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  1. 26 May '14 17:35 / 6 edits
    http://physicsworld.com/cws/article/news/2014/may/26/quantum-accelerometer-is-being-built-for-navy-submarines
    "...
    The device will allow submarines to pinpoint their position underwater to within 1 m after travelling one day, without having to surface to use GPS. This is much better than is possible with current accelerometers, which are accurate to within 1 km after a day's travel.
    ..."

    I am surprised that an accelerometer can be made to be THAT accurate!
    Perhaps one day, in the far future, all humans would have extremely accurate tiny accelerometers and rotation detectors implanted into our heads to artificially give us massively accurate sense of balance and incredible navigation skills?

    it also says:

    "..
    ...
    The team is now working on shrinking the optical and electronic components of the accelerometer so that it can fit into about 1 m3. While this would make it suitable for naval use, the device would have to be further miniaturized – to the size of a beer can, for instance – before it could be sent down an exploratory bore hole to search for oil or other mineral deposits. Other possible applications that could emerge in a 5–10-year timeframe include "gravity scanners" that can peer into sealed containers and create density maps of their contents.
    ..."

    -that could also be useful.

    the link also mentions some research into the more theoretical physics:
    "...
    in another recent development in the field, physicists in Germany and the US have used atomic interferometry to measure the effects of gravity on two different atoms: rubidium and potassium. The experiment found that the acceleration due to gravity experienced by both types of atoms is the same to one part in 10 million. This is the latest verification of the universality of free fall, which is a cornerstone of Einstein's general theory of relativity. The rubidium atoms are more than twice as massive as their potassium counterparts. If they were seen to respond differently to gravity, it could point physicists towards a quantum-mechanical theory of gravity.
    ...."
  2. Standard member DeepThought
    Losing the Thread
    26 May '14 20:46
    Originally posted by humy
    http://physicsworld.com/cws/article/news/2014/may/26/quantum-accelerometer-is-being-built-for-navy-submarines
    "...
    The device will allow submarines to pinpoint their position underwater to within 1 m after travelling one day, without having to surface to use GPS. This is much better than is possible with current accelerometers, which are accurate to within 1 ...[text shortened]... ly to gravity, it could point physicists towards a quantum-mechanical theory of gravity.
    ...."
    It's nice to see a precision test of the principle that a body falling in a vacuum accelerates independently of its mass.
  3. Standard member sonhouse
    Fast and Curious
    26 May '14 21:52
    Originally posted by humy
    http://physicsworld.com/cws/article/news/2014/may/26/quantum-accelerometer-is-being-built-for-navy-submarines
    "...
    The device will allow submarines to pinpoint their position underwater to within 1 m after travelling one day, without having to surface to use GPS. This is much better than is possible with current accelerometers, which are accurate to within 1 ...[text shortened]... ly to gravity, it could point physicists towards a quantum-mechanical theory of gravity.
    ...."
    Doesn't that universal thing fail when two bodies the same mass come together?

    Like Earth has G = 9.8 M/S^2 and if another Earth mass planet was to fly down here, wouldn't the closing distance be 19.6 M/S^2?
  4. 27 May '14 06:11
    Originally posted by sonhouse
    Doesn't that universal thing fail when two bodies the same mass come together?

    Like Earth has G = 9.8 M/S^2 and if another Earth mass planet was to fly down here, wouldn't the closing distance be 19.6 M/S^2?
    Each would be free falling in a gravity well that gives all objects (even atoms) an acceleration of 19.6 M/S^2.
    The acceleration is independent of the object being accelerated, but dependent on the objects creating the gravity well.
  5. Standard member DeepThought
    Losing the Thread
    27 May '14 11:25 / 1 edit
    Originally posted by sonhouse
    Doesn't that universal thing fail when two bodies the same mass come together?

    Like Earth has G = 9.8 M/S^2 and if another Earth mass planet was to fly down here, wouldn't the closing distance be 19.6 M/S^2?
    The figure of 9.8 N/Kg only applies at the earth's surface, in your example the planets would be torn apart by tidal forces long before then. The force from earth 1 exerted on earth 2 is -GM^2/r^2 (and vice versa) so the acceleration of earth 2 is -GM/r^2. But this is relative to the centre of mass of the two earth system, so the rate at which their relative velocity changes is -2GM/r^2.

    In the case of an atom falling towards the earth, the centre of mass of the earth atom system is experimentally indistinguishable from the centre of mass of the earth, so the Earth's acceleration is virtually zero and the atom's acceleration is 9.8m/s^2.
  6. 27 May '14 14:49
    Originally posted by DeepThought
    It's nice to see a precision test of the principle that a body falling in a vacuum accelerates independently of its mass.
    Yes... We do that all the time... We call them satellites.

