Also, I assume that these waves are very gentle sinusoids? Could the opposite — a high-amplitude gravitational square wave — be possible? What would it do to the things it passes through?
The gravitational way has a direction (say, z) in which its propagating. Within the plane perpendicular to that direction (x-y), a circular ring of particles will at at one moment experience squeezing in one direction (x) and stretching the perpendicular direction (y). As the wave passes through and you move from the peak of the wave to the trough, the directions reverse, so the first direction (x) stretches and the other direction (y) squeezes. By "stretching" and "squeezing" I mean instantaneous additional (positive and negative) acceleration on top of the (much, much larger) acceleration from the background gravitational field provided by the Earth.
Here's a visualization:
https://www.researchgate.net/publication/313828462/figure/fi...
Just as a child can swing their legs at the resonant frequency of a swing to pump up their sinusoidal amplitude, a very weak gravitational wave can pump up a ring oscillator if it's oscillating at the ring's resonant frequency.
Exactly square gravitational waves are of course not possible, just as for electromagnetic waves. (They would have infinite energy at the corners.) But in principle you could get a close approximation. However, spacetime is incredibly stiff, and I think all the known real-world sources produce pretty smooth waves. I presume most violent events are mergers of existing black holes, and essentially always result from a smooth in-spiral rather, say, a sharp collision event. This is what the "chirp" signal looks like to the LIGO detector:
https://www.youtube.com/watch?v=TWqhUANNFXw
The effects of a square wave would be roughly as you would expect: instead of smoothly pumping up an oscillator, it would give it a sharp kick, just as with electromagnetism.
If this analogy holds, then can it be taken further? Acoustic waves dissipate their energy insofar as they trigger plastic deformation in a material. Could gravitational waves plastically deform... spacetime itself? Or would they just be deforming the material? Or is gravitational energy not dissipated into other forms of energy at all?
(This is waaaay outside my expertise though, so take my answer with a grain of salt. Everything I've said in this thread is basically based off the rudimentary understanding from taking a GR course in grad school. I've never done research on this topic.)
Extremely weakly. The mechanism is similar to acoustic waves, but the coupling constant is so small, that the amount of dissipated power would be insignificantly small.
In theory, you can use gravitational waves to extract energy. For example, you can wait for the "compression" part of the wave, and push a cart uphill during it. Then let it slide downhill when the compression peak passes. You'll be able to extract some useful energy, because the distance that you pushed the car uphill will be shorter than the "normal" distance.
You can make more elaborate systems on this principle. E.g. a tuned resonator: two orbiting masses with a period selected to match the frequency of the gravitational waves.
If all of that energy could be harnessed; it would be sufficient to power a small toaster oven.
Do you have a source? Sounds like an interesting calculation.
W = 32 * G^4 / (5*c^5*r^5) * (M1*M2)^2 * (M1 + M2).
It's actually kinda amazing that once you substitute in all the values and do the math, all the scary large powers just somehow cancel out to leave a small macroscopic number (I remember getting around 60 watts).
Edit: note, that both objects radiate. So it's 60 watts for both the Earth and the Sun, around 120W in total.
Light up 1 room with an incandescent bulb, or your entire flat with LED bulbs nowadays. I would be very interested in seeing some napkin math, based on power efficiency progress and "rate of technological innovation", that attempted to project when we could feasibly run the equivalent of our present-day human civilization purely off of gravitational waves/radiation.
(Eventually after all the typing below I think that I traced the 32/5 and (M1*M2)^2 * (M1 + M2) to (eqn 16) in Peters & Matthews 1963 maybe? (e=0, a->r) https://doi.org/10.1103/PhysRev.131.435 (stick sci-hub.se in front of that if you need to). The authors take an approach comparable to the textbooks below.)
Super-quick textbook review. Practically all of them start with the quadrupole moment and try to justify an energy which is quadratic in derivatives of that while still within a linear theory. Carroll and Mathtias Blau take slightly different-from-each-other paths through the transverse-traceless TT-gauge to P = dE/dt = -2/5 \frac{G^4 M^5}{r^5} (c=1) for a circular equal-mass soft binary. Wald uses the radiation gauge and so eqn 4.4.58 looks fairly different, and comes with the amusing Waldian line "A lengthy calculation (where many terms which integrate to zero are dropped) yields the final result,". Sigh. Blau's development looks a lot like MTW, but the latter gives us .... Exercise 36.6 ("Apply the full formalism ... to a binary star system with circular orbits. Calculate ... the total power radiated; the total angular momentum radiated ..."). Gee, thanks thick textbook. FWIW, 90 seconds of that (mostly trying to make sure both sides have the same dimension rather than extracting a power in watts because bad/lazy reasons and anyway I always think you had about the right order of magnitude) doesn't take me to anything like the form of your calculation.
