"Dark matter": methinks not

[umop_ap_sdn]

I tried posting this over at the Bad Astronomy board, but that thread pretty much died without giving a satisfactory answer so I hope the knowledge of Dopers can address my questions on this matter (no pun intended). I would have posted this in GQ but it seems to warrant a good debate.

It is my understanding that astronomers claim all galaxies are made of roughly 90-95% “dark matter”, which has never even been directly observed.

According to the cover story of the October 2003 issue of Discover magazine (my new favorite issue), The Pioneer 10 and 11 spacecraft, among others, are inexplicably slowing in their motion away from us. Specifically, it is a slight but consistent acceleration towards the Sun which has not been explained by any conventional means. This site has more technical information not presented in Discover.

The Discover article explains how physicist Moti Milgrom and his colleagues have proposed a “tweak” to Newton’s laws of gravity so that at a particular amount of acceleration, the force becomes stronger than normal. The article claims that this tweak, known as Modified Newtonian Dynamics or MOND, correctly explains not only the motions of the Pioneer and other spacecraft but the orbits of galaxies. Dark matter, they say, is not needed (I am ignoring the negligible stuff like planets;) ).

According to the site linked above, “Dark matter or modified gravity, such as Milgrom’s MOND model, fail because there should be observable effects on the orbits and distances of Earth and Mars as well as elsewhere in the solar system.” Aren’t they? On the aforementioned Bad Astronomy thread, somebody helped me figure out that the corresponding distance from the Earth to induce MOND is on the order of 13,900,000 miles. The biggest perturbations in our solar system affect gas giant planets (Cite) and even though they are much more massive than the Earth, their perturbations of each other should involve MOND. Indeed, they seem to be large perturbations for objects so much smaller than the Sun; why are they so big if not for MOND, and is the mechanism well understood? It seems doubtful Milgrom et al would have ignored this point in proposing their theory.

We do not have any satellites that I’m aware of that are 13,900,000 miles from Earth but we do have the newly discovered Sedna which is far enough away from the Sun to exhibit the effects of MOND. Apparently, its orbital velocity must be much faster than expected for that distance since it is estimated to be near the perihelion of a highly eccentric orbit. I’m told its orbital parameters can be derived from a mere 3 observations. Having worked with calculations involving orbital parameters myself, I must ask what these calculations are and whether they would still work if MOND were invoked. With MOND, Sedna’s orbital velocity would be faster than predicted by Newton’s laws without the need for a highly eccentric orbit.

Pointing a telescope at any square degree of sky reveals it to be loaded with stars, both nearby and faraway. Most of these are in a small patch of our own galaxy which also contains so much dust and gas as to obscure most of its stars from our view. If there were dark matter in anything like the proposed quantities streaming around us, we’d see it obscure something. Also, some of it would eventually find its way into our Solar System. Why is it that astronomers continue to believe in a metaphorical IPU when there is new evidence that appears to disprove it?

[j_sum1]

Good questions cityboy. I have often wondered why the proposed dark matter theories are so widely accepted instead of reviewing thoughts about the big bang (which was IIRC the reason for suggesting dark matter in the first place.)

I lack the expert knowledge to answer your questions, but I am interested in answers put forward by knowledgable Dopers.

[Angua]

Let me start with saying that I am an astrophysicist, so I should know a little bit about this.

MOND works on the premise that Newtonian dynamics is modified slightly, instead of force being proportional to acceleration, its proportional to acceleration squared, beyond a certain critical acceleration.

Now, the only situation where MOND works better than dark matter theories is in calculating the rotation curves of galaxies, and that’s it. MOND goes horribly horribly wrong when we try and use it to predict structure formation in the universe. Hence, why we astronomers need dark matter. Yeah, we can’t see it, and have very little idea as to what it is, but we can measure its effects.

Ever heard of gravitional lensing? This is when you have a galaxy (or other bright object) behind a large astrophysical mass, say a cluster of galaxies. Due to general relativity (which we believe to be true, as we’ve done lab tests that confirm the theory), the light from the background object is lensed, and we end up with multiple images of the background object. We can use the configuration of these objects to calculate the lensing mass - i.e. the mass of the cluster or whatever’s causing the lensing. We can then use optical and X-ray astronomy to calculate the mass of radiation emitting matter. This is invariably less than the lensing mass, even when we take effects of other objects behind the lensing mass into account. We need extra dark matter in order to cause these lensing effects. Simple MOND does not work.

