Talking about Sean Carrol, I'd also recommend his latest book, Something Deeply Hidden - a fascinating, and very readable, exploration of the theories, history and even philosophy of modern quantum mechanics.
> Just as light can be bent and magnified when it passes through the gravitational fields of galaxies and other massive objects, gravitational waves should be warped in the same way, too.
So, imagine you have a massive celestial body floating out in space, with a large gravitational field. Its gravitational field is always propagating. Now, take that celestial body, and make it completely and instantaneously disappear. There's now a gravitational differential between the now-gone body, and its previously propagated gravity field. You should be able to detect that if you're close, say through tidal differences.
Very similar happens with black holes colliding, except the gravity differential comes from the two black holes oscillating near each other, close to the speed of light.
Edit: this obviously isn't exactly how this works, since it makes a lot of assumptions, such as the ability to instantaneously remove something. So, don't think of this as how "things actually work", but as a model to help build your intuition.
Be careful with that example. You probably know this, but stars can't disappear instantaneously, and so if you start with that assumption it's easy to get paradoxical results from relativity.
That doesn't mean there's anything wrong with the model. It's just GIGO.
A blackhole traveling at near light speed is pretty darned close to the analogy of a massive object instantaneously disappearing, similar to fictional spaceships engaging their warp engines.
Of course, it's not actually disappearing, just moving, but the original point was about detecting sharp changes in the gravitational waves. A quick Google search tells me that gravitational wave red shifting is a thing, and I imagine that with blackholes it's a very important phenomenon and area of study. And I would guess that there can also be interesting second-order effects that such a blackhole's movements have on the propagation of gravitational and electromagnetic waves from other objects.
> with blackholes it's a very important phenomenon
Black holes can lense gravitational radiation emitted by background systems.
Most background systems we are likely to detect soon will involve black holes. But these are black holes in some sort of mutual orbit, rather than black holes simply moving across some system of celestial coordinates.
For black holes that are moving linearly at near the speed of light, the black hole's effect on the metric elongates like a pencil, with the field weak outside and growing strong towards the centre of the "lead" or graphite. This is similar to Lorentz-contracting the near region around the black hole, and one can generalize a bit and say that as the boost between an observer and any object increases, the object thins. In the ultra-ultrarelativistic limit, the object and all the strengthening-towards-infinity field values around it become infinitely thin.
As one's speed relative to a black hole gets very close to c, the black hole becomes quite easy to model as an exceptionally high-energy massless particle.
You get this effect when your small space capsule whizzes by our galaxy's central black hole at speeds near that of light too, and your small momentary perturbation basically affects the black hole not at all. Because Lorentz contraction is reciprocal, whizzing a black hole -- even a large one -- at ultrarelativistic speeds past the International Space Station is going to have very little effect on it.
Tossing a large black hole past the ISS at low speeds compared to light will really mess up the neighbourhood of the solar system, but your space capsule can pretty safely manage a slow-compared-to-light hyperbolic orbit around a large black hole without much problem (ignoring any accretion disc and twin "paradox" issues).
An object moving and an object vanishing are the same from the perspective of wave propagation, the only difference is that one event will have a more dramatic (therefore easier to visualize) effect.
If the sun instantaneously vanished, we would see it disappear at the same instant as its gravitational effect stops, 8 minutes after the actual event occurred. For those 8 minutes while the light and gravitational information are in transit, the Earth will continue to revolve around a visible (though now nonexistant) sun.
In the same way as if the sun suddenly jerked ten million miles to the south, we would see it move at the same instant as its gravitational force vector changed, 8 minutes after the actual event occurred, but that's harder to keep in your head.
Exactly. As soon as you magically remove the gravitational body, you are magically removing the waves too according to GR. There is no such thing as curved spacetime without mass. (Except for the cosmological constant, but that's different.)
