I am reading a book by Carlo Rovelli, Seven Brief Lessons On Physics, and would like to check if I have understood something. Apologies if my question is badly phrased, feel free to edit where appropriate.

I am not a physicist, just an enthusiast.

It was this excerpt that made me think gravitational waves had something to do with time.

The heat of the black holes is like Rosetta Stone of physics, written in a combination of three languages-Quantum, Gravitational and Thermodynamic - still awaiting decipherment in order to reveal the true nature of time

And the following that made we wonder if they were to do with space as well? My understanding is that space and time are synonymous?

The heat of black holes is a quantum effect upon an object, the black hole, which is gravitational in nature...

It was the next line, following that inspired my question was this,

... It is the individual quanta of space, the elementary grains of space, the vibrating 'molecules' that heat the surface of black holes

Talking about the gravitational field being space-time

The gravitational field, as we saw in the first lesson, is space itself, in effect space-time

Text referred to in the 'First Lesson'

Einstein had been fascinated by this electromagnetic field and how it worked... soon came to understand that gravity, like electricity must be conveyed by a field as well... the gravitational field is not diffused through space; the gravitational field is that space itself

Question: Is space-time 'made of' gravitational waves? Is that field it's fundamental building block?

It seems to me from all of this that space-time is indeed

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    $\begingroup$ "Are space and time 'made of' gravitational waves?"...that's not a meaningful question. $\endgroup$ – ACuriousMind Feb 2 '16 at 22:29
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    $\begingroup$ Nor is that a meaningful response $\endgroup$ – 3therk1ll Feb 2 '16 at 22:31
  • $\begingroup$ It's like asking: "Is air made out of sound?" What meaningful response would you give to that? $\endgroup$ – ACuriousMind Feb 2 '16 at 22:32
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    $\begingroup$ Well instead of posting a pointless retort I might make an effort to explain why the question lacks meaning and what may have been a better way to ask the question. $\endgroup$ – 3therk1ll Feb 2 '16 at 22:36
  • $\begingroup$ news and books written for the large public are painful. As physicists, the authors must be clear and avoid poetry and speculations. But don't blame them, it is the law of this genre of literature $\endgroup$ – user46925 Feb 2 '16 at 23:18

I'll try to boil down several of your questions and answer what I think is most fundamental, and hopefully clarify things in the process:

Gravity is completely synonymous with the shape of spacetime across all 4 dimensions (3 space, 1 of time). The reason we speak of spacetime is thus: When you (having negligent mass) stand in a "gravity field" such as that caused by a massive object such as the Earth, you notice 2 things:

First, that space seems to have a direction, i.e. objects will "fall" towards the center of the dominant mass

Second, that your watch will tick somewhat more slowly than it did when you were far away from this gravity well.

These phenomena of acceleration (a gradient in space) and changes in the rate of your watch ticking (so a gradient in time) is why we don't speak of space and time as separate entities - they are inextricably linked, and movement through one affects your movement through the other. The stronger the gravity field you're exposed to (so the more massive the object you're near), the slower your watch will tick when compared to someone standing safely outside the field. Spacetime is the fabric of the universe - so far as we know, there's nothing "under" it, nothing that it is "made up of" - and GR treats it as such. So when you think of spacetime, think of it as a landscape of hills and valleys in both 3D space and in time, all caused by the various masses that reside there, and understand that those hills and valleys are gravity.

Now, gravity waves, then, are ripples in this landscape that can be caused by an accelerating massive object. So if two black holes accelerate and smash into each other, some ripples in spacetime will travel out from the disturbance and we may be able to detect it.

With our understanding of what gravity is above, what would a gravity wave be expected to 'look like?'

We'd expect to be able to pick it up one of two ways: Either by a change in space (seen as a distance fluctuation between 2 points in our detector) or a change in time (an aberration in the rate a very reliable clock is ticking). Which now makes perfect sense, because gravity is simply the shape of space and time, bound up together.

  • $\begingroup$ Brilliant explanation, thank you for taking the time! $\endgroup$ – 3therk1ll Feb 3 '16 at 12:11
  • $\begingroup$ @RichC My pleasure! Glad you found it helpful, explaining these concepts is a tough tightrope to walk - too much analogy and you stop doing the actual science justice, not enough explanation and it can be impenetrable. $\endgroup$ – JPattarini Feb 3 '16 at 12:24

Good question, Rovelli in his book Quantum Gravity writes:

1.1.3 GR is the discovery that the gravitational field and spacetime are the same entity. What we call 'spacetime' is itself a physical object, in many respects the same as the Electromagnetic Field.

Hence gravitational waves, as the EM field has waves; he adds:

We can say that GR is the discovery that there is no spacetime at all; what Newton called 'space' and what Minkowski called 'spacetime' is unmasked; it is nothing but a dynamical object - the gravitational field - in a regime where it's dynamics are ignored.

For Newton, space and time are seperately absolute, the background stage where actual physical objects change and move - ie motion or dynamics happen; this is also true in Special Relativity, where Minkowski showed that though space and time were separately relative, they were jointly absolute.

Thus the EM field whose dynamics, it's motion is waves, rests on this 'stage' - space and time, or spacetime.

Einstein showed that the EM field doesn't rest, in sense on anything; in the sense that what it rests on is a stage, or is static or absolute - spacetime itself is itself in motion.

It's as though one looked at the sea, and see the waves moving on the sea; and see beneath it the solid firmament of the seabed; and then turning away, and then looking again, one sees the seabed is not solid, but has melted into something liquid which undulates as much as the sea itself; thus, everything is in motion and nothing is at rest; everything flows as Heraclitus said.

But given that we, ourselves are in space and time, and experience it; what would be this experience when it 'waves'; Rovelli, quite carefully and rightly sidesteps this issue here - in the quote above he writes 'space time is the Gravitational field ... in a regime where it's dynamics are ignored'; that is where we don't have waves or they're so small they can be neglected; this is exactly our experience, we live locally, in flat space; and we cannot live elsewhere ie where spacetime is 'wavy' in a manner sensible to us.

One might, align with Popper and say this renders this question 'meaningless'; but it might be safer to say, it is open and possibly answerable; but not in our current interpretative horizon ie the limit to which we can push our interpretations to.

  • $\begingroup$ Thanks for that explanation too, I will look into the book you mentioned. $\endgroup$ – 3therk1ll Feb 3 '16 at 12:12

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