When Does the Future Stop Being Mapped Out in Special Relativity?

[paulselhi]

I have a basic grasp of special relativity and understand why different observers will have different perspectives on the timing of events. My question is this, if Observer 1 sees two events occurring which change in time i.e of 2 lights opposite each other with the same color which change simultaneously, so red-red blue-blue green- green, and an observer moving at speed relative to Observer 1 may see red-blue blue-green green-red due to the change in angle of his time slice

Both obervers are justified in saying that their observations are correct ( equivalence principle ?)

Does this mean that for a moving observer 2 the observations of observer 1 are his past, and for observer 1 they are his present and the observations of observer 2 are observer 1's future

So in effect Observer 1's future as "already occurred" as observer 2 is experencing it.

Am i living in someone elses past and someone elses future ? If so when does the future stop being mapped out ? Is/Was the whole of all time ( past future and present) concieved at once ?

Clearly i am blaberring now and there is a good reason why the future is not already mapped out ! Please enlighten me !

 

[bobc2]

I have a basic grasp of special relativity and understand why different observers will have different perspectives on the timing of events. My question is this, if Observer 1 sees two events occurring which change in time i.e of 2 lights opposite each other with the same color which change simultaneously, so red-red blue-blue green- green, and an observer moving at speed relative to Observer 1 may see red-blue blue-green green-red due to the change in angle of his time slice

Both obervers are justified in saying that their observations are correct ( equivalence principle ?)

Does this mean that for a moving observer 2 the observations of observer 1 are his past, and for observer 1 they are his present and the observations of observer 2 are observer 1's future

So in effect Observer 1's future as "already occurred" as observer 2 is experencing it.

Am i living in someone elses past and someone elses future ? If so when does the future stop being mapped out ? Is/Was the whole of all time ( past future and present) concieved at once ?

Good job, paulselhi! Not everyone notices that implication of special relativity. You have nailed it pretty well. And yes, it is all of time as described by Hermann Weyl (Einstein's close friend and colleague at The Princeton Institute for Advanced Studies).

Clearly i am blaberring now and there is a good reason why the future is not already mapped out ! Please enlighten me !

You are not blabbering at all, paulselhi. However, that kind of discussion is sometimes not regarded as appropriate for this forum.

 

[e^(i Pi)+1=0]

Note that the farther away the observers are, the greater their discrepancy, even if they are moving very slowly relative to one another.

 

[paulselhi]

Does the speed of light limit mean that the possible future-past range may be limited?
So if observer 1 sees an event happening there would be a limit as to how much into the past or future another observer can reside since these differences in observations will be based on the other observers speed relative to observer 1

And as .. err.. the last poster mentioned that distance is relevant, would the size on the universe put a cap on this as well

Thanks BobC for turning me on to Herman Weyl

"The objective world simply is, it does not happen. Only to the gaze of my consciousness, crawling upward along the life line of my body, does a section of this world come to life as a fleeting image in space which continuously changes in time." Hermann Weyl, Philosophy of Mathematics and Natural Science (Princeton University Press, Princeton, 1949)

"What has been will be again, what has been done will be done again; there is nothing
new under the sun. Is there anything of which one can say, "Look! This is something
new?" It was here already, long ago; it was here before our time."

King Solomon

 

[bobc2]

Does the speed of light limit mean that the possible future-past range may be limited?
So if observer 1 sees an event happening there would be a limit as to how much into the past or future another observer can reside since these differences in observations will be based on the other observers speed relative to observer 1

And as .. err.. the last poster mentioned that distance is relevant, would the size on the universe put a cap on this as well

Thanks BobC for turning me on to Herman Weyl

"The objective world simply is, it does not happen. Only to the gaze of my consciousness, crawling upward along the life line of my body, does a section of this world come to life as a fleeting image in space which continuously changes in time." Hermann Weyl, Philosophy of Mathematics and Natural Science (Princeton University Press, Princeton, 1949)

"What has been will be again, what has been done will be done again; there is nothing
new under the sun. Is there anything of which one can say, "Look! This is something
new?" It was here already, long ago; it was here before our time."

