This article is new (just one-two weeks ago) but the latest research based on is from 4 years ago. Poor choice because few months ago a study (https://journals.aps.org/prd/abstract/10.1103/PhysRevD.107.1...) reopened the possibility of being dark matter afterall.
If I understand that paper, it's still the case that there's zero evidence for dark matter annihilation from AMS positrons. What it says it that (a certain type of) such evidence could, in principle, exist—it contradicts a previous theory conclusion that dark matter and pulsar positrons would have indistinguishable energy spectra. I.e. the OP blogpost is still completely accurate. (??)
(Of course—just to clarify if we all have a shared understanding—nothing in OP rules out the possibility of dark matter annihilation; it just says there's no evidence for it currently in AMS positrons. And to clarify shared understanding of a different point—the OP research isn't about positron energy spectrum features; it's a separate question about their total luminosity. The intro section of your paper discusses the distinction: "In addition to energetic arguments, the positron spectrum has long been discussed...")
No, the conspiracy doesn't belong here. There's a vibrant mainstream world trying to pin down or disprove dark matter, but there's a phenotype of cynic that's really common here on HN that precisely deserves the snark - low effort comments dismissing mainstream research while clearly not understanding it.
I think that mainstream research in physics has lost a lot of credibility after spending decades pursuing things like string theories and super symmetry. Not that it was anything wrong with any of those at first, but they weren't presented as hypotheses, they dominated research way too much, and it took way to long to admit they were dead ends (most still haven't, but instead silently started working on different stuff).
Broad brush much? You're also completely ignoring the difference between experimental and theoretical physics, which... both have a lot of things to say about the search for dark matter.
Like... sure, no field is perfect, but saying that a whole gigantic collection of specialties has "lost a lot of credibility" after one particular mathematical framework took up (in your opinion) too much oxygen strikes me as lazy and not particularly useful.
So - why does this add to the discussion? How would I be better served by focusing on your disappointment than trying to learn what actual researchers are saying and doing? Why should I trust in the cynicism of HN comments when I can go read about what experts are actually focusing on/debating/excited about? There's so much new data coming in these days, pedantic "skeptics" just strike me as increasingly annoying and useless.
You classify them as *pedantic* skeptics, while I say their skepticism is well founded. I used to be experimental physicist myself and have followed how, in particular high energy, physics has been presented by practitioners for at least three decades. And after seeing how all that focus and excitement have been unfounded more than a few times, you do get jaded.
Lots of people, physicists included, have criticized the funding allocation for high energy physics... that wasn't what we were talking about. Ironically, a lot of that criticism points out that clever astrophysical studies can be done more cheaply, including studies that work towards constraining what we loosely call dark matter.
No offense, but you seem to be changing the subject (former experimental physicist or no).
No offence, but I haven't changed the subject. I'm not talking about the cost of accelerators when I talk about HEP in this case, I talk about predictions of supersymmetry and new exotic particles that might actually make dark matter a real theory. You see, it's all part of the same.
… all it takes to make positrons is particle collisions at an energy greater than 1.2 MeV, something, pulsar magnetic fields can do easily. To make antiproton takes 1500 times more.
There is a great hope, however, that (i) dark matter interacts with itself, (ii) concentrates in certain places (say around black holes) and (iii) dark mater annihilation makes ordinary matter particles we can see. But who knows? Maybe there is the same asymmetry between dark matter and anti-dark matter so that annihilation stoped happening after the Big Ban, but maybe some dark matter particles are their own antiparticles. See
There could be an entire dark sector of the standard model that is completely decoupled from "our" particle world except through the Higgs. In that case future particle accelerators will be our only chance of ever catching a glimpse of this.
Gravity. The entire dark matter theory is based around the observation that current mass/gravity models don't match our observations in the cosmos. As the Higgs field is what gives particles mass, it stands to reason that dark matter does interact with gravity -- it's pretty much the sole reason for its theoretic existence.
In other words, if dark matter didn't interact with/through the Higgs field, it wouldn't explain the observational anomalies and therefore wouldn't have a reason to exist at all, not even in theory.
Mass is a generic property of particles in a QFT. Unless there are symmetry considerations constraining them (gauge bosons like the photon, for instance, must be massless to avoid violating their corresponding gauge symmetries), masses are free parameters and zero is not particularly more natural than any other value.
What the Higgs mechanism does is add effective mass even to particles which are intrinsically massless, so long as they couple to the Higgs field. For the standard model, that means leptons, quarks, and the W and Z bosons. But there are other fundamental particles with intrinsic masses (the Higgs boson itself, for instance), and composite can acquire mass from their binding energy even if their constituents are massless.
> What the Higgs mechanism does is add effective mass even to particles which are intrinsically massless, so long as they couple to the Higgs field. For the standard model, that means leptons, quarks, and the W and Z bosons.
this was sloppy: the W and Z bosons have mass because they mix with the Higgs field (this is the Higgs mechanism, properly speaking), while the mass acquired by leptons and quarks is due to interacting with the remaining Higgs component in the usual way.
What exactly is mass from a Higgs boson point of view? I mean my layman definition is mass is the amount of matter, but that doesn’t seem like the thing that the Higgs boson is involved in
Characterizing what things "are" in a physical theory is not really possible: ultimately physics is only concerned with (and only has access to) what things do. When we say that "The X-ion has mass m", in the context of a quantum field theory, we mean that
- The magnitude of its 4-momentum is m (better known as E^2 = p^2 + m^2c^4)
- There's an X^2 m^2 term in the Lagrangian of our theory (there are some formal complications here but they're physically irrelevant), which in extremely loose terms means that in the absence of interactions X will behave like a collection of harmonic oscillators with mass m.
- The strength of its interaction with gravitational fields, neglecting relativistic effects, is proportional to m.
and so on for every other place "m" shows up. The meaning of mass, if you want to assign it one, is the correspondence between all these different quantities. And their meanings, in turn, are other correspondences, all the way down. To get actual physical meaning out, you need to find some place in the theory which you can identify with observational results (the typical case for particle physics is scattering amplitudes and collider experiments).
Ideally this is enough of a foothold to interpret everything: the readouts when we objects a and b together match the calculated scattering amplitudes for particles A and B, so we hypothesize that a is an A and b is a B. Now if we smash objects b and c together and get results that match theoretical predictions for B and C, we can be more confident b is a B and hypothesize c might be a C, and so on. But there's no law that says this must be possible, and no guarantee that every term in our theories points to some particular thing in the world.
> In other words, if dark matter didn't interact with/through the Higgs field, it wouldn't explain the observational anomalies
That conclusion seems premature to me. Not only because we lack a quantum theory of gravity but also because (classical) gravity doesn't care where your particle' mass comes from. Heck, it mostly looks at your particle's total energy (mass + kinetic energy + addition d.o.f.) to determine how spacetime should curve.
