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

[nitsuj]

a reply for garbages & giggles,

being overly subtle, I am saying EM away from you is length; EM towards you is time. The length measurement is the past tense of the time measurement.

Less subtle, and more "scientific" Length & time are defined by the path of a photon perpendicular to...the path of a photon. aka one light second per second.

This is also the presentation of a spacetime diagram, specifically when natural units are used. i.e. time axis perpendicular to length axis.

Are we done playing elusive debater?

1.) i don't care for the "rules" of using a FoR for a photon. 'cause I posit that a photon cannot measure time. Perhaps better said as the distance it travels is purely spacelike. Your point here seems to get into the technicalities of applying theory. I have no formal education here so...but see the last comment of this post, I think it's relates to this.

2.) ...

3.) I don't know of equation for measure of time and length from the perspective of a photon. perhaps this imagery would help, two photons; one is 299,xxx metres behind the other. what is the interval between the two? Is it purely spacelike no matter the relative velocity of whoever measures it? Why can there not be a time component in that interval?
(this is just by my possibly wrong reasoning, I don't know the simple math of calculating intervals, but assume that since nothing goes faster then c, the separation between the two is of purely length, no cause from the trailing photon can effect the leading photon no matter how much time is given)

4.) lol, this one is funny, Peter the fundamentally different things are null & spatial, the "same thing" is the path of a photon, hence "fundamentally different perspectives of the same thing." (I know a photon doesn't have an FoR, let's play pretend like they did when defining the metre & second)

next.) I can't help you understand it any further then; if you can't catch up to a beam of light the interval must be purely spatial. for some reason, in the next part you reason exactly what I've been saying. Yay we agree!

Answer.) it's yes & it would be perpendicular to each other, where the distance of the path of a photon are equal parts. i.e. one meter up for a time measure, 1/299,xxxx of a second for a length measure, and there is a meter and a second defined. they are of the same distance (interval) but of different units, referred to as natural units. As defined, separated merely by orientation relative to what an observer calculates as a null path, where time & length are zero. yay invariance!

This from wiki seems to address that technical stuff from point one that I am unable to address; "Massless particles like the photon follow null geodesics. Spacelike geodesics exist. They do not correspond to the path of any physical particle, but in a space that has space-sections orthogonal to a timelike Killing vector a spacelike geodesic (with its affine parameter) within such a space section represents the graph of a tightly stretched, massless filament"

Yay! I learned a new word, I should have be saying "orthogonal" instead of "perpendicular".

Maybe you can help me understand something better, this back and forth regarding the FoR of a photon, worldlines ect, does addressing those "issues" come from the "killing field" stuff? I can't understand the wiki entry well enough to know. In this context I amount "killing field" to the myth buster line "I reject your reality [photons measure of time/length] and substitute my own [observers measure of time / length]". Is that what the "killing field" does?

 

[PeterDonis]

being overly subtle, I am saying EM away from you is length; EM towards you is time. The length measurement is the past tense of the time measurement.

Less subtle, and more "scientific" Length & time are defined by the path of a photon perpendicular to...the path of a photon. aka one light second per second.

This is also the presentation of a spacetime diagram, specifically when natural units are used. i.e. time axis perpendicular to length axis.

Are we done playing elusive debater?

I'm not "debating", I'm trying to understand what you think you're saying. None of the above helps much. The "time axis" is always perpendicular to the "length axis", regardless of what units you use, so that doesn't tell me anything useful. If you think "EM away from you is length, EM towards you is time", then does that mean if I shine a flashlight at you, I think it's length and you think it's time, while if you shine a flashlight at me, you think it's length and I think it's time? That makes no sense. Basically you are saying things that I can't make sense of in terms of any sort of standard physics. The only reason I keep bringing it up is that it seems clear to me that you do have something to say; you are just saying it in a way that I can't understand.

1.) i don't care for the "rules" of using a FoR for a photon. 'cause I posit that a photon cannot measure time. Perhaps better said as the distance it travels is purely spacelike. Your point here seems to get into the technicalities of applying theory. I have no formal education here so...but see the last comment of this post, I think it's relates to this.

