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The Nanohertz Gravitational-Wave Detection Explained
- mariushop 3y agooutstanding delivery
- zadwang 3y agoExcellent illustration. After reading it I thought this looks like the PHD style. And I checked the author, who IS Jorge Cham. About 22 years ago I was reading his Piled higher and deeper, PHD, series and bought several books of his. It is a great feeling to see that he is still doing comics. Thanks Jorge!
- leetrout 3y agoI had never heard of "cosmic strings". https://en.wikipedia.org/wiki/Cosmic_string https://en.wikipedia.org/wiki/Cosmic_string
- chrisweekly 3y agoFantastic explainer. The illustrations are both charming and helpful. It's reminiscent of other excellent multi-modal "explainers", e.g. Julia Evans (https://jvns.ca https://jvns.ca), https://betterexplained.com https://betterexplained.com, Randall Munroe (https://xkcd.com https://xkcd.com), http://inception-explained.com http://inception-explained.com ... And on this topic of mixing text with imagery, I highly recommend Nick Sousanis's amazing "Unflattening" (https://archive.org/details/unflattening0000sous https://archive.org/details/unflattening0000sous)
- dilaver 3y ago"First, [gravitational waves] have really long wavelengths." Why?
- speff 3y agoλ = v / f The velocity (v) is the speed of light for gravitational waves, which is already a really big number. If the frequency (f) is based on the period of the black holes which circle around each other, then I assume one rotation happens over a long period of time. Long period -> low frequency -> small denominator which makes the wavelength (λ) even longer.
- geon 3y agoSome rotations are fast. LIGO detects gravitational waves at 200-10000 Hz. That’s kind of fast. Just imagine a couple of black holes orbiting each other that fast.
- walnutclosefarm 3y agoBut the higher frequency waves detected by LIGO are not caused by two bodies orbiting their common center of mass at a distance, but rather by two much smaller masses - a few to a few tens times the mass of our sun - than the ones described in the cartoon. These masses orbits have decayed and they are spiraling into each other merging. The Short waves we detect only occur in the final bit of the spiral and merge - we see only the final milliseconds of the merger, and just a few wave crests.
- ooterness 3y agoThe idea of something with ten solar masses moving so fast still terrifies me. It's just mind-boggling to think about something at that scale completing an orbit in the blink of an eye. I am reminded of Randall Monroe taking about the sheer energy of a supernova: > Which of the following would be brighter, in terms of the amount of energy delivered to your retina: > A supernova, seen from as far away as the Sun is from the Earth, or > The detonation of a hydrogen bomb pressed against your eyeball? > Applying the physicist rule of thumb suggests that the supernova is brighter. And indeed, it is ... by nine orders of magnitude. https://what-if.xkcd.com/73/ https://what-if.xkcd.com/73/
- defrost 3y agoNot a bad question at all really. In principle, gravitational waves could exist at any frequency. The speed, wavelength, and frequency of a gravitational wave are related by the equation c = λf, just like the equation for a light wave. For example, the animations shown here oscillate roughly once every two seconds. This would correspond to a frequency of 0.5 Hz, and a wavelength of about 600 000 km, or 47 times the diameter of the Earth. https://en.wikipedia.org/wiki/Gravitational_wave https://en.wikipedia.org/wiki/Gravitational_wave To get a short wavelength requires a high frequency, to get an observable gravity wave requires a very large mass. We haven't yet seen a Super Massive Black Hole orbiting with (say) an Mercury orbit radius at a thousand times a second. There is another quote from that wikipedia article: Stephen Hawking and Werner Israel list different frequency bands for gravitational waves that could plausibly be detected, ranging from 10^−7 Hz (very slow) up to 10^11 Hz (very very fast). The faster the wave the shorter the wavelength and the greater the difficulty in detection (as the amplitude likely lessens and with distance falls below our current direct means). You'd have to chase the Hawking-Israel paper for their thoughts on short wavelength high frequency gravitational wave sources and how they believe they might plausibly be detected .. I anticipate some devil in the detail. http://library.lol/main/F92F35CD83F13A6021FC2385BBA171B0 http://library.lol/main/F92F35CD83F13A6021FC2385BBA171B0 ( perhaps wikipedia misquoted that high frequency )
- tzs 3y ago>> The speed, wavelength, and frequency of a gravitational wave are related by the equation c = λf, just like the equation for a light wave [...] > To get a short wavelength requires a high frequency For light that is assuming a vacuum. The more general equation is c' = λf where c' is the speed of light in the medium the light is traveling through. c' <= c. Hence, for light, you can get a shorter wavelength without having to raise the frequency if you work with the light in a medium with a lower c'. Is there anything similar with gravitational waves?
- dotnet00 3y agoProbably not because the medium is the same spacetime everywhere and there isn't anything we know of that slows down or blocks gravity.