I work with clusters of galaxies - gravitationally bound groups of galaxies, and the total mass of galaxies and hot gas alone in these clusters is simply not enough to cause a gravitational potential that will keep them all bound together. There must be something else causing the gravitational potential, and again, the only explanation we have is dark matter, MOND goes horribly wrong trying to explain clusters.

So, there you have it, why are dark matter theories more widely accepted than say MOND, or revising the big bang? Becuase dark matter theories are workable within our current knowledge. They explain more things than say MOND.

[j_sum1]

Thanks Angua for the explanation. Am I right in guessing that MOND is a bit of a leap into the realms of empiricism without a whole lot of thought to the mechanism underpinning the theory?

Are their any other competing theories that propose mechanisms other than the gravitational effect of dark matter?

[Angua]

@j_sum1

Thanks Angua for the explanation. Am I right in guessing that MOND is a bit of a leap into the realms of empiricism without a whole lot of thought to the mechanism underpinning the theory?

Yeah, that’s about right. It was pretty much invented to mach the galactic rotation curves, and no one’s managed to get rid of it yet.

Are their any other competing theories that propose mechanisms other than the gravitational effect of dark matter?

Not that I’m aware of.

[Inoshiro]

@cityboy916

On the aforementioned Bad Astronomy thread, somebody helped me figure out that the corresponding distance from the Earth to induce MOND is on the order of 13,900,000 miles.

I’m a little confused. At that distance the gravitational force exerted by the earth is very weak. I calculated the acceleration due to earth’s gravity at that radius to be 7.967 * 10^-7 m/s^2 (using a = (G*m_e)/r^2, where G is the gravitational constant, m_e is the mass of the earth and r is the radius). Is this MOND supposed to kick in at very low gravitational accelerations? This implies that it cuts out at higher values. But this contradicts something Angua said:

MOND works on the premise that Newtonian dynamics is modified slightly, instead of force being proportional to acceleration, its proportional to acceleration squared, beyond a certain critical acceleration.

[Angua]

@Inoshiro

I’m a little confused. At that distance the gravitational force exerted by the earth is very weak. I calculated the acceleration due to earth’s gravity at that radius to be 7.967 * 10^-7 m/s^2 (using a = (G*m_e)/r^2, where G is the gravitational constant, m_e is the mass of the earth and r is the radius). Is this MOND supposed to kick in at very low gravitational accelerations? This implies that it cuts out at higher values.

That’s right. MOND works for very small accelerations. I suppose using “beyond” implied that there was a lower limit to where MOND works as opposed to an upper limit. Sorry for causing confusion.

[Inoshiro]

@Angua

That’s right. MOND works for very small accelerations. I suppose using “beyond” implied that there was a lower limit to where MOND works as opposed to an upper limit. Sorry for causing confusion.

That would explain the Voyager reference too, yes. Thanks Angua.

[umop_ap_sdn]

@Angua

Now, the only situation where MOND works better than dark matter theories is in calculating the rotation curves of galaxies, and that’s it. MOND goes horribly horribly wrong when we try and use it to predict structure formation in the universe. Hence, why we astronomers need dark matter. Yeah, we can’t see it, and have very little idea as to what it is, but we can measure its effects.

I didn’t know it actually works better for rotation curves. This must mean that either A.) it’s correct internally within galaxies, or B.) it reflects the statistical distribution of dark matter. Except that dark matter doesn’t fit the actual curves quite so well.

So from what you’re saying it sounds like the observed gravitational pull between galaxies drops off at the rate one would expect based on the Newtonian model for galaxies that are 10-20 times heavier than the total number of stars we can observe in them (even after accounting for the obsuring effects of dust and gas). Could it be that MOND ceases to apply below an even smaller acceleration, i.e. as acceleration decreases, the force of gravity changes from being proportional (to acceleration) to being proportional to the square and then back to being proportional again? The multipliers for higher and lower non-MOND acceleration would be different, so the net result would be that a galaxy of mass M at intergalactic distance r in my proposed model would have the same pull as a galaxy of mass (say) 10·M in the current dark matter system.

