Pioneer Anomaly Solved By 1970s Computer Graphics Technique

http://www.technologyreview.com/blog/arxiv/26589/

by _b8r0 • 15 years ago

218 80 15 years ago

jarin2• 15 years ago

Doesn't it seem like a slowdown caused by infrared light emitting from one part of Pioneer and reflecting off of another part of Pioneer is kind of like powering a sailboat with a giant fan attached to the back of the sailboat?

I'm not a physicist by any means, but doesn't conservation of momentum apply to photon emission and absorption/reflection as well?

dexen10• 15 years ago

The original direction of the (infrared) light is irrelevant, only the end result matters -- the `net force', with reflections and losses factored in. And factoring in the reflections is exactly what the article is about.

So it's not like a sailboat with a fan, more like a jet airplane with thrust reversers [1] engaged. While the original thrust is ordinarily pointed rearwards, it's redirected when the reversers are engaged and the `net force' (original thrust sans losses) is pointing frontwards.

----

[1] http://en.wikipedia.org/wiki/Thrust_reverser

jarin2• 15 years ago

Ah, that totally makes sense. Apologies if that was a dumb question, I am a high school dropout after all. :)

ars5• 15 years ago

The reason it's like a jet engine is that the engine "creates" wind - by burning fuel, the hot exhaust is the new wind. If a jet engine only moved wind (like a turbo prop), then a thrust reverser would not work.

ars5• 15 years ago

You can power a sailboat with a giant fan. What's the problem? Don't point it at the sail though - just point it away from the boat. :)

But anyway the difference here is that the "wind" is not being "pulled" from external sources (like real wind), it's created by Pioneer. And that makes a huge difference.

With wind, the fan pulls the wind (which creates force), and also pushes the wind (which also creates force).

Next you impact the wind into the sail, which subtracts from our force (since the wind is pushing in the wrong direction).

And finally the wind "bounces" off the sail, also subtracting, and leaving you with no net force.

i.e. +2 then -2.

With pioneer one step is missing - the force from pulling the wind (since the photons are created on the fly). So you have +1 then -2, leaving you with a net force of -1.

PS. If you don't understand why pulling and pushing the wind counts as two forces, think of a compressor which sucks in air and stores it. It still causes motion by pulling the air, but it doesn't push it.

omellet• 15 years ago

Yes. But the equipment on the back is radiating heat (roughly speaking) in every direction, as I understand it, so absent a reflector, the equipment would have a net zero effect on thrust. But because some of the heat is being reflected, it's contributing a non-zero amount of thrust.

bhickey• 15 years ago

Wouldn't ambient occlusion have been a better technique?

CountHackulus• 15 years ago

Yeah, I'm not sure that Phong Shading is really the takeaway here. It seems that it's more like they went from a local "illumination" technique to a full global "illumination" technique to take into account secondary bounces.

lutorm2• 15 years ago

This was my impression, too.

megrimlock• 15 years ago

Ambient occlusion (as the term is typically used) is a way to approximate local reflection of uniform inbound (aka ambient) light. It does this by measuring how much of the exterior environment is visible at each point, as a way to modulate an ambient lighting term.

It doesn't really simulate how light bounces locally around an object, nor light emitted by an object. Rather than a light transport simulation it's more of a clever CG approximation hack that looks great and is pretty cheap.

But to the real point I think you were trying to make: yes, there have been a lot of strides in simulating light since the 70's, and more advanced techniques exist that should give more accurate results.

jvandonsel• 15 years ago

It's a bit disappointing, actually. I was hoping for another revolution in Newton's laws.

ars5• 15 years ago

So was I. But there is still hope - these results have not been verified yet. :)

CoffeeDregs• 15 years ago

Agreed. Against all reason, I was kinda hoping that the Pioneer Anomaly would somehow lead us to FTL travel... Something like: "Hey, I think I figured out the Pioneer Anom... [pause...] Ooooohhhh!"

JacobAldridge2• 15 years ago

Well, the results have yet to be verified. Maybe it still will lead to FTL travel, and someone will solve this problem last year.

jdavid• 15 years ago

I was hopping that this would lead to us sending out plenty of extra-solar-system probes so we could perform more wacky measurements like the pioneer ones.

http://www.wired.com/wiredscience/2011/03/vanishing-dimensio...

I think the only qualitative way to guarantee research progress is through the expansion of the things we can sense/ perceive. For example, if we want to understand space better, I don't think another near earth telescope is the answer, but rather several that float at the edge of our solar system.

I am also excited about IPN- the Interplanetary Internet by Tim Burners Lee. So that there is a standard way for these crafts to communicate and relay messages as they get really far out.

http://www.slideshare.net/vinodquilon/interplanetary-interne...

run4yourlives• 15 years ago

Occam's Razor strikes again!

CWuestefeld• 15 years ago

"The most exciting phrase to hear in science, the one that heralds new discoveries, is not Eureka! (I found it!) but rather, 'hmm... that's funny...'"

- Isaac Asimov

copper4• 15 years ago

> Of course, other groups will want to confirm these results and a team at the Jet Propulsion Laboratory in Pasadena, which has gathered the data on the probes, is currently studying its own computer model of the thermal budgets.

Here's to hoping that the numbers match. Phong shading to calculate numerically the effects of a what is almost a solar sail to decelerate Pioneer - now that's cool science!

dexen10• 15 years ago

It also goes to show no scientific research (if performed rigorously and honestly) should ever be derided as being of little use. One wouldn't expect a graphics (!) algorithm to solve a (seemingly) physical problem.

Same as one wouldn't expect some obscure branch of algebra to give birth to modern asymmetric cryptography ;-)

hollerith5• 15 years ago

I am not persuaded. You are essentially saying that any attempt to steer (rigorous, honest) research or research funding in the direction of maximum usefulness is futile. Pointing out instances in which predicting the effects of research is tricky does not get you all the way to convincing me of that.

hollerith5• 15 years ago

>It also goes to show no scientific research (if performed rigorously and honestly) should ever be derided as being of little use.

You changed this sentence after my reply (in a sibling to this comment).

MichaelGG2• 15 years ago

Yes, it's quite awesome. But to be fair, it's a graphics technique designed to calculate a physics problem.

archgoon2• 15 years ago

Group Theory is no more an "obscure branch of algebra" than "algorithms" is an obscure branch of Computer Science.

PaulHoule2• 15 years ago

This is a classic example of how the physics community has been failing for the last 30 years.

First of all, there's an extreme focus on papers that have come out in the last 1.5 years, so that a lot of very interesting older work is invisible.

Secondly, physicists don't look outside the discipline, despite the fact that we often use inferior techniques. Back in the 1990's, Mark Newman and I were both working at Cornell and both of us were aware that the techniques physicists were using to evaluate power law distributions were bogus. Well, I was a timid grad student and, despite being one of the best physicists of his generation who already had written half of an excellent textbook and had a stellar research record, Mark was a postdoc who spent most of his two years in absolute anguish about how he was going to find his next job.

Mark wrote a paper about this ten years later, after physicists had published thousands of bogus papers using bogus statistics. It's a tragedy that neither Mark, myself or some other young turk didn't write it earlier -- and it wouldn't have been hard to do it all because it would mainly be a review paper of what was already in the statistics literature.

dexen10• 15 years ago

> Secondly, physicists don't look outside the discipline, (...)

Pardon? Phong shading is a simplified model of results of a physical process -- light reflections off certain kinds of surfaces (metallic ones, IIRC). It's imprecise, it has its limitation etc. Contrast that with the very tiny, precisely measured acceleration of Pioneer spacecrafts. It is not very common to get good results from applying a coarse tool to a fine problem.

copper4• 15 years ago

To nitpick, the discpline of physics covers everything - or it damn well ought to :) While it seems obvious after reading the article that solving the rendering equation is indeed a promising way to solve the Pioneer anomaly, I'm sure that I wouldn't have come up with the idea.

starwed• 15 years ago

> It is not very common to get good results from applying a coarse tool to a fine problem.

From the blog post, that isn't really what happened here -- the problem was accounting for reflections, and a technique was needed to estimate such effects. It doesn't matter that the anomalous acceleration was small compared to the total acceleration. It was of the same order as these reflection effects, and so only a "coarse" accounting of them was needed.

PaulHoule2• 15 years ago

A big part of physics is the art of approximation. It's almost never possible to have a perfectly 'exact' description of a situation.

For instance, in introductory physics we have students work a number of problems involving objects falling under the influence of gravity. Air resistance is rarely considered, and if were to be considered, approximations of some sort would be required, since there's no complete theory of turbulence.

Everything that involves a complex computer simulation, say molecular dynamics or fluid flow, involves approximations.

Even when you look at the simplest calculations involving elementary particles, all of these are intellectually justified by the renormalization concept that assumes that, at some very small length scale, the laws of physics that we know will break down -- but we know that QED, QCD and such are very good approximations.

The first virtue of an approximation is that it captures the qualitative character of a problem, after that, it's a matter of adding an increasing number of decimal places of quantitative accuracy.

More sophisticated models of radiation transport exist (these are critical to the development of H-bombs, etc.) and an obvious follow-up to this paper would be to use a better radiation transport code to validate the result.

copper4• 15 years ago

There's also the example is about how one medical researcher apparently rediscovered the trapezoidal rule, and managed to get a lot of citations to his discovery.

wlievens2• 15 years ago

Do you have a cite on this? Would love to read more.

