7 ms·

Gravity's Oldest Puzzles

186 points by wmeredith 11y ago

hamiltonians 11y ago
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.
- ajkjk 11y ago
  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.
  - phkahler 11y ago
    Thermal expansion?
    - ajkjk 11y ago
      I can't tell what your actual question is.
      - phkahler 11y ago
        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.
- Retric 11y ago
  The neutrino error was very much people saying, we don't think this is accurate, but we don't know why and we are not going to hide this data. I saw a video of the press release and the guy looked like he really did not want to be saying those words. Now, after they said that plenty of people ran with it way to far. However, I feel the team did the right thing.
  - deciplex 11y ago
    Didn't the guy end up having to resign because of the media circus? It was a pretty sad story IIRC.
    - sohkamyung 11y ago
      Nautilus has an interview with him (Antonio Ereditat) [ http://nautil.us/issue/24/error/the-data-that-threatened-to-break-physics http://nautil.us/issue/24/error/the-data-that-threatened-to-... ]. From his point of view, he did the 'right thing' by announcing the apparent FTL result and inviting people to find out what was wrong.
      - deciplex 11y ago
        He absolutely did the right thing. But in the end I think he resigned because the media circus that followed the announcement meant he could no longer contribute effectively to the project. https://en.wikipedia.org/wiki/Faster-than-light_neutrino_anomaly#Discussions_within_the_OPERA_collaboration https://en.wikipedia.org/wiki/Faster-than-light_neutrino_ano...
        plonh 11y ago
        The media circus died down in a few weeks. Why wouldn't he be welcomed back? Why is science drowned in political crap?
        acqq 11y ago
        Read his statement here: http://www.lescienze.it/news/2012/03/30/news/opera_ereditatos_point_of_view-938232/ http://www.lescienze.it/news/2012/03/30/news/opera_ereditato... What happened before is described in the Nautilus article: "In March of 2012, as the dust settled, the OPERA collaboration held another vote, this time to determine whether the collaborators had confidence in the leaders of the experiment. Each member institution got one vote. The tally ended at 16 to 13 favoring no confidence with several abstentions, well short of the two-thirds majority required to impeach the leadership, but enough to send a strong message. Both the OPERA leader, Ereditato, and the experimental coordinator, Dario Autiero, resigned." He's, as far as I understand, now the director of the Laboratory for High Energy Physics at Bern University: http://www.lhep.unibe.ch/modx/index.php?id=86&uid=4 http://www.lhep.unibe.ch/modx/index.php?id=86&uid=4
stephengillie 11y ago
This rambling reminiscence missed a few of the bigger gravity study missions, such as GRACE.
- ajkjk 11y ago
  Do you mean for the word 'rambling' to take its negative connotations here?
  - sethrin 11y ago
    'Starts With A Bang' is generally lowest-common-denominator physics writing. I'd imagine any negative connotations to be well earned.
    - ajkjk 11y ago
      "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?
      - LanceH 11y ago
        It misses on the standard HN article by actually getting to a point within the first 3 paragraphs.
        kristianp 11y ago
        Indeed, so many articles posted really are rambling. This one isn't.
      - tjradcliffe 11y ago
        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.
deleted 11y ago
[deleted]
foobarian 11y ago
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
- matt_kantor 11y ago
  This is mentioned in the article: > The pioneer probes were nuclear powered through a radioisotope thermoelectric generator (RTG). These RTGs usually give off heat uniformly, but the design of Pioneer meant that some of the radiant heat was reflecting off the probe resulting in a thermal recoil.
- akallio9000 11y ago
  So if the spacecraft could be stationary, it would pick up 400km/year acceleration via nuclear power?
  - clort 11y ago
    No, your maths is wrong.. in fact after 10 years the acceleration had provided a 1km/hr velocity, resulting in a 400km displacement from its projected position.
  - david-given 11y ago
    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.
hyyypr 11y ago
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?
- danbruc 11y ago
  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.
- abstractbeliefs 11y ago
  The same reason as when you throw a ball into the air, and it slows down despite moving away from the Earth (until it eventually stops and starts to fall again).
Pyxl101 11y ago
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.
- beyti 11y ago
  possibly the best article I've read for a long time in terms of "knowledge gain/required knowledge". totally agreed

