What is Spacetime: Definition and 1000 Discussions

In physics, spacetime is any mathematical model which fuses the three dimensions of space and the one dimension of time into a single four-dimensional manifold. Spacetime diagrams can be used to visualize relativistic effects, such as why different observers perceive differently where and when events occur.
Until the 20th century, it was assumed that the three-dimensional geometry of the universe (its spatial expression in terms of coordinates, distances, and directions) was independent of one-dimensional time. The famous physicist Albert Einstein helped develop the idea of space-time as part of his theory of relativity. Prior to his pioneering work, scientists had two separate theories to explain physical phenomena: Isaac Newton's laws of physics described the motion of massive objects, while James Clerk Maxwell's electromagnetic models explained the properties of light. However, in 1905, Albert Einstein based a work on special relativity on two postulates:

The laws of physics are invariant (i.e., identical) in all inertial systems (i.e., non-accelerating frames of reference)
The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.The logical consequence of taking these postulates together is the inseparable joining together of the four dimensions—hitherto assumed as independent—of space and time. Many counterintuitive consequences emerge: in addition to being independent of the motion of the light source, the speed of light is constant regardless of the frame of reference in which it is measured; the distances and even temporal ordering of pairs of events change when measured in different inertial frames of reference (this is the relativity of simultaneity); and the linear additivity of velocities no longer holds true.
Einstein framed his theory in terms of kinematics (the study of moving bodies). His theory was an advance over Lorentz's 1904 theory of electromagnetic phenomena and Poincaré's electrodynamic theory. Although these theories included equations identical to those that Einstein introduced (i.e., the Lorentz transformation), they were essentially ad hoc models proposed to explain the results of various experiments—including the famous Michelson–Morley interferometer experiment—that were extremely difficult to fit into existing paradigms.
In 1908, Hermann Minkowski—once one of the math professors of a young Einstein in Zürich—presented a geometric interpretation of special relativity that fused time and the three spatial dimensions of space into a single four-dimensional continuum now known as Minkowski space. A key feature of this interpretation is the formal definition of the spacetime interval. Although measurements of distance and time between events differ for measurements made in different reference frames, the spacetime interval is independent of the inertial frame of reference in which they are recorded.Minkowski's geometric interpretation of relativity was to prove vital to Einstein's development of his 1915 general theory of relativity, wherein he showed how mass and energy curve flat spacetime into a pseudo-Riemannian manifold.

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  1. SD das

    I GR -- about spacetime and time travel

    if GR is correct about spacetime (expanding of space) then i think time travel in back (back to the past) is impossible... suppose before 2 billion years ago (from now) the total space (volume) of universe was "X" so, it's clear that the space (volume) of the present universe must be greater...
  2. S

    A Foliation of spacetime into spacelike hypersurfaces

    A spacetime can be foliated by spacelike hypersurfaces which are related by an isometry ##\partial_t##.1. Why is ##\partial_{t}## called an isometry of the spacetime? 2. How are the spacelike hypersurfaces related by the isometry ##\partial_{t}##?
  3. SD das

    I Consequence of curvature of spacetime

    I was wondering that consequence of curvature of spacetime caused by an entire galaxy! Can anyone suggest?
  4. SD das

    I Does a black hole cut the fabric of spacetime as it changes

    Einstein's general theory of relativity describes the stars distort spacetime fabric and it appears a black hole makes a hole in the spacetime fabric. It is evident that the size of a galaxy does not change due to gravity but in large scale the distance between two distant galaxies increases due...
  5. V

    A How do magnetic fields curve spacetime?

    According to the Einstein field equations, matter and energy both curve spacetime. I'm wondering how magnetic fields contribute to the curvature of spacetime. I have a few questions: 1. Does a magnetic field in a current-free region of a curved spacetime still satisfy Laplace's equation? Or is...
  6. dykuma

    I Can Warping Spacetime Create a "Warp Bubble" in GR?

    First of all, I would like to admit that my understanding of general relativity is limited, though this will hopefully be remedied this summer when I take a GR class. I think we have all seen a sci-fi movie where the characters are stuck in a space where they are trapped in a sort of "warp...
  7. R

    A Is spacetime orientation a convention?

    I thought this was an easy enough question but then I managed to obtain arguments both for and agaisnt it and confused myself. Is there a clear demonstrable answer to this question in relativistic field theory? Is spatial and temporal orientation a convention?
  8. S

    I Rindler coordinates in Minkowski spacetime

    In an inertial coordinate system in two-dimensional Minkowski spacetime, the metric takes the form $$(ds)^{2} = - (dt)^{2} + (dx)^{2},$$ and in an accelerating coordinate system in two-dimensional Minkowski spacetime, the metric takes the form $$(ds)^{2} = - R^{2}(d\eta)^{2} + (dR)^{2}.$$ The...
  9. P

    I Multivariable Calculus Project: Spacetime/Black Holes

    I am a student currently taking both Multivariable Calculus and Differential Equations. Instead of a final exam my teacher assigned a final project for Multivariable, and I chose to do something with Spacetime/Black holes. Within the scope of <100 hours of work, is there anything I can do with...
  10. S