So let me get this straight: you have no formal education, you don't understand any of the "technicalities of applying theory", and yet you think you can just choose to "not care for the rules" of the theory? No wonder you're not making sense.

I'm sorry if that comes off as snarky, but please understand that the theory has rules, and standard terms, and standard ways of talking about things, for a reason: so that the theory can make accurate predictions, and so that its concepts can be talked about with clarity and precision. You appear to be trying to express your thoughts in your own terms, using your own version of the theory, and it's not working well; but clearly you have *some* thought behind them. For example, you say "a photon cannot measure time...the distance it travels is purely spacelike". A photon's worldline is not spacelike; it's null; and it's the fact that it's null that accounts for why the "time measured by a photon" is zero (not the best way to put it, IMO, but a lot of people do use that phrase to refer to the null worldline). The "distance it travels" can be interpreted as a spacelike line, as I said in previous posts, but that spacelike line tells you nothing useful about the physics; in particular, it doesn't tell you that the photon's actual worldline is null, so it doesn't tell you anything about whether or not the photon can "measure time" in the sense you're using the term.

I would strongly recommend that you try to learn more about the standard theory and the standard terms. Even if you're going to end up deciding that you don't entirely accept the standard theory and the standard terms, you will find it a lot easier to communicate what you disagree with if you know the standard theory and the standard terms.

3.) I don't know of equation for measure of time and length from the perspective of a photon. perhaps this imagery would help, two photons; one is 299,xxx metres behind the other. what is the interval between the two? Is it purely spacelike no matter the relative velocity of whoever measures it? Why can there not be a time component in that interval?

Whether or not the interval has a time component does depend on the relative velocity of the measurer. By specifying that "one is 299,xxx meters behind the other", you are implicitly specifying that the measurer measures that length as the space component of the interval; you are also at least strongly implying that the time component of the interval is zero for that measurer--because specifying a distance between two moving objects is normally taken to mean "distance at the same time, according to the measurer".

A measurer that was moving relative to the first one would then see a nonzero time component to that specific interval (that is, the interval between the two specific events implied by your description--events on each photon's worldline that are "at the same time" according to the first measurer); but he would also see a *different* space component. However, a measurer moving relative to the first one would find it more natural to measure a *different* interval, one between events on the two photons' worldlines that were "at the same time" according to *him*, not the first measurer. By the relativity of simultaneity, these will be a *different* pair of events; to the second measurer, *this* interval, between that pair of events, will have a zero time component (and a space component different from 299,xxx meters); but *this* interval will have a nonzero time component (and a space component that is still different from 299,xxx meters) to the *first* measurer.

In summary: an "interval" is a Lorentz interval between a specific pair of events; for any timelike or spacelike interval there will be one particular FoR (one particular measurer) in which only one component (time for timelike intervals or space for spacelike intervals) is nonzero. (For timelike intervals this is called the "rest frame"; for spacelike intervals there is no simple term in common use, but "simultaneous frame" would seem to me to be a good term for it). For null intervals, the time and space components in any FoR must be equal, but their actual magnitude will vary from frame to frame.

(this is just by my possibly wrong reasoning, I don't know the simple math of calculating intervals, but assume that since nothing goes faster then c, the separation between the two is of purely length, no cause from the trailing photon can effect the leading photon no matter how much time is given)

This is false; the leading photon could certainly interact with something that could then propagate back in the other direction and meet the trailing photon.

4.) lol, this one is funny, Peter the fundamentally different things are null & spatial, the "same thing" is the path of a photon, hence "fundamentally different perspectives of the same thing." (I know a photon doesn't have an FoR, let's play pretend like they did when defining the metre & second)

You are still missing my point. The path of the photon, meaning its worldline, is null; it is *not* spatial, and no amount of change in perspective will make it spatial. When you talk about a "spatial path of the photon", you are talking about a *different thing*--a spacelike line, the projection of the photon's worldline into a particular spacelike surface, that has nothing to do with the photon's physics.

next.) I can't help you understand it any further then; if you can't catch up to a beam of light the interval must be purely spatial. for some reason, in the next part you reason exactly what I've been saying. Yay we agree!

