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I'm new to thinking about special and general relativity and I have no formal training as a physicist. However, I've been doing a bit of thinking about spacetime recently. I was wondering if "gravitational waves" provide evidence that spacetime accurately describes the structure of the universe. Or, in other words, do "gravitational waves" provide evidence that time exists as a 4th dimension in the universe (and what that implies about past, present, and future events in 3D space all existing with equal reality)?

Please keep in mind, I'm not asking for opinions (or dogmatic assertions) about the accuracy of universe-as-spacetime-continuum theory per se. I'm asking if "gravitational waves" logically imply the universe-as-spacetime-continuum theory.

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  • $\begingroup$ Are you asking that time exists as a spatial dimension or not? $\endgroup$
    – Ari
    Mar 18, 2016 at 3:34
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    $\begingroup$ Gravity waves are another verification of a prediction made by General Relativity. Previous confirmations of General relativity include the precession of the orbit of Mercury, bending of starlight by the Sun's gravitational field, time dialation for GPS satellite signals and various other previous confirmations for predictions made by General relativity. So far, the theory has held up to scientific scrutiny for the past 100 years. Fundamental to GR is that spacetime is a four dimensional non-Euclidian manifold which includes time as one of the dimensions. $\endgroup$
    – Peter R
    Mar 18, 2016 at 3:52
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    $\begingroup$ The geometric treatment of time in relativity is a convenience that should not deflect from the fact that time is fundamentally different from space. Gravitational waves change absolutely nothing about that. $\endgroup$
    – CuriousOne
    Mar 18, 2016 at 3:52
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    $\begingroup$ For there to be "evidence" of "time existing as a 4th dimension", you have to say what it means for "time to exist as a 4th dimension". If you just mean that general relativity's description of spacetime as a 4D manifold with one time-like direction is correct, well, we had sufficient evidence for that even before detecting gravitational waves. $\endgroup$
    – ACuriousMind
    Mar 18, 2016 at 11:04
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    $\begingroup$ What is the difference between a mathematical model that is useful in the sense that it predicts every possible measurement result correctly and something that "accurately describes the universe"? At the end of the day, all our theories are just mathematical models that are useful to predict what happens. That's what physics is about. $\endgroup$
    – ACuriousMind
    Mar 18, 2016 at 23:36

2 Answers 2

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The successful observation of gravitational waves$^1$ provides very strong support for the statement that Einstein's theory of general relativity accurately describes the behavior of masses interacting in our Universe. The dimensional nature of time plays an important conceptual role in the arguments which lead to general relativity --- and, for that matter, special relativity, which has had many, many, many more successful tests.

Any future theory of gravity must now predict gravitational waves, in addition to describing Mercury's orbital precession, centimeter-scale details of the libration of the moon, the aberration of starlight by the sun, gravitational lensing, length contraction and time dilaton, etc.

I am too cautious to rule out the possibility that some future paradigm shift will treat the time dimensions more differently from the space dimensions than relativity does. But even if that happens, the relativity we have now will remain a powerful tool for thinking about the behavior of the world, just like Newton's outdated laws are such useful conceptual tools and give such useful approximations that most students still spend a year doing Newton before moving on to Einstein.


$^1$ Note that gravity waves are a little more prosaic.

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  • $\begingroup$ What does "centimeter-scale details of the libration of the moon" mean? Google finds me nothing. $\endgroup$
    – KDecker
    Mar 18, 2016 at 12:46
  • $\begingroup$ @KDecker That the moon is getting farther from the Earth by about 3.8 centimeters per year. $\endgroup$ Mar 18, 2016 at 14:36
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    $\begingroup$ @KDecker Newton's gravity and Einstein's gravity predict the location and orientation of the moon due to its libration with differences of about ten meters. In the last ten years it has become possible to measure the moon's location with millimeter precision, which means the difference between classical and relativistic gravity has now been tested to five significant figures. A review. $\endgroup$
    – rob
    Mar 18, 2016 at 14:49
  • $\begingroup$ Libration is a balance-like moving of the moon within the Earth's gravity field. [en.wikipedia.org/wiki/Libration] Wikipedia $\endgroup$ Mar 18, 2016 at 14:51
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Gravitational waves (GW) is one more phenomenon just like many other relativistic phenomena that confirm existence of spacetime (whatever it is defined as). Spacetime has already been confirmed for example, by atomic clocks having different tick rates on earth, and in orbits. There have been number of experiments including one done by NASA about frame dragging. GW was not necessary to prove spacetime. It was a prediction made by GR, which is now proved to be a fact per the recent observation.

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