The Murky NY Times Op Ed on Dark Matter

Appropriate for General Readership

[Apologies: due to a computer glitch, the figure in the original version of this post was not the most up-to-date, and had typos, now fixed.]

On Tuesday, the New York Times Editorial page ran an Op-Ed about dark matter… and although it could have been worse, it could certainly have been better.  I do wonder why these folks don’t just call up an expert and confirm that they’ve actually got it right, before they mislead the public and give scientists a combination of a few giggles and a headache.

Here is the last paragraph from the Times:

This experiment is probing a major hole in the way we understand the cosmos. Roughly speaking, the force of gravity in the universe can be explained only by a corresponding amount of mass, or matter. Some undiscovered mass — dark matter — must exist in order to explain gravity, but no one has seen any traces of it. Those traces, when they are finally found, will be exotic particles left over from the Big Bang. In the tale we tell about everything we know, scientists have now brought us to the edge of the deep, dark woods. They, and we, are waiting eagerly to see how the rest of the story goes.

Ok, out comes the professorial red pen.

First, a relatively minor point of order. “…the force of gravity in the universe can be explained only by a corresponding amount of mass, or matter…” This isn’t great writing, because mass and matter are not the same thing. Matter is a type of substance. Mass is a property that substance (including ordinary matter, such as tables and planets) can have. Mass and matter are as different as apples and applets. You can read about these distinctions here, if you like. The author is trying to evade this distinction to keep things simple: the more correct statement is that gravity (in simple circumstances) is a force exerted by things (including ordinary matter) that have mass.

But here’s the real offending remark: “Some undiscovered mass — dark matter — must exist in order to explain gravity, but no one has seen any traces of it.” Dark matter is most certainly not needed to “explain gravity” in some general way; there’s not one bit of truth in that remark. For instance, the gravitational pull of the sun on the earth (and vice versa), and the pull of the earth on you and me (and vice versa), has absolutely nothing whatsoever to do with dark matter, nor is dark matter needed to explain it.

What the author should have said is: since the 1960s we have known that gravitational forces on large astronomical scales seem to be stronger than we can account for, and so either our equations for gravity are wrong or there is matter out there, pulling on things gravitationally, that we cannot see with any type of telescope.  The reason the latter possibility is taken more seriously than the former by most experts is that attempts to modify gravity have not led to a convincing case, while the evidence for additional “dark” matter has grown very strong over recent decades.

Here’s one of the several arguments that suggest the possibility of dark matter… the simplest to explain. Experts study the motions of the stars in our own galaxy — the star city known as the Milky Way — and also study the motions of stars in other galaxies. [The overall motions of galaxies themselves, inside giant clusters of galaxies which can be found in deep space, are also studied.] Now what we ask is this; see Figure 1. Supposing all of the matter that is out there in the universe is of a type that we can see in one way or another: stars, gas, dust of various types. Then we can figure out, just by looking with a telescope and doing simple calculations, roughly how much measurable matter is in each galaxy, how much mass that matter has, and where it is distributed inside the galaxy.  We can next use that information to figure out how hard that matter pulls on other matter, via the force of gravity. And finally — crucially! — we can calculate how fast that pull will make the matter move, on average.  And what do we find when we measure how fast the stars are moving? Our calculations based on the matter that we can see are wrong. We find that the stars in the outer edges of a galaxy, and the galaxies inside clusters, are moving much, much faster than our calculation predicts. (This was discovered in the 1960s by Vera Rubin and Kent Ford.)  It’s as though they’re being pulled on by something unseen — as though the gravity on the stars due to the rest of the galaxy is stronger than we’ve guessed. Why is this happening?

Fig. 1:

Fig. 1: One of several lines of evidence in favor of the hypothesis of dark matter is that stars in the outer regions of galaxies move much faster than would be the case if the galaxy was made only from what we can see.

One possibility is that there is matter out there that we can’t see, a lot of it, and that matter is inside galaxies and inside clusters of galaxies, exerting a pull that we haven’t accounted for properly. A huge “halo” of dark matter, in this view, surrounds every galaxy (Figure 2).

Clearly, this isn’t the only logical possibility. Another option is that there could be something wrong with our understanding of gravity. Or there could be some other new force that we don’t know about yet that has nothing to do with gravity. Or maybe there’s something wrong with the very laws of motion that we use. But all attempts to make sensible suggestions along these lines have gradually run into conflicts with astronomical observations over the recent decades.