King Solomon

Those are pretty heavy quotes there, paulselhi. I was familiar with the Weyl quote but did not realize King Solomon was so knowlegable about special relativity. How did you ever come up with that one? Anyway, nice work.

 

[nitsuj]

So in effect Observer 1's future as "already occurred" as observer 2 is experencing it.

Am i living in someone elses past and someone elses future ? If so when does the future stop being mapped out ? Is/Was the whole of all time ( past future and present) concieved at once ?

Clearly i am blaberring now and there is a good reason why the future is not already mapped out ! Please enlighten me !

In my opinion this is no different then saying my future experience of the sun / moon / anything not in the same location as me has happened prior to my experience of it. whoopty doo
(yes this is Galilean transformation, but what do I care of the measure of someone elses proper time/length?)

The next step in the concept is to realize that past, present and future are relative terms, all from the perspective of assigning an arbitrary "now".

The only part of the future that's "mapped out" is it will become your "now" & then your "past".

When thinking of SR you should ditch the common conceptions of past / future and now. And think more in terms of cause & effect. From there (cause & effect) consider the constancy of the laws of physics and relativity of simultinaity.

Cause (present) and effect (future) all according to the laws of physics (mapped out). Relativity of simultinaity; "now" is a relative term, and inturn "future" & "past".

 

[paulselhi]

I assume that is KS from Israel and not some enlightened Rasta who has been " 'erbing it out"

 

[bobc2]

In my opinion this is no different then saying my future experience of the sun / moon / anything not in the same location as me has happened prior to my experience of it. whoopty doo

I think he hit on something far more profound than that.

(yes this is Galilean transformation, but what do I care of the measure of someone elses proper time/length?)

His insight is directly related to Lorentz transformations, not Galilean. And of course no one has to care about implications of special relativity. Some are passionately curious about them and others shrug their shoulders. And no value judgement is involved.

(When thinking of SR you should ditch the common conceptions of past / future and now. And think more in terms of cause & effect.

But, cause & effect just becomes a definition with Weyl's model. One event does not cause another event in the sense that you imply, because it is all there at once. As Weyl said, "The objective world simply is, it does not happen..."

... relativity of simultinaity.

But, it was the relativity of simultaneity with the different angles of the slices of the 3-D cross-sections across the 4-D universe that let to his recognition of the implications of special relativity.

 

[nitsuj]

It is not more profound, that is the problem. Thinking it is profound. The profoundness is a consiquence of having a brain, in that we can consider other FoR and think but they have a "now" momnet too and that persons "now" moment is in my past, wow cool!"

That is my point of who cares if another observers proper time length is of different measurements then mine, what is the implication in the context of past & future? The words their "now" is my "past"? It is of no consiquence to cause/effect.

So no his insights are not strickly lorentz, it is strickly about overextending a "now" momment to beyond local, where it starts to loose meaning in the more common sense of "now".

These are measurements we are talking about. You're stretching this to the universe is static via words such as "now" past future.

A quote such as "The objective world simply is, it does not happen..." without strickly defining the words I think is nonsense.

The universe IS 4D, there are no "now" "slices", so anything you derive from these "slices" is not speaking of the "nature" of a 4D continuum.

The direction of this discussion is transparent bobc2,

 

[nitsuj]

it was the relativity of simultaneity with the different angles of the slices of the 3-D cross-sections across the 4-D universe that let to his recognition of the implications of special relativity.

He/she needs to recognize the implication of SR in this context is redefining the concepts of past, present & future. Specificaly "now" / relativity of simultinaity.

 

[bobc2]

From "The Fabric Of The Cosmos" - Brian Greene (pg 141 hard copy):

"A particular moment can no more change in time than a particular location can move in space: if the location were to move, it would be a different location in space; if a moment in time were to change, it would be a different moment in time."

"Under close scrutiny, the flowing river of time more closely resembles a giant block of ice with every moment forever frozen into place."

