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B Why is every event in spacetime limited to our present point

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  1. Jun 2, 2017 #1
    I never got an answer to my original question

    Why can't an event where light from that event hasn't reached our present point in space and time, reach a past point in space and time in 4D space?

    http://curious.astro.cornell.edu/ab...ace-the-same-as-looking-back-in-time-beginner

    When we look at other planets we're looking into the past!

    If an Alien today 10 billion light years away sent us a message today, why wouldn't reach into our past?
     
    Last edited: Jun 2, 2017
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  3. Jun 2, 2017 #2

    Nugatory

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    Because the light will take ten billion years to get here, so if it leaves today it will reac us ten billion years into our future.
     
  4. Jun 2, 2017 #3

    Dale

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    By "a past point" are you specifically asking about past points where we existed, or are you talking about just any past point, including past points far removed from where we were.
     
  5. Jun 2, 2017 #4
    I'm talking about past points where we existed. This could also include other past points.

    If an event is outside of our present past/future worldine, why couldn't light from that event reach a point that includes our past? Why are all events in spacetime only allowed to reach our present position.

    Look at this pic:

    PNniV.png

    We're the Observer but why are all events that occur outside of our past and future light cone limited to the present position of the observer?
     
  6. Jun 2, 2017 #5

    Nugatory

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    First we have to define "past" and "future" in 4D spacetime.

    The easiest way to visualize this is to draw a space-time diagram, with the x-axis horizontal and the t-axis vertical. In such a diagram (for simplicity let's do this assuming flat space, no gravity, so that we can use special relativity instead of the much more demanding general theory) the light emitted at any event follows a 45-degree path slanting up and left and up and right; we call the downwards-pointing wedge between those two lines with its point at the event the "future light cone" of that event. We can extend both of these lines downwards as well to form an upwards-pointing wedge with its point at the event; we call that wedge the "past light cone". Everything in the past light cone is the past of the event and everything in the future light cone is in the future of that event (which is, of course, why we gave them those names).

    Now, we and all other observers are going to be following our own paths through spacetime. Assume for simplicity that we're all moving at constant speeds, no acceleration (acceleration just complicates the picture without changing any of the basic principles) and these paths wil be straight lines. They will also be at an angle less than 45 degrees from vertical; any shallower, and we'd be moving at the speed of light or faster relative to someone else, which is impossible. (These paths are called "worldlines").

    Now pick an event on my worldline - it's the event when I said "NOW!". At that moment, my past is everything in the past lightcone of that event and my future is everything in the future lightcone of that event. Look at the diagram, remember that light always moves up the page at a 45 degree angle, and you'll quickly agree that if light from some event hasn't yet reached me, it cannot have reached any event in my past.
     
  7. Jun 2, 2017 #6

    Dale

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    In order for a signal from an event outside of our past light cone to reach reach us in the past then either we or the signal would have needed to travel faster than c.
     
  8. Jun 2, 2017 #7

    Nugatory

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    They aren't; that diagram is somewhat confusing.

    Suppose that we the observer uttered the word "now!" right at that point in the middle where the tips of the cones touch. The horizontal plane is all of the events that we will say happened at the same time that we said "now!", meaning that if they happened ten light-seconds away the light from them will reach the observer ten seconds after he said "now!", if they happened 20 light-seconds away the light from them will reach the observer 20 seconds after he said "now!", and so forth.

    However, nothing requires that all the events outside of the two lightcones lie in that plane, and indeed they don't. Our observer will say that everything above that plane happened after he said "Now!" and everything below it happened before. However, for the events outside the past and future light cones other observers in motion relative to our observer will not necessarily agree about whether they happened before or after he said "now!". Only for events in the past and future light cones will observers agree about the relative ordering.
     
  9. Jun 2, 2017 #8
    I think it could reach our past line cone based on things like simultaneity.

    So 2 observers can see events in different order.

    Say observer A sees:

    Kennedy election
    Cuban Missle Crisis
    Kennedy assassination

    Observer B sees

    Kennedy election
    Cuban Missle crisis

    He doesn't see Kennedy assassination.

    Does Kennedy assassination have to occur for observer B?

    So our past light cone only has subjective meaning to observers at each present point in space that they're experiencing.