So in your lexicon, "the interval must be purely spatial" is equivalent to "the path of the photon is a null line". Hmm. At least you appear to agree with what I said.

Answer.) it's yes & it would be perpendicular to each other, where the distance of the path of a photon are equal parts. i.e. one meter up for a time measure, 1/299,xxxx of a second for a length measure, and there is a meter and a second defined. they are of the same distance (interval) but of different units, referred to as natural units. As defined, separated merely by orientation relative to what an observer calculates as a null path, where time & length are zero. yay invariance!

A "null path" does *not* mean time and length are zero; it means "length in time" and "length in space" are equal (speaking somewhat loosely). The rest of what you wrote just seems like a long-winded way of saying what I just said in the previous sentence.

This from wiki seems to address that technical stuff from point one that I am unable to address; "Massless particles like the photon follow null geodesics. Spacelike geodesics exist. They do not correspond to the path of any physical particle, but in a space that has space-sections orthogonal to a timelike Killing vector a spacelike geodesic (with its affine parameter) within such a space section represents the graph of a tightly stretched, massless filament"

Yay! I learned a new word, I should have be saying "orthogonal" instead of "perpendicular".

Yes.

Maybe you can help me understand something better, this back and forth regarding the FoR of a photon, worldlines ect, does addressing those "issues" come from the "killing field" stuff? I can't understand the wiki entry well enough to know.

A Killing vector field is a very general concept in differential geometry; it has nothing specifically to do with the case of a photon. The only reason the concept was brought up in the Wiki entry in reference to spacelike geodesics is that it's a lot easier to give a physical meaning to spacelike geodesics if the spacetime as a whole has a timelike Killing vector field. The best simple way I know of to picture what that means is that the spacetime is stationary: i.e., the metric "looks the same" at all times. Since the metric determines which curves are timelike, spacelike, and null, that also means that curves of each type "look the same" at all times; for example, if null lines (photon worldlines) are 45-degree lines on a spacetime diagram at one time, they are 45-degree lines at all times. That makes it easier to picture what's going on in the spacetime.

In this context I amount "killing field" to the myth buster line "I reject your reality [photons measure of time/length] and substitute my own [observers measure of time / length]". Is that what the "killing field" does?

No; see above. But there is no such thing as a "photons measure of time or length", because photons don't have a standard FoR in which they are at rest.

 

[bobc2]

1.) i don't care for the "rules" of using a FoR for a photon. 'cause I posit that a photon cannot measure time. Perhaps better said as the distance it travels is purely spacelike.

nitsuj, you seem to be expressing the same idea that Brian Greene expressed ("The Fabric Of The Cosmos" pg. 49):

"Moreover, the maximum speed through space is reached when all light-speed motion through time is fully diverted into light-speed motion through space--one way of understanding why it is impossible to go through space at greater than light speed. Light, which always travels at light speed through space, is special in that it always achieves such total diversion. And just as driving due east leaves no motion for traveling north, moving at light speed through space leaves no motion for traveling through time! Time stops when traveling at the speed of light through space. A watch worn by a particle of light would not tick at all..."

 

[PeterDonis]

nitsuj, you seem to be expressing the same idea that Brian Greene expressed ("The Fabric Of The Cosmos" pg. 49):

And as I've pointed out in previous threads where this has come up, you have to be very careful drawing deductions from this way of thinking about it. Greene's way of putting it makes it seem as though moving at the speed of light, c, is a limiting case of moving at speeds closer and closer to c, but still less than c. However, this is not true. Moving at .999999999999c is not "very close to" moving at c; there will be some inertial frame in which the object that we see as moving at .999999999999c is at rest, and is therefore just as "far away" from moving at c as we are, viewed from our rest frame. And in that object's rest frame, we appear to be moving at - .999999999999c (i.e., the same speed in the opposite direction), so it is we who appear to be moving "close to" c, even though to us we are at rest.