- deleted 3y ago[deleted]
- bloopernova 3y agoWhat an utterly delightful and eminently understandable presentation of a complex subject! I know that it would take decades or centuries to gather enough data to make discoveries. But I wonder if one day a scientist will be looking at these results and see something that makes them think "that's odd..." And suddenly our understanding of the universe is turned upside down yet again!
- p1mrx 3y agoAustralian scientists vindicated as new pulsar data proves South is up.
- layer8 3y agoIt would be nice if there was a plain-text version for accessibility.
- NickC25 3y agoI don't have a deep understanding of physics and would have no chance at understanding the details of this if it was in a traditional academic paper, and not presented so well. This is absolutely fantastic!
- jack-bodine 3y agoIf gravitational waves can be detected by looking at pulsars, then what is the purpose of LIGO and ground-based gravitational wave observatories? Is there any difference in the waves detected by LIGO and those from observing pulars?
- venusenvy47 3y agoThose two methods observe waves of much different wavelength/frequency, and the mechanisms that create such different wavelengths are assumed to be different. So cosmologists are studying different things by looking at different wavelengths.
- magicalhippo 3y agoAs mentioned in the sibling comment, different frequencies means probing different things. I found this talk[1] rather nice to explain the NANOGrav experiment, and at 6:45 there's a very nice plot that shows where NANOGrav fits in compared to the other gravitational wave experiments and which type of sources the various experiments can probe. [1]: https://pirsa.org/20100068 https://pirsa.org/20100068
- _Microft 3y agoLIGO detects gravitational waves in the frequency range of hundreds of Hertz (10^2 Hz), which are produced in the last moments of the in-spiraling of merging neutron stars or blackholes. Nanohertz are 10^-9 Hz which relate to a rotational period of decades of years. https://en.wikipedia.org/wiki/Gravitational-wave_astronomy https://en.wikipedia.org/wiki/Gravitational-wave_astronomy
- Cerium 3y agoFor one we didn't have a way to measure the speed of gravitational waves before [1]. [1] https://physics.stackexchange.com/questions/622729/did-ligo-measurements-prove-that-the-speed-of-gravity-equals-the-speed-of-light https://physics.stackexchange.com/questions/622729/did-ligo-...
- brookst 3y agoReally great explainer! Naive question: how do we know it is gravity specifically distorting space? Could there be other forces or phenomena not related to mass?
- teraflop 3y agoThe theory of general relativity says that "gravity" and "distorted space(-time)" are one and the same phenomenon. So it's a question of whether you believe that theory is correct. We've known for a long time that GR predicts that when black holes and/or neutron stars collide, they should emit gravitational waves with a very specific shape. The LIGO and Virgo observatories detected faint signals that seem to match this shape, which gives us confidence that the theory is a correct explanation of the data. This adds to the mountains of other evidence for GR, and complements it by showing that GR still holds in environments where space-time curvature is extremely strong. Technically, we don't know for sure that it's impossible for GWs to be produced by other sources than moving masses, but we have neither theory nor evidence to suggest such a thing is possible.
- aaron695 3y agoVery well done So pulsars are not regular at the tick level as they can spin faster and slower but have 100 nanoseconds accuracy (Way worse than an atomic clock) when doing an average profile - https://astronomy.swin.edu.au/cosmos/p/Pulsar+Timing https://astronomy.swin.edu.au/cosmos/p/Pulsar+Timing This is a cool line - "In effect this allows changes in the relative distance between the pulsar and Earth to be computed to an accuracy of 30 metres (~100 feet)"
- beders 3y agoI still don't understand what the carrier of gravitational waves is. How are the waves propagated? Space is not an elastic fabric or water where water molecules transfer potential energy. Aren't gravitational waves a hint that spacetime is not fundamental, but an emergent property of something else?
- deleted 3y ago[deleted]
- dotnet00 3y agoSpace is like a fabric, it's a literal compression and stretching of space as the wave propagates through it. The distances physically get shorter and longer as the wave passes by. It doesn't necessarily mean that spacetime cannot be fundamental, as there's no rule that says that space itself must emerge from some sort of underlying medium (eg the way fabric is made of threads).
- beders 3y agoWhat is stretching/compressing it? Spacetime is warped by mass but no mass is transmitted. So what exactly is being carried along?
- pseudosudoer 3y agoYou're essentially asking the same question after it's been answered. Space-time is the carrier that acts like a fabric, and some large cosmological mass is distorting that fabric. The distortion of the fabric of space-time ripples throughout the entire universe at the speed of light, and spreads it's effect proportional to the inverse square related to distance.
