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B Can there be many Worldlines in 4D spacetime?

  1. Jun 2, 2017 #1
    Can many worldlines occur in 4D spacetime where observers in each worldline think they're the "real" worldine? Is there a mechanism preventing many worldlines from occurring in 4D spacetime. Einstein said this in his book Relativity:

    Look at this scenario.

    Event A occurs outside of the light cone of earth. Light from this event reaches earth in it's past light cone.Let's say 1986. This event causes NASA to move the Challenger launch to a later date. In this worldline Challenger was a success and never exploded.

    So in this worldline observers think it's "the real worldline" and Challenger never exploded.

    Is there anything scientifically wrong with this scenario? Is there a mechanism we know in science that would stop other worldlines from occurring in 4D spacetime?
     
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  3. Jun 2, 2017 #2

    Ibix

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    I think you are misunderstanding the term "worldline". Einstein's maths can be interpreted as saying that the universe is four dimensional - three spatial dimensions and one time-like dimension. So something that looks like a point, but exists for sone time is a line in spacetime. This is called a worldline. It has nothing to do with alternate histories which are pure science fiction.

    I also suspect that you don't quite understand light cones, since the Earth doesn't have a past light cone. The Earth at a specified time has a light cone.
     
  4. Jun 2, 2017 #3
    You didn't really answer the question. You just said you don't understand this or that. You said alternative histories is pure science fiction but many theoretical Physicist accept just that (many worlds). From Max Tegmark to Brian Greene. Here's a video of Brian Greene talking about time.



    Worldlines are not just represented as points in space. A 2D line is just used even though this equivalent to a 3D object in spacetime.

    300px-Brane-wlwswv.png

    So again I ask, Is there anything scientifically wrong with this scenario? Is there a mechanism we know in science that would stop other worldlines from occurring in 4D spacetime?
     
  5. Jun 2, 2017 #4

    PeterDonis

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    These two statements are contradictory. Light can't travel faster than light, but it would have to to go from outside the Earth's past light cone at some event to inside it.

    As Ibix explained, the term "worldline" doesn't mean what you appear to think it means.

    What I think you are asking here is whether events in 4D spacetime are fixed, or whether there can be different "versions" of them that somehow exist alongside each other--for example, in one version the Challenger explodes, in another version it doesn't. This would amount to there being different "versions" of 4D spacetime itself, since in any given 4D spacetime, all of the events are fixed--they only happen one way. In (classical) relativity, there can only be one "version" of 4D spacetime--we figure out which one it is by making observations and matching them up with mathematical solutions of the relevant equations.

    If we ever figure out a theory of quantum gravity, it could involve assigning probability amplitudes to different possible 4D spacetimes; but in any given 4D spacetime, all of the events would still be fixed. And we haven't figured out a theory of quantum gravity, so we don't know whether this idea of there being different "versions" of 4D spacetime actually works.
     
  6. Jun 2, 2017 #5
    You said these statements are contradictary but how? As I showed with the Brian Greene video and this isn't something new, an event can occur outside of our present light cone but light from the event is on a plane with our past.

    4D spacetime is fixed but why does that mean events that occur in space and time are fixed and not relative as explained in this video.



    Again, is there a mechanism that prevents different worldlines occurring for the same event in 4D spacetime?
     
  7. Jun 2, 2017 #6

    PeterDonis

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    Is a pop science source, not a science source. And from a source, Brian Greene, whose pop science presentations have probably spawned more misunderstandings in PF threads than anyone else's.

    This "plane" is spacelike. Light can't travel on a spacelike trajectory. Light from such an event will not reach us until some time in our future; at the event you are calling the "present", we have not yet seen it and don't know of its existence.

    Pop science sources are not valid as a basis for discussion here. You need to look at a textbook on SR.

    If you mean, can different worldlines--the paths through spacetime of different objects--intersect at the same event, of course they can. Any pair of curves can intersect at some point.

    But you appear to mean something different by "different worldlines occurring for the same event", and I don't fully understand what. I suspect you have some misunderstandings due to trying to learn actual science from pop science sources. Once again, you should look at a textbook on SR.
     
  8. Jun 2, 2017 #7
    Why is every event in the whole of spacetime limited to being exactly aligned to our present point in space and time? Why would 4D spacetime look at events in space and time as the past, present and future? Why would it make that distinction?

    Why couldn't an event that's not aligned with our present point in spacetime be aligned with a past point in spacetime thereby causing a worldline with a different history to occur and what mechanism stops this from occurring?
     
  9. Jun 2, 2017 #8

    PeterDonis

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    I don't know what this means. Every event is a distinct point in a 4-dimensional manifold, spacetime. They're not "aligned" to anything.

    Who said it did? You are the one introducing a distinction between past, present, and future. Where are you getting it from?

    Again, I don't understand what you mean by "aligned".

    Here's a suggestion: instead of using vague ordinary language, use precise math. Give me the coordinates, in some inertial frame (any one will do, pick whichever one works best for you), of all of the events you are talking about. Describe any relationships between them (such as "aligned") in terms of those coordinates. Then ask your question in terms of those coordinates and relationships defined in terms of the coordinates. If you can't or won't do this, I'm afraid I don't see how your question can be answered, since as it stands it's too vague for me to know what it means.
     
  10. Jun 2, 2017 #9
    You said:

    So now Brian Greene isn't science but you are LOL? Also, it wasn't just Brian Green, there were like 4 or 5 Physicists saying the same thing in the video.

    Also, we look at Planets today but they're planets in the past we don't look at them in the same present. You don't look at yourself in the mirror from the same moment. It's you from the past.

    So again I ask:

    Why couldn't an event that's not aligned with our present point in spacetime be aligned with a past point in spacetime thereby causing a worldline with a different history to occur and what mechanism stops this from occurring?
     
  11. Jun 2, 2017 #10

    Nugatory

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    Closed pending moderation
     
  12. Jun 2, 2017 #11

    PeterDonis

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    I didn't say Brian Greene wasn't science, period. I said the particular source you were referencing was pop science and not a valid basis for discussion here at PF. The PF rules are quite clear about that.

    If you want to see actual science done by Brian Greene, go look up his peer-reviewed papers. There are plenty of them on arxiv.org. If you can find one that says the things you're saying, then by all means link to it.

    Same comment. They all have published papers on arxiv.org. Some of them probably have published textbooks. Go read them and see what they say.

    Sorry, if you're not going to do what I asked then your question is too vague to answer. If you are not capable of doing what I asked, then you need to learn enough SR from a textbook to be capable of it. Then you can come back and pose your question in a precise enough manner to get an answer.

    In the meantime, this thread is closed.
     
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