Is the Future Mapped Out ?

bobc2

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.

Yes. And of course it is.

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

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.

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

bobc2

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

With all due respect, outside the light cone is your "red herring". What shape is the light cone in three spacial 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 spacial dimensions. Said differently photon does not "experience" time, it never has more then one absolute set of spacial 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[s]" of EM visually.

bobc2

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

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

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.

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.

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.

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.

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

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

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.

yuiop

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 travelling 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

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. Im 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 Im idealizing here, i know it can't physically happen & mathematically infinite is not zero.

3.) Im 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 travelled 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

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.

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

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?

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.

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.

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.

nitsuj

a reply for shits & giggles,

being overly subtle, Im 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

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.

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.

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

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.

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.

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.

Yes.

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.

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

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

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.