Rather than flip through other textbooks, let me rely on my maybe-shaky memory and say that most of them, at least the modern editions, follow the TT gauge approach and come up with an equation in a form similar to Carroll.
What stands out here is that Earth-sun has a large mass ratio and noncircular orbit, and it's not really a binary system anyway, and so will defy these textbook schemes. Secondly all of these take P in the far field, because linearization. (Compare that with your edit).
I also got way off into the weeds wanting to work with chirp mass (rather than q=m_1/m_2) which is what GW obs data analyses use because extracting the individual masses is hard. I also know a bit about EMRI BHBs (extreme mass-ratio inspiral) and in those dissipation is dealt with differently from textbooks even for soft binaries (e.g. soft->hard roughly PN & perturbative methods, EOB, GSF, numrel) absolutely none of which is of immediate practical use here.
So that's some of what motivates my question about the origin of your calculation.
ETA: I think most of what happened here is that my brain reads equations as words, and I simply forgot that I could actually rearrange teh ltteres! TGIF :-)
(I guess such a material would interact with gravitational waves as if it had more mass than it does, without that affecting its inertial mass?)
But yes, spacetime itself is jiggly like Jell-O.
As we can't visualize 4d spacetime, most analogies will be wrong. But as photons don't experience time themselves, thinking about the geodesic path is probably less error prone than thinking of it as squishing physical objects.
Objects being pulled towards slower flows is the intuition that matches the math best for me.
From my perspective, it takes about 8 minutes for a photon from the sun to hit my eye. From the perspective of the photon, a little time has passed, no? Doesn't the atmosphere and passing through my glasses slow it down a wee bit? Can the photon "know" that its position has changed between emission and absorption? From the photons point of view, I must be very, very close to the sun, right?
From the point of view of the photon, "forwards" is, like time, a null[0] dimension.
[0] I may be using that word imprecisely, but I can't think of a better one.
> Doesn't the atmosphere and passing through my glasses slow it down a wee bit?
When a photon is travelling through anything other than vacuum, it's not "slowed down." It's repeatedly being absorbed and re-emitted. (Or rather, it's being absorbed, and new photons that happen to be mostly equivalent are being emitted.) The refractive index of a material is effectively a measurement of the likelihood of absorption, times the average per-particle time-delay between absorption and re-emission.
I think an answer to that question depends of what you mean by total duration of the universe.
If the photon never gets absorbed by anything, then it goes in forever. If it exists then the universe still exists forever since at least one photon exists forever.
Massless particles being required to travel at the speed of light is perhaps a lens to think about it.
Some quotes from this thread:
> photons [have] no concept of time
and earlier
> Nothing that travels at light speed experiences time. For a photon, emission and absorption is a single event.
and other commenters in the same thread
> Photons ... "subjective" time is zero. In Einstein's theory of special relativity, the faster you go the slower your proper time appears to an external observer
> time within the photon's own reference frame is not advancing at all
and even Don Lincoln in a linked video in this thread: "we have to be careful since the equations of relativity don't apply for travelling at the speed of light, but hopefully you see that this limit trick allows us to get arbitrarily close. So I think we can see that a photon experiences no time ..." Thus everyone above is in good company with these slogans. However, Don Lincoln almost certainly knows he needs to correct s/the/these/ (in the context of the lim v->c analysis in the video), and that his conclusion needs to be understood as "no proper time" in that context. But we also all know that it's a youtube pop sci outreach video, not a university lecture or crucial vital factual no-fake-news hackernews thread.
So, let me make the counter-propostion: photons evolve on their worldlines, so must experience some time.
Additionally, elastic Rayleigh scattering supports the idea that there may be one or more point-coincidences along the worldline of a photon. There can also be non-scattering point-coincidences where the photon's momentum energy is some fraction less than 1/1 of the energy-density (the stress-energy) at some point in spacetime along its worldline even if the photon does not interact non-gravitationally with the rest of the stress-energy there (e.g. at that point there could be one or more of a neutrino, free neutron, dark matter particle, or another photon). We should be able to describe such a point-concidence in coordinates adapted to our photon's worldline, just as we could adapt them to e.g. the free neutron's worldline.