So worldlines that are null (that "move at c") are separated by a discontinuity from worldlines that are timelike (that "move at less than c", so there is a frame in which they are at rest). Greene's way of putting it obscures this fundamental discontinuity.

 

[nitsuj]

nitsuj, you seem to be expressing the same idea that Brian Greene expressed ("The Fabric Of The Cosmos" pg. 49):

yea, that was when I said I think block universe is a valid concept when describing the universe from the perspective of a wave (specifically c) post #22.

 

[nitsuj]

Peter you are terrible at interpreting my posts ( and not the really poorly worded parts ).

ME: (this is just by my possibly wrong reasoning, I don't know the simple math of calculating intervals, but assume that since nothing goes faster then c, the separation between the two [two photons, a distance apart one trailing the other along a plane] is of purely length, no cause from the trailing photon can effect the leading photon no matter how much time is given)

YOU: This is false; the leading photon could certainly interact with something that could then propagate back in the other direction and meet the trailing photon. (whoopty doo Peter)

That is not at all what I said. You changed the simple description completely.

With absolute certainty, and clear as day to this layman, No cause from the trialling photon can effect the lead photon, that would be faster then c, how can you not visualize this?(clearly I am assuming you have education in physics, a safe assumption for the most part from what I've read)
So it's your interpretation of my straight forward statement that was "false".

"A "null path" does *not* mean time and length are zero; it means "length in time" and "length in space" are equal (speaking somewhat loosely)."

It doesn't "mean" that at all. Nor does it "mean" time is zero. What I said is it is where time and length are zero. All roads lead to Rome, in the case of c & observer measurements of time & length. But, From that null line, length and time are "separated" into equal parts, graphically orthogonal is observed rest relative to c. Because of that I can decide what unit I want c to measure, depending on orientation this means I can choose c to measure just length in which case I very safely assume the time component is zero.

It is how we measure length and time strictly via c. Because every observer measures c to the same value it makes for some counter intuitive results when comparing observations of length and time. This in turn I think speaks volumes of the nature of time and length.

What I take of those unusual results from comparative measures, is that time and length are different perspectives of the same thing, that null line.

Peter all my posts are clearly (does it matter?) not for comparison to theories. Its not that I don't subscribe to that validity of mainstream theories, and it's not that I've said anything proven wrong by theories, but when it comes to "it's not true a photon can't measure time, it's non-zero." I find funny, and ignore it.

"If you think "EM away from you is length, EM towards you is time", then does that mean if I shine a flashlight at you, I think it's length and you think it's time, while if you shine a flashlight at me, you think it's length and I think it's time? That makes no sense."

Well of course that makes no sense.

What does make sense is if you run from a flashlight beam you will never ever out run it, It is "destined" to be in your future , Conversely if you try and catch up to a flashlight beam you will never ever catch up to it, it is "destined" to be in your past (how cool that you can see it , oh wait you could never).

That is how to interpret the length measurement is the past tense of the time measurement.

 

[PeterDonis]

ME: (this is just by my possibly wrong reasoning, I don't know the simple math of calculating intervals, but assume that since nothing goes faster then c, the separation between the two [two photons, a distance apart one trailing the other along a plane] is of purely length, no cause from the trailing photon can effect the leading photon no matter how much time is given)

YOU: This is false; the leading photon could certainly interact with something that could then propagate back in the other direction and meet the trailing photon. (whoopty doo Peter)

That is not at all what I said. You changed the simple description completely.

Ah, I see, I switched "leading" and "trailing". Sorry, you're right; the leading photon can causally affect the trailing, but not vice versa.

"A "null path" does *not* mean time and length are zero; it means "length in time" and "length in space" are equal (speaking somewhat loosely)."

It doesn't "mean" that at all.

