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.
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.
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?
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.
Readit News
This line needs to be made into a motivational poster, or a statistics meme.
https://www.preposterousuniverse.com/podcast/2018/11/26/epis...
I'm gonna need a refresher course on that.
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.
That doesn't mean there's anything wrong with the model. It's just GIGO.
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.
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 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.
If future advances in this field permit we may even use those to communicate. Who knows, perhaps someone is already talking in that language.
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.
“...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.
https://en.wikipedia.org/wiki/Hierarchy_problem
Every time you lift an object, you are overcoming the gravitational force of an entire planet. Gravity is that weak.
Meanwhile, the incredible destruction caused by a nuclear bomb is the product of the energy stored in just a handful of atoms.
[1] https://backreaction.blogspot.com/2019/09/whats-up-with-ligo...
[2] https://news.ycombinator.com/item?id=20881986
Not to say that this isn’t exciting, but it seems long overdue for us to stumble upon something major that is unaccounted for.