Relativity gives us (for all practical purposes, fapp) total coordinate freedom. Point-coincidence physics are invariant under changes of coordinates.
So we can label any curve any way we like, without changing the physics of anything touching that curve.
Proper time \tau solves the timelike geodesic equation, which makes \tau handy for labelling points along a timelike geodesic, but \Delta\tau = 0 on null geodesics, so is not suitable for them.
There is a unique labelling of points along a null geodesic that does solve the geodesic equation, and that is the affine parameter. See https://physics.stackexchange.com/questions/17509/what-is-th... to save me a bunch of typing. Note that as the third answer says one can use the affine parameter to calculate and explain the gravitational or cosmological redshift as a consequence of the null geodesics picked out by the Einstein Field Equations.
That (and the equivalence principle) is also a satisfying way of understanding the relation E = hf (see the first equation at <https://en.wikipedia.org/wiki/Photon_energy#Formulas>) in a lab-scale patch of flat spacetime.
Otherwise, how do you explain any \Delta f if photons "have no concept of time"? You and others in this thread appear to have been arguing that in Special Relativity the standard inertial frame for massive particles is inappropriate for showing the time-evolution of massless particles. That's true. But the point of relativity is that we can deploy (fapp) any system of coordinates and if we are doing covariant physics (i.e. using tensors; one might start with chapter 11 of J.D. Jackson's textbook which is freely available online (and 2nd ed is on the Internet Archive) and is very widely used in teaching) then it almost doesn't matter what system of coordinates we use.
Almost: we can choose practically useless coordinates, like labelling a curve in a non-monotonic way, or labelling points non-uniquely. In fact, any f(\tau) does both of those on a null geodesic: every point gets labelled with a 0. That's not the photon's fault, that's the fault of trying to use an inappropriate system of coordinates. To be fair, such coordinates seem like obvious choices by a person familiar with their use in inertial frames for massive objects, but who then may be misled into thinking the inappropriateness of the coordinates for objects on null geodesics determines the physics of those objects.
Unfortunately, this mistake is very common, and has led to poor slogans which have been repeated many times by several people in this discussion.
If one wants to sloganize, "proper time is inappropriate for photons because they are massless" (cf. §1.2 on the inverse square law and photon mass in Jackson) "but just as nobody's proper time is preferred in relativity, neither is any proper time; and for photons affine time is a useful substitute".
> Massless particles being required to travel at the speed of light is perhaps a lens to think about it.
Indeed, and I just did that for you, although Jackson and I would flip that around to say that c is the speed of massless particles and experimentally (and for theoretical reasons) photons are massless.
Forgive the lazy \latex anyone who actually sees this, including future me.
"If u is the tangent vector to a curve, a tensor Q is said to be parallel propagated along the curve if \nabla_u Q = 0. If the tangent vector is itself parallel propagated, \nabla_u u = 0 (tangent vector "covariantly constant") the curve is a geodesic, the generalization of a straight line in flat space. If x^\alpha(\lambda) is the geodesic (with u^\alpha = dx^\alpha / d\lambda) then the components of the geodesic equation are
0 = (\nabla_u u)^\mu = \frac{du^\mu}{d\lambda} + u^\alpha u^\beta \Gamma^\mu_{\alpha\beta}.
"Here \lambda must be an affine parameter along the curve; for non-null curves this means \lambda must be proportional to the proper length.
"If a curve is timelike, u is its tangent vector, and a := \nabla_u u = Du/d\tau, then a vector V is said to be Fermi-Walker transported along u if \nabla_u V = (u \otimes a - a \otimes u) \cdot V."
See also problem 7.11 and its solution.
Also of interest is Matthias Blau et al. 2006, "Fermi coordinates and Penrose limits". doi:10.1088/0264-9381/23/11/020 hep-th/0603109 which adapts Fermi coordinates to null geodesics. (abs. "(Fermi coordinates are direct measures of geodesic distance in space-time)... We describe in some detail the construction of Fermi coordinates" §4, "We now come to the general construction of Fermi coordinates associated to a null geodesic \gamma in a space-time with Lorentzian metric g_munu. Along \gamma we introduce a parallel transported pseudo-orthonormal frame ... Fermi coordinate are uniquely determined by a choice of pseudo-orthonormal frame along the null geodesic \gamma" "For many (in particular more advanced) purposes it is useful to rephrase the above construction of Fermi coordinates in terms of the Synge world function").
https://www.quantamagazine.org/gravitational-waves-should-pe...