It most certainly does. You've already said you don't want to go into the math, so it would be advisable for you not to make blanket statements that contradict what the math says. I've already given the math once, but I'll repeat it briefly: for any null vector, in any given inertial frame, the "length in time" (i.e., the time component of the vector) is equal to the "length in space" (i.e., the square root of the sum of the squares of the space components). That's the mathematical condition for a path to be null. It does *not* mean "time and length are zero"; the time and space components I referred to above are nonzero. (A vector with time and length both zero would be the zero vector, which doesn't represent anything physically.)

Most of the rest of your post is basically expounding on the same mistake, but there is one part at the end that's worth further comment:

What does make sense is if you run from a flashlight beam you will never ever out run it, It is "destined" to be in your future , Conversely if you try and catch up to a flashlight beam you will never ever catch up to it, it is "destined" to be in your past (how cool that you can see it , oh wait you could never).

I see what you are saying here, but I don't think it's a very useful way of saying it (meaning the bolded parts). First of all, before the flashlight beam has caught up to you, it is not "in your past", at least not as that term is usually used in relativity (meaning "in your causal past", or "inside your past light cone"). The flashlight beam can't affect you causally at all until it catches you, so until it does, it's outside your past light cone.

Similarly, once the flashlight beam passes you, it's true that you can't catch up to it, but that doesn't mean it's "in your future", meaning "in your causal future". Once the beam has passed you, not only can you not catch up to it, but you can't even send another light beam to catch up to it (as you correctly pointed out to me, see above). So once the beam has passed you, it's outside your future light cone.

So except for the instant at which the light beam passes you, it is neither "in your past" nor "in your future"; it is "elsewhere", in the region of spacetime that is spacelike separated from you. The event on your own worldline where the light beam passes you can be in your future or your past, depending on which event on your worldline is being looked at, but that's not the same as the light beam itself being in your future or your past.

That is how to interpret the length measurement is the past tense of the time measurement.

Even if, for the sake of argument, I ignore the issues I just pointed out, I don't see the connection here at all.

 

[nitsuj]

I think you did a switcheroo again.

Flashlight beam scenario, is you running from beam it's in your future (towards you is what's important, and the fact it will always "reach" you).

You running towards (chasing) the beam it's in your past...man. It can't be caught up to.

The beam cannot go "past you" as that is not a cause effect scenario, it is "elsewhere". Only the photons traveling on the same spatial axis as you can be considered. This would be the "tip" of the cone, the area (you'll be mad if I get this wrong) called zero. (were we getting the measure of time / length is zero, and the unit time length is zero mixed up? null line is zero unit for time/length, calling it "can't measure time" maybe misleading, idk)

I don't think you are visualizing the "light cone" the same as me.
Yes in 2D outside the light "cone" is "elsewhere", In 3D that is greater then c interval...away! from you continuum. From there it is the direction of the EM relative to you that is "past/future". This is semantics, even I can tell, "past", "future", "elsewhere", comon.

So I call greater than c interval continuum away is the "past". Greater than c interval continuum towards is the "future". ( Peter I don't mean this applied macroscopically/holistically, of course as we both have pointed out direction is important and each FoR's Point of view is valid)

I agree all I am talking is Causality in the context of "past/future" & things that go c. As I am defining it future changes with motion out of one plane(axis) of em into another.

How else can we define past/future but with cause/effect & c?

I can totally appreciate your last comment, it is a rather odd way I had interpreted the measurements of time & length, and that's how I got "there".

 

[Spourk]

Isn't this all just simply explained by the time it takes the light of an event happening to reach the observer?

Jack and Jill observe an event that took the light almost the same time to reach them.

It just happens to hit Jack a few minutes quicker, and he's pretty damn talkative.

To Jill, if they could talk fast enough, Jack can see the future.

But neither is true, they are just at different paths of where the light happens to hit, and the event itself happened, but is still in both of their pasts. In other words, the event happening isn't seen by anyone in the future per sae, but because on person had a little distance on the other, they saw it happen first, because the of the travel time of the light from the event.

A bit of wine and extrapolation here, but it seems pretty simple if you just think about an event happening and a bit of time for that light to hit someone, depending on where they might be standing when the light hits them.

If it was a mac truck it might be better that Jack was there first...

