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Block Time vs Q. Indeterminacy

  1. Nov 19, 2011 #1
    Block Time in relativity where past, present and future exist all the time seems to be in conflict with quantum indeterminism where the latter states the future is uncertain. So why do physicists still competely believe in Block Time? What is the consensus about this in the physics community at the present time?
     
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  3. Nov 19, 2011 #2

    pervect

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    I don't believe that special relativity necessarily implies block time. Furthermore, I don't think that quantum mechanics implies non-determinism.

    For instance, you can find some discussion of quantum determinism on wiki,

    http://en.wikipedia.org/w/index.php...60792599#Other_matters_of_quantum_determinism

    I wouldn't go so far as to say that the wave function is reality (as the wiki article on quantum determinism discusses), I take the position that it could be and it wouldn't matter. In short, I don't think determinism is a testable theory, it's a non-testable philosohpical issue.

    It's really hard to say if this is a "consensus" position or not, but it's mine, and I don'tthink it's terribly uncommon.
     
  4. Nov 19, 2011 #3

    ghwellsjr

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    Isn't there an oxymoron going on here?
     
  5. Nov 19, 2011 #4
    But the lorentz frames and the way they are boosted and lorentz transformation automatically create block time. Block time is the heart and meat of special relativity. How can you create SR without implying block time?
     
  6. Nov 19, 2011 #5

    ghwellsjr

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    That's a new one on me. That's like saying that since my cell phone has a calendar in it the goes up to the year 2069, then the next 58 years already exist. Maybe you could explain how you arrived at this conclusion because I don't see anything that would suggest block time. Also, maybe you should expand on what you mean by block time, just to make sure we're all on the same page.
     
  7. Nov 19, 2011 #6
    Block time means the past, present and future are just worldlines which don't flow but already exist. This is how physicists can contemplate how making spacetime loop (like rotating black holes or universe) can entail time travel because you can visit the past.

    This is in contrast to old views of space and time where the present exist, the past has happened, and the future is still to come.
     
  8. Nov 19, 2011 #7
    In addition to the above. Look at the illustration of Block Time by Paul Davies here:

    http://www.nikhef.nl/pub/services/biblio/bib_KR/sciam14327034.pdf
     
  9. Nov 20, 2011 #8
    I find block time a useful concept but I don't believe the world is actually like that.
     
  10. Nov 20, 2011 #9

    ghwellsjr

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  11. Nov 20, 2011 #10
    Well said, stglyde. I think many physicists recoil at the thought of a block universe because some of the implications are quite unwelcome to our thinking. Subjectively, I don't like the implications, but at the same time I have never found a way to objectively discount what you have just stated.

    On determinism, one should consider that the 4-dimensional filaments making up the fabric of the 4-dimensional universe do not necesarily have to obey laws of physics at the sub-microscale. In the block universe, filaments are woven into the fabric of the universe (to borrow Brian Greene's language) in a way that manifest physical laws to us observers. The filaments are not positioned along the world lines in response to forces, etc., rather the illusion of forces, masses, etc., are a result of the filament configurations. For whatever reason these filaments are layed out along the 4th dimension in a very special and precise way.

    Then there is conscousness and time, about which physics has little to say.

    And by the way, ghwellsjr was quite incorrect in implying there was an oxymoron in your earlier statement. A more natural characterization of time than what he probably had in mind is correctly implied in your comments.
     
    Last edited: Nov 20, 2011
  12. Nov 20, 2011 #11
    I agree with you.

    Doesn't block time come up because we can convert time into the same unit of measure as the other 3Ds? And graphically it's typically represented as time in meters to distance in meters. So graphically how long is a meter?; Very little time.

    But in 4D, spacial xyz coordinates already account for position, I don't think time can still be considered a forth distance measurement with 4D space time.

    Is the block universe concept considered 2D + 1 time dimension? Otherwise in what sense is a "right now" slice of space 3D?
     
    Last edited: Nov 20, 2011
  13. Nov 20, 2011 #12
    Yea that was funny.
     
  14. Nov 20, 2011 #13

    PeterDonis

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    I see Davies uses the term "block time", but I'm not sure it means what he seems to think it means. :smile:

    Consider this quote:

    To me, this does not suggest that my "future" already exists; all it suggests is that the question "I wonder what Commander Jones is doing at Alpha Base now" is not well-defined, even though our intuitions suggest that it should be. I can change which event on Commander Jones' worldline corresponds to my "now" by changing my state of motion, but no matter how I change it, I can't bring what Commander Jones is doing "now" into my *causal* future, which is what would be required to say that my future must already exist. No matter how I change my state of motion, whatever Commander Jones is doing "now" will be outside my future light cone. So what he is doing "now" can have no causal effect on what *I* am doing "now", and vice versa. So the question "what is Commander Jones doing now?" is physically meaningless; I can label any event on Jones' worldline that is spacelike separated from me as "what Jones is doing now" and it will make no difference at all to the results of any observation I make or any experiment I run.

    However, Davies does bring up a different issue that strikes deeper:

    Julian Barbour has written a number of papers expanding on this kind of thinking:

    http://platonia.com/papers.html

    However, even this view does not imply that my future "already exists" in the sense of being fixed. All it implies is that time is not something that "flows"; it's just a dimension along which we can assign coordinates to events. It does not require that events with coordinates that we, at this particular point on Earth's worldline, would label as "future" must be "determined" from our viewpoint "now". There can still be quantum indeterminacy; it just shows up as a statistical variation in relationships between types of events, looked at over the entire 4-dimensional continuum.

    For example, there might be lots of events, at various different coordinate "locations", that can be described as "spin-up electron going into spin-measuring device oriented left-right", and lots of events at coordinate "locations" very close to the first set that can be described as "electron coming out of the spin-left side of the spin-measuring device" or "electron coming out of the spin-right side of the spin-measuring device". Quantum indeterminacy just means that the two types of "electron coming out" events are randomly associated with the single type of "electron going in" event, with 50-50 odds, if we look at the entire collection of such event pairs over the entire spacetime.
     
    Last edited: Nov 20, 2011
  15. Nov 20, 2011 #14
    The block universe requires the 4th dimension actually be a spatial dimension. You could not have various 3-D cross-sections required by special relativity without that. The 4th dimension is identified with time because time seems to flow as the observer's consciousness appears to move along his world line (along his rest 4th dimension) at the speed of light.

    Let's say everyone is required to move along the interstate from point A to point B at exactly 60 mph. Then, at every mile from point A there is a time marker on the side of the road, i.e., a clock, that reads out the time lapsed from point A. And instead of looking at an odometer, you look at your watch to keep track of time lapsed from point A (no distances are displayed). Now, you've turned the highway (your 3D world line, so to speak) into a time dimension in exactly the same sense that the 4th dimension is regarded as a time dimension.

    So, the 4th dimension is a time dimension only in that sense. But, more fundamentally, the 4th dimension is a spatial dimension.

    All of the 4-dimensional filaments strung along the 4th dimension that make up objects are just all there. On a macroscale the pattern posses unique forms that allow a description in terms of laws of physics. And from these laws you can predict the future 3-D cross-section organization of the objects. However, you cannot rely on these overall patterns to predict the future 3-D cross-section organization of objects down to the level of individual quark filaments, electron filaments, and photon filaments.

    So, the 4-dimensional filaments are all there. You could say that the future is determined, but the rules we physicists have only work for predicting the future on a macro scale. We do not have rules capable of predicting the future of individual elementary particles. We can only predict statistically what to expect of a sizeable group of 4-D elementary particle filaments. And that's not because the filaments have not extended far enough along the 4th dimension; it's just because the filaments down at the sub-micro scale are not layed out with a fixed pattern reflecting the rules recognized at the macro level.

    We have the wave functions, but a wave function is not, in this view, an actual object--it is a mathematical description which, when complex conjugate squared, allows you to make statistical predictions about the configuration of some 3-D cross-section of 4-D filaments at some point in the future.

    One of the mysteries of the sub-micro patterns is that, even though we don't recognize a specific pattern conforming to specific laws of physicis, the wave functions have a very detailed and specific phase in it's description of the individual 4-D filament shape in 4-dimensional space.

    Thus, we have two major mysteries of physics: 1) The many different cross-section views of 4-dimensional space (attended by the constant speed of light) and 2) The double-slit experiment of QM.
     
    Last edited: Nov 20, 2011
  16. Nov 20, 2011 #15

    PeterDonis

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    The fact that the time dimension has the opposite sign in the metric to the three space dimensions indicates that it is not quite the same as a spatial dimension. It is *like* a spatial dimension in some respects (for example, it can be measured in the same units as spatial dimensions by using the speed of light as a conversion factor), but it is not a spatial dimension. It is a dimension of the overall 4-D manifold of spacetime, topologically speaking, which is sufficient to allow 3-D cross-sections to be cut as you describe; cutting those cross sections does not require that the time dimension be exactly like the space dimensions.
     
  17. Nov 20, 2011 #16
    Hi, Peter. I think you've made very good points with that analysis. The other way to look at it may be seen with the sketches below. First, the left sketch symbolizes red and blue rockets moving away from each other with the same speeds relative to the black "rest" system. We have the usual time dilation. Red, when at his world line station 9, "sees" blue in red's simultaneous space at blue's world line station 8. But, blue, when at his station 9, "sees" red in blue's simultaneous space at red's world line station 8.

    The 4-D metric, from which the Lorentz transformation time dilation equation follows is derived directly from 4 spatial dimensions. The selection of coordinates in the 4-space is simply selected based on orientations of 4-D world lines in the space. So, we have a right triangle with the blue X4 axis as the hypotenuse. The negative sign in the metric does not arise as a result of some mysterious role of time. It is just related to the orientation of world lines. It just results from the red X4 and blue X1 axes in the position of legs of the triangle. So, when we solve for the red X4 we're just solving for a leg of a right triangle instead of a hypotenuse. Some physicists refer to the red leg of the triangle as "Einstein's Hypotenuse."
    SR_Coordinates_PythagoreanTheorem.jpg
     
    Last edited: Nov 20, 2011
  18. Nov 20, 2011 #17

    PeterDonis

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    No, it isn't, because the sign of the "time" term is opposite from the sign of the "space" term. In 4 spatial dimensions, the sign of all the terms would be the same and the metric would be positive definite. In 4-D spacetime, the metric is not positive definite. That makes a big difference.

    Yes, the labeling of events by coordinates respects a standard R4 topology; coordinate 4-tuples that are almost the same represent points that are "close together" topologically, meaning they lie in small open neighborhoods of each other.

    And look at how the axes are tilted relative to the black axes. If both dimensions were "spatial", so the metric was Euclidean, the red and blue sets of axes would each be perpendicular to each other, as the black axes are. But they're not. Minkowski spacetime is not Euclidean space. It's a different geometric object.
     
  19. Nov 20, 2011 #18

    PeterDonis

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    Also, I can play the same trick with Euclidean space, to make it look like the "metric" has a minus sign for one dimension. But that requires me to mix coordinates from two different frames on one side of the equation, which means it isn't a metric any more; it's just an equation that's been rearranged. A valid metric expression involves coordinates that are all from the same frame. For example, write the equation for the blue X4 in terms of the black X4 and X1; it will have a minus sign. *That* is the metric. I can rearrange that equation to move the negative term to the other side, but all that shows is that I can do algebra; it doesn't change the intrinsic geometry of the manifold.
     
  20. Nov 21, 2011 #19
    That's not the way the block universe works. We begin with a Euclidean R4 manifold and freely select an initial set of coordinates using blue X1 and red X4 (blue and red X2 and X3 are the same). Do not regard the blue and red as belonging to two different coordinate systems at this stage.

    Using these coordinates we have a distance along the blue X4 direction using Pythagorean theorem. Having these relationships we can now freely select a new set of coordinates, i.e., the blue coordinates X1, X2, X3 and X4. The Minkowski metric follows.

    We have had no need to make any reference to time. It is all spatial. Now, if you wish to introduce a concept of time as an observer's consciousness moving along his X4 world line at the speed of light, you are free to postulate that and then investigate the implications and the consistency with special relativity theory.

    Again, I don't like the concept at a subjective level. I'm just trying to do the best job I can at explaining the 4-D spatial universe concept. There are probably others on the forum who can make it more clear (and perhaps present it more accurately) than I. The link to Paul Davies's article given in the earlier post is good. I visited Davies's place at Arizona State University and hoped to meet him, but he was out of town on that day.
     
  21. Nov 21, 2011 #20
    Surely you meant "our eternally static worldtubes had such and such a 4D spatial separation at that region of eternally existent Platonia"?:rofl:
     
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