Besides, as the sibling pointed out, the Higgs mechanism is only responsible for generating the mass of the W± and Z bosons. Other masses are unaccounted for and have nothing to do with the Higgs.
An experimentalists would tell you its still an open possibility because we haven't seen enough Higgs particles to claim we know how they all decay. An anomaly in decay rates, lets say particles disappearing, or taking too long/short to decay might hint at some new physics.
Good question, I've understood dark matter to be a kind of "we haven't proved its existence/discovered definitive examples yet but the existence of dark matter seems to explain many behaviors we see in the universe" thing, so it's possible the true mechanism would "suddenly" be discovered and the hypothesis of "dark matter drives space expansion" be abandoned just as quickly.
I think this is a good example of the whiplash science seems to go through. If you have studied science (or even remember high school science) you know it's the scientific process working as intended. In the eyes of the public, though, science keeps "changing its mind" as though its a monolithic structure of truth which keeps lying to us.
I'm not sure there's a solution to this issue short of science journalism adoption an addiction to qualifying statements -- the ones everyone hates in ChatGPT but from a liability standpoint are required so nobody tries a new home blood chelation therapy hallucinated by an LLM.
And some scientists aren't exactly helping by making it seem as if this inherent conflict between 'this' and 'better' isn't part and parcel of the whole but instead driven by personal conflict. The damage done like that is likely long term fairly massive, hopefully prospective scientists won't be discouraged by these vendetta like phenomenon.
As a complete lay person, I’ve always understood dark matter to be our generation’s aether. Obviously there’s an undetectable medium in space for light to propagate through, how else could it work? We know from all our best observations and models that everything propagates through a medium.
Dark matter has a similar vibe, we need it for our best models measurements and understanding to work. But that doesn’t mean there isn’t a crucial detail we’ll discover later that makes dark matter sound as silly as aether. Or we could find a way to directly detect/measure it! That too would be cool
Disproving the existence of a medium with certain expected properties and behaviours - one of which was that light travelled through it like waves through water.
The reality is that light travels through spacetime and has completely unexpected and non-intuitive properties, one of which is an absolutely constant velocity.
This doesn't mean spacetime isn't a medium of some kind, it means spacetime isn't a medium of any familiar or intuitive kind, and the old waves-on-water metaphor is too simple to explain it.
QFT suggests spacetime is filled with fields of all kinds and particles are excitations of these fields.
But what these "fields" actually are, and what they're made of, and why there are certain kinds of fields and not others, and why they operate with relativistic geometry, is a complete mystery.
No it was a serious theory that seemed obvious and logical at the time. But now it seems so obviously wrong that it’s almost laughable anyone could ever take it seriously.
See also: phrenology. Same phenomenon, different field.
"But now it seems so obviously wrong that it’s almost laughable anyone could ever take it seriously"
Was it really that laughable?
"Whatever difficulties we may have in forming a consistent idea of the constitution of the aether, there can be no doubt that the interplanetary and interstellar spaces are not empty, but are occupied by a material substance or body, which is certainly the largest, and probably the most uniform body of which we have any knowledge.”
(James Clerk Maxwell)
We discovered there is no true vacuum. And we still don't know how exactly electromagnetic waves travel, or how gravitaion works. So the old modell of aether is clearly wrong, but before we have not come up with a consistent model explaining all of it, I would not call it laughable. It made sense at the time. And to me there still is some appeall to the basic concept, that there must be something allowing the spreading of the various waves. Or is that question solved by now?
Perhaps laughable is a stretch. I remember aether as an example of the scientific method working in a high school class. We had a theory, it seemed reasonable, it fit observations, and then we disproved it. This is good.
The laughable part of my memory probably comes from being 15 when I learned about this and the whole class thinking “wow look at those fancy scientists, they didn’t even know basic things we all learn in middle school! ha ha”. Dumb kids be like that sometimes :)
BUT aether was also used as an example of failing Occam’s razor. It added weird unmeasurable just-so variables/explanations to existing theories so they could expand to fit new measurements. This rarely leads to a theory that stands the test of time.
In this way dark matter, in my lay-person view, feels similar. We don’t know what’s going on, so we say it’s gotta be some new “thing” that just happens to be invisible and undetectable except by how it just happens to make our existing models/math/explanations work. That seems fishy to me as a non-expert. Kinda like when an engineer says “i’ve tried everything, it’s gotta be a compiler bug” … it usually isn’t a compiler bug.
edit: Point is that when things don't fit together, just adding more <stuff> rarely works long-term. You need a new model. And personally I'm excited to see what we think about dark matter in 20 years.
> just happens to be invisible and undetectable except by how it just happens to make our existing models/math/explanations work.
That describes all of physics: there's no such thing as "direct" detection in the folk "I know the billiard ball is there because I can see and feel it" sense. It only feels like there are because you've been using your models of human-scale physics for so long that you trust them completely and automatically. The only people operating in everyday life the way astronomers have to professionally are infants: for them, it's not obvious that there are such things as rigid bodies, that sight and touch should correspond in the way that they usually do, and so on. Not because they're stupid, but because if you want to reconstruct the whole world from a bunch of low-rez sense data you're going to need a ton of it, and they don't have that much yet.
> Point is that when things don't fit together, just adding more <stuff> rarely works long-term. You need a new model.
Adding new stuff is coming up with a new model. Particular dark matter candidates aren't things like "there's some stuff out there that makes things happen for no particular reason, case closed": that would get you gently corrected out of an undergrad scientific writing class, let alone a real journal. They're things like
"If we suppose the CP-symmetry violation term of QCD is the result of a spontaneously broken symmetry (resolving the strong CP problem), the allowed masses and EM-coupling strengths of the corresponding pseudo-Nambu-Goldstone boson are consistent with the required properties of cold dark matter, but would give off photons in extremely intense magnetic fields, so here are some proposed experiments:..."
> This rarely leads to a theory that stands the test of time.
1. The simplest explanation is usually the best one.
2. The simplest explanation is usually the easiest to comprehend, discuss, and-- if applicable-- test. This leads to faster iteration times on the path toward a fuller understanding of the phenomenon in question. Consequently, it's best to tend toward the simplest explanation, even if that choice is no better than a coin's flip over a more complicated competing explanation for a particular paper.
Number 2 seems intuitive and supported by the best practices in programming. Well, let's say the least bad practices in programming. :)
Number 1 seems ambiguous enough to lead to some kind of cargo cult. Perhaps greatly increasing iteration time after the simplest explanations have been ruled out? E.g., reluctance to fund research into a more complicated explanation in such a case?
If you think of 'Aether as a possible medium that we haven't discovered yet and that may not exist' then you wouldn't be too far off. The difference between that and pseudo-science is obvious: the existence of Aether can be proven (or disproven), the typical pseudo-science stays very far away from naming any testable hypothesis and instead focuses on how the proponents of pseudo-science are being repressed. But let me point you to a youtube link that will lay it all out in great detail...
Dark matter is just a theory hallucinated by computational models. It is and has always been defined as nothing but "where matter could be to explain the gap between model and data" but no good reason is ever given as to why it could be matter and not anything else. You can do the same thing with the solar system to "explain" a geocentric universe if you wanted.
> no good reason is ever given as to why it could be matter and not anything else
Man, astrophysics and finance are not this forum’s strong suit. See the bullet cluster observations [1]. Explain it simultaneously with most galaxy’s spins, but also NGC 1277’s [2], without using dark matter.
But armchair physicists love to shriek 'bullet cluster' every time because they aren't aware of the extent of their ignorance. Considering your HN bio I'd comfortably classify you as "knows just enough to be dangerous". Fair, given your generalization knowing nothing about me?
More reading for your benefit, from a working astrophysics professor specializing in ultra-diffuse galaxies:
Lots of ground between settled science and “no good reason is ever given.”
There are strong arguments for dark matter (broadly, not just Lambda CDM). They are inconclusive. The bullet cluster is less strong evidence for dark matter than good evidence against common armchair refutations. (I agree with your author that it doesn’t rule out MOND as is commonly claimed.)
The arguments inevitably and only boil down to "look, our model converged". The point is, dark matter is not a theory, just a supposition that can always be "proven" with conveniently arbitrarily flexible models (no one can see it so it might be anywhere!).
There is no positive theory that has been brought out to explain dark matter, only mere deductive hypothesis about where it would be. Deduction needs a culprit so people keep reaching for different kinds of particles. Jury's out on whether they exist but keeps the experimentalists employed, so at least it's worth that much.
There have been actually many positive theories trying to explain what particles dark matter could be, they've all been proven wrong.
To me dark matter came about from this,
"As we understand gravity we can postulate how galaxies rotate given an estimate on its mass, galaxies do not rotate like this"
In this statement there are three elements;
1) Our understanding of gravity
2) Our estimates of the mass of galaxies
3) Our ability to determine how galaxies rotate.
At one point in time this last 100 years, we had "solved" gravity with regards to our solar system, and we were finding so many new particles that #2 seemed like a great explanation. However, we are now left with no room in our understanding of particles, I think its time to look at the other elements.
Put it this way, if we had never observed the galaxy, but developed the standard model in isolation. Then we looked at the stars and tried to define gravity, I'm not sure we'd be so quick to introduce a new type of matter to define gravity.
> There have been actually many positive theories trying to explain what particles dark matter could be, they've all been proven wrong.
Not at all. Ordinary neutrinos and MACHOs (black holes, rogue gas giants, extremely faint dwarf stars) are mostly ruled out. Supersymmetry is not looking promising but certainly not proven wrong. Sterile neutrinos and axions are very much live candidates.
> However, we are now left with no room in our understanding of particles
There's plenty of room. Quantum gravity is obviously the elephant in the room, but even aside from that, and off the top of my head: the standard model doesn't account for neutrino masses ,matter/antimatter asymmetry, or why the lepton masses are related, and it gets the magnetic moment of the muon wrong. The existence of physics beyond the standard model is certain. We just don't know what it is yet.
>Quantum gravity is obviously the elephant in the room,
Isn't that just our understanding of gravity not being complete.
> the standard model doesn't account for neutrino masses ,matter/antimatter asymmetry, or why the lepton masses are related, and it gets the magnetic moment of the muon wrong. The existence of physics beyond the standard model is certain.
That seems to be a big leap, is not the existence of flaws in our understanding of gravity also certain?
> Isn't that just our understanding of gravity not being complete.
No. It's that, but it's not just that: if a QFT in 4-d spacetime has a coupling constant with a negative mass dimension, it has infinitely many free parameters, which means you can only use it below a given energy scale. We live in a 4-d spacetime, and the mass dimension of Newton's constant is -2, so either the true theory is not a QFT, or it's an infinitely complicated QFT we can never actually find.
> That seems to be a big leap, is not the existence of flaws in our understanding of gravity also certain?
Yes, both GR and the standard model are known to be incomplete.
> It's that, but it's not just that: if a QFT in 4-d spacetime has a coupling constant with a negative mass dimension, it has infinitely many free parameters, which means you can only use it below a given energy scale.
Huh.
Do you know of any videos that go into more depth? (My level is the "PBS Space Time" and Sabine Hossenfelder videos as I don't do this professionally).
Fair enough. I think it’s qualitatively different from SUSY, where I tend towards your conclusion of jobs programme masquerading as theory. (I’m much less convinced dark matter is a novel particle.)
> no good reason is ever given as to why it could be matter and not anything else
The options are 1. matter; 2. energy; 3. new physics.
We'd expect pure energy to not stay still for long enough to do anything.
Lots of people are looking for new physics, and would be anyway even if it weren't for all the things that made us look for dark matter in the first place, because of the whole "relativity and quantum mechanics don't play well together, and neither is sufficient by itself" problems.
Dark matter requires something new, given it can't be explained by baryons or black holes, so it too needs some new physics — though as "matter" it would be in the particles-and-fields area rather than the how-does-spacetime-even area which the not-actually-matter solution would be.
Dark matter is more complicated than that. It would be nice if it were as simple as you claim it is, but it isn't.
From the '30s through to the '70s, evidence was piling up that what we saw and what our models predict were incompatible. Zwicky's application of the virial theorem to a galaxy cluster, and various astronomer's calculations of galaxy rotation curves implied that either there was a bunch of stuff we couldn't see, or general relativity was wrong, or both. MOND was born in this era to explain that general relativity was wrong. (note when I say "wrong" I mean in the same way F = Gmm/r^2 is wrong: it's correct in the limit, but wrong in the extremes. Newtonian gravity is wrong at very high acceleration, and MOND implies General Relativity is wrong at very low acceleration) If science had stopped in 1985, you'd be correct: we couldn't tell the difference between dark matter as particles, (CDM, cold dark matter) dark matter as heavy dark objects, (MACHOs: brown dwarfs, black holes) or dark matter as a new gravity model. (MOND: modified Newtonian dynamics)
But science did not stop in 1985.
First and most obvious is gravitational lensing and the bullet cluster. This is well trodden ground, so I won't get too much into it: but the bullet cluster shows us that whatever dark matter is, it has momentum. Some MOND theories do predict something like that, and are compatible with the bullet cluster, other models are not compatible with that, and are falsified by the bullet cluster.
