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

Time frames and general relativity

  1. Mar 14, 2015 #1
    I stumbled upon this video:

    which seems to claim that past present and future exist at the same time due to the fact that observes in different frames would not agree on the order of events. The video claims that there are frames in which Mozart is alive.
    I don't think that this is correct, since the speed of light prevents us from communicating with these frames, they may as well be considered non-existent. But do they really exist ?
  2. jcsd
  3. Mar 14, 2015 #2


    User Avatar
    Science Advisor

    "Now" is not a universally agreed term anywhere except "right here" in relativity. This means that the video is correct - there are places where "now" (any way you define it) one can choose a "now" where Mozart is still alive.

    Since Mozart died 223 years ago, anyone moving in the right direction at a high enough velocity (relative to Earth) at a point at least 223 light years from Earth can claim that Mozart isn't dead "now" (although they probably won't have heard of him - so perhaps better to say that they could claim that 5th December 1791 by our primitive Earth calendar hasn't passed yet). The speed of light prohibits you from communicating with them "now". By the time you (or anyone since 1791) manage to communicate with them, they will agree that the date of his death has passed. Necessarily so, or relativity would be full of paradoxes and we could make a killing on the stock market.
  4. Mar 14, 2015 #3
    The video is saying that past present and future exist simultaneously and that is what I'm having problems with.
  5. Mar 14, 2015 #4
    What the video is trying to explain is that there are frames in which the event of, say, you waking up in the morning is simultaneous with some point in Mozart's life.
    You, on the other hand, would argue that this event, of you waking up in the morning is a few hundred years in the future of Mozart's life. This does not mean that past, present and future exist all together in any meaningful way, it simply means diferent observers have different notions as to which events are simultaneous or not.
  6. Mar 14, 2015 #5


    User Avatar
    Science Advisor

    Greene is describing something called the "block universe" model. Despite the certainty with which the various commentators are (or have been edited to appear to be) speaking, it's not completely clear that this is reality. It's certainly a very helpful model for visualising what goes on in Special Relativity, though.

    In the block universe, time doesn't flow. The universe is rather like one of those simple flip-book animations, where there's a picture on each page and you flip the pages to make the picture appear to move. There's no-one flipping the pages, however. You get the impression that time passes because you "now" do not remember events in the future, but do remember events in the past, and events in the past include you a second ago when you hadn't quite finished reading this sentence, and you a few more seconds ago when you hadn't even started reading this reply. Time is not an illusion, but the "flowing" feeling of time is (in this model at least). The pages of my flip-book are the slices that Greene was talking about - and not everyone agrees which direction the pages go...

    I'll just repeat what I said at the beginning: this is a very good visualisation tool for special relativity. It's not completely clear that this is what is really going on, however. Research into what lies beneath relativity is ongoing, and is likely to be ongoing for many years yet.

    The important thing to remember is that "past", "present" and "future" do not always have universal definitions is relativity. There is some flexibility. The example in the video of the alien jumping onto his bike is a good one. As he's sitting there, his "now" includes an Earth guy at a gas station. When he's pedaling away from Earth, his "now" includes Beethoven. But the thing to remember is that for such a tiny speed to make such a huge difference he must be millions of light years away. If he looks through a telescope at Earth he'll see dinosaurs. He'll see the same dinosaurs when he's on his bike. The point is that he cannot see Earth "now". It's impossible. Nothing travels faster than light. So whether "now" includes Beethoven or Bananarama or Bruno Mars doesn't matter one bit. All he sees, and all he can see, is the same dinosaurs going about their business.

    It does turn out that the alien cannot define "now" to include the dinosaurs he's looking at, or any earlier (or at least, not coherently). The reasons for that require a bit more explaining than I'm willing to do at twenty past midnight. You could look up "light cones" if you want to know more, and feel free to ask follow up questions.
    Last edited: Mar 14, 2015
  7. Mar 14, 2015 #6


    User Avatar
    Science Advisor

    ...although you would have to be hundreds of light years away from Earth for there to be such a frame. You would need to be outside the future light cone of the event of Mozart's death, speaking formally. If you have heard of the death of Mozart then his death is in your past and there are no frames that will make you waking up in the morning simultaneous with Mozart's death.