 

[PeterDonis]

I'm taking things from your post slightly out of order, because this comment confused me:

The beam cannot go "past you" as that is not a cause effect scenario, it is "elsewhere". Only the photons traveling on the same spatial axis as you can be considered.

I thought that's the case we were discussing: the beam passes you at some event on your worldline. If the beam never crosses your worldline at all, you never observe it, so who cares?

I think you did a switcheroo again.

Flashlight beam scenario, is you running from beam it's in your future (towards you is what's important, and the fact it will always "reach" you).

You running towards (chasing) the beam it's in your past...man. It can't be caught up to.

By "it" here you appear to mean "the event where the beam crosses your worldline". With that interpretation, I agree with the above: if you are running from the beam, the event where it catches you is to your future; but if you are running after the beam, the event where it caught and passed you is in your past. However, as I pointed out in my last post, the event where the beam passed you is *not* the same as "the beam" itself; it's only one event on the beam's worldline.

This would be the "tip" of the cone, the area (you'll be mad if I get this wrong) called zero.

If by "zero", you mean "the event where the beam passes you, to which we assign coordinates t=0, x=0", then I agree. (I'm leaving out y and z spatial coordinates because, as I said above, I thought we were discussing the case where the beam passes us, so we can observe it as it does so; and in that case only one spatial dimension is relevant, the one along which the beam moves relative to us.) This event is the "tip" of two light cones, the past light cone and the future light cone. (The same would apply to any event, of course, but each event has its own unique pair of light cones.)

(were we getting the measure of time / length is zero, and the unit time length is zero mixed up? null line is zero unit for time/length, calling it "can't measure time" maybe misleading, idk)

From the above, the only thing that is zero about the "tip" of the light cone is the time *coordinate* (and the space coordinate too, of course). The only "length" or "measure of time/length" that I can see here is the trivial statement that the event at the "tip" of the light cone is zero distance in time/space from itself.

I don't think you are visualizing the "light cone" the same as me.

None of the things I've said about light cones are dependent on any particular "visualization". Light cones are well-defined physical objects in spacetime and their properties are invariant; they do not depend on how the light cones are visualized.

Yes in 2D outside the light "cone" is "elsewhere", In 3D that is greater then c interval...away! from you continuum. From there it is the direction of the EM relative to you that is "past/future". This is semantics, even I can tell, "past", "future", "elsewhere", comon.

Nor do the properties of the light cones depend on how many spatial dimensions we include. The only thing that affects is the aptness of the term "cone" to describe them. If we are restricting to one spatial dimension, then the "light cones" look like two intersecting lines, t = x and t = -x (the t > 0 portion is the "future light cone", and the t < 0 portion is the "past light cone"). If we add a second spatial dimension, then they actually do look like cones (but the common axis of the cones is the time axis; the two spatial dimensions define a set of stacked planes that cut circular cross sections from the cones, except for the "tip" at t = 0, x = 0, which cuts the "cones" at that point only). If we add the third spatial dimension, of course we can't directly visualize what the light cones look like. But in *all* of these cases, the physical properties of the light cones are the same: the region of spacetime inside the future light cone is the "future", the region inside the past light cone is the "past", and the region outside the cones is "elsewhere".

So I call greater than c interval continuum away is the "past". Greater than c interval continuum towards is the "future".

No, this is wrong. The meaning of "future" and "past" doesn't depend on how many spatial dimensions we include. See above.

I agree all I am talking is Causality in the context of "past/future" & things that go c. As I am defining it future changes with motion out of one plane(axis) of em into another.

The definition of "past" and "future" as I've given it above applies to a single event. However, one can also make fairly strong statements about the entire worldline of a timelike observer. Take any event E on such a worldline, and consider the future light cone of that event. No matter *what* the observer does after passing through event E, no matter how he changes his motion, two things will always remain true: (1) His worldline will remain within the future light cone of E; and (2) the future light cone of every point after E on his worldline will be contained within the future light cone of E. All he can change by changing his motion after event E is which particular *portions* of E's future light cone he explores. So in one sense, even though he can "change his future" by changing his motion, in another sense he can't; he can't change his "set of possible futures" at a given event once he has passed through that event.

