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B "Spacetime" (what is "it"?)

  1. Mar 14, 2016 #1
    I've been trying to learn about cosmological expansion (some weeks ago), I think I understand as much as any lay-person could, regarding why everything is moving away from our galaxy. However I still don't understand what spacetime is. The fact that space can deform indicates to me that spacetime is not not a metaphysical thing, but something that has tangible, observable properties. Such as in observing closer galaxies lens more distant galaxies, or that the Alcubierre drive is theoretically possible...or moreover that a 'big rip' could tear the electrons from atomic nuclei, if expansion accelerates.
    So if the space part of spacetime itself is getting bigger, and for the aforementioned reasons space is presumably more than just a vacuum of quantum mechanical fluctuations (with various standard model fields in it). Then does humanity actually know what spacetime is, or is it still more or less a mystery?

    Thank you


    P.S. I forgot, of course gravitational waves.
     
    Last edited: Mar 14, 2016
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  3. Mar 14, 2016 #2

    phinds

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    Spacetime is a framework in which things happen. It is geometry. It is NOT a "fabric" or other pop-science nonsense. Expansion is just things getting farther apart, not space "stretching" or "expanding". The "big rip" is not in the cards based on our current understanding of cosmology and even if it were that would not make spacetime anything other than a framework. Light does not get "bent" when it passes a massive object, it follows a straight line, BUT ... that "straight line" is in Riemann Geometry (which describes spacetime) not Euclidean Geometry which is why it is described as "bent" from a human point of view.
     
  4. Mar 14, 2016 #3

    FactChecker

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    There is a trade-off between a person going through time and the same person going through space (at a fast speed). To understand that, time and space need to be put into the same coordinate system. It is similar to the trade-off that a person traveling at a constant speed in two physical dimensions. Suppose a person is traveling at a constant 50 miles per hour. If he goes North at 50 mph, then you know he is not going East or West at all. Likewise if he is going East at 50 mph, you know he is not going North or South at all. In space-time we are always going at the speed of light. If we go at the speed of light in physical space, then you know that we are not moving in time at all. Likewise, if we go full speed in time, then you know we are not moving in physical space at all.

    Far from metaphysical, the distortion of space-time is very real. It explains gravity. Einstein's General Relativity is all about that. That is about as "down-to-Earth" as you can get. (Pun intended.)
     
  5. Mar 14, 2016 #4

    Dale

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    In science a thing is identified with its measurable properties. So an electron is a thing with a certain combination of measurable properties including mass, spin, and charge.

    Spacetime is the thing with measurable properties known collectively as the metric. It includes all of the normal spatial geometry (distance, angles, curvature) as well as similar concepts for time (duration, relative velocity)
     
  6. Mar 14, 2016 #5

    bhobba

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    Due to the symmetry properties of space and time as measured using rulers, clocks or whatever, its turns out its like space by itself - it can in a sense be 'rotated' so you cant consider them separate. If you have a stick and you cant get it through a door you can rotate it to get it through. The same with space and time.

    Here is the technical detail:
    http://www2.physics.umd.edu/~yakovenk/teaching/Lorentz.pdf

    Length contraction, time dilation etc is just like rotating a rod to go through a door - except its 'hyperbolic' rotation.

    Thanks
    Bill
     
  7. Mar 15, 2016 #6
    H'mm, yeah I see how light can bend around a galaxy, yet still be traveling 'straight', due to spacetime being a curvature of the geometry itself.
    However, (and I don't mean to irritate, but) it is the other implications of proper distances getting bigger (over "time"?) or possibly in a big rip scenario tearing electrons from nucleus. I understand how that sentence is true, just not like how can this 'geometry' becomes this fabric of creation that is tangible with real consequences. And is presumably has properties as I address in my response to Dale below.

    That is actually a really good way of putting it, nice sort of vector-ie way of putting it.

    I sort of understand that, how would you define it? I understand that is has these quantum fluctuations (creation and destruction of particles spontaneously, really fast) as well as these fields through it (Higgs, electromagnetic...and others? Sorry, what are they?)
    I'm trying to get my head around the QED and QCD vacuum wiki pages.
    Presumably these quantum fields in the standard model are are of this spacetime geometry, I.e. when the light travels around a galaxy, the electromagnetic field is distorted by the mass of the galaxy it passes around so that 'straight' from the perspective of the light or the EM field is around the galaxy?


    Thanks all!
     
  8. Mar 15, 2016 #7

    Dale

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    GR is a classical (non quantum) theory. None of this is going to help you understand GR.
     
