Gravity Probe B | Hacker News Reader

Gravity Probe B

en.wikipedia.org

70 pointslelf15 comments

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.

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.

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?

I think they'd probably be spherical. They are super dense, so there's not much space between the atoms. That's what makes them dense and heavy. So imagine a bag of popcorn with all the little popcorn bits popping out at weird spots. Then take it and crush it in your hands to make a circle. It's still the same mass (weight), but now it's more dense since it's smaller. And as you squeeze the popcorn it sticks out less. That's mostly because you're squeezing the popcorn, and applying a force to drag things to the center. There's still gravity inside a star, so that's what gravity is doing. And the more dense it is, the more tightly packed, and so the gravity will keep it tightly together. So that's why I think it'd be spherical.

Yes magnetic fields and spin are expected to deform neutron stars slightly.

Ah yes good point. I didn't take into account the rotation of a star in my answer around its axis. But yes you're right. And that's why the earth is also kind of a spheroid (squished sphere).

Einstein you crazy bastard, you were so ahead of your time. I wish we could have met. I think you're quite a Feynman, if you catch my meaning.

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...

«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!

"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."

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.

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.

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.

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...

General relativity remains a marvel of abstract, intuitive thinking that has led to one of the most verified physical theories ever. I'm not sure there is any other theory that stands up to it in this regard. Other thoroughly verified theories, like the standard model, had an extremely tight iterative loop between theory and experiment during their development.

I'm having a hard time understanding this Wikipedia article. Can someone explain like I'm five what this probe actually did/proved? Thanks!

Reply on news.ycombinator.com