    They are all constantly falling in a vacuum.
  7. 27 May '14 14:51
    Originally posted by DeepThought
    The figure of 9.8 N/Kg only applies at the earth's surface, in your example the planets would be torn apart by tidal forces long before then. The force from earth 1 exerted on earth 2 is -GM^2/r^2 (and vice versa) so the acceleration of earth 2 is -GM/r^2. But this is [b]relative to the centre of mass of the two earth system, so the rate at which t ...[text shortened]... he earth, so the Earth's acceleration is virtually zero and the atom's acceleration is 9.8m/s^2.[/b]
    An incoming planet would only be torn apart by tidal forces if it's orbiting the earth.

    If, it's coming [effectively] strait in it just piles into us.
  8. Standard member DeepThought
    Losing the Thread
    27 May '14 15:40
    Originally posted by googlefudge
    Yes... We do that all the time... We call them satellites.

    They are all constantly falling in a vacuum.
    Hardly, the atmosphere persists out to about 10,000 km, the orbit there is still some atmosphere. There is also the solar wind and there are gravitational perturbations from the moon and the sun. There is just too much going on for a precision test of the principle, which the article said was to one part in 10 million.
  9. 27 May '14 18:26
    Originally posted by DeepThought
    Hardly, the atmosphere persists out to about 10,000 km, the orbit there is still some atmosphere. There is also the solar wind and there are gravitational perturbations from the moon and the sun. There is just too much going on for a [b]precision test of the principle, which the article said was to one part in 10 million.[/b]
    Surely the orbits of the planets are known to that accuracy?
  10. 27 May '14 18:32
    Originally posted by googlefudge
    An incoming planet would only be torn apart by tidal forces if it's orbiting the earth.

    If, it's coming [effectively] strait in it just piles into us.
    Surely the near edge would be accelerated so much faster than the outer edge that it would get torn apart? It would only start piling into us once contact is made, but prior to that it should experience very significant tidal forces.
  11. Standard member sonhouse
    Fast and Curious
    27 May '14 18:52
    Originally posted by twhitehead
    Surely the near edge would be accelerated so much faster than the outer edge that it would get torn apart? It would only start piling into us once contact is made, but prior to that it should experience very significant tidal forces.
    But with 12,000 km diameter planets coming straight at each other, the final approach would be coming so fast I doubt there would be much TIME for such things as tidal stretching. It would be an awesome site though

    They say that is how we got our moon, a Mars sized body whacking into Earth and leftover debris forming the moon. And I understand it was less than 25,000 km from Earth ATT and receding at about 2 cm per year.
  12. Standard member DeepThought
    Losing the Thread
    27 May '14 19:30
    Originally posted by twhitehead
    Surely the orbits of the planets are known to that accuracy?
    The positions of the planets probably are known to that precision - since that is measured from the solar system's centre of mass. Mars' orbital speed is 24km/s, one part in 10 million of that is 2.4mm/s I doubt it's velocity is known to that precision. Getting to actual accelerations, the acceleration due to the Sun's gravity for Mars' at Aphelion is 21 mm/s^2. Which means that they'd have to be able to measure a difference in acceleration of the order of nanometres per second per second to test to the same precision.
  13. Standard member DeepThought
    Losing the Thread
    28 May '14 13:17 / 4 edits
    The caveat to what I'm saying is that the group who did the gravity test gave their result in terms of the Eötvös ratio and I have no idea what that is. I looked up the abstract, by following the above link you can get to it at http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.203002
    We simultaneously measure the gravitationally induced phase shift in two Raman-type matter-wave interferometers operated with laser-cooled ensembles of Rb87 and K39 atoms. Our measurement yields an Eötvös ratio of eta_{Rb,K} =(0.3±5.4)×10^-7. We briefly estimate possible bias effects and present strategies for future improvements.

    Does anyone know what an Eötvös ratio is?
  14. Standard member wolfgang59
    Infidel
    01 Jun '14 07:38
    Originally posted by DeepThought
    The caveat to what I'm saying is that the group who did the gravity test gave their result in terms of the Eötvös ratio and I have no idea what that is. I looked up the abstract, by following the above link you can get to it at http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.203002
    We simultaneously measure the gravitationally induc ...[text shortened]... d present strategies for future improvements.

    Does anyone know what an Eötvös ratio is?
    No, but I'm going to remember that for Scrabble!
  15. 01 Jun '14 13:29
    Originally posted by sonhouse
    But with 12,000 km diameter planets coming straight at each other, the final approach would be coming so fast I doubt there would be much TIME for such things as tidal stretching. It would be an awesome site though
    Relative velocity is not important - unless we take relativity into account, but assume that we are not dealing with relativistic speeds. Just before impact, the near sides of the planets would be experiencing significantly more acceleration than the far sides. Both the planets would disintegrate.