Fig. 2:  The visible part of every galaxy is believed to lie roughly at the center of a much larger halo of dark matter.

Fig. 2: The visible part of every galaxy is believed to lie roughly at the center of a much larger halo of dark matter.

Meanwhile, during those last few decades, a simple version of the “dark matter” hypothesis has passed test after test, some of these tests being very complex and subtle. For example, in Einstein’s theory of gravity, gravity pulls on light, and can bend it much the same way that the lenses in eyeglasses bend light. A galaxy or galaxy cluster can serve to magnify objects behind it, and by studying these lensing effects, we again conclude there’s far more matter in galaxies and in clusters than we can see.  And there are other arguments too, which I won’t cover now.

So while an explanation for the fast motion of stars inside galaxies, and galaxies inside clusters, isn’t 100% sure to be dark matter, it’s now, after many years of study, in the high 90%s. Don’t let anyone tell you that scientists rushed to judgment about this; it has been studied for decades, and I can tell you from experience that there’s a lot more consensus now than there was when I was an beginning undergraduate 30 years ago.

Those traces, when they are finally found, will be exotic particles left over from the Big Bang.” Will they? Will the dark matter turn out to be particles from the Big Bang? Not necessarily. We know that’s one possibility, but it’s not the only one. Since I explained this point last week, I’ll just refer you to that post.

Now here come the big meta-questions: should the New York Times be more careful about what it puts on its editorial page? Should its editors, who are not scientists, talk broadly about a subtle scientific topic without fact-checking with an expert? What are the costs and benefits when they put out oversimplified, and in some ways actually false, information about science on their editorial page?

72 responses to “The Murky NY Times Op Ed on Dark Matter

  1. Perhaps the solution to journalists publishing misleading and imprecise scientific articles is a certified certificate program in science journalism, which could be offered by various colleges and universities, or even conducted at the news agencies. Editors would require such certification of their science writers. The editors would also be required to have this certification.

    • Of course we all know that people with degrees and certificates in science (never mind “science journalism”) never write (or, God forbid, publish) nonsense (and those of us who grant such certificates are particularly sure of that – after all, it’s one of the things that makes our job important). Well, some might say, there is “wrong” and there is “not even wrong” and replacing the former by the latter (particularly when enhanced by an impressive sounding degree) may not necessarily be a change for the better.

  2. Would the world expert allow a lay person to ask a simple question ?
    All of what you said here does not PROVE that DM IS the solution , all evidences are converged into DM only in our mind , but all that evidences could very easily fit a new modified theory of gravity , not Mond , by something much more sophisticated ….
    We are missing something conceptual here of which if found ,DM , as a solution will be obsolete ….so maybe you are repeating what NYT is doing.

    • You are constantly using the word “proof” (or in this case prove) as if you had no idea of what it means. Nothing is ever strictly “proved” outside of mathematics (and the significance of mathematical proofs in dealing with the physical world is arguable). Instead in the real world one lives by computing and often simply “judging” probabilities. The probability that among the many wannabe Einsteins that one comes across on physics blogs (I mean particularly the completely unknown ones who claim that in their spare time they have worked out the Theory of Everything that all the recognized “world experts” have missed) there is a genuine case is no doubt greater than zero, but it would be foolish for anyone to bet anything (including any valuable time) on that.
      The same applies to the probability of your judgement being better in this case being better than that of the “world expert” you are addressing.

  3. The fact Matt. As per today is simply that NO one can adopt ANY solution w.r.t. The behavior of the universal entities be it DM , Mond or you name it,
    It is simply too early, saying otherwise is mere prejudice.

    • Carl 'SAI' Mitchell

      DM has the problem of not having been directly observed in a convincing manner. But it’s still the best explanation we have for several issues, some of which can’t be explained by a modified theory of gravity. For example, the anisotropy of the cosmic microwave background is not explainable by modifying gravity, and is explained by dark matter. It is possible to have two theories, one for the gravitational effects and one for the nongravitational effects, but that would be more complicated than just assuming the simplest theory that fits all the evidence. So we go with dark matter, for now, until something disproves it, we find some, or we get a better theory that accounts for all observations.

      • That model that can explain all the facts with the least amount of assumptions is the corner stone of the scientific method, Occam’s Razor.

