# Is the Future Mapped Out ?

by paulselhi
Tags: future, mapped
Physics
PF Gold
P: 6,157
 Quote by nitsuj 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.

 Quote by nitsuj "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:

 Quote by nitsuj 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.

 Quote by nitsuj 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.
 P: 1,098 I think you did a switcheroo again. Flashlight beam scenario, is you running from beam it's in your future (towards you is whats 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 Im 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".
 P: 52 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 differant 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...
Physics
PF Gold
P: 6,157
I'm taking things from your post slightly out of order, because this comment confused me:

 Quote by nitsuj 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?

 Quote by nitsuj I think you did a switcheroo again. Flashlight beam scenario, is you running from beam it's in your future (towards you is whats 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.

 Quote by nitsuj 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.)

 Quote by nitsuj (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.

 Quote by nitsuj 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.

 Quote by nitsuj 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".

 Quote by nitsuj 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.

 Quote by nitsuj I agree all I am talking is Causality in the context of "past/future" & things that go c. As Im 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.

 Quote by nitsuj 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).
P: 1,098
 Quote by Spourk 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 whats in her future, this is often termed "who cares?" lol.

More specifically it is explained by causality.
P: 1,098
 Quote by PeterDonis 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 gotta read what I say before commenting on it Peter, why do you think I said only the EM on the same spacial 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 wanna 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).
P: 1,098
 Quote by PeterDonis 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?
Physics
PF Gold
P: 6,157
 Quote by nitsuj 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).

 Quote by nitsuj 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.

 Quote by nitsuj Peter I do wanna 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).

 Quote by nitsuj 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).

 Quote by nitsuj 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.

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

 Quote by nitsuj 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.

 Quote by nitsuj 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.

 Quote by nitsuj ?properties of a light cone?
I was mainly referring to the fact that the light cones define causal boundaries, as above.
P: 1,098
 Quote by PeterDonis 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,
 P: 887 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.
P: 848
 Quote by 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.
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.
Physics