    Also, gravity waves could tell us if events are happening around us but just haven't reached our now so to speak. So these events could reach a past now and the order of events can be seen differently.
     
  10. Jun 2, 2017 #9

    PeterDonis

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    Only if those events are spacelike separated. Events that are timelike separated, which all of the ones you give are--since they all involve the same person, Kennedy, and a person's path through spacetime must be timelike--will appear in the same order to all observers.

    Yes, because you said A saw it, and A and B are in the same spacetime. The only difference is that, from what you describe, B hasn't seen the light from the Kennedy assassination yet; but he will.
     
  11. Jun 2, 2017 #10

    Nugatory

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    The light cones are defined by whether a light signal could in principle have made its way from the source event to the observer, not whether one actually did. This removes the element of subjectivity from the definition - in flat spacetime all observers everywhere will in principle be able to receive a light signal from all three of the the election, missile crisis, and assassination events.
     
  12. Jun 2, 2017 #11

    PeterDonis

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    Yes, because light takes time to travel--exactly the amount of time in the past that we see those other planets, because that's how long it took the light from them to get to us. And since nothing can travel faster than light, no causal influence from those planets from any event that happened there that we are just seeing happening (because the light is just reaching us) could have reached us any faster than the light did. So nothing that happened on those planets at the time we are seeing them, could have affected anything that is in our past at this instant--since that would require a causal influence to travel faster than light.

    Of course things happened on those planets earlier than when we are seeing them--light left those planets a day before the light we are seeing now, and that light arrived on Earth yesterday and so could have affected something that happened and is in our past now. But the ordering is still the same: what we saw happening on those planets when the light from them reached us yesterday, couldn't have affected anything that happened on Earth before yesterday.

    In short, the ordering of events on some distant planet that is sending causal influences our way will be the same as the ordering of whatever events here on Earth are affected by those causal influences. Causal propagation can't change the ordering of events.
     
  13. Jun 2, 2017 #12

    Nugatory

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    This would be a good time to mention that if two events are spacelike-separated, neither of them lies in either the past or future light cones of the other. Referring back to the diagram in @quantumfunction's post #4 of this thread: If we start at the event at the apex of the cones where the observer said "Now!", the events that are spacelike- separated from that event are the ones that lie outside of both light cones. These are the ones that different observers will disagree about whether they happened before or after the "Now!" event.
     
  14. Jun 2, 2017 #13

    PeterDonis

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    Gravity waves travel at the speed of light. So the same rules apply to them as apply to all other causal influences: they can't change the ordering of events.
     
  15. Jun 2, 2017 #14

    PeterDonis

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    @quantumfunction I asked you in the previous thread to assign coordinates to events and phrase your question in terms of those coordinates. I still think you should do that. You are using vague ordinary language and it appears to be misleading you. Forcing yourself to precisely specify everything in terms of coordinates will fix that.
     
  16. Jun 2, 2017 #15
    Again, you say B hasn't seen the light from the Kennedy Assassination but he will but why will he?

    Why is observer B bound to the worldline of observer A? He doesn't even have to see these events in the same order.

    Here's more from Dr. Sten Odenwald.

    http://www.ws5.com/spacetime/

    So again, why is observer B bound to the observations of observer A?

    The Kennedy assassination isn't in the future light cone of observer B. Observer B could be a friend of Lee Harvey Oswald and decide to stop him when he finds out his plans.

    What mechanism in physics says observer B has to experience the same thing as observer A? Spacetime doesn't evolve, it exists but objects in space and time experience the evelotion of events. There's nothing that says all of these events have to occur in the same way for all observers. Einstein said:

    This would mean all events that can happen must already exists in 4D spacetime even though observers experiencing local events are under illusion that there's this distinction between events and there isolated experience is the sum of all that can happen for all observers.
     
  17. Jun 2, 2017 #16

    Nugatory

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    Try drawing the spacetime diagram that I suggested above.... Please.
    If you work through the math, or draw and play with with the visual picture that the spacetime diagram provides, you will see that all observers see all timelike-separated ("timelike-separated" refers to a pair of events such that one of the is in the future light cone of the other) in the same order. JFK's assassination is in the future light cone of the missile crisis and the election; the missile crisis is in the future lightcone of the election; therefore all observers everywhere will agree about the relative ordering of those three events.
     
