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A Rule of Thumb for Scientific Papers?

March 16, 2009 on 11:32 am | In Scientific papers |

There have been a number of readers who have pointed me to scientific papers, many of which make outrageous claims. There are many different levels of outrageous, ranging from least to most outlandish:

  1. Plausible — explaining a newly observed phenomenon based on a theory with lots of supporting evidence.
  2. Speculative — based on an untested theory with marginal but inconclusive evidence.
  3. Hopeful — based on an untested theory with no real supporting experimental or observational evidence.
  4. Stabbing in the dark — simply adding X new parameters (or wiggle-room) to a theory in order to possibly explain X or fewer new results.
  5. Stabbing in the dark without a knife — advancing a theory that is known to conflict with experiments on many fronts in order to explain one new observation.

Those that fall into the first category make me very happy as a scientist, and are the papers I strive to write when I do my research work. Why? Because the laws of nature tend to be very simple, and most things can be understood without having to resort to new physics. This interacting pair of galaxies (a.k.a. the antennae galaxy) is an excellent example:

Many of the scientific papers I see today (and most of the ones I get sent) fall into the third category. But they upset me greatly, because the authors treat it like it’s in the second category. One of the big things you may have heard about is the PAMELA data:

People have been publishing papers left and right trying to explain the “observed excess” in terms of dark matter that annihilates with itself. Could dark matter annihilate with itself? Sure! We don’t see any evidence of this, yet, but it could, in principle. If you look at the PAMELA data above, there are extra high-energy positrons, to be sure. But, and this is very important, we know that most of it is not caused by annihilating dark matter. There is some astrophysical phenomenon making anti-electrons (i.e., positrons) that we do not understand, but the observed amount is too great to fit in with what we know about dark matter. We also have seen this for many years; the PAMELA data is nothing new.

But the big problem with these explanations is that there is no observed excess of anti-protons! That would be compelling evidence, but we don’t see any evidence for it at all. Yes, I think anyone can write a bunch of papers explaining phenomena that doesn’t happen, but why would you, and how would that possibly qualify as interesting science?

But there is a surefire way to identify a scientific paper that falls into the worst two categories up there. You don’t even need to get past the title of the scientific paper to do it:

Does the paper you’re reading end in a question mark? If so, the answer is almost always “No!” But they make interesting, imagination-capturing claims, and so they get media attention, even though they’re mostly scientifically baseless. Let’s take a look at some of the astrophysics papers that come up (my comments in parentheses):

  • Could some black holes have evolved from wormholes? (Could a wormhole ever even — realistically — exist in our Universe? The current evidence says no.)
  • Could the Pioneer anomaly have a gravitational origin? (Let’s see, did gravity just randomly decide to turn the volume up in the 1980s? No? Surprise, surprise!)
  • Do We Live in the Center of the World? (Oh my, Copernicus! Do you think the answer to this one might be “No?” Just maybe? Would this paper ever have garnered anything other than laughs if Andrei Linde weren’t one of the authors?)

You get the point. Seems like an easy lesson to learn, yes? Well, apparently, not for many scientists, because today, this paper came out:

Do we live in a “Dirac-Milne” universe?

For a little more information on what a Dirac-Milne Universe is, take a look at this, from the abstract:

…an unconventional cosmology, the Dirac-Milne universe [is] a matter-antimatter symmetric cosmology, in which antimatter is supposed to present a negative active gravitational mass.

How simple. Except for one little thing: we’ve observed antimatter! And it has a positive mass! That’s been measured! So what this paper says is, if the laws of physics were entirely different, which they aren’t, we could imagine a Universe where some of the more complicated things we see were explained, even though it jettisons simple, valid, and accepted explanations for many other things.

How is this acceptable science? How is this deemed interesting? How am I the only one grossly outraged by this? I thought the whole point of science was to investigate and understand what occurs naturally in the Universe, not to invent rules that we know are incorrect and ask, “What if…?” I mean seriously, you may as well write a “scientific paper” about time machines:

And I would be fine with this if the authors would just own up to the fact that this falls into category 5, but they don’t. If you’re going to go off into fairy-land, don’t try to convince us that it’s reality, or that it’s even plausible as reality. This is a lesson that I wish scientists would learn, but that it’s absolutely vital that science journalists learn. I can spot it for astronomy/astrophysics/physics, but who’ll do it for health, for biology, and for the environment?

Keep that in mind whenever you read about science, including at the latest Carnival of Space, where someone thinks I should write about Venus all the time!


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  1. After pulling apart a paper in journal club, we often find that one we thought was in category 1 at first glance is instead in category 2, or sometimes 3. Good characterization!

    Comment by Nicole — March 16, 2009 #

  2. This article has been added to the Astronomy Link List.

    Comment by Astronomy Link List — March 17, 2009 #

  3. Even your scientist writer, Ethan, is guilty of writing papers that fit into his definition of a surefire way to identify a scientific paper that falls into the worst two categories up there:

    “Can Electric Charges and Currents Survive in an Inhomogeneous Universe?”


    Comment by Craig — March 17, 2009 #

  4. Sir, I abide by your general rule
    that every poet is a fool
    but you yourself may serve to show it
    that every fool is not a poet

    Craig, what’s funny is that the conclusion I came to in my paper is that the answer to my question is “No.”

    Comment by ethan — March 17, 2009 #

  5. Hi Ethan,

    My point was simply that you have experience with some of your different levels of outrageous :-)

    We should catch up sometime!

    Comment by Craig — March 17, 2009 #

  6. Hi Ethan,

    Apologies for my ignorance, but which experiment are you referring to that has proven conclusively that antimatter has positive mass?

    I think that is important because the model in the paper which you are complaining about does seem to agree pretty well with the cosmological data.



    Comment by Sabbir — March 19, 2009 #

  7. Sabbir,

    One very simple experiment that has been done many times is to create the element “positronium”. It’s one electron and one anti-electron. It’s a relatively “long-lived” unstable element; it lives for about a millionth of a second. That’s a lot, considering that some particles decay in 10^-24 seconds!

    From the equivalence of mass and energy (E = m c^2), we know that if the anti-electron had the negative mass of an electron, there would be no net energy of this element. But what we see is that when the electron and positron meet up and annihilate, they produce two photons of exactly the rest-mass-energy of the electron. That tells us that the positron has the same exact mass as the electron.

    We understand antimatter very well; we have created about 10^14 antimatter particles at Fermilab alone.

    Also, their predictions for nucleosynthesis, as well as for the baryon-to-photon ratio, are definitely *not* in agreement with the experimental data. Helium-3 and Deuterium are MORE abundant than lithium by a lot, not less.


    Comment by ethan — March 19, 2009 #

  8. Hi Ethan,

    The problem with your positronium example is that in all theories of antigravity that I am aware of, the principal of equivalence fails for antimatter, so that E=mc^2 no longer holds (in fact E=-mc^2 in general, for antimatter in such theories), so your conclusions are model-dependent. It is not really fair to discard the proposed model on the basis of the application of a principle which does not hold in the context of that model.

    We do understand many properties of antimatter very well - unfortunately, I don’t think that their gravitational mass is one of them. We have certainly created huge amounts of antimatter, but unfortunately not enough of them at once in the same place in a stable, neutral charge state that we could actually check which way they fell in a gravitational field.

    As point of fact, we even have difficulties with normal matter - several of the experimental measurements of the mass squared of the neutrino give negative results, and in general, results from different experiments have tended to disagree with each by a statistically significant margin. (BTW, take a look at particle data group’s section on neutrino mass for an example of what scientists do when they are confused but are too embarrassed to publicly admit it).

    Do the negative mass-squared measurements for the neutrino mean that the neutrino has complex mass? Probably not - in my mind it is more likely that the model is wrong, so investigating alternative models should be encouraged if they show signs of promise.

    Best wishes,


    Comment by Sabbir — March 20, 2009 #

  9. Well, there’s an easy test we can do. We’ve created neutral anti-hydrogen in the lab. We can put it in a vacuum under the influence of gravity, and see if it falls down at 9.8 m/s^2 or falls up at that speed.

    I had thought that we had already tested that, but perhaps I am mistaken.

    We still have a lot to learn, but you must be very careful for cosmology if you want to introduce things with a negative energy, as cosmological expansion at early times are well-constrained by observations of nucleosynthesis (which the paper conflicts with significantly) and the cosmic microwave background (which they do not address). Additionally, there must be sufficient room in this theory for baryogenesis to take place, and I am not sure that would work in the same fashion.

    In any case, my conclusion is that I am extremely skeptical that you can just ascribe a negative gravitational energy willy-nilly to antimatter with no motivation, screw up the abundance of the light elements, and then claim to explain the observed accelerated expansion.

    Comment by ethan — March 20, 2009 #

  10. Hi Ethan,

    I think a healthy scepticism is fine, but I think that being open-minded to alternative theories is likely to be more productive than ignoring them in the long run - at the worst, the models will turn out to be wrong, but if they happen to be on the right track (any new model is unlikely to work perfectly straight out of the box), but are discarded without any consideration or discussion, then that may turn out to be a fatal missed opportunity that is never recovered.

    One of the authors, Gabriel Chardin, has been working for many years on HEP experimental observations (particularly CP violation, which he has shown can be explained by the presence of antigravity), and has a good publication track record, so he has not come out with these ideas from thin air. I think he should be given a chance to investigate his ideas further, even if they seem to turn their nose up at some of the current mainstream beliefs.

    Best wishes,


    Comment by Sabbir — March 21, 2009 #

  11. Sabbir,

    I don’t mind when people put forth theories that are out of the box. In fact, *I* do it sometimes in my research. The possibility that antimatter is gravitationally repulsive is interesting and should be investigated.

