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Space and time

  1. Jun 17, 2008 #1
    In physics space and time (or spacetime) are talked of as real entities that can be stretched and bent, but is there a justification for thinking in this way? I see space and time as relations between objects or events: you need at least two points to have a distance between them and two events to have a time elapsed. So is spacetime really a "thing" that can be curved or merely a relational concept?
  2. jcsd
  3. Jun 17, 2008 #2
    The only justification necessary for a physical theory is that it meets the criteria.
    In General Relativity (GR), the geometry of spacetime is be effected by matter.
    Similarly, the motion of matter is effected by the geometry of spacetime.
    This deep and unexpected insight made by Einstein has been verified by experiment.
    If you accept that GR is a correct theory, then spacetime has geometry.
    Should this fit your definition of a "thing" then yes, spacetime is a "thing".
  4. Jun 17, 2008 #3
    The justification is that a photon (i.e. light) despite having no mass can have its path bent by gravity (This is how GR was originally proved, by observing that the ligth from stars as seen behind the sun during a solar eclipse (because you can actually make out these stars during a solar eclipse where they're drowned out otherwise) were 'bent' as the passed near the sun. (there are of course many many other results but that's a quick and easy one).
  5. Jun 17, 2008 #4
    No, "space" and "time" are not the names of any things. They are nouns because this is forced on us by the grammar of our language, but not all nouns are simply a person, place, or thing.

    When a physicist says that spacetime stretches and bends, he just means that gravitational motion can be explained using a geometric theory wherein masses always move in straight lines but the coordinates we use to measure that become distorted. Instead of "the bending of spacetime" we could talk about "the distortion of spacetime coordinates."
  6. Jun 17, 2008 #5
    I think that the geometry is a mathematical construct and isn't the same thing as the physical reality. We may know empirically that objects will follow a curved path but we do not know that this "spacetime" is something there that is curved. I think that you need at least two (pointlike) objects or events for the concepts of space and time to make sense. Without anything in the universe would it make sense to say that space or time existed? There would be nothing to have a distance or time between. So what I am asking is whether you think spacetime exists independently of the objects within it or only comes into existence as a relation between them.
  7. Jun 17, 2008 #6
    Except relativistic time distortion is seen all the time in experiments.
  8. Jun 17, 2008 #7
    true but is an apple really an apple or do we just call it that?
  9. Jun 17, 2008 #8
    I don't think that existential philosophical questions should be discussed in the context of physics.
    We are best to avoid bickering philosophy and semantics.
  10. Jun 17, 2008 #9
    Have you read a Brian Greene book? Popular physics is all about metaphysical ramblings.
  11. Jun 17, 2008 #10
    ... which is why I hate popular physics.
    Anyone interested in learning something solid should check out Lee Smolin.
  12. Jun 17, 2008 #11
    Leonard Euler said:

    "Although to penetrate into the intimate mysteries of nature
    and thence to learn the true causes of phenomena is not allowed
    to us, nevertheless it can happen that a certain fictive
    hypothesis may suffice for explaining many phenomena."

    It sounds like you are afraid of what might be true, and you would prefer to keep your head in the sand. I also think we should avoid bickering, but what is "bickering philosophy?" In case you think philosophy is a waste of time, then I would ask what isn't a waste of time?

    That's silly. Only if we had convincing fake apples would we ever wonder if a particular apple was really an apple. And if we had convincing fake apples strewn about then the question "is that a real apple?" is perfectly sensible.

    Anyway, "apple" is a simple noun that refers to something. What about "pain"? If you think that "pain" refers to a private sensation that only one person is experiencing, then how could you have learned the correct use of the word "pain" at your mother's knee?

  13. Jun 17, 2008 #12
    What does the fact that time distortion is seen in experiments have to do with anything? All this means is that the relation between two events is different in different depending on the velocity of the observer. The very fact that it is called a "distortion" is what is misleading you into thinking that time is something tangible, rather than a relation between two events.
    I also don't see why I can't discuss existential topics in the context of physics - this is the philosophy forum, and it's in a physics forum. Lastly, I don't see how this is any more a metaphysical rambling than the standard interpretation of spacetime as something which can be curved and distorted, has anyone actually seen this spacetime?
  14. Jun 17, 2008 #13
    One can get a fair amount of metaphysical mileage out of all things not GUT (or TOE). Lee Smolin is just a proponent of another type of GUT.
  15. Jun 17, 2008 #14
    Well no one is thinking there is some 'tangible' space-time which can physically be 'stretched'. Rather we say that empirically we believe that the correct predictive physical model of events is one where mass causes a change in the geometry of the mathematical space-time construct through which we abstract our real-world variables. You can take from that what you will
  16. Jun 17, 2008 #15
    I totally agree with everything madness says (uh oh), and I want to point out that the word reify does not exist for no reason.

    Reify: To regard or treat an abstraction as if it had concrete or material existence.
  17. Jun 17, 2008 #16
    Also I take offence at your recommendation of Brian Greene and popular physics for its "metaphysical ramblings", I am nearly finished my mathematical physics degree and have no need for popular physics.
  18. Jun 17, 2008 #17
    Oh, come on, that's a personal attack and totally uncalled for.
    I didn't notice that this thread is under the General Discussion > Philosophy heading.
    I take it back, this is the perfect place to bicker philosophy.
  19. Jun 17, 2008 #18
    You should read some of Feynman's lectures. He was not a fan of these kinds of discussions. He takes some reasonably humorous jabs at them.
  20. Jun 17, 2008 #19
    Then why didn't you answer the question "no", instead of saying "the justification is..."

    Fair enough, I apologize.

    I tried to read Feynman's lectures, but he dodges all the good questions. Very cookie-cutter...

    I did find him humorous in high school, and discovering his books back then made me major in physics, but now I find him to be full of hot air.
    Last edited: Jun 17, 2008
  21. Jun 17, 2008 #20
    And whether you're talking about the 'space-time distortion' of a mathematically abstract frame work intended to model the real-world or a real-world distortion doesn't make a lick of difference in terms of how we understand the universe.
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