In fact this weak coupling is what makes GWs so interesting for observational astronomy: They propagate from the source to our detectors virtually unchanged. (This is in contrast to EM radiation, which is very easy to scatter.) For example the farthest we can see back with EM radiation is about 200k years after the Big Bang, when the plasma of the early universe recombined into neutral hydrogen. By contrast gravitational waves can see back to the Big Bang itself, so it is a truly unique source of information as compared with light.
Gamma ray burst is twice as far away? It's four times dimmer. A thousand times as far? A million times more dim. Gravitational wave signal from <event> is twice as far away? Makes it twice as hard to detect. A thousand times as far away? Only a thousand times as hard to detect.
The linear drop-off you're referring to is when we look at it in terms of field strength (in this case the spacetime strain). Since power is proportional to field squared, this implies a linear drop-off in the field. It just so happens that for GWs it's easier to detect the field, whereas for (most) EM radiation it's easier to detect power.
There are field-detection methods for EM radiation as well, which are useful for weak signals. Homodyne and heterodyne detection are good examples.
Looks like these things haven't detected a real gravitational wave, but if a strong enough one at the right frequency came through, they might start ringing like (very quiet) bells!
So is it possible that a passing gravitational wave could initiate some natural process that otherwise might have not happened?
You mean something like tectonic event trigger or something physics specific?
I assume the gravity wave could push the reaction to initiate by warping a subatomic element (like an electron orbital) into an otherwise impossible configuration on a scale of picometers for a split second.
To even detect these, we need to observe multiple pulsars over long periods of time in order to find minute effects only visible at galactic scales due to the nanohertz frequency of these waves. In other words, spacetime is being stretched and compressed at subatomic scales on a sinusoidal wave with a period of a month or so.
It’s a bit like asking if cosmic rays from the Pinwheel Galaxy are affecting cancer rates.
Thank you for that context!
I don’t know the answer but it’s an interesting question
However there's nothing that we know of that could happen close by, so the risk is near zero. Apparently the black hole at the center of the milky way is going to merge with another super massive blackhole in Andromeda in 4.5 billion years or so.
The real question is, is the change meaningful? If you have a tsunami but it doesn't change anything meaningful, is it an interesting observation outside of the event itself?
In English, why does stiffness correlate to smooth waves? What does stiff spacetime mean? I'd have thought a square wave would be "stiff" as it's quite the opposite of smooth.
The two things are kind of related, though. One "natural" way to create a square wave in nature, is to "interrupt" a material transmitting a sinusoid wave, at the peak of its transmittance. And one way to do that, is to break through the modulus of elasticity of the material transmitting the wave, such that it switches from the elastic-deformation domain (transmitting the wave) to the plastic-deformation domain (ripping apart.)
Imagine a speaker cone tearing at the peak of a high-amplitude drum beat. The cone pushes "out" — and then doesn't push back "in", because instead the air behind it rips forward through it. The air created by the speaker cone wants to rush back "in", but now there's no longer a speaker cone acting as a waveguide for the inward flow, so the natural turbulence cancels out much of the "falling" energy of the wave, making it look much more like a square-wave drop.
I believe that the GP is saying that, because spacetime has such a high effective "modulus of elasticity", we haven't yet observed any practical way to perturb so as to create the conditions that would generate a gravitational square wave.
A stiff material tends to dampen high frequencies, simply because it cannot deform fast enough to follow the wave's shape. In a way, the medium acts as a low-pass filter; compare, fow example, how fast you can clap your hands in air vs in water: the stiffness of water slows down your movements so you cannot reach high clapping frequencies.
[0] https://en.wikipedia.org/wiki/Square_wave?useskin=timeless#C...
All materials roll off their frequency response. But this is backwards - stiffer materials have higher resonant frequencies given equal density. It’s basically a word to describe a high spring constant.
Gravitational waves cause space itself to stretch in one direction and simultaneously compress in a perpendicular direction. In LIGO, this causes one arm of the interferometer to get longer while the other gets shorter, then vice versa, back and forth as long as the wave is passing. The technical term for this motion is "Differential Arm" motion, or differential displacement, since the arms are simultaneously changing lengths in opposing ways.