    A ADM mass of a spacetime and mass of the associated matter

    The ADM formalism gives a definition for the energy (Hamiltonian) of a static, asymptotically flat spacetime. This energy can be equated to the mass of the matter (for example, a black hole) which resides in this spacetime...
  11. A

    I Understanding ISSS from Minkowski Space to Poincaré Group

    I try to understand where the intrinsic spin symmetry of spacetime (ISSS) is established. I read articles but still do not understand how to put together all the information to make a clear picture of where ISSS comes from. Minkowski space - Lorentz group O(1,3) - Covering SO(1,3) with SL(2)c -...
  12. binbagsss

    Schwarzschild spacetime proper time to fall radially inward

    Homework Statement Question attached My method was going to be: set ##r=R## and solve for ##n(R)## set ##r=2GM## and solve for ##n(2GM)## I was then going to integrate proper time ##s## over these values of ##r##: ##\int\limits^{n=cos^{-1}(\frac{4GM}{R}-1)}_{n=cos^{-1}(1)=0} s(n) dn ###...
  13. K

    I Can spacetime exist in superposition?

    It's well known that a single particle can exist in superposition, but what about the gravity of the particle? Is the gravity also in superposition? I suppose this makes it difficult to write a wavefunction, since we can't express it in terms of a field over a single spacetime. But what if we...
  14. Anujkumar

    I Spacetime Relativity: Universe Expansion & Our Place in It

    When the universe is expanding then why not we are expanding along with it
  15. mpolo

    I Gravity: Contraction of Space?

    Specifically, what I am wondering is gravity considered to be a physical contraction of space? For instance the space between any two points A and B shrinks as gravity grows stronger. Is this the right concept?
  16. M

    B Spacetime same size during Big Bang up to now?

    I read somewhere here (but couldn't find the exact message) where it was mentioned the size of spacetime during the Big Bang is the same size as it is now. But the metric expands. However semantically even though the metric expands, spacetime doesn't expand. But spacetime should get bigger now...
  17. M

    Exploring the Relationship Between Bohmian Mechanics and Spacetime

    If the world is really described by say Bohmian Mechanics or Objective Collapse. does it mean Spacetime is also objective or a substance in some way or absolutely no relationship between the quantum mechanism and spacetime? But then if no connection.. how can one have a wave function that is...
  18. LarryS

    I QFT in Euclidean or Minkowski Spacetime

    Forgetting for the moment about curved spacetime, does the relativistic QFT in use today by experimental physicists live in Euclidean spacetime or Minkowski spacetime. Thanks in advance.
  19. T

    I Path of Light in Curved Spacetime with Metric g

    Suppose we have a curved spacetime with metric g, how can we find out the path of light throughout that space?
  20. itssilva

    Lack of evidence for symmetric partners and extended spacetime?

    Some motivation: It's relatively easy to postulate "supersymmetric theories" - e.g., you can build one by simply monkeying around with the harmonic oscillator H = p2+x2 and linear combinations of x and p using Grassmann numbers - that, AIU, is NOT what one usually refers to as SUSY, but...
  21. Chris G

    A Q: Black hole event horizon vs distortion effects

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  22. LarryS

    I Spacetime Metric: Which signature is better?

    It seems that, in general, mathematicians prefer the (-,+,+,+) signature for the Minkowski spacetime metric while most physicists prefer the (+,-,-,-) signature. Is there any evidence that Nature actually prefers one over the other? As usual, thanks in advance.
  23. Arman777

    I Can Spacetime be a Reference Frame?

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  24. FallenApple

    I Strange Spacetime behavior inside Event Horizon

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  25. Dheer Parikh

    B Exploring Spacetime: Can We Time Travel?

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  26. arupel

    A How was the value of the spacetime curvature obtained?

    I received a reply about the value of space time curvature. Evidently it is not zero but an almost unconceiveably small number. I was just wandering how this number was gotten.
  27. B

    B Is the Cosmological Red Shift Evidence of Time Slowing Down?

    It is my understanding that most cosmologists prefer to describe our universe in terms of space-time as opposed to simply time alone or space alone (or in discrete combination). Why is it then that when we observe a red shift in distant galaxies or stars, we describe this to a physical expansion...
  28. J

    B Lorentz transformation formulas for arbitrary distances

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  29. Ibix

    I Null geodesics in Schwarzschild spacetime

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  30. E

    B Visualizing 3D Contours of Spacetime

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  31. P

    B How do we study space-time singularities?