Ever heard of gravitional lensing? This is when you have a galaxy (or other bright object) behind a large astrophysical mass, say a cluster of galaxies. Due to general relativity (which we believe to be true, as we’ve done lab tests that confirm the theory), the light from the background object is lensed, and we end up with multiple images of the background object. We can use the configuration of these objects to calculate the lensing mass - i.e. the mass of the cluster or whatever’s causing the lensing. We can then use optical and X-ray astronomy to calculate the mass of radiation emitting matter. This is invariably less than the lensing mass, even when we take effects of other objects behind the lensing mass into account. We need extra dark matter in order to cause these lensing effects. Simple MOND does not work.

I have heard of gravitational lensing (the light is converged, making it look as though it originated some distnce “outward” from the actual source, right?) but not sure I follow what you’re saying about the mass of the emitting matter vs. the lensing mass. Specifically, why the former is always less than the latter. ISTR experiments done during solar eclipses where distant stars more massive than the Sun, or even galaxies, were observed demonstrating the Sun’s gravitational lensing.

So, there you have it, why are dark matter theories more widely accepted than say MOND, or revising the big bang? Becuase dark matter theories are workable within our current knowledge. They explain more things than say MOND.

Not sure what dark matter has to do with the Big Bang. Thought it was only necessary for the rotation and interaction of galaxies. But like you said, MOND explains galactic rotation better than dark matter, so here we have one theory that does more and one that does better.

We also have our own solar system for testing the behavior of microgravity (nanogravity?) - can anyone enlighten me on the planetary perturbations or the orbit of Sedna, specifically, would the MOND go horribly wrong there too or is it workable?

[dalovindj]

Isn’t there a difference between Dark Matter and Dark Ennergy? Dark Matter is suspected to be failed stars, ejected planets, WIMPS, brown dwarfs, and such. Things that are composed of matter but that are too dark to see with our telescopes. Dark Energy, on the other hand, is suspected to be a repulsive force, perhaps akin to Einstein’s Cosmological Constant. Is this discussion suggesting that neither DE or DM is responsible/exists? Or are you proposing that there isn’t as much dark matter as they think while allowing for dark energy?

Either way, it seems that DE and DM aren’t really descriptive. They are names for things that we don’t know what they are. I got hit in the head with Dark Matter, but when I turned on my light it was a baseball. It’s not like once I turned on the light, the baseball didn’t exist anymore. I simply understand it better. DE and DM are ways of saying “We don’t know, really, what is causing these effects. When we find out we will give them proper names.” I never really thought of it as a definitive theory. So, if it turns out that MOND (which is new to me) is the culprit to whatever degree, then DE can be described (perhaps partly as MOND). What I’m saying is it really wouldn’t disprove anything, rather it would give a name to a phenomenon that we don’t quite understand.

Forgive me if I’m being overly simplistic. This thread is covering fairly complex stuff . . .

DaLovin’ Dj

[bonzer]

Angua’s nicely summarised the case for dark matter, so I’ll only address the specific misconceptions in cityboy916’s posts.

@cityboy916

This site has more technical information not presented in Discover.

The link is to Barry Setterfield’s site. Thus the first thing to say - and it can’t be said often enough - is that Barry Setterfield is an idiot. Worse, he’s an idiot with an agenda.

He does at least link to the most recent paper by the Pioneer tracking team (a fairly large pdf). The immediate question here has to be whether the Pioneer anomaly is actually good evidence for new physics. This paper actually concludes that while the anomaly isn’t yet understood, the explanation is likely to be mundane:

Currently, we find no mechanism or theory that explains the anomalous acceleration.What we can say with some confidence is that the anomalous acceleration is a line of sight constant acceleration of the spacecraft towards the Sun … Until more is known, we must admit that the most likely cause of this effect is an unknown systematic. (We ourselves are divided as to whether “gas leaks” or “heat” is this “most likely cause.”)

The detailed discussion makes clear that these two possible causes are at least plausible: they can generate accelerations on the probe of the right order of magnitude. My bet is that, given we’re talking about an object that hasn’t been in a lab for 30 years and is now a hell of long way away, there’s just some overlooked minor effect that’s yet to be accounted for. As we’ll see, even Milgrom doesn’t think that the Pioneer issue is decisive.
This review also specifically address MOND. At least in one of its forms. As this lecture by Milgrom explains, there are two versions of his hypothesis. Crudely you can regard these as interpreting the proposal as modifying the strength of the gravitational field or modifying the way bodies interact with gravity. In the first , as Milgrom explains in the same lecture, the consequence of a Pioneer-style acceleration translates to a similar small additional effective acceleration throughout the Solar System. He concedes that under the first version, this is entirely inconsistent with the data from the inner Solar System. The Pioneer paper expands on the details here. In particular, it points out that ranging data from the Viking missions pins down the orbits of the Earth and Mars to a limit about 2 orders of magnitude smaller than the Pioneer anomaly would allow. Not going to fly.
The alternative is that bodies with different masses and trajectories can interact with the gravitational field differently. Milgrom suggests:

It may well be that the modification enters the Pioneers motion, which corresponds to unbound, hyperbolic motions, and the motion of bound, and quasi-circular trajectories in a different way.

