Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Andromeda paradox

  1. Jun 13, 2008 #1

    "Two people pass each other on the street; and according to one of the two people, an Andromedean space fleet has already set off on its journey, while to the other, the decision as to whether or not the journey will actually take place has not yet been made. How can there still be some uncertainty as to the outcome of that decision? If to either person the decision has already been made, then surely there cannot be any uncertainty. The launching of the space fleet is an inevitability."

    If so, how is this compatible with free will of Andromeda citizens, random events or quantum mechanics? One observer seems to live in universe where the andromedans are predestined to launch the space fleet.

    Either the paradox is flawed, or there is something seriously wrong with the relativity of simultaneity.
  2. jcsd
  3. Jun 13, 2008 #2


    User Avatar
    Science Advisor

    This is more a matter of philosophy than physics, but it's important to distinguish between the philosophy of time known as eternalism, which rejects the notion of an objective present and says that all events are fixed in 4D spacetime, and the idea of determinism, which says that later events could in principle be predicted with total confidence given complete knowledge of earlier events (say, all events on some 3D spacelike slice of 4D spacetime). Eternalism does not logically imply determinism, and thus it is quite possible for an eternalist to believe in "quantum randomness" in the sense that there could be a random element to nature which would make it impossible to predict later events even given the most complete information possible about the state of the universe at some earlier time. As a comparison, one can illustrate the time-evolution of a 1D cellular automata in terms of a 2D diagram in which each horizontal section represents the state of the cellular automata at a particular time, like the ones here. If the automata's rules our deterministic, then we can confidently predict what any lower horizontal section will look like based on knowledge of a higher one; if the the automata's rules contained a random element, then we might not be able to do this. But either way, the 2D diagram is a fixed thing from our perspective, with "later" and "earlier" events just being at different positions on the vertical spatial axis.
  4. Jun 13, 2008 #3
    The only real paradox here is that the basis for the Theory of Relativity is the result of the Michelson-Morley Experiment (the very basis that actually refutes the Theory of Relativity).
  5. Jun 13, 2008 #4
    Oh man, arbol, I hadn't realized you were one of those, I will completely stop paying any attention to your posts from now on :smile:
  6. Jun 13, 2008 #5
    Originally Posted by arbol
    The only real paradox here is that the basis for the Theory of Relativity is the result of the Michelson-Morley Experiment (the very basis that actually refutes the Theory of Relativity).

    That's not a paradox its a contradiction.

  7. Jun 13, 2008 #6


    User Avatar

    Staff: Mentor

    The paradox is not flawed, it's just that like most paradoxes in science, it is just an apparent paradox.
    "Seems to" is exactly the correct way to put it. He only seems to.

    If a person were traveling toward some distant planet at just a smidge under the speed of light, then every event in the future of that planet is in the present moment (or very, very near future) for that spaceship. Well - every moment, of course, up until the spaceship smashes into that planet.
  8. Jun 13, 2008 #7


    User Avatar
    Science Advisor
    Homework Helper

    Hi Ookke! :smile:

    No … according to the first person, the decision has been made, but he doesn't yet know what it is.
    But neither person knows the result of the decision.
  9. Jun 13, 2008 #8
    I do not know what you mean by one of those. But if by one of those you mean one of those who, after determining that by the result of his or her physical experiment, his or her theory needs to be re-evaluated, yes, I am definitely one of those.

    Against my better judgment, I suggest that, if you which the time t, in the stationary system, the ray of light, emitted by the moving system, takes to move along its path of length L, to be

    t = (L + D)/sqrt(c^2 - v^2),

    I will regretfully will say, "Let it be so". I will miss you. :smile:
    Last edited: Jun 13, 2008
  10. Jun 13, 2008 #9


    Staff: Mentor

    The paradox is flawed. Let's say that the Adromedans have a really fast government, so immediately as soon as they decide to get us they turn their lasers on to "extra crispy". The observer who said that they made the decision on 1 Jan 2000 will determine that since Andromeda is 3M light years away we will fry on 1 Jan 3002000. The observer who said that they made the decision on 2 Jan 2000 will determine that since Andromeda is 3M light years - 1 light day away we will fry on 1 Jan 3002000. Also, because of time dilation they will agree on the date that the andromedans made the decision according to the Andromedan calendar. There is no disagreement about any outcome.