- Sharlin 3y agoThe surface of water is warped by a rock thrown into a pond, but no rock is transmitted by the waves. The electromagnetic field is disturbed by accelerating charges but no charges are transmitted by the EM waves. In any case, mass is not the only thing that warps spacetime. The components of the stress–energy tensor [1] include energy (which includes mass) density, energy flux, momentum density, and momentum flux, the latter of which is composed of pressure and shear stress components. [1] https://en.wikipedia.org/wiki/Stress%E2%80%93energy_tensor https://en.wikipedia.org/wiki/Stress%E2%80%93energy_tensor
- eande 3y agoIs there a similar illustration to the Quantum Entanglement phenomena? Something I yet to have to fully understand how that is possible?
- turndown 3y agoSay your car and another car got in a crash, but you both panicked and drove away(that is, you did not inspect the damage.) You get home and finally summon the courage to look at your car. It's busted. With that information you can infer that the other car is busted too, and you don't need to ever come in contact again with the other car to know this fact only that at some point in the past your car and theirs interacted.
- swid 3y agoThis explanation infers a hidden variable, which is not how quantum entanglement works. IE - it sounds like the damage occurs at the time of impact, and you just decided to look at a later point in time. But that isn’t what happens with quantum entangled particles. The particles will have opposite spin when the entangled state collapses, but measurement will affect the angle of the spin in a way that proves spin is not preselected when the particles were initially entangled and local to one another. In your story, looking at your own car would have an observable effect on the entangled car, even though it is far away. But you also cannot even tell it was entangled with the other car without comparing the two over more traditional channels! Edit: Since a graphic was requested, I found this image hosted by JPL, which shows a related phenomenon (quantum teleportation) to what I described, and hopefully dispels the car crash explanation which doesn’t imply “spooky action at a distance”. https://www.jpl.nasa.gov/news/researchers-advance-quantum-teleportation https://www.jpl.nasa.gov/news/researchers-advance-quantum-te...
- whoknowsidont 3y agoI've always wanted to ask this really dumb question, so I'll just ask it here (laugh or answer, your choice): Would there be gravitational waves so huge that we'd have trouble detecting them? As in, if there was a huge, massive gravitational wave and we were in the process of "riding" it how would we ever know or detect such a thing? Would this possibly (not plausibly) be an answer to some of the weirdness and contradictions we have observed in astronomy over the past few decades?
- krastanov 3y agoYou can see "sensitivity" plots for the various methods we have for observing gravitational waves. The horizontal axis is usually in terms of the frequency of the wave and the vertical axis is how sensitive we are to them. This article is about "nanohertz" waves, which means their wavelength is roughly 10⁹sec × speed of light, i.e. 30ish light years. So, it seems we can detect really "big" gravitational waves. If you look through the aforementioned plots you will see that sensitivity drops off significantly for "more astronomical" wavelengths, so there are definitely scales at which we can not detect, as you suggested. There might be some interesting ways to introduce very long gravitational waves as solution of current discrepancies in our understanding of cosmology, but they are also probably introducing more discrepancies than they solve. I would guess people have considered these ideas, because "cosmological scale general relativity" is pretty heavily researched and mainstream.
- raattgift 3y ago> so huge that we'd have trouble detecting them Gravitational waves can be arbitrarily long because patterns in the cosmic microwave background suggest the early universe had gravitational radiation with wavelengths of arbitrary size, including quite long waves. The metric expansion of space has stretched those "primordial gravitational waves" (PGW), so even short-wavelength-in-early-universe PGW can have periods of many years at about the present size of the universe. Additionally, most models of cosmic inflation stretch PGW, with some of the stretched wavelengths being made longer than the distance to the cosmic horizon (Initially, before inflation, PGWs can have practically unlimited length or can be almost arbitrarily short, with the shortest winding up not so short once inflation has ended), so at modern times the super-horizon-length primordial waves would have periods of many billions of years. Any masses in a ~binary orbit (stars to galaxies, down to grains of dust and up to collections of galaxies like in the local group, <https://en.wikipedia.org/wiki/Local_Group https://en.wikipedia.org/wiki/Local_Group>, the "dumbbell" shape is relevant) generates gravitational waves with a frequency comparable to the orbital period. In our sky there are galaxies that appear to be in mutual orbits with orbital periods of many millions of years. The pulsar timing arrays in the cartoons linked at the top of the page look for gravitational waves with periods of months to years. It takes several orbital periods to be statistically comfortable that a gravitational wave with that period (nanohertz frequency) has been detected. Those are probably generated by supermassive black holes in hard mutual orbits. If we soften such an orbit, increasing the semimajor axis (or radius for a circular orbit), the orbital period grows, so we have to watch the array of pulsars longer (and be more wary of so-called red noise and other long-term signal contamination). > some of the weirdness and contradictions we have observed in astronomy Unlikely. Do you have any particular weirdness and/or contradiction in mind?
- zackees 3y ago[dead]