 

[bobc2]

The universe IS 4D, there are no "now" "slices", so anything you derive from these "slices" is not speaking of the "nature" of a 4D continuum.

Special relativity gives us many "now slices." But the "now slices" for different observers moving at different velocities cut the 4-D universe at different angles--that was the clue that led to the understanding of a 4-dimensional continuum.

That's why Brian Greene, in his book "The Fabric Of The Cosmos" spent so much time developing the loaf of bread analogy with the slices of bread sliced at different angles. He associated physical reality (the entire 4-dimensional universe) with the whole loaf of bread and associated the different slices with the different 3-D universes that different observers live in at any given instant of time (different "now slices").

And that's why so many physicists have been captivated by the concept of the block universe. Paulselhi, I think you would be interested to google "block universe."

 

[bobc2]

Note that the farther away the observers are, the greater their discrepancy, even if they are moving very slowly relative to one another.

That's a really good point, e^(iPi)+1=0 (how did you come up with that interesting handle?). This was dramatized in Roger Penrose's famous Andromeda Paradox shown below (my graphics version).

Bill and Ruth walk past each other. Since they are in relative relative motion, they are living in different instantaneous 3-D worlds. In Bill's world the Andromeda leaders are meeting to discuss whether to attack earth. In Ruth's world they have already made their decision and the attack is under way.

 

[e^(i Pi)+1=0]

It's Euler's equation.

http://en.wikipedia.org/wiki/Euler's_identity

 

[nitsuj]

Special relativity gives us many "now slices." But the "now slices" for different observers moving at different velocities cut the 4-D universe at different angles--that was the clue that led to the understanding of a 4-dimensional continuum.

That's why Brian Greene, in his book "The Fabric Of The Cosmos" spent so much time developing the loaf of bread analogy with the slices of bread sliced at different angles. He associated physical reality (the entire 4-dimensional universe) with the whole loaf of bread and associated the different slices with the different 3-D universes that different observers live in at any given instant of time (different "now slices").

And that's why so many physicists have been captivated by the concept of the block universe. Paulselhi, I think you would be interested to google "block universe."

As if the analogy you quote is the one that got me to stop "learning" SR from Brian Greene's poetic physics books.

I'm pretty sure he "spent so much time developing the loaf of bread analogy" because it sells books. It only confuses the reality that physics "describes".

Lastly, if this is how you believe it is (block universe) then this perspective of a block universe has to also be true for every single bit of anything there is in the universe (that's subject to SR type physics) does it not? Given the relativity of simultaneity this can't be true.
How could anything move? (note spacetime is isotropic)

 

[PeterDonis]

Both obervers are justified in saying that their observations are correct ( equivalence principle ?)

Actually, no, this is just the principle of relativity.

So in effect Observer 1's future as "already occurred" as observer 2 is experencing it.

No. For the time ordering of events to be observer-dependent, the events must be spacelike separated; each must be outside the other's light cone. That means there can be no causal connection between the two events. So what observer 2 is experiencing can't directly be observer 1's "future", because observer 1 can causally affect his future.

As nitsuj has commented, Brian Greene's talk about the "block universe" obfuscates this crucial point.

 

[PeterDonis]

Bill and Ruth walk past each other. Since they are in relative relative motion, they are living in different instantaneous 3-D worlds. In Bill's world the Andromeda leaders are meeting to discuss whether to attack earth. In Ruth's world they have already made their decision and the attack is under way.

But this is only true if the distinction between Bill's world and Ruth's world has no causal consequences; in other words, Bill can't causally affect the Andromeda leaders' decision any more than Ruth can, even though in Bill's world the decision hasn't happened "yet" but in Ruth's world it has "already happened". So this presumed distinction actually has no physical content.

 

[bobc2]

As if the analogy you quote is the one that got me to stop "learning" SR from Brian Greene's poetic physics books.

I'm pretty sure he "spent so much time developing the loaf of bread analogy" because it sells books. It only confuses the reality that physics "describes".

Actually, Brian Greene does one of the better jobs of describing what's going on with special relativity for the layman--and also for some physicists who have not thought too much about some of its implications.

Lastly, if this is how you believe it is (block universe) then this perspective of a block universe has to also be true for every single bit of anything there is in the universe (that's subject to SR type physics) does it not?

Yes. And of course it is.

Given the relativity of simultaneity this can't be true.
How could anything move? (note spacetime is isotropic)

That's the whole point. Nothing does move in the block universe model. And it's the relativity of simultaneity that implies the block universe.

 

[bobc2]

Actually, no, this is just the principle of relativity.



No. For the time ordering of events to be observer-dependent, the events must be spacelike separated; each must be outside the other's light cone. That means there can be no causal connection between the two events. So what observer 2 is experiencing can't directly be observer 1's "future", because observer 1 can causally affect his future.

This is a popular objection to the block universe model. However, it is totally irrelavent. Theoretical physicists are interested in a model that represents the entire universe. The range of modeling would certainly not be limited to one's personal light cone.

And when paulselhi was talking about observer1's "future" I think he was referring to the future 3-D cross-section cut across the 4-D universe (not the directly experienced future that he actually experiences with light transmittal information). Just as when talking about an observer's "Now", a convention is typically adapted that assumes we've accounted for all of the distracting details such as speed of light time delays before actually "seeing", etc. Brian Greene makes all of that very clear in his book. So, we know what he is talking about when he talks about the "future" and the instantaneous 3-D cross-section (or slice of the loaf) that an observer "sees" in language that is understood in the context of Greene's discussion. And we know what Roger Penrose is talking about when he compares the events in the andromeda galaxy for Bill and for Ruth in our sketch--the inability of Bill or Ruth to influence events is irrelavent to the fundamental issues related to 4-dimensional existence. These issues are very profound for many physicists.

We easily have a concept of the sun existing "right now", regardless of not receiving light for another few minutes. We have a concept of stars existing "out there" even though it may be years before their existence is confirmed with the arrival of the light.

We don't ignore knowledge about the content of the distant reaches of the universe because it is not in our light cone. We don't ignore the theory of quarks because they are not directly observable. We pursue string theory even though it may never be possible to confirm the theory by experiment.

The block universe is implied by special relativity. It is a logical model. That does not make it a fact of external objective reality. It's not a concept embraced by every physicist. Some physicists don't even embrace any kind of external objective physical reality.

As nitsuj has commented, Brian Greene's talk about the "block universe" obfuscates this crucial point.

No. The block universe concept is the crucial point. The outside-of-light-cone issue is a red herring.

 

[bobc2]

...So this presumed distinction actually has no physical content.

The physical content is manifest directly and is profound. Whether Bill or Ruth can influence the Andromeda decisions has nothing to do with whether events really exist in Andromeda. Physicists are interested in external physical reality that is independent of observers. What exists "out there" does not depend on observers. The existence of the early universe did not and does not depend on observers. The existence of objects on the other side of the moon does not depend on observers. Just because the event of a rock rolling down the side of a mountain on Mars "right now" cannot be affected by us does not have anything to do with the reality of the existence of the rock or the event.

The no-physical-content presumption just because there is no possibility of intervention from outside the light cone is a red herring argument.

 

[nitsuj]

The outside-of-light-cone issue is a red herring.

With all due respect, outside the light cone is your "red herring". What shape is the light cone in three spatial dimensions?

removing any of the dimensions from the 4D continuum is an "abstract". With in this context of reducing dimensions of spacetime, the whole is more then the sum of it's parts.

Outside the light cone in 3D is ">c interval" away from me, continuum.

Here is some poetic physics bobc2, EM propagates towards the observer at c, the length measurement is the past tense of the time measurement*. Measuring the two dimensions, length & time simultaneously is a coordinating event, that is assuming "perpendicular" (?) measurements of time & length. Speed is the assumed "now" along some arbitrary spatial axis, one of an infinite number of possibilities (just ask pi) in isotropic space; motion is relative.