General Relativity admits general curved vacuum metrics (vacuum meaning: no matter anywhere), and many of them are useful theoretical approximations to real astrophysical systems. Famous ones include the Schwarzschild and Kerr metrics (both of which have T^{\mu\nu} = 0, where T is the stress-energy tensor), de Sitter and anti-de Sitter space, and Minkowski space. Useful ones include vacuum pp-waves, used in studying gravitational radiation from the perspective of an observer at large distance from the source. There's even the Sexl ultraboost, which can approximate ultrarelativistic motion between a black hole and a low-mass observer.
These are usually probed by adding test masses of some sort, letting them evolve along available trajectories. Some such test masses are pointlike, neutral, and nearly massless; others are some sort of classical or quantum field. In most cases, the goal is to keep T^{\mu\nu} negligible.
One can alternatively be lead by the stress-energy tensor, and may be tempted to call T^{\mu\nu} the matter tensor in that case. One typically chooses some vacuum background -- Minkowski space, usually, but any background can be used -- and then uses perturbation theory to capture how the chosen matter alters that background curvature. This is very common in cosmology.
> Except for the cosmological constant, but that's different
No, it's not different; one has flexibility to move the cosmological constant into the RHS for calculational convenience without having to change its interpretation as part of the background curvature: https://en.wikipedia.org/wiki/Lambdavacuum_solution
Gravitational lensing occurs because spacetime is curved/stretched by an amount relative to it's distance to a massive object which changes the local geometry.
Since light and gravitational waves both propagate through spacetime, both will try to go straight but will get bent since they're traveling through curved spacetime.
It's sort of analogous to how your path gets bent as you try to walk straight along the earth, making you walk in a really big circle. Except that in General Relativity, time is getting bent too and trajectories aren't through space, but spacetime.
The part I'm finding hard to wrap my head around is that gravitational waves _are_ the disturbances in the curvature of spacetime. Wouldn't that change how they're affected by the curvature of spacetime? Maybe I'm mixing the static and dynamic aspects of the field.
A ripple of water traveling along the surface of a vortex still experiences the curvature of the vortex, even though both are disturbances. (this is a terrible analogy, please take with a ladle of salt)
Firstly, somewhat nontechnically: if you think of a binary as being two ends of a barbell with an infinitesimally thin bar and heavy weights, and put the observer in the plane of rotation and far from the binary, sometimes one of the weights on the end of the "bar" is closer, sometimes one eclipses the other so the weights line up, sometimes both are equidistant. What a pair of accelerometers at the observer reports depends on the geometry, and in particular the orbital phase. We have, in effect, a giant Cavendish experiment. If we put a large compact mass between the binary and the observer, then the light image of the binary forms a https://en.wikipedia.org/wiki/Einstein_ring . Since experimentally we know that accelerometers point to the visible (and radio and so on) image of a massive system, we would expect that gravitational radiation of all sorts gravitationally lensed just like light. We usually just make a formal analogy on that, which I'll deal with a few paragraphs further down.
Slightly technically: the "curvature of spacetime" in your question is the metric tensor field ("the metric") which fills the whole of spacetime. Yes, when we have a binary like above, the metric is dynamical. We can deliberately fix a background metric chosen for calculational or conceptual ease, and capture much of the metric dynamics as small perturbations on the background.
In this picture, we usually get gravitational waves by imposing a coordinate time on a relevant part of the spacetime, and then finding which small perturbations obey an appropriate time-dependent massless wave equation.
More physically, what this picture is saying is that in the case of a single binary, if an observer stays at one spatial location and from time to time checks its accelerometers, and they will point to the retarded positions of the objects in the binary. If you put a gravitational lense between the binary and the observer, one is free to encode the lense as a perturbation, or one can add it into the background. (The former is more popular for reasons I'll explain further below.)
Alternatively, it might help to think of gravitation as a gauge theory, wherein one can only measure potential differences between two points in spacetime rather than some absolute potential.
Let's start by defining a background wherein the potential is everywhere identical and calling that the vacuum expectation value (vev). If our background has some static spherical mass on it, we can measure a potential difference between fairly-close-to-the-mass and far-from-the-mass[1]. The potential difference is not time-dependent.
As we add any such mass we may update our vev a little, but at enormous distances from all the masses, it's only a very little, so we can largely take a value much closer to our set of masses.
But when we add in more than one mass, we lose time-independence.
If the initial conditions are picked so the masses do not orbit or twist around each other, our masses will obey Raychaudhuri's focusing theorem, and we can reason using shell theorem or Birkhoff's theorem, and expect no gravitational waves detectable outside the collapsing system. On the other hand, if we let our masses fall into orbits, we get gravitational waves.
In the focusing-only case we end up seeing vev - potential_near increasing during the collapse, where near is at a large finite radial distance (in Schwarzschild coordinates) from the collapsing mass, and we measure vev at radial infinity. In the orbiting case, vev - potential_near has a long term tendency to increase, but will vary slightly depending on the orientation of the measurement points to the binary.
A large mass -- like a galaxy cluster -- far from the source is just a region where the potential departs from vev. Helpfully, when the lense mass is that large and the GW frequency is high (e.g. near the end of a binary inspiral, or quantitatively greater than ~ 1 Hz), we can draw a direct formal analogy with electromagnetism: the geometrical optics approximation [2]. Following gravitational wave analogy with https://en.wikipedia.org/wiki/Geometrical_optics we map the difference in potential from the vev to a difference in index of refraction and let the GW plane wave refract. It's useful to remember here that GWs are modelled in a linearization of the Einstein Field Theory and so treating galaxy clusters as linear media or galaxies as nested shells of linear media is a reasonable approach. Unfortunately, in general wave optical solutions are only obtainable numerically.
Using the full GR, you have a metric with pretty much no symmetries and good luck with the calculations. Your starting point for extragalactic GW sources would be a "swiss cheese" like Einsten-Straus 1931 with multiple vacuoles and then combine that with a pp-wave, probably perturbatively, ideas which have been explored in the literature although usually in cosmological/primordial GW contexts [3]. (Compare with a more formal statement of the approach in previous paragraphs, e.g. in §1.2 at http://aether.lbl.gov/www/classes/p139/homework/hw12.pdf )
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[1] If we are sufficiently close to Newton, by splitting spacetime according to a slow-moving-compared-to-light observer not very close to any compact massive objects then we can can use : https://en.wikipedia.org/wiki/Gauss%27s_law_for_gravity#Pois...
Oh yes, me too. I learnt that gravitation bends space-time and that's why light rays seem to be bent. If gravitation is just waves too, then how does that work and who bends the gravitational waves. This gets even more puzzling if we assume gravitation is mediated by particles[1].
On the other hand if gravitation is just wave and not particle it would be completely unlike the other fundamental interactions including those mediated by photons, aka light.
[1] LIGO detected gravitational waves but we don't know if gravitation particles (gravitons) exist. Detection of gravitons might not be practically possible.
> I learnt that gravitation bends space-time and that's why light rays seem to be bent. If gravitation is just waves too, then how does that work and who bends the gravitational waves.
I think your confusion comes from your thinking that the same word ("wave") is always used for the same effect. In your few sentences the "wave" is the word which is used for completely different effects and scales:
For the gravitational waves that we measure we don't have to worry about some single "gravitation particle". The gravitational wave detectors don't have to care "if gravitation particles (gravitons) exist" as that's on completely another scales, you can imagine these (measured) waves as being created by such an immense number of "gravitons" that these are surely not seen.
If you want some analogy: you know that for people to think easier about the spacetime curvature due to the gravitation one says imagine a 2D membrane with a ball on it (representing a star or a black hole or some other big object) curving the membrane. Now what are the events LIGO detects: imagine two balls, rotating one around another, resulting in the ripples on the membrane, just like two fast boats circling produce interesting waves on the surface of the lake. LIGO detects such ripples.
So the curvatures due to the masses always exist, but the huge masses moving around produce the ripples in the spacetime (it's just the shape of the curvature that changed in the different time points). That is not "gravitation is just waves too" that's: we see the ripples independently of the existence or non-existence of the gravitons on the quantum level.
To go back to the "boats on the lake" example, you detect the waves of the whole lake surface, and on that level to observe these waves it's irrelevant to you that the water is made of molecules, that the molecules are made of the atoms, that the atoms are made of the particles, and that there are the experiments that demonstrate the quantum nature ("wave"-like nature) of the said particles.
The difference in the orders of magnitudes between the waves LIGO detects and quantum "wave"-like particles is immense, so big that there aren't any simple examples I can imagine.
Gravitational waves can be measured and can transmit information. If the sun disappears, you don't immediately know about it or feel the gravity loss for about 8 minutes, otherwise that would break c and would imply that you could signal information faster than c.
I recommend watching videos by Rana Adhikari if you want to know about gravitational waves. I went down a rabbit hole about a year ago with his video's, he is increasingly engaging. Also even in a short time his predictions for the field are coming to pass.
The fundamental thing about gravitational waves is that those are a new form of data that the universe has always been sending towards us, and that we can read now. It's like being able to read the radio waves coming from the universe for the first time, not just the visible spectrum, or close to the visible spectrum.
If future advances in this field permit we may even use those to communicate. Who knows, perhaps someone is already talking in that language.
Communicating with gravity waves would be very inefficient given how weak Gravity is. I don't see any reason why you would want to use gravity for communications... but who knows.
I am a complete neophyte regarding these topics. I did notice that the detected events (black holes merging, neutron stars merging) seem really exotic, while I assume gravitational waves are all around us all the time. Is our instrumentation for detecting gravitational waves simply very insensitive compared to the instrumentation used to detect electromagnetic waves? Or are they harder to detect for some more fundamental reason?
Gravitational waves are hard to detect because of their scale, speed and required precision.
Some of the most precise lasers built are repeatedly bouncing light over miles in a tunnel to track changes in arrival significantly less than a trillionth of a second different.
Doing this while filtering out noise from miscalibrated sensors or just small vibrations outside the chamber is HARD, but improving. It gets better as more come online around the world to confirm detections and aid in better directional targeting.
As far as I understand it, the primary reason is that gravity is a much weaker force than any of the others, so detecting gravitational effects is difficult unless the scale of the event is colossal. These instruments are extremely sensitive; the effects they detect are just that small.
It may be weak, but we can still measure it quite accurately:
“...Concluding on a lighter topic, let me remind the GGP community of what I recall as probably the most memorable moment of the first campaign. It occurred at the GGP Workshop in Munsbach Castle, 1999, when Virtanen was describing the effect of snow cover on the residual gravity at Metsahovi. He showed a figure of gravity increasing by about 2 microgal over a 4-h period as men shoveled snow from the roof of the SG station, when a member of the audience asked why there was an interruption in the rise of gravity, Heikki said this was a 'tea break'...”
D. Crossley, in Journal of Geodynamics 38/3-4 (2004), p. 234.
Gravitational waves are indeed everywhere. If the early universe underwent inflation, they should be everywhere. Any object orbiting another object emits gravitational waves. These exotic events, however, are the only gravitational waves that are observable using the terrestrial light interference technique (LIGO and Virgo). A space based mission would be sensitive in a different frequency range (satellites bouncing light off one another over millions of kilometers). The primordial gravitational waves from inflation might be observed indirectly -- there was much excitement about the BICEP telescope observing indirect evidence of these gravitational waves, but I believe that other explanations have come forth.
I've been super excited about primordial gravitational waves for a while now. I have high hopes that they will lead to new physics. Just have to wait 15+ years for LISA...