How else can we define past/future but with cause/effect & c?

The light cones are certainly intimately connected with causality, and for any physically reasonable spacetime there will be a continuous choice throughout the spacetime of which half of the light cones is "past" and which is "future". However, the light cones themselves, plus causality, do *not* tell us which direction (which half of the light cones) is future and which is past. We have to add that to the model "by hand", so to speak; normally we do that by specifying in which direction observers in the spacetime experience time (i.e., the "past" direction is the one observers remember events from, the "future" direction is the one they don't remember but can only anticipate).

 

[nitsuj]

Isn't this all just simply explained by the time it takes the light of an event happening to reach the observer?

And whether it will reach you,
yea it is, pretty lame definitions of past and future eh? because you can't go c it is definitive. Jack and Jill are in different places so their "past / future" are not comparable, there is no causality as you mentioned Jack can't talk "fast enough" to inform Jill of what's in her future, this is often termed "who cares?" lol.

More specifically it is explained by causality.

 

[nitsuj]

I'm taking things from your post slightly out of order, because this comment confused me:

[ME]"The beam cannot go "past you" as that is not a cause effect scenario, it is "elsewhere". Only the photons traveling on the same spatial axis as you can be considered."

I thought that's the case we were discussing: the beam passes you at some event on your worldline. If the beam never crosses your worldline at all, you never observe it, so who cares?

you got to read what I say before commenting on it Peter, why do you think I said only the EM on the same spatial axis can be considered? Of course you cannot observe it if it doesn't "reach" you, so we don't consider it, it is "elsewhere" like with the light cone. (the light cone diagram does place events off the world line and still labels it in your causal future, that assumes light reaches you) Wiki had a funny term for this, calling farther into "elsewhere" becoming more "unphysical", I would also call the "past /future" as "unphysical".

Peter I do want to get this right, along the null line are proper time and proper length null? But the coordinates have a value whether it be zero to whatever those values are still of a physical meaning.

Of course past present doesn't depend on how many dimensions there are, interpreting how it "plays out" in 3D does. And it has to be defined by what you can and can't causally effect. That is separation by a distance, besides the shape it's not as if it's much different than the light cone (oh and that in 3D it's continuum).

 

[nitsuj]

None of the things I've said about light cones are dependent on any particular "visualization". Light cones are well-defined physical objects in spacetime and their properties are invariant; they do not depend on how the light cones are visualized.

Nope, see in spacetime there are actualy 4 dimensions, how many with the light cone? It's an abstract of spacetime, not a physical object of spacetime.

?properties of a light cone?

 

[PeterDonis]

Of course you cannot observe it if it doesn't "reach" you, so we don't consider it, it is "elsewhere" like with the light cone. (the light cone diagram does place events off the world line and still labels it in your causal future, that assumes light reaches you)

Events off your worldline can be in your causal future (at a particular event) if they are inside your future light cone (at that particular event). That does not mean that light signals from those events can reach you; it means that light signals from you (at that particular event) can reach events in your causal future.

The set of events from which light signals can reach you (at a particular event) is your causal past (at that particular event).

Wiki had a funny term for this, calling farther into "elsewhere" becoming more "unphysical", I would also call the "past /future" as "unphysical".

I would be very careful about reading very much into the Wiki discussion.

Peter I do want to get this right, along the null line are proper time and proper length null?

No. Along a null line the Lorentz interval is null, but that interval is neither a proper length (a proper length is a spacelike interval) nor a proper time (a proper time is a timelike interval).

But the coordinates have a value whether it be zero to whatever those values are still of a physical meaning.

Yes, of course the coordinates can be given a physical meaning (assuming a standard inertial coordinate system).

Of course past present doesn't depend on how many dimensions there are, interpreting how it "plays out" in 3D does.

No, it doesn't, except in the trivial sense that you can only talk about events "playing out" in the dimensions you include in your model.

