[HN Gopher] Pulsars, not dark matter, explain the Milky Way's an...
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Pulsars, not dark matter, explain the Milky Way's antimatter
Author : PaulHoule
Score : 189 points
Date : 2023-10-01 13:00 UTC (9 hours ago)
(HTM) web link (bigthink.com)
(TXT) w3m dump (bigthink.com)
| forgotpwd16 wrote:
| 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.
| perihelions wrote:
| 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...")
| xqcgrek2 wrote:
| Dark matter people can always wiggle out of any non-detection
| or standard physics models by adding more parameters.
|
| They've been doing this for 40 years and at this point it's
| just sad and pathological science.
| Obscurity4340 wrote:
| What is the relationship between pulsars and quasars and also
| black holes?
| superjan wrote:
| 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.
| pitaj wrote:
| 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.
| denton-scratch wrote:
| > Pulsars(rotating neutron stars)
|
| Thanks.
|
| 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.
| ben_w wrote:
| > But don't essentially all neutron stars spin?
|
| 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
| superjan wrote:
| 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.
| Obscurity4340 wrote:
| Thanks for edifying me, I appreciate it
| Frankmartin321 wrote:
| [dead]
| thedudeabides5 wrote:
| Couldn't it also be there's a lot more black holes than we
| thought?
| ianburrell wrote:
| 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.
| thedudeabides5 wrote:
| 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."
| goller wrote:
| The author, Ethan Siegel, has a really great podcast[1] I started
| listening to during covid. Highly recommended!
|
| [1]: https://www.startswithabang.com/podcasts
| sockaddr wrote:
| I LOVE his content, super knowledgeable, friendly to guests,
| interesting topics, good production quality, everything. But I
| have one question:
|
| Does he ever stop to breathe when he talks?
| ck2 wrote:
| if you want a crash course, PBS Space Time is very high quality
| education
|
| https://www.youtube.com/@pbsspacetime/search/?query=dark+ant...
|
| (just wish I could sort by date on youtube, bet that's on
| purpose)
| devoutsalsa wrote:
| You can sort by latest & oldest. Oldest ad a sort option was
| removed for a while, but it's back.
| layer8 wrote:
| The problem is navigating the middle range.
| fnordpiglet wrote:
| 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.
| 0967604717 wrote:
| [flagged]
| tleilaxu wrote:
| Is this really such a definitive result, as the source suggests?
| forgotpwd16 wrote:
| >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.
|
| The source doesn't show it as definitive.
| I_Am_Nous wrote:
| 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.
| jacquesm wrote:
| 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.
| Swizec wrote:
| 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
| tekla wrote:
| Are you trying to discount Aether as a pseudoscience versus
| a serious theory?
|
| You're italicizing "Obviously" which makes me think that.
| Swizec wrote:
| 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.
| j16sdiz wrote:
| Literally every Lorentz-violating theory introduce an
| aether like term.
|
| And, yes, they are published on peer reviewed journals
| every years. Not mainstream, but there are reputable
| physicist taking it seriously
| jacquesm wrote:
| 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...
| hutzlibu wrote:
| "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?
| Swizec wrote:
| > Was it really that laughable?
|
| 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.
| miramba wrote:
| I never heard of aether before, so here is a reference for
| anyone else:
| https://en.m.wikipedia.org/wiki/Luminiferous_aether
| nobodyandproud wrote:
| That's surprising.
|
| Proving/disproving the existence of a medium was the
| whole point of the Michaelson-Morley experiment.
| TheOtherHobbes wrote:
| 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.
| Cacti wrote:
| 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.
| jacquesm wrote:
| 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.
| Misdicorl wrote:
| 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
| jameshart wrote:
| 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.
| SoftTalker wrote:
| So it's cold matter, basically? Matter that's at or close
| to absolute zero temp?
| ianburrell wrote:
| 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.
| jameshart wrote:
| 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.
| SoftTalker wrote:
| 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.
| jpk wrote:
| 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.
| JumpCrisscross wrote:
| > _Dark matter is largely an instrumentation problem_
|
| The bullet cluster refutes this hypothesis [1].
|
| [1] https://en.m.wikipedia.org/wiki/Bullet_Cluster
| Cacti wrote:
| 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.
| JumpCrisscross wrote:
| I'm not claiming those observations prove dark matter.
| I'm saying they are problematic for the claim that this
| is an instrumentation issue.
| JumpCrisscross wrote:
| > _the hypothesis of "dark matter drives space expansion"_
|
| This is dark energy [1].
|
| [1] https://en.m.wikipedia.org/wiki/Dark_energy
| uoaei wrote:
| 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.
| nwallin wrote:
| 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.
| mr_mitm wrote:
| 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.
| uoaei wrote:
| 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?
| ben_w wrote:
| > 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.
| wanda wrote:
| 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.
| nobodyandproud wrote:
| > 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.
| wanda wrote:
| > 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.
| nobodyandproud wrote:
| 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.
| uoaei wrote:
| 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
|
| No they don't.
| freedomben wrote:
| > _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.
| uoaei wrote:
| 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.
| wanda wrote:
| 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.
| wanda wrote:
| 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.
| forgotpwd16 wrote:
| >"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.
| uoaei wrote:
| 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?
|
| Transparent straw man. Barely worth acknowledging.
| JumpCrisscross wrote:
| > _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.
|
| [1] https://en.m.wikipedia.org/wiki/Bullet_Cluster
|
| [2] https://en.m.wikipedia.org/wiki/NGC_1277
| uoaei wrote:
| Far from settled: https://arxiv.org/abs/1003.0939
|
| 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:
|
| https://tritonstation.com/2016/12/23/crater-2-the-bullet-
| clu...
|
| https://tritonstation.com/2017/03/06/lcdm-has-met-the-
| enemy-...
|
| https://tritonstation.com/2016/07/30/missing-baryons/
| [deleted]
| JumpCrisscross wrote:
| 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.)
| uoaei wrote:
| 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.
| jboy55 wrote:
| 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.
| JumpCrisscross wrote:
| 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.)
| ksherlock wrote:
| It's a better explanation than dark matter (aka cosmic fudge)
| mannykannot wrote:
| As it shows the observations of positron flux are consistent
| with well-established physics, I would guess that it is pretty
| solid.
|
| To be clear, this result does not rule out some form of dark
| matter as the cause of the phenomena it was posited to explain
| in the first place.
| bizzahbatool wrote:
| [dead]
| [deleted]
| perihelions wrote:
| https://arxiv.org/abs/1903.05647 ( _" Detection of a g-ray halo
| around Geminga with the Fermi-LAT and implications for the
| positron flux"_)
|
| ( aka. doi.org/10.1103/PhysRevD.100.123015 )
| tshadley wrote:
| So this is old news?
| forgotpwd16 wrote:
| The study up to which article covers, yes. But it's still an
| open research topic.
| tambourine_man wrote:
| 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.
| PaulHoule wrote:
| ... 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
|
| https://profmattstrassler.com/articles-and-posts/relativity-...
| sigmoid10 wrote:
| 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.
| codethief wrote:
| Why through the Higgs?
| abakker wrote:
| 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?
| tux3 wrote:
| This doesn't really answer your question, but there is already
| an extra term! (only sometimes it is zero)
|
| E^2 = (mc^2)^2 + (lc)^2
| lamontcg wrote:
| 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).
| spanglasaur wrote:
| I was going to comment this as well. Here is a thorough
| explanation of the missing term: https://physics.stackexchang
| e.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!
| simonh wrote:
| Distance from the Big Bang? So zero then.
|
| 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.
| [deleted]
| johndunne wrote:
| 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.
| FetusP wrote:
| Intro college Physics is reformatting formulas to solve for
| variables in other formulas, and inserting them into each
| other.
|
| So even if c = (w+x+y+z), e=mc^2 and e=m(w+x+y+z)^2 would still
| be equivalent.
| layer8 wrote:
| 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.
| abakker wrote:
| Oh, I was not trying to be original or second guess experts.
| It's just hard to google a concept like this and get useful
| answers.
| marcosdumay wrote:
| 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.)
| V__ wrote:
| > light always move on the same speed, on any direction and
| for every observer has been tested again and again
|
| I might be wrong or misunderstanding, but I think that hasn't
| actually been tested ever. Only two-way speed of light is
| known. [1]
|
| [1] https://www.youtube.com/watch?v=pTn6Ewhb27k
| marcosdumay wrote:
| 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.
| ajuc wrote:
| > Can you explain it?
|
| 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.
| j16sdiz wrote:
| We can measure the speed from A to B back to A.
|
| We can't measure the speed from A to B directly (without
| sending something from B back to A wrt speed of light).
|
| https://en.wikipedia.org/wiki/One-way_speed_of_light
| bitvoid wrote:
| 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]
|
| 1: https://en.m.wikipedia.org/wiki/One-way_speed_of_light
| marcosdumay wrote:
| Oh, ok.
|
| 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.
| j16sdiz wrote:
| The two way speed are the same along the y axis versus the
| x axis. We have no ways to measure the one way speed.
|
| we can't falsify it. According to some HN-er, this must be
| unscientific
| ben_w wrote:
| > 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).
| lamontcg wrote:
| 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).
| tekla wrote:
| Everything you've proposed has already been tested non stop for
| a hundred years.
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