Second is baryon acoustic oscillations. (BAOs) In the few minutes after the Big Bang, the universe was, to a first approximation, a roiling sea of photons. There were electrons, protons, and the odd helium/lithium nucleus, but because charged particles interact via the electromagnetic force, they were being tossed about on the sea of photons. Baryons could not form overdensities because they were charged, and if anything thought about clumping up, the photons would scatter them. But baryons were not the only objects with mass: there was also dark matter. Dark matter could form clumps, and clumps formed by clumping dark matter would be able to clump normal matter. Eventually, the universe cooled enough that normal matter could clump properly, and at the moment the universe cooled enough to be transparent, the cosmic microwave background (CMB) was born. In order for the CMB to look the way it does, there are very tight bounds on how dark matter has to behave, and wouldn't ya know it, these bounds are compatible with the bounds on CDM in order to explain galaxy clusters and galactic rotation curves.
Third is the detection of ultra diffuse galaxies. These galaxies are remnants from a galaxy merger, which spilled off some of its normal matter but none of its dark matter, creating a galaxy with negligible dark matter. In other cases, these collisions create massive blobs of dark matter, but with little to no normal matter. These galaxies falsify MOND. For MOND to be correct, these galaxies cannot exist.
Forth is LIGO/VIRGO and the neutron star-neutron star collision a few years ago and the associated gamma ray burst. Many MOND theories predict that gravity travels slower than light. However, GW170817 shows that gravity travels at the speed of light. Some MOND theories are compatible with this, others are not.
In general, theories of MOND comes in two flavors: those that are compatible with the bullet cluster, and those that are compatible with GW170817. None of them, AFAIK, are compatible with both.
So if you want a MOND theory with no CDM, that's fine, but you have a number of hurdles to jump. You need to create a theoretical framework which is compatible with both the bullet cluster and GW170817, which nobody's been able to do. You need to show that ultra diffuse galaxies are a sensor or interpretation error; those galaxies are significantly closer or farther than currently believed. You need to come up with an entirely new mechanism that explains BAOs. It's not impossible, it is just extraordinarily difficult.
Exactly. You can do all this and you'll end up with a super convoluted theory that basically says: the laws of physics have conspired to make everything look like as if there was dark matter.
The laws of physics (of the time) conspired to make everything look as if there was phlogiston, too.
Is chemistry "convoluted" to you? The concept of oxidation? Combustion? These are big words with lots of implications, are we sure they're warranted given how easy it is to chalk it up to phlogiston?
> The laws of physics (of the time) conspired to make everything look as if there was phlogiston, too.
No, they conspired to make it look as if some substance was transferred between fuel and air during combustion, and between the air and the lungs during respiration, which is true. It happens to be absorbed from the air and not by it, but until and unless you actually devised an experiment to test that, there's no way you could have known. Anyone absolutely convinced of the existence of oxygen in 1600 was being just as unreasonable as someone absolutely convinced of phlogiston - they just got lucky.
The good reason for adding dark matter, that you say is absent, is one that is more a question of philosophy of science, in that adding more mass accounts for the observed behaviours without changing the known laws of physics.
The known laws of physics have been formed on math that checks out and is consistent with all our other observations, and has made many predictions that have checked out and even formed the basis for technology that we use every day.
The way science works is that we form mathematical models of physical behaviour, we test model against real world data, and if the model is consistent with reality, and predicts further behaviours that we then can test for, the theory behind it holds water and we have something to work from.
If you like, you can think of it as building trust in a model, having courage in a theory isn't a mistake, it's how science has been built. Of course finding the errors and new laws is important, but you have to conclusively rule out the established theory first.
This is how we got to Newtonian mechanics instead of firmaments, elements, worlds of forms and mythologies, and how we got to medicine instead of humours, phlegms, biles and alchemy.
Adding mass that can't be seen preserves the body of theory of the standard model and doesn't raise any questions of why GR/QM work correctly for things like GPS etc.
In other words, dark matter is an answer that doesn't require going backwards.
Saying that gravity behaves differently to what we previously thought means that the standard model is only coincidentally right or only right in particular places, and from there where does the scepticism end? Where do you even start unravelling the tapestry?
Think about it in a diagnostic analogy. If your patient is critically ill, and you don't know why, you will prioritise testing for conditions that fit the symptoms and can actually be treated/cured. Because if it isn't treatable, the truth of what caused it isn't that important.
Occam's Razor as an argument against dark matter, but saying that gravity behaves differently — when our theories do not otherwise predict that it should behave differently — is actually less simple than saying there is more mass than can be detected via EM interaction.
The other point I would make is that dark matter can explain most if not all of the otherwise problematic observations, which makes it preferable over modifying the laws of physics, because as I understand it, doing this doesn't account for all of the problematic observations.
Again with an analogy to medicine, it is less likely to be three unrelated, coincidental conditions in one patient than a single condition if both diagnoses explain the same symptoms.
To frame everything I've said in a medical analogy, we have essentially treated for the condition we think it is, and we're trying to figure out why the condition has presented differently to typical cases, rather than ruling out the diagnosis and saying it is something else entirely — because the treatment is working. That is, the empirical evidence we have suggests that our diagnosis is correct, but we don't know everything there is to know about the condition.
Our GPS works, gravitational lensing has been observed, gravitational waves have been detected, we power our homes with nuclear reactors, we calibrate our most accurate clocks based on quantum mechanics, and so on. Particles we then predicted would exist have since been detected.
The patient had a fever. We treated the patient with antibiotics, and they got better, so we have reason to believe it's a bacterial infection — we just can't see the bacteria in the blood work. So the next logical step is to think of what presents and responds like a bacterial infection but isn't bacterial, or otherwise speculate that we have discovered something that does this, rather than question whether we understand the human body or whether thermometers work.
If in trying to confirm this discovery, we find that actually we don't really understand the human body or that our instruments are broken, that's when we should start looking to re-assess the laws of physics.
The standard model has no useful purpose if we don't place some trust in it to find new things. If we threw out our scientific models every time we encountered something we weren't expecting, we wouldn't make any progress at all.
The only reason dark matter raises so many eyebrows is (a) the popular press just loves to pick at it because it's an easy target with great headlining when your scientists are saying the majority of the universe is "missing"; and (b) because the breakthroughs of today are framed as being incremental compared to the big eureka moments of the 19th and 20th centuries which saw us leap from Newtonian mechanics to GR and QM. But between Newton and Einstein et al, there were centuries of incremental refinements/improvements on Newtonian mechanics and early modern astronomy, so why is there such impatience because we haven't found dark matter in barely a hundred years?
By definition, dark matter is going to be hard to detect because the only means of detecting it is merely enough to suppose that it exists, i.e. it interacts gravitationally but not electromagnetically, so we can only detect its gravitational influence on celestial bodies. Of course, it's the odd behaviour of celestial bodies that led us to suspect that dark matter was a thing in the first place, so this isn't very helpful.
I should think that, in order to prove dark matter exists, we shall have to imagine an edge case of what an extreme concentration of dark matter would do to nearby celestial matter and how that might be distinguished from conventional phenomena. Easier said than done, the universe is full of bizarre phenomena, much of which can be explained by GR and QM, and any remainders probably defy any remotely intuitive reasoning.
Alternatively, we shall have to imagine what phenomena might occur in situations where dark matter is absent, and where something can be modelled mathematically as being conclusively due to a lack of dark matter i.e. if gravity were to be different to Newtonian/GR, the phenomenon would never be seen, we can then say with confidence that dark matter is real.
To confirm it beyond any useful doubt, I suppose we would need to create conditions under which dark matter would form and observe the phenomena that occur, or build some kind of instrument that can detect gravitationally as accurately as we can detect electromagnetically. Again, easier said than done, EM force has a fundamental particle that we know very well, while gravity ... well the jury's still out on that. The graviton even if it were a thing would not be a particle in the same way, you can't have a quanta of gravity, when gravity is more of an emergent property of the geometry of spacetime? You can do the math as though it has a force carrier, but this isn't something that you expect to be able to manipulate as a particle in application.
Anyway, I'm away on a pretty hefty tangent now. The point is that it's more constructive to suppose that there is dark matter, since alternative theories (a) also include dark matter, to a lesser extent and (b) do not account for the observed behaviours without in some ways failing to predict behaviours we know and understand with known physics.
> in that adding more mass accounts for the observed behaviours without changing the known laws of physics
I disagree with statement in that I feel this is an incorrect interpretation of what transpired with physics.
Classical physics, modified and tweaked over the centuries, worked well and it’s still valid for the domains where it was already conceived and tested for.
The cracks in the model
formed when we pushed experimental boundaries.
Very high speed and extremely tiny were both new, but most (all?) of the vetted modern models will simplify down to
classical physics when in every day conditions.
The new boundary condition is galactic scale mass and distance but with mostly (?) non-relativistic speeds and probably subtle GR gravity conditions.
MOND? Darkmatter? It’s good science explore all avenues, not shutdown a discussion until conclusive evidence and lack of rebuttal shows otherwise.
Otherwise it’s not science. These days, I’ve started to suspect that it’s not scientists that have such a black and white view and certainty.
> It’s good science explore all avenues, not shutdown a discussion until conclusive evidence and lack of rebuttal shows otherwise.
I couldn't agree more. I don't think anyone should claim to be certain about any of our understanding of the universe. I just say that it isn't very useful to think that way, we make more progress if we have the courage to trust in a theory and dare to be proven wrong, than to go back to the drawing board when we get stuck.
For uncertain things, as long as the courage extends to all feasible (meaning, not outright disproven like flat-earth…) models then we’re in agreement.
What bothers me generally, is how one speculative theory
dwarfs others when the scientists themselves will admit that there’s far
more wiggle-room.
This has implications in funding and brainpower, hindering progress in the long run.
Yes, in case someone asks: The contrarian side where alt-theories include the outright disproven is also worrisome; but in HN at least everyone seems pretty bright and attentive to good science; far smarter than yours truly, certainly.
The flaw in this reasoning is ironically a philosophical (epistemological) one: by what authority is it said we "know" the laws of physics? We "believe" theories, even go so far as to sometimes call them "laws", but as we've seen with Newtonian interpretations (the "law" of gravity) they can obviously be superceded by more elegant, positive (not merely deductive) theories, i.e., general relativity. Who is to say our current understanding is the correct or best one? Granted, it's a good place to start for the experimentalists, but for some reason theoreticians have also drank the Kool-aid rather than honestly examining the other proposed theories.
> The known laws of physics have been formed on math that checks out and is consistent with all our other observations
You will of course note that "all the other observations" conveniently reside in the limit of high-mass-density regions of spacetime, where other theories also expect the current best theories of physics to hold. Where the confusion still lies is in the low-mass-density regions. At least other theories posit some explanation besides "there's still mass it just exists in other dimensions". Sounds like sci-fi crackpottery when put so plainly, but I'm sorry to say this characterization is accurate enough for our needs here.
> where does the scepticism end?
Strange application of slippery slope fallacy. It obviously ends where the theories still hold, i.e., high-mass-density regions. This is IMO enough of a response to most of your "philosophical" arguments.
No one's denying that particles exist. I'm only arguing for a theory that actually posits something besides "oops there's a gap". You've articulated a reason why dark matter is offered but it is nothing more than a deduction about where matter would be should it exist. I swear I'm going to have to spin up some cycles on the cluster to fit dark matter models on a geocentric universe to get you people to understand the non-reality of any dark matter paper.
Gosh I love being mansplained on this site by people who obviously have no personal experience with this stuff. Really gets me going.
Edit to reply to your edit:
> when gravity is more of an emergent property of the geometry of spacetime?
Emergent gravity and dark matter are incompatible theories in their usual forms, though there's probably ways to mash them together into a chimera. I suggest reading more into emergent gravity -- entropic gravity is interesting but still in its early stages. I'm not advocating for any particular theory, just humility from those who repeatedly insist that dark matter is already correct and we just need to find the matter.
> since alternative theories (a) also include dark matter, to a lesser extent
> Gosh I love being mansplained on this site by people who obviously have no personal experience with this stuff. Really gets me going.
I say this in the hope that it's constructive. You should try not to sink to this level of rudeness and assumption of other people's motives/situation/perspectives. Not only is it a rude assumption about something that (to me) looks to be a good faith attempt at conversation that also clearly took some time to compose, but it's an emotional response that shows that you take it personally and emotionally when you're challenged. I'm not implying that GP is an idiot (I'm too ignorant on this subject to know), but I've been challenged in subjects where I'm well versed by idiots many times and I tend to react the same way that you did. It can be enraging (especially when surrounded by down votes and social/group reinforcement from other idiots), but you immediately lose any power of persusasion with other people when you stoop to that level rather than keeping on the high road and keeping it factual/scientific. IMHO you're rarely if ever going to convince the person you replied to, but the third party observers are often much more persuadable. They're the people I mainly try to write comments/replies for.
I'm not responding to the challenge to my points. I'm responding primarily to the infantilizing tone explaining how science works, and especially triggering is how inaccurate it is yet delivered with such confidence. It often feels like commenters want to cosplay intelligence and see how far they can get -- in other words, there's a lot of bullshitters on this website.
If others cannot judge an argument on its merits, that is not really something I can control. I understand your point about rhetoric re: persuasion I'm just resistant to playing civility games in what should be a facts-based discussion.
I acknowledge I am impatient with those who refuse to offer good faith responses. In my opinion such good faith would mean engaging with the facts of the matter not running through a phil.sci. 101 lecture, however sincerely.
Thanks for your engagement. Your point about third parties is a good one, one I keep forgetting and re-learning.
It was not my intention to trigger anyone. I only wanted to say why dark matter is given the time of day. If I came across as condescending or as a know-it-all, I apologise, it was not what I wanted at all.
I also must apologise for my use of the term "emergent property" regarding gravity — judging by your response, I seem to have alluded (unintentionally) to a whole other theory of gravity; I only wanted to say that gravity itself is the curvature of the geometry of spacetime rather than a force in the conventional way it is described.
Also, regarding alternatives still requiring dark matter, it is my understanding that MOND and its derivatives explain galaxy rotation curves but not other phenomena that dark matter is purported to resolve (galaxy cluster formation/structure, gravitational lensing, CMB). If I am wrong about this, I would welcome correction. On the other hand, if your comment simply meant that there are alternatives to DM and MOND that require no DM, fair enough, I should have been clearer and said that some of the foremost competitor theories still require DM.
But I stress again, I am not fighting DM's corner or saying that alternatives are wrong. My stance on it is irrelevant, and I have no more belief in it than any other explanation, belief is irrelevant and doesn't enter into the matter. I was just saying that I understand why a theory that inflates mass arbitrarily, and understandably ruffles some feathers as a result, is given any credence at all.
Personally, I understand your frustration with DM, it does not seem like very good science to let unexpected or inexplicable observations make us simply add parameters without making further predictions to test if that's the right thing to do. Does seem like we're manipulating facts to fit the theory where we should be altering the theory to fit the facts.
Since DM is a substance that, for all intents and purposes, defies detection by any means at our disposal, it makes no further predictions, it just lets us push the square block into the round hole — what we should be doing is finding the square hole.
> Since DM is a substance that, for all intents and purposes, defies detection by any means at our disposal, it makes no further predictions
This is completely untrue. All serious dark matter candidates are observable. For example:
- MACHOs should show up in gravitational lensing surveys. We did the surveys, they didn't, MACHOs were rejected. Exactly the way it's supposed to work.
- Axions convert to photons in sufficiently intense magnetic fields. ADMX has ruled out part of the parameter space for axions and is undergoing upgrades to test the rest of it.
- Other WIMPs still interact via the weak force, and therefore with nucleons. There are many experiments looking for WIMP scattering. A few of them have gotten signals but not enough to be convincing.
Dark matter candidates are not just "mass with no further properties" sitting out there to make the model fit. They're proposed extensions to the standard model (which is nothing but proposed extensions to quantum electrodynamics which ended up working out), and therefore very tightly constrained by the standards of any other scientific field.
Unfortunately those are all candidates which are conjured ex post facto to explain the "mass with no known properties" that is inferred. As you say, none of them are convincing. It's also just bad science to reach for factors that are just-so explanations of the observed phenomena.
They are not. MACHOs definitely exist, it just turns out there aren't nearly enough of them. Axions were proposed as a solution to the strong CP problem years before anyone went looking for dark matter candidates. Sterile (i.e. right-handed) neutrinos are motivated by the need to explain why left-handed neutrinos, contrary to the predictions of the standard model, have mass. Supersymmetry was originally an attempt at strong-electroweak unification.
At no point did I say the laws of physics are beyond question or doubt, and neither did I say dark matter is correct.
I just said it makes more sense to give DM priority because what is the point of having a model if you don't start from it.
Also why are you being so hostile towards me exactly? I didn't express any love for dark matter or the standard model, and I am not a fan of perpetuating a status quo or any form of academic dogma.
All I said, quite unsuccessfully it would seem, is why people think its likely for dark matter to be there — because if you add in "invisible" mass with existing laws of physics, you get something that looks like what we see through our telescopes. I think many would prefer to suppose that there is non-EM-interacting mass (a lot of it apparently), than there being as yet unknown behaviours of gravity/spacetime geometry, which we like to think we understand pretty well.
Though I agree that we don't understand the universe as well as we like to think; and that the universe is not intuitive at all most of the time; and a preference based on how intuitively likely something seems is irrelevant to what the truth will turn out to be.
Edit: just FYI, I am not downvoting your responses by the way. I am not bothered if you dislike me or disagree with things I have said, though these two things should be distinct from each other.
I'm nobody and I didn't seek to "mansplain" anything whatever this term is supposed to mean. I was just talking, always happy to debate, something I thought people came here to do. I won't make the mistake again.
>"there's still mass it just exists in other dimensions"
What? Dark matter is there. Leading models consider it to be particles that don't interact with e/m field and interact weakly with gravity so is undetected in low-density regions. That doesn't make it other-dimensional.
>Sounds like sci-fi crackpottery
Like the prediction of particles in standard model? Is Higgs boson that went undected for 50 years sci-fi crackpottery?
There're issues with dark matter but it also (sadly some may say) happens to be the best explanation of the observed phenomena since all alternative models fall (plus although simpler at first sight quickly get more complex) in more ways than dark matter.
>I suggest reading more into emergent gravity
Emergent (either entropic or induced) gravity has nothing to do with the comment. It's obvious what GP meant. The correct should've been intrinsic property but this is nitpicking.
Being able to fit models is very far from being able to say "it's there", especially the kinds of models that are being fit. See my responses to sibling comments. "Leading theories", especially given the human impulse toward consensus, got the charge of the electron wrong for a long time before the culture readjusted and decided to look fresh at the problem with new experiments.
> happens to be the best explanation
The people who repeat this in popular science are just repeating what they hear from academics who, surprise!, have invested their entire career and reputation in the scientific community on that being true. Science demands more skepticism and interest in the truth than parroting status quo. It's also technically true only if you assume that explanations built on fitting extremely flexible nonparametric models are theories, but that doesn't seem like a mindset that's very interested in those theories representing truth per se.
> Like the prediction of particles in standard model? Is Higgs boson that went undected for 50 years sci-fi crackpottery?
It would be better to rearrange this to "interact gravitationally, and maybe non-gravitationally at the scale of the weak nuclear force".
In the standard cosmology \Lambda-CDM, cold dark matter ("CDM") is allowed to interact at the scale of the weak nuclear force (wnf), which may allow participation in nuclear interactions. This motivates the search for direct detection in various track and scintillator experiments, where a WIMP (weakly interacting massive particle) could take recoil energy from an atomic nucleus, the latter leaving behind a trail of charged particles and/or emitted photons. There are of course other hypothesized interactions between WIMPs and normal matter which do not literally engage the wnf, but instead have some new interaction at no more than the same energy scale ("weak scale"). And for completeness, there are CDM models which have stronger but rarer individual interactions which average out to the weak scale (or effective collisionlessness) across volumes comparable to the size of galaxies or galaxy clusters.
(In the standard cosmology what matters is that the equation of state for dark matter is 0 or close to it so that expansion dilutes it away like cold baryons; in structure formation and galactic dynamics it's more important that most dark matter is in a halo outside the luminous structure. Both are incompatible with decays or collisions which emit relativstic particles (w > ~ 1/6 leads to early evaporation of overdensities; galactic dynamics is less tolerant of (non-radiative) clumping and other mechanisms which concentrate/gravitationally-collapse halos).
> interact ... with gravity
In a system of coordinates that absorbs linear and angular momentum, all matter -- dark or otherwise -- interacts gravitationally in proportion to its mass (or energy-density).
In General Relativity, this is the universality of free fall, which descends from Newton's Law of Universal Gravitation.
Unless we modify or abandon General Relativity, a dark matter particle and an alpha particle occupying the same starting point and having the same initial velocity will follow the same trajectory together forever barring some interaction (examples: intrinsic: one of the particles decays emitting radiation, or there is some weak scale interaction between them that imparts a recoil on one or both of the initial alpha or DM; extrinsic: scattering of a photon off the alpha, or the alpha captures electron(s) imparting a recoil -- a force that shoves the alpha onto a different trajectory, a non-gravitational acceleration). This is the weak equivalence principle (WEP), <https://en.wikipedia.org/wiki/Equivalence_principle#The_weak...> from a somewhat Fermi-Walker perspective; the "weak" in WEP is unrelated to the weak nuclear force.
Finally, dark matter and baryonic matter (and all other matter and radiation) interact the same way with the curvature of spacetime. If dark matter and baryons interact non-gravitationally at all, the (averaged) interaction is at no more than the weak scale. \Lambda-CDM does not require any non-gravitational interaction between CDM and baryons.
Dark matter is largely an instrumentation problem. The universe is very big, and direct observation of individual objects is largely limited by distance/luminosity, which is problematic when you are looking for lots of old dark stars very far away. The history of astronomy is largely a process of deriving estimates of new phenomena from
what we can observe, and thus underestimating the significance until a better instrument comes along (as at first we only can see the larger objects, which often results in undercounting).
Dark energy is partly instrumental, but largely a theoretical gap in understanding. God only knows what is going on there.
The problem of dark matter isn't best thought of as 'dark matter is theorized as an explanation for observations'. It's better thought of as 'observations of the mass of matter in the universe are inconsistent with observation of the amount of light coming from matter in the universe'.
Matter generally does two things: interacts with other matter through gravity, because it has mass; and emits EM radiation, because it has temperature.
When you look at galaxies and try to figure out how much matter they contain, if you look at the gravity, you get one number for mass that implies one quantity of matter; and if you look at the EM radiation, you get a smaller number that implies a lower quantity of matter.
So the conclusion is, there must be some matter that's causing the gravitational effects, but that's not emitting any EM radiation. Matter that is dark. Dark matter.
I blame George Lucas for a lot of this confusion. When people hear 'dark matter' they think it's dark in the sense of 'mysterious'. It's way more literal than that. It's just matter that's dark.
"Cold" in this context means "a typical particle is moving slow enough that we can safely neglect relativistic effects". Hydrogen plasma on the surface of the sun is "cold", in this sense. Solar neutrinos are not.
I mean, it could be - but you'd need to come up with an explanation for why it isn't warming up at all. Most matter reaches an equilibrium temperature where the outgoing EM radiation equals the incoming EM radiation. This matter isn't doing that.
There are areas of the universe that are "voids" where there's almost nothing, maybe that's where this cold/dark matter is? There's nothing nearby to warm it up? Though this article seems to be specifically about the Milky Way.
Most of the study around dark matter is in the context of galaxies because that's where the gravity/light mismatch occurs. The rotation of galaxies (as influenced by the gravity of the matter in them) indicates there's more matter than what we can see via the light they emit.
That mismatch when observing galaxies is the whole reason we think dark matter is a thing.
Most of the evidence is localized around galaxies (not just rotation curves, but also radial velocities and excess gravitational lensing), but not all. Even uniformly distributed dark matter would still show up as a contribution to the mass density of the universe, which curvature measurements indicate is much higher than baryonic matter alone can account for.
There are potential dark matter candidates are hot like new kind of neutrinos. But observations have ruled those out so the remaining candidates are mostly cold.
Most of the dark matter candidates are not ordinary matter but particles that don’t interact with ordinary matter.
This is unlikely. There are many many theories of how dark matter could be accounted for with ordinary matter that we simply can't detect because our telescopes/etc aren't good enough.
Not all of these theories are completely excluded yet but most have very very thin margins of phase space left to explore (even when combining multiple explanations together). Every time a new telescope comes online we see the phase space diminish rather than hints towards first observations.
We are left with:
A) new particles that don't (or very weakly) interact with electro magnetic fields.
B) New theories of gravity.
C) New theories of the early universe that open up phase space previously thought closed to existing matter contributions
D) better instrumentation that sees actual contribution in the tiny phase space left to ordinary matter and ordinary physics
D is by far the least interesting of these options and so gets very little press. But it gets plenty of academic attention and you can be assured it is not being ignored by scientists
No, the current most reasonable explanation for the dynamics of the bullet cluster are merely consistent with our current best dark matter theories. It is suggestive at best.
That's not what 'Dark' means in this context. It's not that it is too far away to be detected, it is non-luminous (as in: not emitting any electromagnetic radiation) and so it isn't detectable other than by its secondary effects on other objects.
Black holes are another example of something that we can not directly observe using instruments, but that we can observe through their secondary effects. But black holes are part of the cosmic accounting book in an identified manner, dark matter is not.
>If it’s pulsars that are truly generating the positrons that could be responsible for the signal that cosmic ray experiments are seeing [...]
>Whenever there’s an unexplained phenomenon that we’ve measured or observed, it presents a tantalizing possibility to scientists: that perhaps there’s something new at play beyond what’s presently known.
(Is it me or this paragraph that sentence is taken from has a strange flow in it?)
>However, we cannot claim evidence for a new discovery until we first scrupulously and quantitatively account for everything that represents the physics and astrophysics of what’s already known.
YouTube is surprisingly unusable for indexing historical content. There used to be alternative UIs for the site but I assume Google over time killed those off while they dark patterned their own UI into being a TikTok stream of consciousness UI. I don’t mind that flavor for discovery of new content, but it is surprising that they seem to intentionally make older content inaccessible.
PBS space time is a perfect example of how badly it interferes with the quantity of the content. The episodes cross link and build on each other, but finding X episode about Y is almost impossible. You can’t apparently even search a single channels content by string.
Pulsars(rotating neutron stars) and black holes are two possible outcomes when a large star is burned up and implodes. When there’s enough mass, you get a black hole. When there is less, you can get a neutron star: all mass converted to neutrons, as tightly packed as possible. When it rotates, it’s rotation frequency is observable as a train of pulses. A quasar is essentially a galaxy, (long ago and far away), with a supermassive black hole at it’s center. The black hole swallows a lot of mass, causing a lot of high energy radiation, which is how why in the beginning they were mistaken for strange quasi-stars, hence the name.
I was going to ask whether the article was using the term "pulsar" when "neutron star" would have been more accurate; I would have said that a pulsar was a rotating neutron star with a beam that intermittently points at Earth (making the distinction observer-specific).
But don't essentially all neutron stars spin? I don't know how a non-rotating neutron star might form. If that's the case, they could have just said "neutron star", and I wouldn't then have this issue about the subjectivity of what a pulsar is.
Almost certainly, though they do slow down over time. The distinction is indeed observer-specific, which doesn't matter yet, but will eventually matter for things like the "galactic positioning system" that sometimes gets suggested as a galaxy-wide extension of the current research on pulsar-based_navigation: https://en.wikipedia.org/wiki/Pulsar-based_navigation
The neutron stars that the article uses to explain the observed antimatter all must spin to cause the radiation observed. They would appear as pulsars to some observer, not necessarily us here on earth.
To be clear, neutron stars and back holes do not contain all of the mass of the star they were created by. Usually they are only created from the core of the star, but the outer layers get blown into space.
Primordial black holes are dark matter candidate. We should see gravitational lensing events; we see more than expected but not enough. Evidence is inconclusive, and it is possible they exist but not all of the dark matter.
Asked ChatGPT, got these answers but not sure I understand, anyone an expert here who can explain how we can estimate the amount of black holes outside of gravitational lensing?
"The distribution of black holes is inferred from their interactions, like accretion of matter and merging events, which emit observable signals. The total lensing exceeds what's expected from black holes and visible matter. Moreover, lensing often occurs where no black holes are detected, suggesting another form of mass—dark matter—is responsible. The distribution and amount of lensing provide crucial information that, when combined with other observations, suggests the presence of dark matter.
Apologies for the confusion. The distribution of black holes is inferred from observable interactions like accretion and merging events, not primarily from lensing. The lensing exceeding what's expected from black holes comes from comparing the total lensing observed to what could be attributed to known black holes plus visible matter. There's more lensing than can be accounted for by black holes, hence suggesting dark matter as a probable cause."
I’m always a bit unease to click on content from Big Think. Not knowing exactly what the Koch Foundation agenda is feels somewhat unsettling, even though the content seems generally good.
Maybe others can answer this naive question that I have: how do we know that e=mc^2 actually has the correct number of variables? E.g. what if either mass or light speed were composites of some other variables? Say that light speed really had a directional component or a distance from the Big Bang component? Or mass was itself a composite of particles and the forces acting in the nucleus?
Wouldn’t some additional term here be a plausible way of explaining discrepancies in the observable vs predicted universe?
We don't know anything. If you want to know how we are sure of it, we aren't.
But the information that light always move on the same speed, on any direction and for every observer has been tested again and again since the end of the 19th century. Nobody ever saw it fail.
There is of course a huge number of variables nobody tested yet. And we can't know if those are important or not. But we are very logically biased into assuming they aren't until we have a real need for them to explain something.
(Anyway, the thing you are trying to invent is a kind of modified Newtonian mechanics - even though it's not quite Newtonian, but people use that same name. There are other people trying to get a workable theory with similar parameters. Up to now, nobody got any.)
I'm sorry, but I'm not watching a 19 minutes video to discover what you mean by "two-way speed". Can you explain it?
But anyway, we've been comparing the speed of light over different directions since the 19th century. Modern relativity was created because of those measurements, and nowadays you can buy accelerometers and gyroscopes that work based on that constant speed.
It basically means round-trip tests where the source of light and the detector are the same, and the light reflects back from a mirror. One-way would be having the source send the light to the detector directly, but the problem with testing that way comes down to synchronizing the clocks of the source and detector.
I don't know anything about this, I just found a Wikipedia page [1]
Yeah, the one thing we tested again and again is that if there is any difference on the speed of light going and coming back, it must average to the same thing in every direction (actually in every circuit, with any number or directional changes).
There are some experiments that measure one-way speed even on the link you posted. The first measurement of the speed of light was one-way. Those are just low precision ones.
Can't measure one-way speed of light because you need to get the result back somehow (and that's also limited by speed of light). Usually we just send light and wait for it to bounce back.
> we can’t falsify it. According to some HN-er, this must be unscientific
Apparently more than merely "can't falsify it": the maths is such that if it was different, there would never be any observation that could distinguish between them.
That extra step — that it genuinely makes no difference — would, I think, cause it to be unscientific.
At least, unscientific within relativity; given relativity can't be the whole picture (singularities popping out of maths that presumes spacetime is differentiable), we may well find another better model that would allow us to make the test, which in turn allows us to ask the question scientifically.
That said, after watching the Veritasium video ages ago, I was immediately asking myself: what about the CMB? Surely that would have a stonking big anisotropy if the speed of light wasn't close to the same in all directions? (It does have one which is assumed to be because of our motion relative to it, and I don't know enough to guess what to expect of a different-way-speed-of-light-anisotropy other than its existence, and therefore cannot compare and contrast with the observed one).
> I was immediately asking myself: what about the CMB? Surely that would have a stonking big anisotropy if the speed of light wasn't close to the same in all directions?
No, you can always cancel it out by making the whole rest of physics suitably anisotropic too. Which would, of course, be ridiculous.
changes in the speed of light would change spectroscopy, it really isn't observed in our cosmic neighborhood (maybe in the very early universe at very high redshift).
Your what ifs are theories that have been or are being explored. But so far the predictions they made either have been falsified, or they haven’t made any that would make them more plausible overall than the standard model.
For any such ideas that come to mind, you can be pretty sure that many, many physicists have already gone through them.
I was going to comment this as well. Here is a thorough explanation of the missing term: https://physics.stackexchange.com/questions/143652/is-e2-mc2...
The thing that I love about this formulation of the equation is that it directly ties it to the Pythagorean theorem: c^2 = a^2 + b^2 . It really shows you just how pretty that equation really is!
the m in that equation is the rest mass (m_0) which is a proper scalar.
the m in E=mc^2 is the relativistic mass which depends on rest frame and therefore isn't a proper scalar quantity and that isn't a proper tensor equation.
the (E, px, py, pz) energy-momentum 4-vector is a tensor that transforms to any reference frame with a lorentz transform. all observers will agree on the shape of that 4-vector object, although they'll measure the components differently. that equation is just the dot product (rearranged), where the rest mass is the length of the 4-momentum vector (and a proper scalar quantity that all observers agree on is exactly that value in all reference frames). the rearrangement introduces a negative sign which actually comes from the SR metric -- usually diag(-1, +1, +1, +1) but sometimes diag(+1, -1, -1, -1).
But even if say energy or the speed of light are derived from other parameters, that doesn’t make the equation wrong, just incomplete. Anyway for all the phenomena we don’t have complete explanation for, none of them give any indication relativity is incorrect, and therefore there’s no reason to suppose tweaking it would help explain them.
It's a salient question. We don't know. There's still physics we don't fully understand (e.g. dark matter/energy) which could end up requiring adjusting or explaining further out understanding of the constants in e=mc^2 for example.
