeterDonis advises I should not refer to Block Universe in the context of just the past, because using the term Block Universe includes all future light cones-- can't possibly have measurements there).

 

[Dale]

Once using realism as an assumption to prove observable predictions (Bell's inequality) which one cannot prove without making this assumption, realism it is physical hypothesis.

Unfortunately, they (EPR) never define "reality", they just give a sufficient condition.

 

[Denis]

Unfortunately, they (EPR) never define "reality", they just give a sufficient condition.

Once even this criterion is already sufficient to make the point that it is physical, what is the problem? If you add more to a definition, what is already shown to be physical (by allowing to derive, together with Einstein causality, a physical predictions) will not go away.

 

[PeterDonis]

What about quarks?

Google "deep inelastic scattering experiments". They are experiments that made direct measurements of the properties of quarks. Yes, you can't isolate a single quark, but that doesn't mean you can't measure anything to do with quarks.

What about strings?

What about them? If you're saying that string theory currently makes no testable predictions, I agree (although many string theorists might not). But nobody in this thread is claiming otherwise.

What about the relativity of simultaneity and existing 4-D physical universe structure of 12 billion years ago?

What about it? Our past light cone gives us lots of information about it--that's how we have tested the standard hot big bang model of cosmology. What's the problem?

PeterDonis advises I should not refer to Block Universe in the context of just the past, because using the term Block Universe includes all future light cones

That's more or less what I said, yes. What's the problem?

 

[Dale]

Once even this criterion is already sufficient to make the point that it is physical, what is the problem? If you add more to a definition, what is already shown to be physical (by allowing to derive, together with Einstein causality, a physical predictions) will not go away.

You are thinking of a necessary condition, not a sufficient condition.

Since EPR's criterion is a sufficient condition that means that all things that meet the condition are "real" but not that all things that are "real" meet the condition. There can still be real things which are not measurable with the EPR criterion.

Furthermore, I am not sure that the EPR criterion is generally accepted outside of QM foundations, and maybe not even then.