There’s a technical name for this: it’s called vigorous hand-waving. He’s worded this so vaguely, that even he must regard it as a weak argument. This is all so vague that there’s no point discussing the effects of MOND within the Solar System. Milgrom’s defence is that we just don’t know. When he’s prepared to make a prediction on the issue, then there’s something to examine. For that matter, applying MOND to the Pioneer anomaly at all has been an entirely post hoc matter.
For physicists, the truly horrible aspect of the second version of MOND is meanwhile that it utterly trashes the Strong Equivalence Principle. In other words, General Relativity isn’t just slightly wrong, it’s completely wrong.

Apparently, [Sedna’s] orbital velocity must be much faster than expected for that distance since it is estimated to be near the perihelion of a highly eccentric orbit. I’m told its orbital parameters can be derived from a mere 3 observations. Having worked with calculations involving orbital parameters myself, I must ask what these calculations are and whether they would still work if MOND were invoked. With MOND, Sedna’s orbital velocity would be faster than predicted by Newton’s laws without the need for a highly eccentric orbit.

The reduction of three observations to a Keplerian orbit is a standard result in celestial mechanics.
From a strictly mathematical point of view, MOND is currently sufficiently vague that the equivalent question can’t even be precisely formulated. However, to be merely practical, the effects we’re talking about are so stonkingly small within the Solar System that they can make no significant difference to this sort of calculation. You’re barking up a very wrong tree.
Sedna is a one-off example from what’s probably a very large population of which very few have been observed. It may just happen to have an odd orbit. No biggie.

If there were dark matter in anything like the proposed quantities streaming around us, we’d see it obscure something.

The reason for calling it dark matter in the first place is because it doesn’t interact with light. It thus won’t block photons, entirely as part of its original definition.

Not sure what dark matter has to do with the Big Bang. Thought it was only necessary for the rotation and interaction of galaxies.

]

Nope. Historically, in parallel with worries about such matters, there were independent arguments that the total density of the universe had to have been driven close to the critical density by inflation. Since there wasn’t enough non-dark matter to account for this density, these were independent arguments for dark matter. The results from WMAP are a quite stunning confirmation of the predictions this leads to, even if some of the details are still up for grabs.
By contrast, MOND also originated in an era where dark matter was mainly a problem to do with galaxy rotation. Twenty years on, that’s still the best evidence for MOND - as Milgrom admits in the lecture linked to above. That’s no great surprise: it’s an ad hoc hypothesis designed to explain just that particular case. By contrast, dark matter also explains lots of other data that have only become available in the last few years.

[Icerigger]

Since NASA is still in contact with Voyagers 1 & 2 do these probes show tracking anomalies as well, can they be used to explore this phenomena.

[umop_ap_sdn]

I had no idea about Setterfield. :eek:

@bonzer

As this lecture by Milgrom explains, there are two versions of his hypothesis. Crudely you can regard these as interpreting the proposal as modifying the strength of the gravitational field or modifying the way bodies interact with gravity. In the first , as Milgrom explains in the same lecture

I don’t know if it’s a bug in the vB code or what, but your links didn’t work when I tried to click on them, so in case others are having this problem I’ll repost them here.

Had no idea the theory was that vague, but given what little Discover has published on the matter :rolleyes:, the theory as I see it boils down to: the gravitational pull on a sufficiently distant object is greater than that expected by a = (G·M)/r[sup]2[/sup]. More gravitational pull means objects escaping the center of mass are slowed whereas objects orbiting the center of mass are sped up, just as if the central mass had somehow grown heavier.

(Apparently, Discover also misprinted Milgrom’s first name; the lecture you cited refers to him as Mordehai whereas the Discover article calls him Moti.)