As described above, as the lengths of the arms change, so too does the distance traveled by each laser beam. A beam in a shorter arm will return to the beam splitter before a beam in a longer arm--as the wave passes, each arm oscillates between being the shorter arm and the longer arm. When they arrive back at the beamsplitter (where they re-merge), the light waves no longer meet up nicely; they are out of phase. Instead, they shift in and out of alignment for as long as the wave is passing.
Please ELI5 specifically and empiracally what "Space Itself" actually is.
Would it be possible to build a 'galactic clock & Compass' - a "clock" to the regular pulses of a pulsar and the galactic direction the pulsar is in relation to the terrestrial compass (magnetic) on earth...?
What is the pulsar with the most reliable timings?
General Relativity successfully establishes that the presence of mass distorts this, so it defines a mathematical object (the Einstein tensor) that reacts to the distribution of mass and energy and precisely describes the changes to the metric. For example it can model how the mass of the sun distorts the space so that light from distant stars appear to follow a curved path because very close to the sun a curved path is now the shortest path.
The Einstein tensor defines how distances --and time-- are measured and it's the best mathematical model that we have about what "space itself" is. Future theoretical advances could take us forward and demonstrate that space itself emerges from other more fundamental elements, but this needs bridging quantum mechanics and gravity. We don't really know what space is made of, but scientists have precisely modelled how it reacts to mass (and energy) with utmost precision.
NB: At cosmic scales the exercise becomes more difficult, as there is an expansion of the metric of spacetime that is not due to the presence of mass, in fact it is caused by the _absence_ of mass as it seems to be due the energy of empty space: the phenomenon called Dark Energy.
Hope this helps!
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TensorFlow
>*General Relativity successfully establishes that the presence of mass distorts this, so it defines a mathematical object (the Einstein tensor) that reacts to the distribution of mass and energy and precisely describes the changes to the metric.*
This leads me to think that TensorFlow was attempting to map the 'weight' among topics of intersecting interests, sciences, etc... and seeing who the "tensor warping" was most strong with and adding higher eval weights to things that "gravitated" to one another based on the informational difference in distance?
(I dont know the nomenclature, but is that were using 'tensors' in AI/ML/whatever 'weights' come from?
So reasoning about a neural network weights and operations in terms of tensors makes sense and I guess that's what the name Tensorflow comes from.
It helped my put my own internal visualizations to the understanding.
and I had a weird peripheral memory on this from a thought I had whilst driving in 1999 where I was thinking of tensors in this way, but I didnt know what I was just daydreaming about... but apparently, it was einsteins tensors coupled with information theory - and while to me it was a pedestrians take on the premise - it turns out that that day dream was correct!
And it all ties back to when I was ten years old and meditating on the Mind of God -- It all tied into one another - and you gave me some cord to pull these experiences together with understanding which I havent had in 40 years... so that was nice. Thanks.
Anyone else recall daydreams from their past where their later understanding was confurmed, even though you were just "daydreaming at the time"?
Joking aside, thank you for the deeper explanation. The idea that spacetime isn’t fundamental is a very non-intuitive concept given how we’ve evolved to interact with the world. Any suggested reading on this topic for laypeople?
let's think about space like a giant, invisible playground. Normally, it's flat like your bedroom floor, where you can measure how far your toys are from each other with a ruler straight across. That's like when there's no gravity in space.
But guess what happens when something really heavy, like a big bowling ball (that's like a star or planet) comes into your playground? It makes everything around it bend and curve. So, the distance between your toys is no longer a straight line. It's like when you throw a ball, it doesn't go straight, it goes in a curve.
This bending is what a really smart guy named Einstein explained in a thing called General Relativity. He came up with a way to measure how space bends around heavy stuff.
And you know what else? There's this really weird stuff called Dark Energy that's everywhere but we can't see it. It makes space grow bigger and bigger, not because of heavy stuff, but because there's a lot of empty room. Scientists are still trying to understand this, but it's like blowing up a balloon: even though there's no heavy stuff inside, the balloon still gets bigger!
Carlo Rovelli is a bona fide theoretical physicist, very involved in the development of Quantum Loop Gravity (one of the attempted approaches to bridge GR and QM). Turns out he is also a good pop-sci writer, so I would begin there. His book "The Order of Time" deals with the nature of time, which is not about the nature of space, but then again reading it you see that the mental gimnastics are similar.