    But I mind when people want to use a theory like that to explain astrophysics when it presents more problems for astrophysics than it helps explain. Here’s why. If I had an astrophysics theory that said, “hey, if the lifetime of a proton was 10^15 years, it could explain dark energy,” you’d freak out and tell me that the experimental constraints place the life of the proton to be > 10^25 years. And you‘d be right, and that would mean my theory didn’t matter. I also can’t give you anything that gives too many flavor-changing-neutral currents. It’s that simple: there are things you know about particle physics that are true, and all theories have to pass that simple screening before you can ever take them seriously.

    Well, if you want to explain cosmological expansion by giving negative gravitational mass to antimatter, you have to make sure it doesn’t screw up any of the three cornerstones of the big bang theory: the CMB, the Hubble expansion of the Universe, and nucleosynthesis. This theory totally screws up nucleosynthesis. By something like 10 orders of magnitude for some elements. That’s why I turn up my nose at this one, because based on what we know about astrophysics, based on the information in his paper, it cannot be right.

    Comment by ethan — March 21, 2009 #

  12. Hi Ethan,

    Well the same could be said about the standard model, which is out by 120 orders of magnitude out for the cosmological constant - that theory cannot be right either, so we have to keep looking. Back in the 80s, string theory wasn’t right about anything (i.e. it didn’t actually make any predictions, and still doesn’t, pretty much), but it did at least provide a mathematically consistent quantum theory of gravity - which is why it has survived until now.

    I guess different people have different thresholds. I have come across many good ideas in papers which have otherwise seemed quite dubious, so I think it’s still good to keep an open mind, and not risk throwing the baby out with the bathwater.

    Best wishes,


    Comment by Sabbir — March 23, 2009 #

  13. A lot of antimatter particles (positrons and anti-protons) were and are used in particle colliders. I don’t know how much time passes between injecting those particles into a collider and the actual collisions (with normal matter) - but if enough time passes, one should notice if antimatter rises up to to gravity instead of falling down, I’d say.

    Comment by Bjoern — March 24, 2009 #

  14. Hi Ethan,

    you are confusing inertial mass with gravitational mass (active and passive). The positronium experiment proves that matter and antimatter have the same (positive) inertial mass, which gets converted into radiation.

    OTOH the negative energy issue is related to the Friedmann equations, where you have density Rho which refers to gravitational mass-energy (which resists expansion). Regarding this Chardin and Benoit-Levy hypothesize that matter/antimatter has positive/negative active and passive gravitational mass: matter attracts matter, antimatter attracts antimatter, matter and antimatter repel each other.

    There is a paper making the case that this hypothesis cannot be ruled out a priori and so it makes sense to test it with the proposed Antihydrogen Gravity experiment at Fermilab (P981).
    Direct Observation Limits on Antimatter Gravitation
    Authors: Mark Fischler, Joe Lykken, Tom Roberts

    Re CMB, the first accoustic peak of CMB is easily accomodated, but they still have to build a whole theoretical framework for CMB. Re BBN, I am not aware of severe difficulties. Lohiya et al have already dealt with it (see “Nucleosynthesis in slowly evolving Cosmologies”)

    Comment by Parvulus — March 31, 2009 #

  15. […] A Rule of Thumb for Scientific Papers? Starts With A Bang! Some people may think that scientists are perfect beings who write papers that are 100% accurate and based on sound theories. However the truth can be far from that image. In this post, Ethan places some of the papers he has come across in his research into 5 distinct categories. I am quite amazed that a scientific paper that has gone through a rigorous peer review could ever be published if it falls in to his last two categories. […]

    Pingback by Link list – 17th March 2009 | Astronomy Link List — April 7, 2009 #

  16. Hi,

    The matter-antimatter symmetric universe need not include “anti-gravity”. Since the GR universe is everywhere, cannot be escaped and includes permanently embedded information at invariant frames of reference everywhere, the universe has a certain unity about it.

    In 4D we observe motion and change. In reality, the changing shape of space creates our sensation of motion and time. At the big bang, everything was here, where you are and where I am…where the farthest reaches of the universe are “today”.

    If matter and antimatter co-exist in equal amounts at the quark level of scale, everywhere, and collectively occillate 2.8 trillion times per Earth second, both matter and antimatter would not only exist in equal amounts, they both would “pull” downward…they are both “there”, forever. However any given set of 4D coordinates would observe only exactly that…a 4D cross-section of a higher dimensional structure…an exclusively matter or antimatter universe.

    It is generally accepted that the universe must, for many good scientific reasons, exist in more dimensions than the 4 we immediately perceive.

    Engineering good sense requires that the universe have a coherent structure…that is what a quasi-static universe is…such a universe answers very important engineering questions directly and very completely.

    Some of the most important concepts we now accept as scientific “truth” were considered laughable trash for centuries…even millenia.

    Hawking said: “The Universe just IS”. The scientific facts of today are in a state of constant refinement. The Standard Model is excellent, but it is in serious question…most reputable scientists believe there is “new physics” beyond it.

    Who knows what we will learn from the Hadron Collider in the next 5 years? The Tevetron has given us some fascinating peeks at the possibilities.

    Best Wishes, Sam Cox

    Comment by Samuel A. (Sam) Cox — May 19, 2009 #

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