  9. Mar 15, 2016 #8

    pervect

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    I would say that the fundamental observable phenomenon of space-time is the ability to measure distances, and time intervals. You can pretty much regard this as being done with rulers, and clocks. If you wax philosophical, you can probably agonize a lot over what a ruler and a clock really is. As far as science goes, we have an operational procedure based on the SI standard for measuring both.

    I can dig up a quote for the "SI meter" and the "SI second" definition if one is needed, but it should be easy to find.

    Space time is geometry, and we can regard geometry as the study of distances. Angles a a part of geometry, but if we have distances, we can compasses drawing circles, and we can imagine measuring distances along the arcs of these circles, and those define angles, so we can define angles in terms of distances. Thus we don't need to regard geometry as being about angles and distances, since we can define angles in terms of distance. We can regard geometry as being fundamentally about distances.

    It's helpful to introduce the concept of coordinates to talk about geometry, though not strictly necessary. Since it's not strictly necessary, I will avoid it for now.

    Space-time geometry takes a rather funny turn, in that there turns out not to be two separate sorts of distances (spatial distances and time distances, usually referred to as time intervals), but only one sort of "distance", an observer independent interval known at the Lorentz interval. The relationship between the Lorentz interval and the SI concepts of time interval and distance is one of the topics of special relativity, and the reason that we view space-time as a single unified entity rather than two separate concepts of space and time.

    But you can compute the space-time interval knowing only how to measure distances and time intervals. So if you understand distances and time intervals, you have the tools needed to understand the Lorentz interval.

    There are some tricky aspects about distance that I've glossed over, but the main point I'm trying to make is that you can regard space-time as being all about distances, and that we have operational procedures for measuring distance via instruments we call rulers and clocks.
     
  10. Mar 15, 2016 #9

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    If you like that way of explaining relativity, I highly recommend the book "Relativity Visualized" by Lewis Epstein. I got that idea from his book. I really enjoyed reading it.
     
  11. Mar 15, 2016 #10

    PeterDonis

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    "Theoretically possible" in the sense that it is a valid mathematical solution of the Einstein Field Equation, yes. But it is a solution that in all probability does not describe an actual physically possible state of affairs. The examples of gravitational lensing and gravitational waves are better since we know they actually happen.
     
  12. Mar 15, 2016 #11
    How do we know that spacetime is "there" and not just an "illusion" made by our brain? I mean, we "see" things, but we don't actually "see" the spacetime itself. While metric is just a mathematical property of a spacetime model, i.e. differentiable manifold. By the way, is it possible to make a model of the spacetime using some mathematical objects other than a differentiable manifold, or, more generally a topological space?
     
  13. Mar 15, 2016 #12

    jbriggs444

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    What difference does it make?
     
  14. Mar 16, 2016 #13

    FactChecker

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    "spacetime" is the idea that explains the time and distance measurements that we observe in the universe and in physics experiments. If we ignore measurements, what is left to be called "knowledge"?
     
  15. Mar 16, 2016 #14
    Of course, something which is "there" is different from something which is the result of our conception. However, whether that difference has a significant physical effect or not, I don't know. :biggrin:
     
  16. Mar 16, 2016 #15

    Dale

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    I am with jbriggs444 on this. This question and its answer don't matter. Whether it is all in our head or not we have a theory which accurately predicts the outcome of measurements involving clocks and rulers.
     
  17. Mar 16, 2016 #16
    Is there any way to formulate a measurement other than using metric?
    (But this is just a restriction to my earlier question)
     
  18. Mar 16, 2016 #17
    Maybe it's just a matter of philosophy. But to me it's still bothering.
     
  19. Mar 16, 2016 #18

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    It's important to understand that the distortions of space and time measurements apply to ANY method of measurement: the aging of a human, the vibrations of atoms, the time for Mercury to orbit the Sun. It means that we can take an atomic clock to the top of a mountain and it will have run faster when we bring it down and compare it with a lower-altitude clock. The distortion also explains gravity. That is in Einstein's general theory. So it is not just an intellectual mind-game.
     
  20. Mar 16, 2016 #19
    Hm, this is getting further from my question whether we can make a model for spacetime other than using differentiable manifolds. In our solar system scale, GR gave a very accurate prediction. However, there still some mysteries out there, dark matter and dark energy for example. We say that there is a dark matter and dark energy is because we keep GR the way it is, or in other words, we keep a differentiable manifold as our model of the spacetime.
     
  21. Mar 16, 2016 #20

    phinds

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    I don't think dark matter has anything to do with GR. Am I missing something?
     
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