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  32. M

    B Spacetime, quantum field, and aether

    Hey guys, What is the difference between spacetime, quantum field and aether? For a beginner, maybe these terms seems similar because these terms indicating "something" that which has no shape. But what are the differences exactly between these?
  33. V

    I Generalized Tetrad Formalism for Rotating Spacetimes and the Equatorial Plane

    I'm interested in tetrad formalism for describing phenomenons near Kerr black hole. I've read some papers and I have a question about Localy Non-Rotating Frame (LNRF). In all papers is mentioned that most of astrophysically important cases are in equatorial plane (EP) and deals with EP only...
  34. I

    B Spacetime: Understanding Warps, Limits & Potential Energy

    I would like to know about this: Is there a limit to how much spacetime can warp? If there isn't, can there be? If we imagined an isolated object in the universe, how much would it warp spacetime per 1 cubic meter if it had the mass of 1 kg. No other gravitational forces are acting on this...
  35. A

    B Exploring the Mysteries of Time and Spacetime

    Hi. Recently I was watching youtube about time and spacetime. And I have few questions about it. If I well understood no one knows what is the time. Here are no formulas to describe time. If it is true how we know if other watches measure time and not our perception on time which can be...
  36. J

    I On the invariant speed of light being the upper speed limit

    Hello! I have a question that has been bothering me since I first started learning about Special Relativity: Given only the Minskowskian metric and/OR the spacetime interval, how can one reach the conclusion that the speed of light is invariant for every observer and how can one conclude that it...
  37. I

    I Does spacetime have potential energy?

    Does spacetime have potential energy, and does an object with more potential energy have more mass? (E=mc^2)
  38. KetilT

    I What is the Simpler Name for Conserved Property of Spacetime?

    If we have come to realize that energy conservation is not the most general conservation law in our spacetime, isn't it odd that we don't have a simple name for the "real deal"? I bumped into this thought through Noether's theorem, which relates symmetries in fields to conservation of all kinds...
  39. wolram

    B Gravity: Is it Spacetime or Mater-Dependent?

    Do you have to have mater for the gravity field,or is gravity a thing that has all ways been.
  40. SteveManPhy076

    B Black Holes and The End of Spacetime

    Recently I was reading Stephen Hawking's brilliant book "A Brief History of Time" and I believe I can recall him writing multiple paragraphs about falling into a black hole. I think I remember him saying that if you were to fall in all of time begins to collapse and the singularity will always...
  41. F

    B Exploring the Possibility of an Outside Spacetime: Separating Fact from Fiction

    Can spacetime have an outside? I sometimes read stuff where it is mentioned. Is it true?
  42. wolram

    B Is Spacetime Actually a Superfluid?

    http://phys.org/news/2014-04-liquid-spacetime-slippery-superfluid.html What if spacetime were a kind of fluid? This is the question tackled by theoretical physicists working on quantum gravity by creating models attempting to reconcile gravity and quantum mechanics. Some of these models predict...
  43. A

    Worldlines and spacetime diagrams

    Homework Statement An observer on the bridge of a spaceship is undergoing a proper acceleration a, so that the observer’s worldline expressed in terms of the coordinates of an inertial reference frame S is given by t(\tau)=(c/a)\sinh(a\tau/c)x(\tau)=(c^2/a)\sinh(a\tau/c) (a) Draw on a spacetime...
  44. A

    I Gravity Waves: Measure Spacetime Stretch with a Ruler

    Hi all, On the website of gravity waves by PhD comics: At minute 1 and 5 seconds to minute 1 minute 20 secs. "Now if the spacetime between you and me stretched or compressed, we wouldn't notice it if we had made marks on our metaphorical sheet...because these marks would also get...
  45. Uchida

    I Relation between spacetime distortion and light

    Recently, I was thinking about the nature of reciprocity in some cases of natural phenomenon, for example, Newtons Thrid Law of Motion, or, Faraday's Law of Induction. Considering that spacetime distortion, due to a gravity field, leads to a change in the light path, or, a change in light...
  46. A

    I Why isn't spacetime considered as true 4D?

    ...and what is the difference between true 4D and the minkowski space? To me, it would be much easier to see universe as a 4D and us humans just experiencing the dimension of time differently. In my mind i pictured the universe as a complete 4D structure which we humans experience in one...
  47. P

    I Can Hyperbolic Space be affecting our view of the universe?

    Usually when gravitational lensing is discussed, the examples are those of matter bending spacetime into a positive curvature. https://commons.wikimedia.org/wiki/File:Gravitational_lens-full.jpg In these cases, distortion of light is clearly evident as images of galaxies from behind these...
  48. T

    A Does spacetime lose its determinism in third dimension?

    In a lecture from a course in QM, it was mentioned that Shroedinger's equation is deterministic in one and two dimensions. But in third dimension it gives unstable solutions, loosing it's determinism. It was mentioned that "in space of D dimensions Gauss theorem leads to the conclusion that...
  49. P

    B Visualizing Spacetime: 3 Dimensions Not Enough?

    I've tried to imagine and draw the effect of a large object would have on space time in 3d but it just doesn't seem right. Would I be right to assume that maybe spacetime has more than 3 dimensions?
  50. J

    I Spacetime and effects of being super close to the Sun

    I have been wondering about space time and how it works like satillites putting corrections in there clocks by a few nano seconds a day because of gravity or lack of it! But I have also wondered about the effects of being super close to the sun and time that we observe there looking back at...
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