    Nothing that happens on Andromeda today can affect anything that happens on Earth today, so the universe really doesn't care about the order. Only the order of events that could affect each other is important.
  11. Jun 13, 2008 #10


    Staff: Mentor

    Please read the http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html" [Broken] in its entirety.
    Last edited by a moderator: May 3, 2017
  12. Jun 13, 2008 #11


    User Avatar
    Science Advisor

    And why do you believe this to be true? The Michelson-Morley experiment certainly didn't find this result.
  13. Jun 13, 2008 #12
    Assuming that information in our frame regarding the Adromedeans is 3M years old with respect to the Adromedean frame (due to spatial separation), the "extra crispy" laser blast might arrive any time now (given the expedience of the Adromedean government).


    Last edited: Jun 13, 2008
  14. Jun 13, 2008 #13
    I think stillwonder named the group the "relativity buster gang", but I wouldn't know anything about that.

    It's nothing personal, I'm just starting to learn relativity and since you're already denying it, I'm assuming your understanding of the theory is inmense (you'd be surprised, but there's actually people that deny the theory before fully understanding it!)

    So if it turns out you're smarter than Einstein I'm sure I'll hear about you again and maybe even eventually start to comprehend your genius, if it turns out you're wrong (which I know is a tiny, tiny possibility, but hey! we have to consider it), then the subtleties of your errors are probably way beyond me, which means reading your stuff would just confuse me.
  15. Jun 14, 2008 #14


    Staff: Mentor

    :approve: Beautiful response! Very classy and entertaining.
  16. Jun 15, 2008 #15

    Hans de Vries

    User Avatar
    Science Advisor
    Gold Member

    This is what relativity says about cause and effect:

    "The decision propagates with the speed of light and reaches the two people at the same time"

    Reference frames are artificial constructions useful for practical reasons. They can be
    derived and constructed entirely from classical physics. Take for instance the following

    Equal objects which appear to have the same size for the eye (or your camera) are
    said to be at the same distance.

    Now in classical physics the apparent size of an object as you see it depends on
    the angles of the light coming from the object. These angles change if the speed changes.
    If you are following and object then its apparent size becomes smaller if the speed gets

    Thus, in classical physics the apparent size of an object depends on the speed. Objects
    in front of you look smaller while objects behind you look larger. By using the above
    definition, which says that objects which appear to have the same size are at the same
    , you create a new reference frame which is just that of special relativity.

    See also this image here:


    Non-simultaneity, as everything in special relativity, can be explained entirely from
    classical physics.

    Regards, Hans
  17. Jul 10, 2008 #16
    So there is a consensus that relativity of simultaneity is just a mathematical construction that does not have much to do with physical reality. Simultaneity calculations are not necessarily wrong, but should not be taken too seriously either.

    I'm still wondering why this aspect isn't emphasized more in teaching relativity. Keeping mind on actually measurable parts of relativity could be benefical.
  18. Jul 10, 2008 #17


    User Avatar
    Science Advisor

    How do you arrive at that conclusion?

    What do you mean by "simultaneity calculations"?

    So you believe that "parts of relativity could be beneficial" but that it "does not have much to do with physical reality"?
  19. Jul 10, 2008 #18
    I don't want a fierce argument about this, so let's just say that relativity has much to do with physical reality, but there are parts of it, that are just mind play.
  20. Jul 10, 2008 #19


    User Avatar

    Staff: Mentor

    I wouldn't go too far with that - I don't like saying "it does not have much to do with physical reality". It is more a matter of realizing/accepting that what is physically real for one observer may not match what is physically real for another - but that the difference is calculable and the observers can agree on it.
  21. Jul 10, 2008 #20


    Staff: Mentor

    I tend to agree with Ookke on this. As far as I know the only physical implication of the standard Einstein simultaneity convention is that the one-way speed of light is c. Most alternative conventions it is only the two-way speed of light that is c.

    I think that is a reasonable convention, but the fact remains that two events which are simultaneous in some frame cannot be causally connected and so the universe doesn't really care which happened first.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook