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How unreal are time and space

  1. Sep 21, 2012 #1

    mdl

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    from wikipedia:
    I thought information about space (i.e. matter) exists in the universe regardless of humans. it's maybe in holographic, 1D or other form, but it is stored outside humans.
    while information about time is created by humans (by comparing memorised and new information) and exists only "virtually" in our minds.

    why are space and time taken as they were both imaginary (the same way)?
    if all information about space exists outside humans, how can it be imaginary?

    I'm also little confused about whether space-time is a part of the universe or it is only a tool used to describe how space in the universe changes.

    thanks
     
  2. jcsd
  3. Sep 21, 2012 #2

    Simon Bridge

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    Can a memory of events not exist without humans?

    Can we not equally say that positions only exist by humans comparing different locations like they compare different times by consulting memory?

    You have to be careful with your definitions.
     
  4. Sep 21, 2012 #3

    Drakkith

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    Why can't it be both? Our perception of space and time may be unique, but I doubt anyone could claim that spacetime isn't "part of the universe".
     
  5. Sep 21, 2012 #4

    Simon Bridge

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    I'd have said that the tools for describing how space in the Universe changes are math and language ... the space-time is "out there". But I think that gets bogged down in philosophy.

    In fact - that would go with the quotes ... is it possible that OP wants the philosophy forum?
     
  6. Sep 22, 2012 #5

    tom.stoer

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    The idea to get rid of space and time as physical entities has a long tradition and it has influenced Mach and Einstein as well. Einstein started with a Machian point of view where space and time are not physical entities but all what counts are relations (signals, interactions, ...) of physical entities (bodies, fields, ...).

    But as we know Einstein did not "succeed"; the story GR tells us about spacetime is different. Here spacetime seems to be a physical entitry which can exist w/o objects or relation between objects (it cannot be 'observed' w/o test particles, but that's a different story). There are non-rivial vacuum solutions of Einstein's field equations like black hole spacetimes, deSitter universe etc. indicating that (according to GR) spacetime itself is a physical entity (this translates to some quantum gravity approaches like LQG)
     
  7. Sep 22, 2012 #6
    Einstein proved that time was not well-defined, but it works almost perfectly well here on Earth so hardly anyone worries about this.

    As to whether concepts correspond with reality, physicists wisely don't worry about that much. If the math works well enough, fine. It is an applied science with practical results, not philosophy.

    Since space and time are interdependent, they must have equal ontological status. That is, they must be equally "real" or "unreal," whatever that may mean.
     
  8. Sep 22, 2012 #7
    We have two working theories, QFT and GR. And at some deep level we think there is a connection. It occurs to me that without particles one cannot measure space. If nothing whatsoever existed in space, how could you measure the distance between things? And if particles never moved, how could you measure time? So it seems even at the deepest level there must be things moving in order to measure space and time.

    Likewise, if you had no space for things to exist in, how could there be particles? So it seems to me that both are necessary and compliment and complete each other.
     
    Last edited: Sep 22, 2012
  9. Sep 22, 2012 #8
    Actually there is an important solution to GR called DeSitter space which has no mass. I do not understand it. I guess that you can say how a mass would move if it were there. All GR is built on the idea of these insignificant virtual test masses, so why not use the concept when no mass is there? In both cases we just imagine the test mass is present. It isn't necessary to actually do it.
     
  10. Sep 23, 2012 #9

    tom.stoer

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    This is exactly what I mean: Einstein did not succeed in building GR on Machian principles only b/c that would rule out vacuum spacetime like deSitter. And yes, of course you can calculate the geodesics on which test particles move in deSitter spacetime.
     
  11. Sep 23, 2012 #10

    mdl

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    i'll try to simplify my thoughts:

    - space is a link between two points (or objects)
    - time is a link between two events

    if a link between two points wouldn't exist, no information/change could propagate from one point into another - so it must exist. for example there's a link between a tree and an apple on ground, because particles/information can propagate between them.

    but a link between two events doesn't exist or is unnecessary.
    for example link between events "apple is hit by a ball" and "apple falls on ground" doesn't exist. that's why we have developed memory - so that we could create the link.

    that's why i think that time and spacetime can't be physical entities..
     
  12. Sep 24, 2012 #11
    I could say that the apple was the link.

    I've lost interest. This is like the foggy word games played by old-time philosophers where they would argue endlessly about definitions. Eventually everybody got tired of it and they started using more precise definitions to avoid this sort of thing. Mathematics is nice for that. If you will learn the math, you will get your answer. There are dozens of web sites on special relativity.
     
  13. Sep 24, 2012 #12

    Drakkith

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    I agree. Learning the math or at least the terms used in science and philosophy will go a long way in letting your get your point across clearly MDL. You may also develop a new understanding as you learn.
     
  14. Sep 25, 2012 #13
    Sure seems like in GR space and time are as 'real' [or imaginary, if you wish] as anything else.

    Want to create a particle: accelerate. Want to slow down time.....go faster and or find a deeper gravitational well....to change mass to energy...E=mc

    What to change distance...go faster.....

    perception seems to be reality....

    From QM: If a system is in a state described by a vector in a Hilbert space, the measurement [observation] process affects the state in a non-deterministic, but statistically predictable way....again, 'differences' appear....

    I saved this from another discussion:

     
  15. Sep 25, 2012 #14

    Simon Bridge

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    I suspect you are reading more into GR than is there.

    How would you slow time by changing speed?
     
  16. Sep 26, 2012 #15

    tom.stoer

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    I think you should try to understand at least the basic mathematical concepts what spacetime according to GR - and possibly some theories of quantum gravity - are. Then your ideas would have a more solid basis.
     
  17. Sep 26, 2012 #16

    chiro

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    I'd be interested to hear (especially by the physics people) what being physical actually means.

    Does it mean it can have a physical interaction? Does it mean it can be observed? Does it just mean that it can be described in some linguistic context and framework? Does it have to be measureable with specific measuring devices?

    What does it mean for something to be physical in a highly specific (i.e. non-vague) way?
     
  18. Sep 26, 2012 #17

    tom.stoer

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    This is unfortunately not a physical but a metaphysical question; for an answer or explanation on must rely (implicitly or explicitly) on a philosophical position like Platonism, positivism, structural realism, ... In addition one must explain the words "physical", "real", "existing", "measurable" and their relation, which is again beyond physics.

    Spacetime is a good example: its obviously "physical" or "real" in the sense that we are able to use it a mathematical framework to describe theories with experimentally testable predictions. But it is not "real" in the sense that it can be measured w/o referring to (the motion of) test particles or something like that. The mathematical structure of spacetime (the metric tensor in GR) is not an "observable" due to several reasons.

    I am afraid this is not the intended answer ...
     
  19. Sep 26, 2012 #18

    Drakkith

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    Thank you Tom, I spent a few minutes attempting to say exactly what you did, but I suffer from an inability to explain my ideas well, so I had to give up.
     
  20. Sep 26, 2012 #19

    chiro

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    Well if something is so vague that it can't be defined, how is one able to actually discuss anything related to this "something"?

    If physics can't say what something physical actually is, then I think that this is really an embarrassing situation for the physics people.

    For instance, the idea of measurability is where you are able to map some observation of some sort to an element usually in a set of numbers. It's basically identifying something in one language with something in another and in number systems with rank (like the 1 dimensional number systems), we can describe how to classify each element relative to the others by use of ordering and relations like <, >, = and so on.

    Now you can formalize this with sets if you want, but intuitively for most purposes, this is enough to explain in detail what measuring does in an abstract sense using spoken languages as opposed to the mathematical one.

    If you have a situation where physics can't describe what is physical, then IMO you don't really have physics but something way too vague.
     
  21. Sep 26, 2012 #20

    Drakkith

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    Why? Because we recognize that when you get right down to some of the fundamentals it becomes hard to give concrete answers? This has always been the case. Thinking that there should always be a definite concrete answer for everything is simply unrealistic. It ignores a great many other issues that make it hard to answer.

    We don't require that something "be physical" in order to measure it. We can measure where an electron is at, it's velocity, its spin, but whether it is "physical" or not delves into other areas like philosophy. If an electron is composed of nothing but fields, is it "physical"? I don't know. That's probably why we don't have a specific definition for physical that I know of.
     
  22. Sep 26, 2012 #21

    tom.stoer

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    That may be your impression, but please keep in mind that - although lacking a "physical definiton of physical" - the physics community making progress regarding experimentally testable predictions. Einstein and Heisenberg had a debate where Heisenberg intended to ground a theory (of quantum mechanics) purely on measurable quantities. Einstein argued that this approach leads to nothing and that (to some extend) it's the theory which "decides" what is measurable. Now we have a theory of quantumm mechanics based (to a large extent) on mathematical entities which are not measurable (wave functions, density operators, entangled states, time evolution operator, Greens functions, path integrals, ...) but from which we can extract measurable predictions which are proven to be "correct". So the question whether these "not measurable quantities" are "unphysical" is a metaphysical question which seems to be irrelevant for the success of physics.
     
  23. Sep 26, 2012 #22
    But then you also have things that are predicted and undetectable even in principle such as gravitons, dark matter and much of high energy theory.

    Also, the basic wave mechanics of quantum mechanics can already tell you alot, and wave mechanics is pretty easily visualized and even experimentally seen. There's even photos of wavefunctions (given by electron distributions) in metal clusters, graphene and even individual organic molecules.

    The bra-ket notation and matrix mechanics served to *make QM less intuitive and harder to visualize*. However it did clean up alot of the integrals and make certain things easier to calculate. In materials science and chemistry though, wave mechanics is used more often since it its an easier (not easy) way to calculate applied problems such as molecular clusters, polymers, and complicated nanostructures. Bra-ket notation is still important for other uses such as NMR off the top of my head. but if you don't use those specific tools you only need wave mechanics.

    I'm in physics and I'm find that the way quantum mechanics is taught in physics is wayyyy different from the way its taught to chemists and materials engineers, and it is taught that way to make it easier for the theorists, not experimentalists. That's a shame because without experiment, there's no science, industry or technology, just a bunch of arcane symbols. Just my opinion.
     
  24. Sep 26, 2012 #23

    chiro

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    The less specific something is, the more useless it becomes when it comes to not only defining something, but making use of something particular for analysis and inference/decision making of any sort.

    There are different levels of specificity and while I agree that things are always being clarified a constant basis, having some basic idea of what a physical thing is is not much of an ask for something that has been developing for many hundreds of years.

    It's a good idea for the physicists to do this anyway because it helps them clarify what they are looking at and doing this will actually lead to solving the problems that exist.

    You can't solve a problem that you can't define, and science (and mathematics and life in general) is all about solving problems and dilemmas.

    If it's hard to give explicit answers, maybe a definition involving measureability with specific kinds of information can be given about what physics measures and studies. How about something like that?

    Again, the more vague someone is, the more useless they are in making their point.

    But you can constrain the linguistic and informational constraints that are being talked about.

    Everything is information and you can classify information. We create languages that take a subset of all classifications and create ways to subdivide those classifications all the time whether its mathematically or non-mathematically.

    What you have implied above is that there is some kind of line (but you haven't been specific enough) that divides philosophy and physics on the basis that physics is associated more or less with measurability but not with the nature of what information actually is.

    In other words physics considers information, its sources, how to measure them, and subsequent analysis of such information but there is a line drawn where it comes to thinking about what information actually is in terms of some kind of other meaning.

    Now if you want to get more specific, then what you should do is discuss meaning in the context of an interpretation to get that line more formally defined.

    If you making a specific kind of interpretation that is philosophy then so be it, and if it's physical then so be it, but there must be a line that gets more clarified as times goes on.

    Information itself can be taken as simply information in some particular context and it's done all the time through the language that is used: you simply look at the language used and then derive the interpretative context that this particular form of analysis is bound by.

    You take one particular context, you get interpretation and then you get a way to find that line. You take another and you get similar properties for that context. They may overlap, they may not.

    At the very least though, one can consider firstly the information context associated with what physics tries to describe and then work from there.
     
  25. Sep 26, 2012 #24
    Hi mdl,
    "space"* and "time" are human concepts that relate to the "real" physical world in a way that is not fully understood (at least not by all) and which are even open to modification. There certainly exist physical-philosophical papers on that, for example "The evolution of Space and Time":
    http://en.wikisource.org/wiki/The_Evolution_of_Space_and_Time

    *physical space, to be distinguished from mathematical space

    EDIT: IMHO this topic is just in the realm of science
     
    Last edited: Sep 26, 2012
  26. Sep 26, 2012 #25
    time dilation.


    skirting the issue of an exact definition, something is 'physical' if we can measure/observe it. Otherwise it's just theory. A fly in the ointment is that different observers have, in general, different observations.

    For example, in the quantum field theory view, "real particles" are viewed as being detectable excitations of underlying quantum fields. The physical existence of the underlying fields is unproven.
     
    Last edited: Sep 26, 2012
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