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Is there an Instant in Time That's Everywhere The Present

  1. Aug 9, 2013 #1
    Is there an Instant in Time That's Everywhere "The Present"

    We sit in a room. I see you and you see me. We both understand we are seeing each other as we "were" a small time interval in the past. The speed of light is finite and the distance traveled by the light from me to you, and you to me, is nonzero, and it takes time for the light to travel. But does not this demand that the matter that makes you and I up was both physically present at this, say, "common instant of light reflecting from you and me"? Snap your fingers perfectly, an instantaneous snap. Is there so meting physically "going on" on the other side of the Earth then? I say yes, even if I am not observing it. On the moon? Yes. On the Sun? Yes. In a Galaxy billions of light years away? Yes. To me, everywhere at once, there is a physical instant in time that is "The Present."
     
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  3. Aug 10, 2013 #2

    WannabeNewton

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    There is no absolute notion of "the present"; special relativity got rid of that concept.
     
  4. Aug 10, 2013 #3
    There is no problem with a global definition of simultaneity with respect to an inertial observer:

    http://en.wikipedia.org/wiki/Einstein_synchronisation

    However, there is no definition of simultaneity which allows all inertial observers to agree that two arbitrary spacetime events are simultaneous. If you say that two events at (spatially separated) points A and B are simultaneous, there exists an observer with the same synchronization convention who will disagree.

    (Note: the above is true in the absence of gravity, I'm not enough of an expert in GR to know if there are pathological exceptions.)
     
  5. Aug 10, 2013 #4
    If so, there would seem to me a chance that the physical existence of matter throughout the universe could somehow get "out of synch" from location to location. I can't see how this is possible. It seems logical to me, because I believe in physical existence beyond any relativistic observation, all matter must coexist at once in an absolute physical space. This "at once" we call "the present."

    With regards to relativistic observations, consider the following. An n-body system of observers (bodies) is considered. One of the observers observes a nonzero relative velocity for one of the other observers (bodies). To me then, there must be some type of absolute physical motion. You can not have an observed nonzero relative motion without an absolute physical motion, which must have occurred, regardless of observations of it were taken or not. The reverse in not true. If each body in the system is never seeing any relative motion on the part of the other bodies, then the whole system maybe moving and none of n bodies "sees" this. But if any nonzero relative motion is observed, there must have been some type of physical change of motion/location involved. This is Einstein's "external world."
     
  6. Aug 10, 2013 #5

    WannabeNewton

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    There is no external world. There is no absolute velocity. There simply is no absolute notion of simultaneity. There are tons of threads on this and resources throughout the internet which can help you come to terms with that.
     
  7. Aug 10, 2013 #6
    Here is a quote by Einstein:

    "The belief in an external world, independent of the perceiving subject, is the basis of all natural science."

    He said this in refutation of quantum mechanics. If you examine Einstein's Equivalence Principle, it shows what Einstein was talking about. A man is floating in an elevator. He sees one of the walls of the elevator approach him. He has a radar gun, and the radar gun says one of the walls is approaching, due its observed Doppler shift. The man is a physicist, and knows that, well, he would get the same observation if the wall were not approaching him, but somehow he and his radar gun were approaching the wall. How could that be? Perhaps a big mass is actually outside the elevator, and pulling the man to the wall. The internal observations cannot tell. This is what Einstein meant by "independent of the perceiving subject." The perceiving subject is the man, with his radar gun for observing. He is observationally isolated from Einstein's "external world." But note, and this an important note, some type of physical motion occurred. That's the only way the radar records a Doppler shift. Some type of physical "external world" motion occurred. Perhaps there were rocket engines attached to the outer walls of the elevator which fired, and it was not gravity, but external inertial accelerations that produced the interior observations. But something physically, absolutely moved in Einstein's "external world."
     
  8. Aug 10, 2013 #7

    WannabeNewton

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    Acceleration is on a completely different playing field from velocity. There is no such thing as absolute velocity; if you propose there is then how would you detect it using local experiments? Acceleration is of course absolute and can easily be detected using accelerometers carried by observers.

    As for your Doppler shift statement, if a light source is at rest with respect to an inertial observer then trivially that observer detects no Doppler shift. However if the light source is moving with respect to some other inertial observer then this observer can certainly detect a Doppler shift. It is not absolute.
     
  9. Aug 10, 2013 #8

    ghwellsjr

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    This was the common belief prior to Einstein. And it is an appealing concept. The only problem is that there is no way to identify this absolute physical state (or do you know something that the rest of us don't know?) Even if you believe it to be true, you will be hard pressed to build a scientific theory based on it, and since Einstein's theory of Special Relativity is entirely compatible with the notion that there really does exist an absolute physical state, you'd be well-advised to learn it and put your mind at ease.
     
  10. Aug 10, 2013 #9

    WannabeNewton

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  11. Aug 10, 2013 #10

    Nugatory

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    By "then"' you mean "at that exact same time", do you not?

    If so, here's a question for you: What does it mean to say that two events occurred "at the same time"? Imagine two people on opposite sides of the earth snap their fingers. How could you convince me that they snapped their fingers at the same time?

    That sounds easy: I sit down somewhere 10 light-seconds away from both finger snappers. If the light from both events reaches me at the same time (note that this is one event, namely "my eye was just hit by two flashes of light) we'll say that they both happened at the same time, and allowing for light travel time, that time was ten seconds ago.

    But it turns out that other observers using the exact same procedure but moving relative to me will find those two finger snaps not to be simultaneous, and will find events to be simultaneous that I didn't. That is, there is no universal notion of "at the same time".

    Google for "relativity of simultaneity" and pay particular attention to Einstein's train example that comes up when you do.
     
  12. Aug 10, 2013 #11

    Yes.


    I assume you mean the two people on opposite sides of the earth physically snapped their fingers at the exact same time in your mind's eye, as I immediately thought also from your words? If that actually happened as you and I envision, then me trying to convince you of that physical fact seems irrelevant to me, in terms of whether or not it did or did not happen simultaneously. I think both you and I are here envisioning the physical fact is that the snaps were simultaneous in our mind's eyes. Then this is the assumed external world physical truth to me. The snaps were absolutely simultaneous. They can be, can't they? After all, we are discussing physical events which are assumed to have occurred in a physical world, even if not observed.

    Let's say two flashes of light occur at different times, according to at least one observer. Other observers see only a single flash. Were the flashes actually, physically simultaneous? One thing can be sure, at least one physical flash occurred. I next believe if a physical flash must have occurred, in the actual physical space where this happened, either it happened alone or another flash occurred in the same physical space, perhaps at a different absolute location, possibly even at the exact same physical absolute instant in time, or perhaps not. Any and all observations of events cannot tell internally if there were actual, physical singular or multiple flashes You can be guaranteed of one physical fact, though: at least one physical flash occurred. There is necessarily needed to me, a belief in Einstein's external, independent world, to even accept at least one physical flash physically occurred, "independent of the perceiving subject."

    This is a common character I see with all thought experiments on these topics. There is no way to set one up without resolving the paradoxes, which are in fact resolved by the assumed physical setup. For example, the Twin Paradox. The setup words say one of the twins actually moved. The setup words resolve the paradox: It is the twin that moved which time dilated.


    Your words tell me to take my point of view outside the scene and "look down," as I am sure you also did (by "seeing" yourself sit there in your mind's eye). To me, you and I are then "envisioning" in the "external world." In this "external world", I'll assume you mean the finger snaps occurred simultaneously. If that is the physical truth, then that's the physical truth in Einstein's "external world." If every observer actually knew the absolute physical truth, which is in my opinion, necessarily needed to believe in, to accept relative concepts, each could actually compute if they were in fact "faked out" about the timing of a truly, physically simultaneous event.
     
  13. Aug 11, 2013 #12

    ghwellsjr

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    sb635, the question is: are you interested in learning Special Relativity?
     
  14. Aug 11, 2013 #13

    Nugatory

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    That's an interesting question, but I don't know how anyone can answer it without knowing what you mean by "simultaneous". Can you provide a definition of "simultaneous" and "at the same time" that will work for two observers who can see both flashes, or both finger-snappers? This is harder than it sounds; if you haven't already tried googling for "relativity of simultaneity", you should.

    (We had been talking about two people in different places snapping their fingers. Now you're talking about flashes of light; I assume that you mean that the flashes are emitted from different places, both because that's your original question and because there's no problem deciding whether events at the same place happened at the same time).
     
  15. Aug 11, 2013 #14

    PeterDonis

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    Nugatory didn't say both snaps were simultaneous. He just said to imagine that there are two snaps. He didn't say anything about their time relationship. That's the point: if all you know is that two snaps occurred, how do you tell whether or not they were "absolutely simultaneous"? What evidence would you look for?

    No. Simultaneity is relative.

    Yes, but that's a separate question from the question under discussion. Nobody is disputing that there is a physical world that exists independently of our observations. All we are saying is that that physical world that exists independently of our observations does not include "absolute simultaneity" as one of its features.

    What keeps the other observers from seeing the second flash?

    Why only one? You said at least one observer sees two flashes. Doesn't that mean at least two flashes must have occurred, even if other observers don't observe the second one? Didn't you say the physical world exists independently of our observations?

    You are assuming a lot here, though you probably don't realize it. You are assuming that there is an "actual physical space" (as opposed to an actual physical *spacetime*), in which there are "absolute locations", and that there are "absolute instants" of time. All of these assumptions are false. I know they seem obvious to you, but they're still false. If you want to understand how relativity works, you have to drop these assumptions.

    The twin paradox can be stated without saying anything about which twin "moved". I'll agree that plenty of statements of the paradox aren't made as carefully as they should be, since they invite you to make assumptions that don't hold (such as the assumption that there is an absolute fact of the matter about which twin "moved"). Although in some cases, the paradoxes are stated that way on purpose, to see whether you are capable of questioning those assumptions.

    Read what he said again, carefully. He made no such statement, and you are making an unwarranted assumption.
     
  16. Aug 11, 2013 #15
    Are you saying in this external physical world independent of observations, that truly physically simultaneous events somehow can not occur? If you believe in the existence of a physical world independent of our observations, where to me that means events actually do physically happen regardless of being or not being observed, why can't there be two exactly simultaneous events in this physical world, and who cares about if they were observed or not, in terms of whether or not they physically occurred simultaneously?


    Because of their type of absolute motion in the absolute world in which at least one flash occurred. Because of their motion they see the second flash apparently happen, if it did actually happen, at the same point of/in their dilated time.


    Because if at least one flash was seen, there was, absolutely, at least one physical flash in the external, physical world. This fact, that there was at least one physical flash, is directly dependent on at least one relative observation seeing at least one flash. But the reverse is not true. If no observer sees a flash, this does not guarantee that no physical flashes ever occurred. But just one relatively observed flash occurring demands at least one physical flash. And if that's so, then where did it occur? It had to be at some absolute physical location in space at some just as absolute instant in time, if it was an actual physical event independent of observation.


    There may have been more than one flash physically occur, and relative observations can't tell, and will never be able to resolve what is the "absolute truth of the external world." But even if just one flash was observed, there is one absolute thing which can be said: At least one flash physically happened. And the only type of "world" in which events do physically happen is an absolutely physical world, independent of observations, as Einstein stated. And then I think this physical world must be the same physical world for you, for me, for all physical matter existing at the same absolute instant in time that we call "the present."

    To me, the entire concept of motion requires one to accept physical matter absolutely moves from some absolute location in space to some other absolute location in space over some absolute time interval. Motion to me, has to be in fundamental essence, truly absolutely physical. How an observer "sees" that motion progress through time is entirely a function of the observer's physical motion relative to the absolute space time with which this physical motion took place. Motion is physical. How we see it is relative.


    This sounds in contradiction to your previous statement that you believe in an external physical world independent of observations. One can take such a point of view and still completely understand all of relativity. Like I said, the Equivalence Principle demands one accept the existence of a physical, external would "out side of the elevator," as Einstein clearly thought, per the quote I gave. If it didn't, there would not be a way to define what is meant by "inside of the elevator."
     
  17. Aug 11, 2013 #16
    I am conflicted about the nature of a thought experiment. I propose one in which I (its creator) say, A bomb goes off in Earth's atmosphere *at the same time* that a solar flare erupts from the Sun. Doesn't that establish the simultaneity of the events, i.e., that we can all imagine those events happening at the same time, regardless of what different frames of reference might see, i.e., seeing them happen at different times? And certainly they could happen at the same time in this physical solar system, regardless of who might see them happen at different times because of different frames of reference.
    An observer midway between Earth and Sun would see them as the thought experiment dictated... happening simultaneously. No doubt an observer closer to earth would see the bomb explode first and the flare later. But how could that alter the physical simultaneity of the events, happening at the same time as specified, regardless of when different points of view might see them?
     
  18. Aug 11, 2013 #17

    Dale

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    In which frame? The words "at the same time" don't mean anything unless you specify the reference frame.

    Relativity isn't about optical effects. It is what remains after accounting for optical effects. An observer closer to the earth would receive the light from the events, correct for the distance, and determine that they happened at the same time even though he received the light at different times (assuming that they were simultaneous in his frame).
     
  19. Aug 11, 2013 #18

    PeterDonis

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    Yes, because the concept of "truly physically simultaneous events" has no physical meaning.

    Agreed, no argument here.

    Because the actual physical world doesn't work that way. I know your intuition says it ought to, but it doesn't. That's just the fact.

    To be clear: there can be pairs of events in the actual physical world that are simultaneous to a particular observer; but there can't be pairs of events in the actual physical world that are "truly" simultaneous, in any absolute sense. Simultaneity is relative.

    There is no such thing as "absolute motion" in the "absolute world" (by which I assume you mean the actual physical world that exists independent of our observations). The actual physical world just doesn't work that way, even though your intuition says it ought to. There is only relative motion--motion of one object relative to another.

    Yes.

    No. There is no such thing as an absolute instant of time, whether it's the present or any other. The actual physical world doesn't work that way, even though your intuition says it ought to.

    Yes, I understand that this is how it seems to you. But how it seems to you is not how it is; that's what relativity tells us. If you want to understand relativity, you will need to change your concept of motion.

    It's true that there is something absolute about an object in spacetime: it has a worldline, a curve in spacetime that it follows, and that worldline is independent of how any particular observer sees it. But there is no absolute sense in which the object is "moving" or "not moving"; it just has a worldline.

    That's because your concept of "an actual physical world independent of observations" attributes properties to that actual physical world that it doesn't actually have. Saying that the actual physical world exists independently of our observations is not the same as saying there is absolute space, absolute time, or absolute motion. If you think it is, then your concept of the actual physical world is wrong, and you will need to change it if you want to understand relativity.
     
  20. Aug 11, 2013 #19
    Dalespam told me there are no solipsists on this forum?

    If that is the case, and spacelike events, simultaneous with my 'now' event, exist out there, and simultaneity is relative, then 4D spacetime is the physical observer independent 'world' out there.
     
  21. Aug 11, 2013 #20

    Nugatory

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    We've suggested several time already that you google for "Relativity of simultaneity" and Einstein's thought experiment that goes with it. Have you done this?

    I don't think so, or you wouldn't still be talking about "truly physically simultaneous events"... You'd either be saying "OK, now I understand" or you'd be asking follow-up questions about how the relativity of simultaneity argument works.

    This is not strange weird controversial out-on-the-edge stuff that we're talking about. It was discovered more than a century ago, it has been a basic part of a first-year physics curriculum for many decades, and there is no shortage of experimental evidence to confirm special relativity. Not studying it, yet asking questions about "an instant in time", makes about as much sense as trying to design a jetliner but refusing to look at any technology developed since the 19th century.

    So, one more time, as I said back in post #10 of this thread... Google for "relativity of simultaneity" and pay particular attention to Einstein's train example that comes up when you do.
     
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