Outada park, block universe is debunked, as this* can only work via continuum.

 

[nitsuj]

Thinking of block universe some more, I agree the concept is "valid" from the perspective of EM.

If EM takes the shortest path, it's existence then is literally a slice in the three spatial dimensions. Said differently photon does not "experience" time, it never has more then one absolute set of spatial coordinates (that I can think of) but that of it's geodesic path; axis x. (there is no time to "effect" this cause [wave], hello uncertainty, lol sorry pf.)

This is just the perspective of EM, For each existence of a wave, maybe the universe is as if it is like the block universe concept, so each wave's existence is only of length. An "I'm here, then I'm there" type existence. Then upon the wave's next event in the continuum passes the "baton" onto the next bit of whatever, cause effect it sounds like. A block universe continuum from EM's perspective.

Going into the deep, it seems like being human is what adds to this confusion of time & length in relativity. It is where, very literally the two concepts/perspectives "blend", absolute length & time, arbitrary "now" of EM visually.

 

[bobc2]

Thinking of block universe some more, I agree the concept is "valid" from the perspective of EM.

If EM takes the shortest path, it's existence then is literally a slice in the three spatial dimensions. Said differently photon does not "experience" time, it never has more then one absolute set of spatial coordinates (that I can think of) but that of it's geodesic path; axis x. (there is no time to "effect" this cause [wave], hello uncertainty, lol sorry pf.)

This is just the perspective of EM, For each existence of a wave, maybe the universe is as if it is like the block universe concept, so each wave's existence is only of length. An "I'm here, then I'm there" type existence. Then upon the wave's next event in the continuum passes the "baton" onto the next bit of whatever, cause effect it sounds like. A block universe continuum from EM's perspective.

Going into the deep, it seems like being human is what adds to this confusion of time & length in relativity. It is where, very literally the two concepts/perspectives "blend", absolute length & time, arbitrary "now" of EM visually.

You're O.K., nitsuj. And you have carried this deeper than my limited intellect is capable of keeping up. Thanks for your ideas.

 

[PeterDonis]

If EM takes the shortest path, it's existence then is literally a slice in the three spatial dimensions. Said differently photon does not "experience" time, it never has more then one absolute set of spatial coordinates (that I can think of) but that of it's geodesic path; axis x. (there is no time to "effect" this cause [wave], hello uncertainty, lol sorry pf.)

I'm not sure I understand what you mean by this. A photon's worldline is a null line; I don't see how a null line can be described as "a slice in the three spatial dimensions". Can you elaborate?

 

[PeterDonis]

Theoretical physicists are interested in a model that represents the entire universe. The range of modeling would certainly not be limited to one's personal light cone.

I don't disagree with this, but I don't think it means what you think it means. I probably haven't been clear enough about what *I* think it means; see below.

And when paulselhi was talking about observer1's "future" I think he was referring to the future 3-D cross-section cut across the 4-D universe (not the directly experienced future that he actually experiences with light transmittal information).

To be more precise, I believe he was referring to the fact that, for example, the event "Andromeda leaders send the fleet to attack Earth" is to the future of Bill's simultaneous space but to the past of Ruth's simultaneous space. I don't disagree with this.

But paulsehi claimed that this implies that the event "Andromeda leaders send the fleet to attack Earth" has "already occurred" for Ruth but not for Bill. I do disagree with that. The event is spacelike separated from both Ruth and Bill, so neither of them can causally influence it. Therefore, I would say the event has "already occurred" for *both* of them; neither of them can change it, so as far as they are concerned, it's fixed.

However, there is also a different question, about what Bill or Ruth can *know* about what's happening in the Andromeda galaxy "right now". See below.

We easily have a concept of the sun existing "right now", regardless of not receiving light for another few minutes. We have a concept of stars existing "out there" even though it may be years before their existence is confirmed with the arrival of the light.

