
Gravity's Oldest Puzzles - wmeredith
https://medium.com/starts-with-a-bang/gravity-s-oldest-puzzles-68c40889cac4
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Pyxl101
It's nice to read an article that's balanced and does not try to hype
everything up, like so much modern media is doing today. Nice find.

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beyti
possibly the best article I've read for a long time in terms of "knowledge
gain/required knowledge". totally agreed

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hamiltonians
rounding errors, machine errors, so many possibilities for such a tiny
discrepancy. Reminds me of the neutrino anomaly that supposedly disproved
relativity, until it was later discovered faulty wiring was to blame.

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ajkjk
This is a bit different. The neutrino error was when the only data-collection
system was reporting surprising results, which can't really be invalidated
without, well, getting another data-collection system and comparing.

These errors are surprises in the data itself: since the position of the
object can be measured by multiple systems, it's considerably less likely that
they would all make the same mistake. Of course it's possible that there's a
'single point of failure', like the data published back from the probe, that
causes all the calculations to be off by the same amount.

On the other hand, the fact that these same probes/instruments can report
everything _else_ with extraordinary accuracy discounts that theory somewhat.
It's still possible (perhaps the bug / cosmic-ray-induced bit flip only
affects one particular data collection code path..) but it's less likely.

Altogether, my intuition for the neutrino result was "this is probably a
broken experiment", but my intuition for this is "there's an effect that's not
being accounted for". That effect could easily be forgetting to account for
thermal radiation reflection or something else that's relatively
uninteresting, but, I still think there's _something_ going on.

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phkahler
Thermal expansion?

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ajkjk
I can't tell what your actual question is.

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phkahler
It was in response to the parent wondering about overlooked phenomena. Perhaps
sunlight or even earthlight heat the surface of the moon and cause thermal
expansion. That could easily displace things by several millimeters.

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hyyypr
Great article, there's something I didn't quite understand though. Why would
the Sun's gravity make the probe go _slower_ as it moves _away_ from it?

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danbruc
Because the sun attracts the probe (and vice versa). Or equivalently the probe
is climbing out of the gravity well of the sun loosing kinetic energy in
exchange for potential energy in the sun's gravitational field.

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foobarian
The Pioneer anomaly was solved IIRC. The likely culprit was identified as heat
being emitted in a particular direction, accelerating the probe due to
radiation pressure.

[https://en.wikipedia.org/wiki/Pioneer_anomaly](https://en.wikipedia.org/wiki/Pioneer_anomaly)

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akallio9000
So if the spacecraft could be stationary, it would pick up 400km/year
acceleration via nuclear power?

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david-given
ITM 0.1 km/h/year, and yes. The Pioneer Anomoly is a photon drive: the RTG
gets hot, infrared photons get emitted preferentially in a particular
direction (because the body of the spacecraft blocks them), and it's a simple
reaction drive. Nothing fancy there.

You could make it loads more efficient by putting the RTG at the focus of a
parabolic reflector. Now all the photons are being emitted in a single
direction, which means you get much more thrust.

According to this article:

[https://en.wikipedia.org/wiki/Nuclear_photonic_rocket](https://en.wikipedia.org/wiki/Nuclear_photonic_rocket)

...a perfectly collimated photonic rocket runs at about 300MW/N. Assuming
you're New Horizons, you have a mass of about 500kg and a 4kW RTG, so you'll
get a thrust of 0.1mN, which means an acceleration of 5x10^-8 m/s^2. Assuming
my arithmetic is correct (a totally unwarranted assumption!), then over a year
this adds up to a velocity change of 1.5 m/s.

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stephengillie
This rambling reminiscence missed a few of the bigger gravity study missions,
such as GRACE.

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ajkjk
Do you mean for the word 'rambling' to take its negative connotations here?

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sethrin
'Starts With A Bang' is generally lowest-common-denominator physics writing.
I'd imagine any negative connotations to be well earned.

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ajkjk
"I'd imagine"? Did you read the article?

I know a lot of physics, and have strong opinions on physics writing, but I
still think this was a pretty interesting and entertaining article. If we have
problems with it can we say what they are instead of just assuming everyone
agrees that it's bad?

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tjradcliffe
It's a mediocre article. It glosses a bunch of history adequately and then
points out a completely different anomaly. By "completely different" I mean
"has all the signatures of an instrumental or analysis artefact." It's
intermittent (huge red flag) and while the article doesn't say so (additional
red flag) close to the threshold of observation.

There are completely mundane explanations (upper atmosphere models slightly
wrong, unaccounted-for EM effects) so while there may be a fundamental cause
(gravity is doing something exciting) the odds are that it's a boring effect,
just like the superluminal neutrino observations.

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lern_too_spel
Perhaps you should write a textbook like the author has.
[http://www.amazon.com/Astrophysics-Through-Computation-
Mathe...](http://www.amazon.com/Astrophysics-Through-Computation-Mathematica-
Support/dp/1107010748)

I didn't find the writing misleading at all. It didn't overstate the
likelihood that there is actually new physics to be found in these anomalies
and gave a specific example where an anomaly was explained by known physics
that was merely unaccounted for.