And it has to be defined by what you can and can't causally effect.

Yes.

That is separation by a distance, besides the shape it's not as if it's much different than the light cone (oh and that in 3D it's continuum).

I don't know what you mean by this, except that I assume by "in 3D it's continuum" you mean that the "light cone" in 3-D (2 space dimensions plus 1 time dimension) includes a continuous range of directions (a full circle) instead of just two opposite directions, +x and -x (discrete). This is true, but I don't see why it's such a big deal. It doesn't affect anything important except, as I said above, the trivial fact that you can only treat effects in a model that happen within the dimensions you include in the model.

I also don't understand why you appear to be trying to draw a distinction between the light cones and causality. The light cones *define* causality in the spacetime; they define the boundaries that govern which events can causally affect which other events.

Nope, see in spacetime there are actualy 4 dimensions, how many with the light cone? It's an abstract of spacetime, not a physical object of spacetime.

Light cones define causality, as I said above. Causality is a concrete physical property of a spacetime; it's not "abstract" (unless you want to make the term "abstract" so broad that it includes just about anything). Since the light cones define causal boundaries, they are surfaces of one dimension less than the dimension of the spacetime; a light cone in 4-D spacetime (i.e., leaving out no spatial dimensions) is a 3-dimensional null surface; it forms the 3-dimensional boundary between 4-dimensional spacetime regions.

?properties of a light cone?

I was mainly referring to the fact that the light cones define causal boundaries, as above.

 

[nitsuj]

No, it doesn't, except in the trivial sense that you can only talk about events "playing out" in the dimensions you include in your model.

Just want to clarify that "playing out" of course refers to continuum.

I completely agree with all you said about the "mechanics" of light cones,

 

[Khashishi]

It seems that relativity leaves you with three options.
Option 1: block universe, totally predetermined
Option 2: solipsism. The universe unravels around me and none other.
Option 3: many worlds hypothesis. Every observer sees a different universe unfurl. This is basically like solipsism, since my universe unravels around only me, and other universes might as well not exist.

It's the same problem as the problem of quantum collapse. Until I see the Andromedan fleet, the fleet exists in a superposition of traveling to Earth or not traveling to Earth. What constitutes a detection which causes one of these choices to realize? If there is some observer-agnostic mechanism, then this seems to lead to option 1. If the wavefunction collapses when I detect it, then that's option 2. Option 3 is effectively the same as option 2, but spoken with less hubris.

Personally, I think option 1 is correct, and there's something fundamentally missing in our understanding of quantum collapse.

 

[bobc2]

It seems that relativity leaves you with three options.
Option 1: block universe, totally predetermined
Option 2: solipsism. The universe unravels around me and none other.
Option 3: many worlds hypothesis. Every observer sees a different universe unfurl. This is basically like solipsism, since my universe unravels around only me, and other universes might as well not exist.

It's the same problem as the problem of quantum collapse. Until I see the Andromedan fleet, the fleet exists in a superposition of traveling to Earth or not traveling to Earth. What constitutes a detection which causes one of these choices to realize? If there is some observer-agnostic mechanism, then this seems to lead to option 1. If the wavefunction collapses when I detect it, then that's option 2. Option 3 is effectively the same as option 2, but spoken with less hubris.

Personally, I think option 1 is correct, and there's something fundamentally missing in our understanding of quantum collapse.

One of the more interesting posts, Khashishi. And once a block universe is assumed, your last comment is justified. It then prompts a host of interesting implications.

 

[PeterDonis]

It seems that relativity leaves you with three options.
Option 1: block universe, totally predetermined
Option 2: solipsism. The universe unravels around me and none other.
Option 3: many worlds hypothesis. Every observer sees a different universe unfurl. This is basically like solipsism, since my universe unravels around only me, and other universes might as well not exist.

You're leaving out at least one important option:

Option 4: consistent histories. There is quantum indeterminacy at the micro-level, but there is still a consistent past history that all observers who remember that past history agree on. So it's not true that "every observer sees a different universe", at least not in the strong sense in which that implies a form of solipsism.