But we don't know the Sun's state "right now"; we only know it up to eight minutes ago (the light travel time). If the Sun was exploding "right now", we would have no way of knowing about it for eight minutes; so for those eight minutes, our model of the Sun, which does not include it exploding, would be wrong.

We don't ignore knowledge about the content of the distant reaches of the universe because it is not in our light cone. We don't ignore the theory of quarks because they are not directly observable. We pursue string theory even though it may never be possible to confirm the theory by experiment.

You're conflating different issues. The theory of quarks has many observable consequences, which have been confirmed. Even string theory will hopefully have some observable consequences eventually (if it never does, it won't be a proper scientific theory IMO). But the question of whether these general theories have observable consequences is a different question than the question of what knowledge we have of the specific states of individual objects far distant from us. We do not know their states "right now"; we only know their states as they were at the boundary of our past light cone. If those states change outside our past light cone, we have no way of knowing about it until we see light or some other signal (moving at the speed of light) that conveys the information to us. So we cannot claim "knowledge" about what is happening at events spacelike separated from us; we can model what "might" be happening there, based on what we know from our past light cone, but we can't be certain that those models are right, not because of any theoretical issues but simply because the information hasn't reached us yet.

The physical content is manifest directly and is profound. Whether Bill or Ruth can influence the Andromeda decisions has nothing to do with whether events really exist in Andromeda.

But I'm not objecting to the claim that the events in Andromeda "really exist" even though they are spacelike separated from us. I agree that they do; I agree that there is an objective reality out there that is independent of observers.

I am objecting to the claim that we somehow can know for sure *what those events are*, what the objective reality actually is, when the events are still spacelike separated from us. We can't. We ignore this when we are setting up thought experiments because in thought experiments we simply *dictate*, by fiat, what the events are; we dictate, by fiat, that the Andromeda leaders sent the fleet at some particular event, which happens to be to the future of Bill's simultaneous space but to the past of Ruth's. But in the real world we can't dictate that; in the real world, neither Bill nor Ruth would know whether or not the Andromeda fleet was coming for two million years after the leaders decided to send it. So in the real world, any portion of a "block universe" model that is not in our past light cone is tentative. That doesn't mean those portions of spacetime aren't real; but it does mean that we don't know for sure what that portion of reality looks like until we see it.

 

[nitsuj]

I'm not sure I understand what you mean by this. A photon's worldline is a null line; I don't see how a null line can be described as "a slice in the three spatial dimensions". Can you elaborate?

It is a null path if it is measured with time & length "perpendicular to each other. I'd say a null line is more specifically c; it is null from an observers perspective who can still measure time, you cannot measure time along a null path, the interval from that perspective (photons) is purely spacelike. I'm just a layman Peter, I tried to summarize this FoR of a photon as an "I'm here, then I'm there" type existence, a discrete step of sorts.

If I assign the geodesic path of a photon as axis x, the interval is complete from the photons calculation, the one without time, in that sense it is a slice in 3D dimensions, given the path is an arbitrary one of "infinite" possibilities in isotropic space.

In other words if a clock went along with a photons journey, the time measurement would be 0 (i'd guess) so it's coordinates are only spatial, since time = 0. So because a photon follows a geodesic path, it is purely timelike from an observers perspective, It is purely spacelike from the photons perspective, despite that an observer can still measure time. In turn the max "speed" / interval can be calculated by observers. We have the unique opportunity to measure time and length separately but "at the same time", and assume they're absolute. I think the fact we can make these kinds of measurements is proof positive of a continuum.I'm not speaking with any authority here, just a layman so if corrections are in order please be simple (specifically no math, sorry)

 

[PeterDonis]

It is a null line if it is measured with time & length "perpendicular to each other.

The property of being a null line is invariant; it does not depend on how it is "measured". Also, I'm not sure what you mean by "with time & length perpendicular to each other". The "time" axis of any inertial frame is always "perpendicular" to any of the three spatial axes.

if a clock went along with a photons journey, the time measurement would be 0 (i'd guess)

A clock can't move at the speed of light, so there's no way to do this experiment. But what I think you mean is the following:

so it's coordinates are only spatial, since time = 0.

This is not correct. A null line has a nonzero "time coordinate" (meaning its "length" in the time dimension is nonzero). What is zero for a null line is the Lorentz interval, which is t^2 - x^2 (for the simple case where there is only a x spatial coordinate).

So because a photon follows a null geodesic path, it is purely spatial;

No, "purely spatial" (or "spacelike") and "null" describe two fundamentally different types of lines.

 

[yuiop]

This was dramatized in Roger Penrose's famous Andromeda Paradox shown below (my graphics version).

Bill and Ruth walk past each other. Since they are in relative relative motion, they are living in different instantaneous 3-D worlds. In Bill's world the Andromeda leaders are meeting to discuss whether to attack earth. In Ruth's world they have already made their decision and the attack is under way.

The expression "In Ruth's world they have already made their decision and the attack is under way." is slightly misleading, because when Ruth walks past Bill she is no more aware that the Andromedean attack fleet has launched than Bill. The event "launch of attack fleet" is still in Ruth's future at this point although she will become aware of it before Bill because she is traveling towards the event (assuming she continues in that direction). If the fleet has not yet launched from the point of view of an observer at the Andromedean launch site the the event "launch of attack fleet" has not yet occurred from the point of view of any observer in the universe and so the event is not inevitable or mapped out. This puts a cap on how far into the future events can be considered as as mapped out and this cap gets tighter the more local the event is. I do not see anything in these observations that gives a universe a deterministic nature. All we are are talking about is that is observers furthest away from an event are the last to know about it and there is nothing astounding about that.

 

[nitsuj]

The property of being a null line is invariant; it does not depend on how it is "measured". Also, I'm not sure what you mean by "with time & length perpendicular to each other". The "time" axis of any inertial frame is always "perpendicular" to any of the three spatial axes.
A clock can't move at the speed of light, so there's no way to do this experiment. But what I think you mean is the following:
This is not correct. A null line has a nonzero "time coordinate" (meaning its "length" in the time dimension is nonzero). What is zero for a null line is the Lorentz interval, which is t^2 - x^2 (for the simple case where there is only a x spatial coordinate).
No, "purely spatial" (or "spacelike") and "null" describe two fundamentally different types of lines.

1.)Gimme a break peter, i know can't assign an FoR to a photon from a lorentz transform perspective.

1.b.)You asked me what I meant by slice in 3D, that is what I mean; the path of a photon. I am leaving it at that. You can't fool me, you know exactly what I mean with "time & length perpendicular to each other." especially after, oddly as it is, you describe what I meant in the very next sentence.

2.) again gimme a break peter, of course I am idealizing here, i know it can't physically happen & mathematically infinite is not zero.

3.) I am not talking about EM from the perspective of an observer, I'm talking about em from the perspective of em. Yes of course EM from the perspective of a observer is a null path.

4.) actually peter it is fundamentally different perspectives of the same thing. (see metre definition below)

Pretty transparent you are not actually interested in what I meant by a slice in 3D. So to summarize your retorts are:

I don't know what time & length measured perpendicular to each other means.

c is invariant

A null line has a non zero time component

quoted out of context and made an irrelevant point

Thanks for the feedback!

So Peter if EM is not purely spatial from it's perspective, can you catch up to a beam of light? Probably not enough time in the interval for that. How else can a meter be defined by EM but by this kinda of reasoning.

from wiki "The metre is the length of the path traveled by light in vacuum during a time interval of 1  ⁄   299,792,458 of a second." that would be an interval free and clear of any time component, i.e. a length. (the length is assumed to be purely spacelike, defined by the path of a photon, & time to be purely timelike, also as defined by the path of a photon; normally called proper time & length) i.e. measuring time & length "perpendicular" to each other as seen when graphed. So there it is real world proof of the point I have had to make to you.