        • I am not convinced that this view is applicable to the concrete issue here. There is no model that really “explains” the astronomical observations; the dark matter hypothesis postulates the existence of some kind of dark matter, but does not explain what this dark matter is and how it came to existence; the term “dark matter” is merely a placeholder for something unexplained.
          So, the question “dark matter” or modified gravity or a third kind of explanation is still open. All plausible hypotheses need to be investigated, without pre-judice, and hopefully more observations can be used to judge about the theories.
          Concerning Occam´s Razor: Nature is not necesarily working in the way that is most easiest and convenient for us humans to understand. (Example: quantum mechanics.) And looking at the pure amount of assumptions makes the implicit, but non-trivial assumption that all assumptions are qualitatively equal. But it is not a priori clear that all assumptions should have the same weight; and there is no obvious reason why 3 assumptions should be worse that 2 when the 3 are of a kind that everyone can agree with, whereas the 2 might cause a lot of headache (as a hypothetical example).
          So, I do not understand why one of the roads should be selected here; all roads need to be checked (and luckily, this is going on).

      • Indeed, in fact, I think the central point is that every known modification of gravity still requires some dark matter to exist (just less of it).

  4. Hi Matt,
    Maybe you should explain what the harm is in journalistic scientific inaccuracies. I think this goes back to a previous discussion regarding popular science, which can be inaccurate or even completely wrong while attempting to engage and entertain the public. This is a very important subject Matt, yet most of the real scientists I’ve read act as if were merely a pet peeve of theirs, or completely beyond their ability to change. I think it is the responsibility of the scientific community, especially since most of their work is publicly funded, to proactively reach out to all forms of media and offer their services. Maybe you could even put together some kind of worldwide scientific advisory council, and offer those resources to the media. Heck, maybe even start a twitter account. The point is, if the scientific community creates a clearinghouse for science information, and get’s enough scientific icons to endorse it so it will have some credibility, then I believe the media will use that clearinghouse. They are, after all, a mangy lazy bunch.

    Thanks so much for your efforts.
    Sam Hawk

    • I do think that blogs already serve some purpose because the science writers, in my experience, read them extensively. The question of education and certification of science writers is something that has to be dealt with by the journalism community; I don’t think scientists can or should address it directly.

  5. Lucretius : Allow me to remind you of one commonsensical point :
    When many roads to reality are open , and no particular one is known to lead to reality , then it is common sense to refrain from adopting any specific
    raod that in the end may lead you astray , so what was wrong with what I said , ? Or maybe it is some kind of Arrogance ? Maybe.

    • When you have to choose a guide, it is better to choose one who has travelled the route before. Perhaps he will not lead you to Eldorado, which might not even exist, but is less likely to leave you in a swamp.

    • Carl 'SAI' Mitchell

      To continue the metaphor:
      You come to a fork in the road. One way is a wide, paved road that leads straight towards the valley you’re trying to get to, and the other way is a narrow dirt path up into the mountains where a blizzard rages. You don’t know that the paved road goes where you want, and hey, there might be a giant chasm in the way that you can’t see and won’t be able to cross, but chances are it’s the better road than the mountain path.
      You pick the road that is most likely to lead to your destination, based on available evidence.

    • aa.sh:

      If you where on the fifth floor of a building, would you exit via the window or elevator?

      Surely common sense dictates we refrain from adopting any specific theroy of gravity so only the arrogant person would opt for the elevator over the window!

  6. kashyap vasavada

    Matt: I agree pretty much with everything you say. But at least now there is more reporting on science than before.I have data going back 50 years!!! If one has to choose between totally ignoring scientific news and technically wrong reporting, I am not sure, which one I would choose. At least reporting arouses some interest for science in public. Most people do not read that carefully anyway. A case in point , in our house we watch NBC evening news. I have never , if ever, seen Peter Williams mentioning about Nobel prizes in sciences. They spend several minutes in mentioning peace prize.

  7. Let’s bear in mind that in the 1930s both Jan Oort and Fritz Zwicky did propose similar ideas regarding “missing mass” in the galaxies, but these assertions were ignored and forgotten.

  8. We could argue that Zwicky was a rather “colorful” character, but he was just as sharp in his predictions.

    Kind regards, GEN

  9. Professor,
    I believe there may be an error in your RSS feed. Your new posts have stopped showing up on my feed reader, but all of my other feeds are working. Also, when I click on the link at the bottom of this page for “RSS-Posts”, I receive an error message.
    If the problem is on my end, I apologize.

  10. It would be interesting to look at the costs of inaccurate public articles like this as I’m not entirely convinced that they are actually that high. I understand that as a physicist who attempts to better our understanding of the world, this level of misinformation can be frustrating. However, there will always be some people (I’d wager many) who don’t understand and do not seek to understand physics at anything close to a high level. From this perspective I’m not sure what strong costs this level of misinformation brings given that those to whom the knowledge is relevant (the curious, students, policy makers, etc) can always fact check with more in-depth sources.

  11. The keystone behind journalism is a thorough checking of facts and sources, so, the NYT and the authors of the op-ed article have no excluse whatsoever, no “matter” (pun intended) how you slice it.

    Kind regards, GEN

    • I’m not sure if your post was a general observation or a response to mine but I certainly agree that this time around the author of the op-ed and the NYT did not do their jobs well. Furthermore, as Matt and others have pointed out this is lack of fact checking is rather widespread at least in physics and some other technical fields. I won’t speculate about its cause of these errors, but I am curious about the negative impact of this misinformation. If, in fact, it is negligible then perhaps it is not worth pursuing a solution (that is, there is no real problem).

  12. Allow me to differ: that could imply that if many engineers start producing sloppy work it OK (which it certainly is not!).

    As a society, we can’t stay put, passive as lambs, when we witness that members of a profession start slacking in their responsabilities.

    Kind regards, GEN

    • Sure, it’s sloppy journalism as it should have an impact on the reputation of the NYT among those who know better but that’s all.
      When an engineer builds a bridge or a house that collapses and kills someone, that is a different matter altogether.

    • I guess I wasn’t as clear as I could be. My point was that its a matter of scope. If engineers make critical errors that impact many people then that is obviously not okay. However, if engineers make errors that affect no one then it’s not a serious problem in a practical sense (although it remains a fact that they made professional errors). In the same way, I was questioning the implications of this systemic problem in scientific journalism. I’m not giving journalists a free pass, I’m trying to assess the damage and look at whether a system level change in how journalists approach science articles is worth the effort.

  13. Okay, agreed. Responsible scientific journalists should seek out experts before creating blather, but a real problem here is that in some cases I think the journalists actually believe they’re doing this.

    Trust me, I am the last person to defend the media, in fact I spend a great deal of my time discrediting their stories in public fora. However, if the media is not sufficiently knowledgable to distinguish “truth” from “fiction”, what would make anyone think that they’re all of a sudden capable of good judgment when it comes to identifying proper scientific sources for their stories?

    Before you jump all over me and say the answer is obvious, consider this.
    There are some very highly-credentialed scientists out there who are content to feed blather to the media. My best guess is that they believe that simplification is best achieved through distortion and misrepresentation. No matter what is their motivation, they are charasmatic story-tellers who have captured the public’s imagination, and that’s all it takes to get a strangle hold on the media.

    Generally speaking, I agree that the media is a lazy bunch, so they are complacent to run with singular opinion even when there may be contrary sources out there. But to some degree at least, the scientific world needs to police itself if this practice is to ever be curtailed.

    • Police itself? How? By appointed authority, majority vote or maybe citation index?
      Really, do I need to remind anyone that there are scientists with impeccable academic credentials and impressive publications who publicly denounce other scientists, who also have impressive credentials, as charlatans or “crackpots” . How is a mere journalist, even if he has been certified by some academic authority, supposed to decide who to side with? How can one even report on this sort of thing impartially?
      Sure, I think it is probably reasonable to expect a journalist writing on science to know what “mass” means and to have some idea about gravity, but hey, there are serious scientists who differ on the details of the latter. How are you proposing to “police them”? The idea is a complete non-starter, and that even in a politics-free field such as particle physics or GR. Now think about what would happen if you tried to carry such ideas over to other less lucky fields.
      I think any proposed cure based on “policing” is going to make the disease seem like a blessing.

    • Well said Doc. There’s some dreadful garbage out there, and sometimes the people behind it are highly-credentialed scientists. It isn’t always down to lazy/uninformed science writers. But like Lucretius said, “policing” isn’t necessarily going to fix it, so it needs to be constantly opposed. The trouble is that some of the people who oppose it still get things wrong!

  14. Re. Fig.2 :
    1- If we take this figure at face value , shouldn’t the force of gravity be to the outside , as most mass is to the outside ?
    2- According to GR , how would you describe the space configuration due to the shown mass distribution?
    3- Do we have GR equations for the space configuration in case of DM ?
    4- To increase the inward force , shouldn’t DM be concentrated in the core ?
    Thanks

  15. 5- What prevents DM sphere from collapsing to the core ?
    6-What determines the rotation plan of the interior Galaxy ?
    7- How do we know that DM is spherical not a disk or ellipsoid for example ?

  16. 8- for the same sphere of DM , what determines the type / shape of the inside Galaxy ?

  17. 9- In case of fifth Unknown force how much that will affect fig.2
    If not at all , then we are safe , If very much then all what we do now is meaningless .

  18. 10- And last , you will say ; but this is how science proceed , fine , but many scientists take DM as Dogma , sure this is Not how science proceeds.

    • Antti Kangasvieri

      1) no, spherical shell of mass exerts no force on anything inside it
      4) no, there is enough DM in the core.
      5) angular momentum
      6) gravity and mass distribution
      7) we dont know that and nobody is claiming it to be exact mathematical sphere

      2 and 3 are about GR which i will botch terribly if i try, 8 is another one for the pros and not me. 9 and 10 are not questions.

  19. 1- Wrong
    4- That is what you assume
    3- Then why AM didn’t prevent ordinary matter from collapsing
    6-Wishful thinking

  20. Matter “pulls on light?” That sounds like something taken from the NYT article.

  21. The distribution of energy/momentum modifies the geometry of space-time in such a way that light follows a path through that altered geometry that looks like “matter pulling on light” … ;0)

  22. Have fun with dark matter even on your laptop:

    http://disipio.wordpress.com/2013/03/14/the-dark-matter-is-out-there/

    Even if not very accurate, this kind of layman simulation shows to me how simple and elegant the DM-as-a-particle-species hypothesis is. What I’m not buying is that they must also have weak interactions: there are no compelling evidences for this. Does anybody know if there are serious theories of non-weakly interacting DM particles? Let’s say, just gravity + maybe some kind of “dark force” completely decoupled from the SM?

    • Antti Kangasvieri

      Dark matter doesn’t have to have weak interactions. However detecting DM that has no SM interactions is much much harder (technological, not theoretical problem) hence the slightly hopeful weakly interacting massive particle hypothesis.

      • Well, one of the assumptions regarding DM is that it only has weak interactions with ordinary matter, and that is what the WIMPs variety of DM is all about.

        We can’t argue a priori (without experiments to validate/refute that) whether or not weak interactions are a major theme (if at all) for DM.

        Kind regards, GEN

  23. Shame…….both AK and Lucretius are deceiving , what they presented are a spherical HOLLOW shell !!!!!!! While what we have is a sphere filled with DM not a hollow shell , in addition ; if DM is the former then it exerts no gravity on the inside Galaxy , if it is the later then my question No 1 is valid and both their answers are deceiving !!!!!!
    As for AK assumption that DM is not collisional , this is what you assume not a proven fact .
    I like very much RDS question, really on what bases it was Decided that WIMPs are WI ????

    • Of course if you mean a solid sphere (which is usually called a “ball” in mathematics) then it follows by a trivial argument that the gravitational force is a linear function of the distance from the centre (provided that the ball has uniform density).
      But this, like all your other questions, has no relevance to the discussion.

    • They did not attempt to deceive. They just incorrectly assumed you could work through the application to the problem at hand yourself. If they were wrong, then you could try reading the first 5 lines of the Wikipedia page on the Shell Theorem.

      If you’re inside a sphere of uniform density some radius from the center, the gravitational forces of all the material that is at a greater distance from the center than you will cancel out, and all the material that is closer to the center will act as though it is a point source at the center of mass. These are both consequences of the Shell Theorem. The gravitational force will not be outward — the difference between a hollow shell (that you are interior to) and a sphere is that in the latter case there is a force *inward*.

      The only assumption involved with the statement that DM is not collisional is the assumption that we’re looking for particles at all. Accepting that for the sake of argument, the conclusion from the data is that it must have a low interaction cross-section so that it would not condense significantly as is observed. IF DM is particles, THEN they are necessarily non-collisional.

      It is not necessarily the case that DM interacts via the Weak Nuclear Force. It could be the case, without violating the constraints on interaction cross-section implied by the data. And it would be convenient if it interacted with regular matter via a known force.

      Note that most actual experiments looking for WIMPs aren’t assuming weak force interactions, but just *any* force even unknown ones that both dark and regular matter couple to.

    • Antti Kangasvieri

      Like Matt already pointed out to you in earlier post its by definition if we are discussing WIMPs. Says so on model name. Obviously its possible to have particle DM that only interacts through gravity but building real world experiment that could detect such particles seems impossible. It is also possible to have new non-SM interactions for DM particles. Or to have multiple species of DM particles. We simply dont know but the WIMP scenario is testable hence interesting.

      Collisional DM would run into the big problem of no longer matching observation. See for example http://en.wikipedia.org/wiki/Bullet_Cluster. Again, it doesnt have to be absolutely positively never ever colliding, just rarely enough to not mess agreement with observation.

      About the sphere thing… You can stack such hollow shells on top of each other like an onion to build a solid sphere.

  24. I’m a vfx artist in the film industry. Practically every mainstream press article I’ve seen about productions I’m involved with is at least this wrong. Journalists and editors these days don’t run their articles past people who know what they’re talking about, deadlines are too short and that would be admitting they don’t know what they’re doing.

    Makes you worry how wrong every press article you read is. And makes me thankful for blogs like this that really strive to get the best information across in a form that interested non experts like me can understand.

  25. Lucretius : This not for you to say , you are in total biased prejudice against what I write , let he who have the right to judge , judge……Matt. Of course.

  26. Titus : Did you notice how you swallowed your deception?

  27. Regarding my Q 1 it cannot be assumed that the DM BALL mass is concentrated in the center , outer mass will affect the space configuration thus affecting the Galaxy form and behavior , here we have a very complex interaction among masses of various entities and their effect on space geometry , that is if we accept that DM is a BALL , there are studies showing that it as a Disk
    Pick your choice , it’s no rules game.

  28. Matt,

    You have already written the correction, why not send it in as a letter to the editor?

  29. The NYT article could have been written better, I think there is consensus here about that. But I do not share the view that a lot of damage is caused by this type of articles.
    While several comments wrote about “damage”, I hardly see any definition or measure of the damage that has occured here. It does not seem to be a material damage.
    Further, I do not agree that a statement of the kind “the media shall write about the facts and about the truth” is getting the point here. Writing about scientific theories is not about a verifiable truth in the same sense as writing about a fact, such as “a tsunami has hit that country and destroyed 1000 houses”.
    And I have the impression that the NYT article was at least in one point possibly misunderstood, and criticized for an error it did not make. The criticized sentence “Some undiscovered mass — dark matter — must exist in order to explain gravity” was a taken out of the context of its preceding sentence, which talks about “the force of gravity in the universe” (which one may read as “effects of gravity on astronomical length scales”) and not about gravity in general.

    • Writing about *what a scientific theory claims* is a verifiable truth in exactly the same sense as a tsunami, or more directly like getting a quote. Whether the *content* of the quote is correct or not may be up in the air, but “So-and-so said such-and-such” is a verifiable fact. “Latest Theory X claims Y” is as well.

      I will agree that I’m not sure there’s any “damage” done here per se.

  30. Matt, hope this question from an amateur non-physicist isn’t ridiculous: Has anyone examined whether it’s the stars near the galaxy center appearing to go SLOWER, rather than those on the outside edge appearing to go faster? I ask this question because (a) most galaxies have black holes at the center, and (b) to an outside observer, time seems to slow down near the center of a black hole. At least, those are factoids I’ve gleaned from reading the experts in the popular press–heh!

    I appreciate your efforts to make science more accessible to those of us with no hope of mastering the math. It’s amazing the misconceptions I’ve overcome.

  31. When I read Strassler say that there “could be something wrong with our understanding of gravity” I was thinking, approvingly, of how much more satisfactory and useful is this way of speaking than the Bayesian insistence on expressing uncertainty in terms of the probability of some hypothesis or theory being true. Unfortunately, a few lines later, Matt starts giving numbers like 90% sure–. No Matt, don’t… it’s the concept of error of understanding/correct partial understanding, etc. that needs to be developed to illuminate progress of knowledge. Or so I have argued.

  32. This is absolutely the clearest and simplest description of what is currently known about dark matter that I have read. You are very, very good at this.
    That said, I have one suggestion: I think even the most general reader would benefit (even more) from you adding that a widely used name scientists use for one of the previously considered alternatives to dark matter is the acronym MOND, modified Newtonian dynamics. While I appreciate the intention to avoid being too jargony, introducing a short phrase for the reader to use in their heads when juxtaposing the various alternatives to dark matter might help simplify their internal summary, and it might as well be the one that is widely used by those who discuss it.

    The answer to your question is: the NYT should have knowledgeable individuals on staff (like they do with food, travel, real estate, economics, etc.) or skip the attempted science coverage entirely.

  33. Does gravity pull on light or does it follow the curvature of space?

    • @Tony(Rácz)Rotz:

      That is a very good question.

      If we consider that photons do have momentum, we could actually use Newtonian mechanics to see what gravity does to light, but we would fumble just like Einstein did in 1914, when he predicted a deviation of light by the Sun of exactly one half of the real deviation.

      He would have to do a lot of work on his “Entwurf” to realize that there is “another half” of that deviation that is caused by the warping of space-time.

      So, by the end of 1915, Einstein did have the right and complete field equations, and he was able to calculate the right value of the deviation of light done by the Sun in its vicinity.

      He was also able to calculate the right value of the precesion of Mercury’s orbit, which had been alreadyu detected by astronomers during the second half of the XIX century, but could not be either predicted or correctly calculated by Newtonian mechanics, so, it was considered an unexplained anomaly.

      Kind regards, GEN

  34. Tony(Rácz)Rotz

    @ Gastón E. Nusimovich
    Thanks.

    • “Does gravity pull on light or does it follow the curvature of space?”

      Neither. The space around the Earth isn’t curved. See this Baez article and note “not the curvature of space, but of spacetime”. Then take a look at the Riemann curvature tensor on wiki. See the “rubber sheet” picture on the right? It depicts curved spacetime, wherein Riemann curvature is the defining feature of a gravitational field.

      Now imagine you’ve placed a whole lot of parallel-mirror light-clocks in an equatorial slice through and around the Earth. When you plot all the light-clock rates, your plot resembles the rubber-sheet picture because light-clocks go slower when they’re lower. And because you measured those clock rates, it’s a curvature in your “metric”. But it’s important to note that light-clocks don’t go slower when they’re lower because your plot of light-clock rates is curved. In similar vein light doesn’t curve because spacetime is curved. Instead see Ned Wright’s article and note this: “In a very real sense, the delay experienced by light passing a massive object is responsible for the deflection of the light”.

      • I should add that what happens is that the concentration of energy tied up as the mass of the Earth “conditions” the surrounding space, altering its “metrical qualities”. That’s what Einstein said, along with “a curvature of light can only occur when the propagation speed of light varies with position”. Hence light-clocks go slower when they’re lower, and light curves, IMHO like a car veers when it encounters mud at the side of the road. But even though relativists know that the coordinate speed of light varies in a gravitational field, you never read seem to read about this kind of thing in media reportage. So I’d say things are even murkier than people think.

        • Hi duffieldjohn. I’m pleased to see here another contributor to Matt’s blog that has good knowledge on the subject of gravity, relativity and spacetime.

          There is one area regarding your comments that needs some clarification in my opinion. But first we should to be in agreement that the concept of time is something that derives from material motion. Assuming that is agreed, if motion is slowed from a perspective of one viewer because that motion comes from a region that has a stronger gravity than the region that viewer is in (for the sake of simplicity a clock on a planet with a solid surface and another clock at a geosynchronous orbital position. Both clocks are calibrated and when together tic at the same rate.) the viewer at the geosynchronous position will measure that the tics from the surface clock arrive slower than his own clock and visa versa the observer at the surface will find that tics from the geosynchronous clock arrive at a faster rate than do those from his clock.

          Time does not not speed up or slow because of being higher or lower in a gravitational field, what speeds up or slows are the material things that convey time, and that is because all things seem to be affected by gravity, including photons.

          I am more sympathetic to the following comment than I am to modern physics seeming or real regard of photon mass as being zero. In my view if something exists and has momentum it also has mass:

          From Abraham Pais’ book ‘Subtle is the Lord…’: The Science and the Life of Albert Einstein, p. 200:

          “I conclude this section by paying my respects to the German geodete and astronomer Johann Georg von Soldner, who in 1801 became the first to answer Newton’s query on the bending of light [S3]. ‘No one would find it objectionable, I hope, that I treat a light ray as a heavy body…. One cannot think of a thing which exists and works on our senses that would not have the property of matter,’ Soldner wrote.”

      • Did you not read that same Baez article you link to?

        Space is certainly curved near compact massive objects, but that curvature is not equivalent to gravitation, it is caused by gravitation.
        Gravitation is spacetime curvature. This is said clearly three sentences after the one you quote directly.

        The choice of coordinates decides whether to think of how the local curvature of spacetime couples to a test object. In most conventional coordinate systems and metrics a slow-moving massive object passing close to the surface of the sun would be seen by a terrestrial observer as being deflected more along a spacelike axis than along the timelike one (example: Mercury, comets, and so forth) whereas a fast-moving light object (e.g. light, neutrinos, cosmic rays) will be deflected more along the timelike axis than a spacelike one.

        That’s hardly super-surprising; one does not talk about the anomalous periodical redshift of Mercury rather than a delay in its “return” to (x,y,z,t’) because its orbital distance is stretched; likewise, one more often talks about light objects in terms of phase anomalies due to gravitaional lensing because their frequencies are retarded. (Gravitational lensing is a somewhat special case, however, where one wants to think about parallel transport in a sense of modifying the paths that light might take rather than the fate of two particles emitted at a given time; the emission sources are typically noisy in the frequency domain but have a well-characterizable average spatial distribution).

        However, because of coordinate freedom, one can easily choose to think of the gravitational effects of passing near a massive object as being some arbitrary mix of spatial and temporal contraction, including all of one and none of the other. “It took more time to cross the same amount of space” is exactly equivalent to “it crossed more space in the same amount of time”.

        Wright’s article (and some of your text above) contradicts your assertion: “In similar vein light doesn’t curve because spacetime is curved”.

        Following a curved path through flat spacetime can be exactly equivalent to following a straight path through curved spacetime where the thing following is in free fall. There is general freedom of choice there, too.

        • “It took more time to cross the same amount of space” is exactly equivalent to “it crossed more space in the same amount of time”.

          * Oops, it crossed less space in the same amount of time, of course.

        • Yes, I’ve read the Baez article, and much more. And the important point is that spacetime is a mathematical space. It is NOT space, thus the “local curvature of spacetime” can be misleading. Space is not curved in a gravitational field. A beam of light moving between two stars isn’t deflected at all, the “deflection along the timelike axis” isn’t a deflection. That light beam took more time to cross the same amount of space, that’s all. And it didn’t “follow a path through spacetime”, it traversed a path through space. There’s no motion in spacetime, or through spacetime. It is utterly static. You can draw a worldline in it, but nothing moves in it. Not light, not falling bodies, nothing.

  35. Professor, as long as you’re willing to follow up each and every time, I’m happy to look for your correxplanations.

    I have hemophilia, and there is news of a cure from time to time. I have a professional source who tells me the truth if I ask. So the news doesn’t bother me so much, as I know “the truth is out there.”

    The rest of the news has similar qualities the science reporting, by the way. It isn’t just bad science.

  36. I’ve written this before, I think, but it bears repeating. As an occasional writer for children’s science magazines, I often have occasion to interview scientists. I always send my finished article to the interviewee for review before I submit it. Once, when talking about this practice with a “real” science journalist, someone who worked for NPR in this instance, he basically told me that no journalist with any professional pride would ever do such a thing. I said my purpose in writing was to educate, and what is the point in getting the science wrong. He said I had the wrong purpose. That’s the mindset that causes so much error, I believe.

  37. @stephenwhitt, Thank you so much for your contribution. As a writer of scientific material, and as an educator of children, you are one of very few who can offer such a unique and revealing perspective. And you’ve done just that. I applaud you.

    I imagine that this scientific journalist to whom you spoke, the one who is a self-proclaimed expert in “professional pride”, has yet to embrace such notions as “conscience”, “grace” and “character”.

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