186 points by wmeredith 11y ago

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.

ajkjk 11y ago
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.
- phkahler 11y ago
  Thermal expansion?
  - ajkjk 11y ago
    I can't tell what your actual question is.
    - phkahler 11y ago
      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.
Retric 11y ago
The neutrino error was very much people saying, we don't think this is accurate, but we don't know why and we are not going to hide this data. I saw a video of the press release and the guy looked like he really did not want to be saying those words. Now, after they said that plenty of people ran with it way to far. However, I feel the team did the right thing.
- deciplex 11y ago
  Didn't the guy end up having to resign because of the media circus? It was a pretty sad story IIRC.
  - sohkamyung 11y ago
    Nautilus has an interview with him (Antonio Ereditat) [ http://nautil.us/issue/24/error/the-data-that-threatened-to-break-physics http://nautil.us/issue/24/error/the-data-that-threatened-to-... ]. From his point of view, he did the 'right thing' by announcing the apparent FTL result and inviting people to find out what was wrong.
    - deciplex 11y ago
      He absolutely did the right thing. But in the end I think he resigned because the media circus that followed the announcement meant he could no longer contribute effectively to the project. https://en.wikipedia.org/wiki/Faster-than-light_neutrino_anomaly#Discussions_within_the_OPERA_collaboration https://en.wikipedia.org/wiki/Faster-than-light_neutrino_ano...
      - plonh 11y ago
        The media circus died down in a few weeks. Why wouldn't he be welcomed back? Why is science drowned in political crap?
        acqq 11y ago
        Read his statement here: http://www.lescienze.it/news/2012/03/30/news/opera_ereditatos_point_of_view-938232/ http://www.lescienze.it/news/2012/03/30/news/opera_ereditato... What happened before is described in the Nautilus article: "In March of 2012, as the dust settled, the OPERA collaboration held another vote, this time to determine whether the collaborators had confidence in the leaders of the experiment. Each member institution got one vote. The tally ended at 16 to 13 favoring no confidence with several abstentions, well short of the two-thirds majority required to impeach the leadership, but enough to send a strong message. Both the OPERA leader, Ereditato, and the experimental coordinator, Dario Autiero, resigned." He's, as far as I understand, now the director of the Laboratory for High Energy Physics at Bern University: http://www.lhep.unibe.ch/modx/index.php?id=86&uid=4 http://www.lhep.unibe.ch/modx/index.php?id=86&uid=4

stephengillie 11y ago

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

ajkjk 11y ago
Do you mean for the word 'rambling' to take its negative connotations here?
- sethrin 11y ago
  'Starts With A Bang' is generally lowest-common-denominator physics writing. I'd imagine any negative connotations to be well earned.
  - ajkjk 11y ago
    "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?
    - LanceH 11y ago
      It misses on the standard HN article by actually getting to a point within the first 3 paragraphs.
      - kristianp 11y ago
        Indeed, so many articles posted really are rambling. This one isn't.
    - tjradcliffe 11y ago
      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.

deleted 11y ago

[deleted]

foobarian 11y ago

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

matt_kantor 11y ago
This is mentioned in the article: > The pioneer probes were nuclear powered through a radioisotope thermoelectric generator (RTG). These RTGs usually give off heat uniformly, but the design of Pioneer meant that some of the radiant heat was reflecting off the probe resulting in a thermal recoil.
akallio9000 11y ago
So if the spacecraft could be stationary, it would pick up 400km/year acceleration via nuclear power?
- clort 11y ago
  No, your maths is wrong.. in fact after 10 years the acceleration had provided a 1km/hr velocity, resulting in a 400km displacement from its projected position.
- david-given 11y ago
  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.

hyyypr 11y ago

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?

danbruc 11y ago
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.
abstractbeliefs 11y ago
The same reason as when you throw a ball into the air, and it slows down despite moving away from the Earth (until it eventually stops and starts to fall again).

Pyxl101 11y ago

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.

beyti 11y ago
possibly the best article I've read for a long time in terms of "knowledge gain/required knowledge". totally agreed