  18. Jun 2, 2017 #17
    I understand this and I agree with what you're saying but I'm pointing out that the assassination of JFK hasn't occurred for observer B yet so it's in the past light cone of observer A but the future light cone of observer B.

    Why would observer B's future light cone be limited to only the events observer A experienced?

    There isn't any mechanism I have seen in the laws of physics that binds observer B to observer A's past light cone. Are you saying there's no free will? Isn't that at odds with things like the Free Will Theorem of Conway and Kochen?

    If observer B was a friend of Lee Harvey Oswald, are you saying he would have no choice but to allow Oswald to assassinate Kennedy? What force in the laws of physics would stop observer B from stopping Oswald?
     
  19. Jun 2, 2017 #18

    Nugatory

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    Draw that spacetime diagram, include the worldlines of A and B, and mark the events where they each say "Now!" to establish the past and future and lightcones that we're talking about. How are the two "Now!" events related?
     
  20. Jun 2, 2017 #19

    PeterDonis

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    You continue to misuse the term "worldline". It doesn't mean what you are using it to mean. If you would take my advice and actually give a precise description of your scenario in terms of coordinates--or, equivalently, draw the spacetime diagram that Nugatory keeps asking you to draw--you would see the issue; but you seem unwilling or unable to do that.

    No, we're saying that free will can't make the same event happen two different ways.

    In your scenario, A has seen light signals from Kennedy's assassination. That means light signals exist in spacetime that carry information about Kennedy's assassination happening, at some particular event in spacetime. Call that event K. Light signals carrying that information will eventually arrive at every event in the future light cone of event K. A seeing the light signals from event K is one such event, in the future light cone of event K.

    Now, consider the path through spacetime (the worldline, according to the standard usage of that term) of observer B. Either it will eventually intersect the future light cone of event K, or it won't. If it does, then B will see light signals from event K, i.e., he will see Kennedy being assassinated, just as A did.

    If B's path through spacetime does not ever intersect the future light cone of event K (note that the only way this can happen is if B gets in a rocket ship and accelerates fast enough away from Earth that no light signals from event K can ever catch up to him--look up "Rindler horizon"), then B will never see Kennedy being assassinated--but he will never see Kennedy not being assassinated either. He will never see any light signals at all from Earth from the period of time (November 1963) in which Kennedy either was or was not assassinated. So he simply never gets any information at all about such events. This doesn't allow him to conclude anything about Kennedy--it certainly does not allow him to conclude that Kennedy was not assassinated. What happened in that region of spacetime is simply unknown to him.

    In other words, once A has seen light signals that show Kennedy being assassinated, no other observer, anywhere, can ever see any information that contradicts that happening.
     
  21. Jun 2, 2017 #20
    The two "Now" events are only related because they share the same past with Kennedy elected and the Cuban missle crisis.

    Kennedy's assassination is in the future light cone of observer B. There's no way this can be fixed unless there's some unknown force like in 11.22.63 with James Franco.

    This is what nobody is providing. A physical mechanism that says observer B has to see and experience what observer A saw.

    PeterDonis says

    I have never heard or seen anything like this. So observer B sees Kennedy elected and the Cuban Missle crisis but then everything goes dark and earth vanishes? How can anything that happens in that region of space be unknown to him when he's already experiencing events from that region of space?

    Again, Kennedy's assassination is a future event for observer B. His now doesn't contain a past where Kennedy was assassinated. Why does it matter if observer B gets any information from a future event when observer B is occupying his own now? Why does his now depend on information from a future event that hasn't occurred yet for observer B?

    lightcone.png

    Observer B is causally disconnected from events in observer A's worldline.So events in observer A's future light cone can't be the cause of events in observer B's future light cone.

    A separate worldline would have to branch at Kennedy's assassination for observer B. Information from observer A's future light cone would have to travel faster than light to be causally connected to observer B's future light cone.

    Again, what you said:

    Of course he wont because his future is causally disconnected from observer A's future. This is why I keep asking for some force or mechanism that makes sure observer B has to see everything observer A saw. This mechanism would have to be faster than light and it would have to force everyone to act in the exact same way as they did for observer A.

    I don't see anthing like that in the laws of physics.
     
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