So in other words these telescopes are measuring "the dog that didn't bark" rather than actually detecting gravitational waves? Much like we infer exoplanets from deviations of their stars' behaviors from what we would expect given no orbiting planets?
I see what you're aiming at, but technically it's not true. It can be similar though.
The point about how we can only infer their existence from a mass of data is similar to how we talk about deep learning. The pretrained neural net works, but we can't pinpoint why, we can only test it on various data and receive results.
There is no "dog that didn't bark", because we clearly can see the graphs and the correlations.
The way we detect gravitational waves is not "the photon haven't arrived", but "the photon has arrived a few nanoseconds earlier than we expected it". (But multiplied by a few thousands of reflections and detected only by interference pattern, not the photon arrival itself).
So, in a way, it's similar to deviations from paths by which we presume dark matter. But also, it's not that: we are almost certain here that these exact deviations are caused by the gravitational waves, because calculations and because multiple unconnected detectors, and because the form of the graph matches.
A revolution would imply there has been a breakthrough that would potentially lead to new theory. However, as with most modern physics experimental breakthroughs, it seems gravitational waves provide just further support for existing theories (with a few minor exceptions.)
Not to say that this isn’t exciting, but it seems long overdue for us to stumble upon something major that is unaccounted for.
Seems dark energy and matter are what you are thinking about. We knew with good certainty that they exist but nobody knows what they are. It’s a wide open field.
Just having "waves" is not enough for a laser. You need bosons--i.e., quantum particles with integer spin. To see the quantum aspects of gravitational waves that would correspond to this, you would need extremely strong sources--much stronger even than the black hole mergers that LIGO has observed. Nobody has any expectation of observing any quantum aspect of gravitational waves in the foreseeable future, much less being able to control such sources in order to make a laser.
This is giving me chills after reading Dark Forest. I hope this sparks a new era of SETI with gravitational waves as the medium. I believe a civilization advanced enough would use gravitational waves for communication.
What are the advantages of using gravitational waves for communication? They seem incredibly difficult to produce or detect, and don't travel any faster in a vacuum than electromagnetic waves.
Do they potentially propagate through matter with less interference than electromagnetic waves? I have no idea, that would be the only thought.. otherwise yeah I don't see what the appeal would be.. and how the hell do you create them on any small scale that can be actually read by someone, since gravity is such a weak force?
No, both gravitational waves and electromagnetic waves decrease in intensity with the square of distance.
(This is true of any wave in 3D space, because the area covered by a wavefront increases with the square of distance. The inverse square law is a consequence of conservation of energy.)
The energy of the gravitational waves will decrease with the square of the distance, but the amplitude will not. We can detect this in LIGO. This amplitude is also known as "strain". For more discussion check this page: http://spiff.rit.edu/classes/ast613/lectures/grav_i/grav_i.h...
The same is true of electromagnetic waves. The amplitude of EM waves decreases inversely with distance, and the energy decreases with the inverse square.
Your original comparison of EM waves to gravitational waves is only possible because you looked at the power of one, and the amplitude of the other. If you compare apples to apples, they're the same.
A question for typon: in light of this information, do you still believe that advanced enough civilizations would use gravitational waves for communication?
Oh come on, just collapsing some million black hole pairs to get the latest JavaScript based ad across the whole Galaxy. Let's not pretend there are still people with a 56k gravitation modem out there.
Unlike Electromagnetism, gravitation is boring because it is only a positive quantity. If you had positive and negative you could have maxwell's equations for gravitation and gravitational circuits and conductors and stuff.
They're listening on a really noise channel. It seems premature to talk about the frequency of the detected events matching some expected rate when they keep tuning their filters to the expectations.
I recently read an article deploring the missing confirmation of the mergers through light-based astronomy. I can't find the article now, so I'm just going to list another one: https://www.newscientist.com/article/mg24032022-600-exclusiv... Has anything changed from what is described in the article?
This line needs to be made into a motivational poster, or a statistics meme.