Isamu• 15 years ago

> Mark wrote a paper about this

I'm interested. Is it:

Power laws, Pareto distributions and Zipf's law, M. E. J. Newman, Contemporary Physics 46, 323–351 (2005). [link: http://arxiv.org/abs/cond-mat/0412004]

mayank• 15 years ago

Actually, the paper you should be reading is this:

"Power-law distributions in empirical data", Aaron Clauset, Cosma Rohilla Shalizi, M. E. J. Newman http://arxiv.org/abs/0706.1062

lutorm2• 15 years ago

the techniques physicists were using to evaluate power law distributions were bogus.

This sounds interesting, care to be more precise?

burgerbrain• 15 years ago

What do Turks have to do with it?

SlowOnTheUptake• 15 years ago

http://dictionary.reference.com/browse/young%20turk

gammarator• 15 years ago

Agreed that physics would hugely benefit from exposure to modern techniques in computer science, "software carpentry," and statistics. How can that be encouraged, though? It's not in the curricula, and those are ossified and hard to shift. Individual researchers forging out on their own can't move the field much themselves. Summer schools, maybe, which could create ad-hoc communities carrying skills back to their home institutions?

As to the temporal bias in literature awareness: it's sort of unavoidable in any research area of decent size. The scope of the literature is simply too enormous. Certain key papers get codified as canon and others are lost in the fog. The situation isn't helped by the various journal publishers, whose various paywalls prevent an effective biobliometric universe which could be used as a discovery engine for old but relevant papers.

Dabacon• 15 years ago

"Mark wrote a paper about this ten years later, after physicists had published thousands of bogus papers using bogus statistics."

Agreed, but to be fair a lot of non-physicists were also doing this. For example in their excellent review article http://arxiv.org/abs/0706.1062 Mark Newman, Aaron Clauset, and Cosma Shalizi show that a claimed power law in bytes received per http request was wrong. I'm not going to defend the sloppiness of physicists, but I would note that other fields don't fare as well in this either.

And I wish you or Mark had written that paper earlier too!

madaxe• 15 years ago

Agreed - they're still teaching undergrads Fortran '77 in 2011, as there's this weird conception in the physics community that no language since F77 could possibly even be arithmetically correct.

Confusion• 15 years ago

Your reply is premised on this being the right solution to the Pioneer anomaly. That remains to be seen. For instance the comment in http://www.technologyreview.com/blog/arxiv/26589/#comment-23... makes a good point about the method used.

I think your view of 'the physics community' is skewed by your own experiences and does not describe the actual situation in 'the physics community' very well. My experiences, perhaps because I was at an 'university of technology', are entirely different.

Jabbles• 15 years ago

The key conclusion, with error estimates:

We performed 10^4 Monte Carlo iterations, which easily ensures the convergence of the result. The thermal acceleration estimate yielded by the simulation for an instant 26 years after launch, with a 95% probability, is

a(t=26) = (5.8 ± 1.3) × 10^−10 ms^-2.

... These results account for between 44% and 96% of the reported value

a = (8.74 ± 1.33) × 10^−10 ms^-2

(which, we recall, was obtained under the hypothesis of a constant acceleration) — thus giving a strong indication of the preponderant contribution of thermal eﬀects to the Pioneer anomaly.

http://arxiv.org/abs/1103.5222

BoppreH2• 15 years ago

Somebody better update the people using it as an anti-science argument.

Edit: I'm serious. Conservapedia used to have an article about it and how it discredits the scientific model and whatnot. There's a huge knowledge gap between this views and we should do our best to eliminate it.

pohl3• 15 years ago

For reference:

http://www.conservapedia.com/Pioneer_anomaly

JacobAldridge2• 15 years ago

Interesting - Conservapedia throws me a 403 Forbidden when I try to access that link, and any other of their pages.

Anyone else having trouble? (Win 7, FF4, UK based ISP)

spc476• 15 years ago

The following appears to have been recently added to the page: "Most recently, a new model for reflections of heat produced on the spacecraft[5] has been proposed. While this greatly reduces the amount of unexplained acceleration, it does not appear to completely eliminate it.[6]"

[5] http://www.technologyreview.com/blog/arxiv/26589/

[6] http://www.planetary.org/programs/projects/pioneer_anomaly/u...

pasbesoin• 15 years ago

What I appreciate about this is the straight-forward manner in which the story presents the case for always challenging your/the assumptions.

Speculation about a whole new aspect for theoretical models of the universe. Someone's grounded enough to go over the work and realize, 'Hey, you're doing the math wrong!'

I wish science education did a better job of teaching this.

dexen10• 15 years ago

There is a detailed, concise article on how the model was prepared: http://www.planetary.org/programs/projects/pioneer_anomaly/u...

Pioneer Anomaly Solved By 1970s Computer Graphics Technique

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