Comprehension question for two points and a smiley face: Is there a difference in position of arbitrarily chosen axis between measuring time with EM and measuring length with em? If so what is it, Peter? Which direction does length contraction happen in? Is time dilation "all around"?

 

[PeterDonis]

1.b.)You asked me what I meant by slice in 3D, that is what I mean; the path of a photon. I am leaving it at that.

Well, that's not a very clear place to leave it, IMO. :-) If by "path" you mean "path in a 3-D space", that is *not* the same thing as "path" meaning "worldline in 4-D spacetime". The latter is the concept that is used in SR, and all the things I've said, and you apparently agree with, about a photon not having a standard frame of reference, having zero Lorentz interval, etc., only apply to the photon's "path" as a worldline in 4-D spacetime, *not* as a line in 3-D space. The "path" in 3-D space can, in special cases, be thought of as a projection into a 3-D spatial slice of the 4-D worldline, but only in special cases; and in any case the 4-D worldline is what you have to use to do the physics.

You can't fool me, you know exactly what I mean with "time & length perpendicular to each other." especially after, oddly as it is, you describe what I meant in the very next sentence.

I conjectured that that was what you meant, but I'm glad to have confirmation.

3.) I am not talking about EM from the perspective of an observer, I'm talking about em from the perspective of em.

I don't know what "em from the perspective of em" means. To write down any equations for Em at all, you have to define what the symbols in the equations mean. I know how to do that from the perspective of an observer; I don't know how to do that "from the perspective of em". Can you please clarify?

4.) actually peter it is fundamentally different perspectives of the same thing.

No, it isn't. "Spatial" is not just a different perspective of "null". They are physically different and distinct. Spatial curves have a nonzero Lorentz interval (a negative squared interval if we are using a timelike sign convention). Null curves have a zero Lorentz interval. Nonzero is physically different and distinct from zero.

So Peter if EM is not purely spatial from it's perspective, can you catch up to a beam of light? Probably not enough time in the interval for that. How else can a meter be defined by EM but by this kinda of reasoning.

I still don't understand what you're getting at here. I understand the Wiki definition of a meter that you quote next, but I don't understand how you're interpreting it.

The path of a photon, meaning its worldline, is a null line; in the case given in the Wiki definition, it goes between two events in spacetime with coordinates (in a suitably defined inertial frame) (0, 0) and (1/299,792,458, 1), where the (t, x) coordinates are given in (seconds, meters). The "length" you are talking about would be a *spacelike* line going from (0, 0) to (0, 1); the "proper time" you are talking about would be a timelike line going from (0, 0) to (1/299,792,458, 0). Neither of those lines is the worldline of the photon. The spacelike line could be thought of as the "spatial path" of the photon, since it is a projection into the (t = 0) spacelike slice of the photon's worldline; but that spacelike line is *not* the line you have to use if you want to figure out the photon's physics. You have to use its worldline.

If the above was in fact how you were interpreting things, then good; but it wasn't clear from what you wrote before. It the above was not how you were interpreting things, then please clarify further.

Comprehension question for two points and a smiley face: Is there a difference in position of axis between measuring time with EM and measuring length with em? If so what is it, Peter?

Again, I'm not sure I understand what question you are posing.

Take the Wiki scenario above again. Suppose there is a mirror at x coordinate 1 in the given inertial frame. I emit a photon at event (0, 0), and I want to verify that the mirror is exactly 1 meter away. I measure the round-trip light travel time to be 2/299,792,458 seconds, which verifies it.

Conversely, suppose I want to verify that my clock is calibrated correctly, and I know for sure that the mirror is exactly 1 meter away. Again, I bounce a photon off the mirror and verify that my clock measures the round-trip travel time to be 2/299,792,458 seconds, which shows that it is properly calibrated.

It seems to me that these experiments count as "measuring length with EM" and "measuring time with EM". But I did the same thing both times; the only difference was which parameter I took to be known and which one I took myself to be measuring. So I don't see how there's any difference in "position of the axis" between the two measurements. But I'm not sure if that's the question you were asking.