The reduction of three observations to a Keplerian orbit is a standard result in celestial mechanics.
From a strictly mathematical point of view, MOND is currently sufficiently vague that the equivalent question can’t even be precisely formulated. However, to be merely practical, the effects we’re talking about are so stonkingly small within the Solar System that they can make no significant difference to this sort of calculation. You’re barking up a very wrong tree.
Sedna is a one-off example from what’s probably a very large population of which very few have been observed. It may just happen to have an odd orbit. No biggie.

OK, allow me to simplify a little here. For context, my knowledge here is based on 1.) reading the magazine article, and 2.) maintaining a software application that calculates objects’ positions in space based on their orbital parameters. In theory, an object a specific distance from the Sun can be expected to orbit at a particular speed if its orbit were circular. P[sup]2[/sup] is proportional to a[sup]3[/sup]·M[sub]Sun[/sub] where P is the orbital period and a is the semi-major axis. The velocity V = 2 π a / P and is the required velocity to balance the Sun’s gravitational pull against the inertial force (“centrifugal force”) of the object’s momentum. If the Sun’s gravitational pull were greater due to MOND, it would require a greater V (certainly enough to be detectable) and P[sup]2[/sup] would not be proportional to a[sup]3[/sup]·M[sub]Sun[/sub]. In fact, I never did work out the math directly without calculating V, but basically what I did was to square the force of gravity after adjusting it so that Milgrom’s a[sub]0[/sub] = 1.

I realize most comets have highly elliptical orbits and that these are not new or heretofore unknown. I realize that most of the time at any given instant an elliptical orbit does not perfectly balance velocity against gravitational pull, and that’s why they always speed up at periapsis and slow down at apoapsis (sp?), which means the observation of an object with “too much” orbital velocity in a Keplerian orbit must mean its semi-major axis is greater than its observed distance. Since Sedna’s orbit is calculated to be quite eccentric, I must surmise that it is orbiting much too fast to maintain its current distance from the Sun and is expected to recede farther into space after it reaches perihelion. My question is whether the calculations which give its 6 orbital parameters would be severely messed up if the Sun’s gravitational pull at each instant was squared.

Historically, in parallel with worries about such matters, there were independent arguments that the total density of the universe had to have been driven close to the critical density by inflation. Since there wasn’t enough non-dark matter to account for this density, these were independent arguments for dark matter. The results from WMAP are a quite stunning confirmation of the predictions this leads to, even if some of the details are still up for grabs.

I’ll admit a lack of familiarity with the WMAP or any other mathematical model concerning the Big Bang, but if you would elaborate it would make more sense.

@dalovindj

Is this discussion suggesting that neither DE or DM is responsible/exists? Or are you proposing that there isn’t as much dark matter as they think while allowing for dark energy?

Dark energy AFAIK is what’s thought to be causing the universe to expand at an increasing rate. I suppose it could be explained by further modifying gravity so that it eventually becomes a repulsive force, but that’s sure to step on the toes of some well established theories so I won’t advocate it without knowing all the equations involved.

[Angua]

@cityboy916

I’ll admit a lack of familiarity with the WMAP or any other mathematical model concerning the Big Bang, but if you would elaborate it would make more sense.

OK. WMAP measures if there is any anisotropy in the cosmic microwave background. Theoretically, to first order, the CMB ought to be totally uniform. However, to higher orders, there should be small fluctuations, which are the seeds of structure formation.

Now, the height and position of these fluctuations is dependant on the total density of the universe. If we take only visible matter, we cannot, no matter how hard we try, reconcile standard cosmological models with the CMB observations. However, if we include a certain amount of dark matter, we can fit the observations exactly. I’m not exagerating here - the error bars we have on the data from WMAP are absolutely tiny, and require an extremely accurate model. The only model that fits the data is one that involves dark matter.

As for the nature of dark matter, failed stars, brown dwarfs, WIMPS, MACHOs etc probably makes up a small amount of it. As for the rest of it, we simply don’t know what it is. All we do know is that it doesn’t interact with anything in any way apart from gravitationally, which is the only way we can detect it.

but not sure I follow what you’re saying about the mass of the emitting matter vs. the lensing mass. Specifically, why the former is always less than the latter. ISTR experiments done during solar eclipses where distant stars more massive than the Sun, or even galaxies, were observed demonstrating the Sun’s gravitational lensing.

We can use the gravitational lensing observations to calculate the mass required to cause the observations. This turns out to be greater than the matter we can see - i.e. the radiation emitting matter. Hence the argument for non-radiative, gravitationally interacting dark matter.

Now, as for the lensing effects demonstrated during an eclipse, its found that the dark matter distribution really doesn’t affect individual stars. Most galaxies and clusters of galaxies will have what’s called a dark matter halo, which essentially determines the gravitational potential that pulls a galaxy or a cluster of galaxies together. However, the effect of dark matter appears to be negligible within a galaxy, when looking at individual stars.

[umop_ap_sdn]

This might be a silly question, but what about all those neutrinos the galaxy’s stars are putting out, could they be part of this dark matter? We’re getting constantly flooded with the ones coming from the Sun with very few observable effects. IIRC they do have a little bit of mass, and considering all the stars that have existed since the universe formed there must be loads of them swarming around.

[toadspittle]

Something that’s always bothered me about dark matter:

Shouldn’t it, technically, just be called “dark MASS”? Or better yet, “dark GRAVITY”? I mean, all we see are gravitational effects. Right? We don’t know WHERE the gravity is coming from. Seems like verbal laziness on the part of physicists.

[Thaumaturge]

@cityboy916

This might be a silly question, but what about all those neutrinos the galaxy’s stars are putting out, could they be part of this dark matter? We’re getting constantly flooded with the ones coming from the Sun with very few observable effects. IIRC they do have a little bit of mass, and considering all the stars that have existed since the universe formed there must be loads of them swarming around.

The calculated approximate weight IIRC of all the neutrinos in the universe amounted to about 1% of the total mass-energy. Visible matter was around 4%, dark matter 20%, and dark energy the rest. There are a heck of a lot of neutrinos out there, but they don’t weigh very much.

I first liked the MOND hypothesis when I first heard it…it seemed more elegant than proposing imposssible to see massive stuff that was everywhere. However, as stated before, the theories of dark matter perfectly fit the available data. Perfect fits are rare and not to be ignored.

[speculation]

One proposed source of dark matter is matter in parallel universes. Some formulations of string theory allow gravitons to escape the universe. This is why gravity is so weak compared to the other forces- it can escape in other directions that matter and energy cannot. Gravitons in other universes escaping into our universe would cause anomalous extra gravity, without any associated visible matter. Since the other universes could have been formed with entirely different particle properties, the matter might assemble in strange configurations.

The dark matter appears to assemble as a hot gas, and the inferred particle size should be in an energy range that we can detect, but no detector has found a candidate. If the matter was in one or more parrallel universes, we would never be able to detect it beyond it’s gravitational influence.

I like this theory because with one stoke we find the missing mass, and explain the incredible weakness of gravity when compared to the other forces. ( If you don’t think that gravity is weak, remember that a tiny magnet can counteract the gravitational force of the entire earth, with ease).

[/speculation]

[Inoshiro]

@Thaumaturge

[speculation]

One proposed source of dark matter is matter in parallel universes. Some formulations of string theory allow gravitons to escape the universe. This is why gravity is so weak compared to the other forces- it can escape in other directions that matter and energy cannot. Gravitons in other universes escaping into our universe would cause anomalous extra gravity, without any associated visible matter. Since the other universes could have been formed with entirely different particle properties, the matter might assemble in strange configurations.
[/speculation]

Time to build a giant gravity wave generator to talk to the aliens in the parallel universes.

[umop_ap_sdn]

That’s a good idea, Inoshiro.

@cityboy916

In theory, an object a specific distance from the Sun can be expected to orbit at a particular speed if its orbit were circular. <snip> the observation of an object with “too much” orbital velocity in a Keplerian orbit must mean its semi-major axis is greater than its observed distance. <snip> My question is whether the calculations which give [Sedna’s] 6 orbital parameters would be severely messed up if the Sun’s gravitational pull at each instant was squared.

(“Squared”, of course, meaning proportional to the square of acceleration ala MOND.)

So, is this a phenomenally dumb question or does nobody know for sure?

[Thaumaturge]

I guess we’d just have to wait and see if Sedna orbits as predicted according to the laws as we know them or not. Unfortunately, any deviations are likely to be very small. And because Senda is so far away, it orbits very slowly, so it would take a long time for the deviations to show up. As far as I know, we only have data for about 3 years. That’s including the photographs in which it was discovered, and some photographs from a few years ago it was noted on after it’s discovery. It won’t even reach perhihelion for another 72 years, and is calculated to take 10,500 years to complete one orbit, though we won’t have to wait that long to find any anomalies. But I assume we’d have to know about it’s history for a bit longer than we do to find any.