    And now I really need to sleep. Buenas noches.
  8. Mar 14, 2015 #7
    Yes, I wasnt considering any relative distances, or space-time separations between the events, just assuming the necesary conditions for a correct causal description were met. I was trying to illustrate the point of the non-absolute character of simultaneity in relativity. As you correctly point out, if you are causally connected to any of the events in Mozart's life, then that event is in your absolute past as there's no way to make any pair of causally connected events simultaneous.
    Last edited: Mar 14, 2015
  9. Mar 14, 2015 #8


    User Avatar
    2016 Award

    Staff: Mentor

    What you say is basically correct. There are just two corrections I would make:

    (1) The speed of light prevents us from communicating with these events, not "frames". The key point is that spacelike separated events are outside each other's past and future light cones, so neither one can send light signals to or receive light signals from the other. But this is true regardless of what frame you choose.

    (2) Rather than saying that events spacelike separated from us here and now are "non-existent", I would say they are "not fixed" or "causally disconnected" or something like that. See further comments below.

    The term "really exist" is not a physics term, it's a philosophy term, and it's a shame that physicists like Brian Greene insist on using it as though it were a physics term. (This is by no means the first, or even the fiftieth, thread here on PF triggered by one of Greene's videos or PBS specials.)

    The root of the problem here is that our pre-relativistic intuitions want to classify events into "past", "present", and "future". But in relativity, there are really four classifications: "past" (events in our past light cone), "present" (here and now--but the "here" part is crucial), "future" (events in our future light cone), and "elsewhere" (events spacelike separated from us here and now--there isn't really a standard term for this, but Roger Penrose calls it "elsewhere" in some of his books and I think it's as good a term as any). Our intuitions are not used to dealing with "elsewhere" events, so it's tempting to try to classify them in one or more of the other three categories; but that's a mistake, and Greene's video illustrates why it's a mistake.

    Also, once we realize that "elsewhere" is there, our intuitive categorization of what events "exist" breaks down as well. Intuitively, we think the past "existed, but no longer exists", the present "exists", and the future "doesn't exist yet, but will exist". But if we try to say that "elsewhere" exists, we run into problems, as you correctly point out. However, if we try to limit the "present" to just "here and now", and say that that's all that "exists", we put ourselves in the position of saying that, for example, the Sun "doesn't exist", because the Sun "right now" is spacelike separated from us here and now; the best we can do is to say that the Sun "existed" when it emitted the light we are seeing here and now. That doesn't seem very workable either. The best way out of all this is to admit, as I said above, that the question of what "really exists" is not a physics question. Physics can tell us what events are or are not causally connected to what other events, and that's enough.
  10. Mar 15, 2015 #9


    User Avatar
    Science Advisor
    Gold Member

    Some authors call it "elsewhen"!
  11. Mar 16, 2015 #10
    From the end of the video...

    "With this bold insight, Einstein shattered one of the most basic concepts of how we experience time - the distinction of past, present, and future, he once said, is only an illusion, however persistent."

    This Einstein quote didn't spring forth from a physics lesson, he composed it in a thoughtful personal letter to console the grieving widow of a colleague.

    "Now he has departed from this strange world a little ahead of me. That means nothing. People like us, who believe in physics, know that the distinction between past, present, and future is only a stubbornly persistent illusion."

    What may have been shattered were prior concepts of how physicists define, measure, and calculate time, but the basic concepts comprising how we experience time continue unchanged ("persistent illusion"), and remain part of the fundamental philosophical mysteries of consciousness outside the scope of modern physics.
  12. Mar 18, 2015 #11
    It does sound as if that video pushes Minkowski's block universe interpretation and sells it under the Einstein label as established physical fact... Why that is not correct is briefly explained in the following post on forum policy:
    https://www.physicsforums.com/threads/what-is-the-pfs-policy-on-lorentz-ether-theory-and-block-universe.772224/#post-4859428 [Broken]

    By searching this forum for "block universe" you can find back old discussions which clarify the general consensus here. :smile:
    Last edited by a moderator: May 7, 2017
  13. Mar 18, 2015 #12


    User Avatar
    Science Advisor
    Gold Member

    A rare example of a paper discussing these issues that is published in a peer reviewed physics journal is the following, which proposes a framework for modifying the BU interpretation to fit plausibly with quantum mechanics. My favorite gedanken experiment in it is a heavy mass with thrusters on opposite sides, turned on and off by a controller which is responding to signals from the decay of a radioisotope, as a demonstration of the implausibility of treating the the whole spacetime manifold as existing in the context of quantum mechanics (the evolution of curvature is, in principle, undetermined until the detector has responded to a given decay).


    and an associated philosophy essay:

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook