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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.
     
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