[HN Gopher] Gravity Probe B
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Gravity Probe B
Author : lelf
Score : 40 points
Date : 2022-08-14 15:09 UTC (7 hours ago)
(HTM) web link (en.wikipedia.org)
(TXT) w3m dump (en.wikipedia.org)
| purplejacket wrote:
| Wikipedia: <<At the time of their manufacture, the gyroscopes
| were the most nearly spherical objects ever made (two gyroscopes
| still hold that record, but third place has been taken by the
| silicon spheres made by the Avogadro project). Approximately the
| size of ping pong balls, they were perfectly round to within
| forty atoms (less than 10 nm). If one of these spheres were
| scaled to the size of the Earth, the tallest mountains and
| deepest ocean trench would measure only 2.4 m (8 ft) high.>>
|
| I wonder how this compares to the sphericity of a neutron star.
| btilly wrote:
| We've never been up close, but the neutron star is probably
| smoother.
|
| A neutron star has a diameter of about 20km and surface
| irregularities are currently estimated at under 1 mm. (See
| https://www.livescience.com/millimeter-tall-neutron-star-mou...
| for verfication.) Scale that up to the Earth's diameter and
| irregularities are on the scale of 64 cm.
| dmurray wrote:
| Are neutron stars spherical or do they tend to become oblate
| spheroids (even if perfectly smooth) because, like the Earth,
| they spin on an axis?
| dzdt wrote:
| The data analysis done to try to salvage results from this
| mission were nothing short of heroic.
|
| See [1] for more detailed discussion of how the Gravity B team
| pushed on with analysis in the face of systematic errors much
| larger than the effect they were trying to measure, to come out
| in the end with a believable measurement of the incredibly subtle
| effect of gravitational frame dragging.
|
| [1]
| https://ddd.uab.cat/pub/tesis/2016/hdl_10803_400663/chev1de1...
| nine_k wrote:
| <<As it was anticipated that "anything could go wrong", the
| final part of the flight mission was calibration, where amongst
| other activities, data was gathered with the spacecraft axis
| deliberately misaligned for 24 hours, to exacerbate any
| potential problems. This data proved invaluable for identifying
| the effects.>>
|
| One should always be open to the possibility of a failure, and
| do everything to meet it well-prepared!
| akkartik wrote:
| _" An unusual feature of the mission is that it only had a one-
| second launch window due to the precise orbit required by the
| experiment."_
| wcoenen wrote:
| Instantaneous launch windows are not really that unusual.
|
| For example, all the missions to the International Space
| Station are like that. They launch when the launch point passes
| through the plane of the ISS orbit.
| a9h74j wrote:
| Back in the day, I heard it said of the Fairbanks group, that
| they could have a collective publication career if they did no
| more than publish all the systematic effect discoveries they made
| in attempting very difficult experiments.
|
| Same group that "measured" a magnetic monopole and fractional
| electric charge, IIRC.
| mjs wrote:
| Gravity Probe B is my favourite example of scientists spending a
| lot of money and effort to verify something almost everyone
| believed to be true, because science.
| hcrean wrote:
| If they had been wrong it would have catapulted humanity
| forwards in understanding. Instead it just affirmatively ruled
| out a big class of future mistakes...
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(page generated 2022-08-14 23:01 UTC)