I also found useful contributions from regular contributors at /r/cosmology (thank you /u/jazzwhizz) but it's less straightforward and alas, Reddit has its own issues.
I think they push string theory too much, and try too hard to braid it into the fabric of our societies, with their little shops and what not... It gets everywhere, and tomorrow is their favorite day "Friday"!
Same thing in general relativity: the metric tensor measures the failure of closed loops on each axis to not close perfectly, the way they would in Euclidean space.
Basically even as a small creature on earth you can 'figure out' about the curvature by carefully measuring small-ish loops. The same is true for spacetime, but the loops' deformities are even smaller.
Take a straight line down from the "north pole" of your ball to its equator. Draw another straight line around a quarter of the equator. Draw a third line back to the pole. You've just drawn a triangle with 3 straight lines and the angles add to 270 degrees.
A non straight line is just not the shortest distance between two points on that surface.
Shortest distance between two points is what it is.
Imagine two people standing some distance apart from each other at the equator. They both begin walking in straight-line paths due south. At first, their paths are parallel. But as they move toward the south pole, they begin to drift closer to each other, as though their paths were curving towards each other. When they reach the south pole, they bump into each other. But they were both walking straight forward following the shortest path to the south pole the whole time. The curvature of the surface causes their initially-parallel paths to converge.[1]
On a plane (which has Euclidean geometry), initially-parallel paths never converge.
[1] Don't take this too literally; the real planet Earth is three-dimensional, and its gravity keeps us on the surface. But mathematically, it's possible to describe a curved two-dimensional space without referring to any higher dimensions. When I talk about "the surface of a sphere", that's what I mean -- the surface is the entire 2D space.
Space-time is 4d array: array of framebuffers. You can stretch your mathematical model all day long, but you knowledge must be mapped to reality somehow. In model we have space-time, while in real world we have "physical vaccum" ("something nothing" or "phaccuum", for short). I prefer to name that thing "ether", because I like that word.
In spherical geometry, the equivalent of a straight line is a great circle. There are no parallel great circles. That's why I used the phrase "initially parallel" -- at the starting point, both people's paths are at a 90-degree angle to the great circle connecting their locations.
I didn't want to get into "locally flat" vs. "globally curved" in something that started as an ELI5 thread.
Yes, of course. If we substitute parallel lines with straight lines in spherical geometry and mix 2D and 3D spaces, then our mental model will be nonsensical but cute.
We found no evidence of fourth dimension in the real world, so we cannot map this cute mathemagical model to reality.
If the 'tether' is a gravitational link (meaning that the teather, is a constant pull against the trajectory, regardless of the trajectory, the object will continue to curve around center.
There’s another aspect to this: the expansion coefficient. One such model is that the coefficient depends only on time: as time passes, distances increase. To model this, draw the same line on the flat sheet of material, then expand it uniformly in all directions. The distance is still a straight line, but the line is longer after the expansion.
Gravitational waves really do change the time it takes for light to travel between two points. We use light travel-times to measure distances, thus we say that the distance between the points has changed.
If it feels counterintuitive for spacetime to be changing, that's good. It is outside our human experience and perception. The strongest gravitational waves ever observed by scientists passed through everyone who was alive in 2015. None of those people noticed before the instruments registered a detection.
IIRC, the reason no one noticed is that even the strongest gravitational waves are only going to "stretch" space by something less than the diameter of a hydrogen atom.
Edit: from wikipedia LIGO page: "(interferometers) are capable of detecting a change of less than one ten-thousandth the charge diameter of a proton"
I dont know why I remember the human hair analogy, perhaps I am confusing it with something else?
If the yardstick is light-speed, is there any meaningful distinction between saying that space itself changed and that there are local perturbations to the speed of light?
The thing I struggle with is that we normally think of things occupying space. If space itself gets distorted, then the size of those things should change, too. Or is that mental model a useful but ultimately incorrect way to think about the world?
When the light merges back together, if the two paths traveled took exactly the same distance (or an even multiple of the wavelength at least), then the beams add together constructively and you put back together the light from the laser.
But if one path becomes longer or shorter the other, the light is out of phase with itself (peaks of the waves no longer line up with each other) and you can detect the interference between them.
LIGO can detect a change in distance of less than one ten-thousandth the charge diameter of a proton.
https://en.wikipedia.org/wiki/LIGO#/media/File:Gravitational...
There’s nothing to really see as such.