Has Gravity Probe B been a waste of money?

  1. Garth

    Garth 3,513
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    After three months in orbit, following its launch on 20 April, Gravity Probe B has now entered its science phase. This success comes after about 40 years of a long saga of planning and construction and three unsuccessful cancellations by NASA in ’89, ’93, and ‘95.
    The project went through several external reviews of its scientific merit and technical readiness while its price tag kept on growing from around $130 million to $700 million.
    The experiment has launched four almost perfectly spherical super-cooled gyroscopes into a polar orbit to test whether they precess according to the predictions of GR. It is measuring two key precessions, a north-south geodetic precession caused by the curvature of space-time and a frame-dragging or gravito-magnetic precession in an east-west direction.
    Many cosmologists, such as Kenneth Nordtvedt, have said that the experiment was worth doing when it was first planned in the 1960s, but that today the result is a foregone conclusion. If so then GPB will have been a colossal waste of money that sapped funds from other more worthy programmes.
    The subject of this thread is to question whether Kenneth Nordtvedt is correct or not. At the heart of the issue is whether GR and the concordance model of cosmology is so robust that it needs no further testing, or whether there are genuine grounds for questioning it. We have an opportunity to debate these issues before the results of the experiment become known in 2006 and unequivocally settle the matter one way or the other.
    So the question is, “Has the GPB money been well spent or has it been a waste of money?”
     
  2. jcsd
  3. Chronos

    Chronos 10,134
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    Interesting question. I think it is worthwhile. The pioneer anomoly is still troubling and this may shed some light on it. Also, checking out the frame dragging prediction in GR is of considerable interest. It has not yet otherwise been tested with any precision AFAIK.
     
  4. chroot

    chroot 10,426
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    Well, nothing in science is really a foregone conclusion. It's poor scientific practice to just assume the results of experiments. Although admittedly general relativity now has enough evidence to convince nearly anyone, it is entirely possible that we'll discover a new phenomenon in these very precise experiments.

    - Warren
     
  5. Gravity probe B has not been a waste of money.If it confirms frame dragging then we
    will know for sure that the fundamentals of GR are right.This means that problems such as anomalous star velocities in galaxies and also anaomalously high galaxy velocities in galaxy clusters must be explainable in terms of the presence of
    additional mass such as dark matter.So resources can be focused on research into dark matter,dark energy etc.Once we know the fundamentals of GR are right we can start asking why we can't get quantum mechanics to "mix" with it.A positive result confirming frame dragging from gravity probe B will focus minds.
     
  6. turbo

    turbo 7,366
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    I agree that it is worthwhile and that GR can benefit from the verification. On this same subject, I read somewhere that Kip Thorne thought the GBP project is still worthwhile science. He also said that most theorists expect GR to fail at some level, and predicted that the failure could happen in a surprising manner.

    What will happen, though, if (for instance) the frame dragging effect causes the gyroscopes to deflect significantly more or less than predicted? Would physicists say "back to the drawing board with GR" or would they demand another very sensitive test (presumably by another method) as verification? That could get pretty expensive. :yuck:
     
  7. I don't think they would get money for another test and I don't think they should.
    A significantly different result than expected would mean that theorists have got a lot of work to do to justify another handout of cash.
     
  8. Garth

    Garth 3,513
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    Of course checking any scientific theory has "value", but if you were allocating limited funds would you have given the money to GPB or to another project, say looking for life on Mars?
    Kenneth Nordtvedt's point was that GPB was checking the Robertson parameter 'gamma', but that had now been evaluated to a high degree from timing radio pulses close to the Sun.
    How valuable is the "value" of double checking GR, how might GR fall short and how significant would that be?
    Garth
     
  9. chroot

    chroot 10,426
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    Well, it's difficult to compare the use of funds across different branches of science. If you restrict your attention to physics alone, I believe GPB is a worthwhile physics experiment. If you want to consider astrobiology in the same breath, the water gets considerably muddier.

    - Warren
     
  10. turbo

    turbo 7,366
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    You're right in that respect. What's the cost/benefit ratio associated with what is assumed by many to be a trivial confirmation of GR? GR's problems (requiring dark matter to be put in by hand with very specific distributions and densities that are very dependent on the purposes it needs to perform in each circumstance) arise at galactic scales and galactic cluster scales. These are complex domains with high concentrations of matter. GR seems to work very well in simpler environs, like the Solar System, where the relevant masses are dense spherical objects (and a lot fewer of them), so I fully expect the probe's results to be confirmatory to a high degree of confidence. The double-check could result in a level of over-confidence in GR that is unwarranted given it's predictive performance at very large scales.
     
  11. Nereid

    Nereid 4,014
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    Well, the $$ has been spent, so the only way to answer the question is by defining - before starting to answer - how you would decide what constitutes 'well spent' and 'a waste of money' (and any other alternatives).

    One approach is to 'second guess' the decision makers at each milestone of the project ... given what they knew at those times and NOT what we know now was the decision to continue reasonable? The only realistic answers to such questions must be couched in terms of the relative merits of competing proposals/uses for the $$ at the time.

    Another - similar - approach is to ask, again at each milestone, whether the scientific objectives of the project could realistically have been attained more economically by spending the remaining $ on a different project.

    These things are hard to stop - rarely is the case for stopping so overwhelming, and the further along a project gets, the more is invested in it (not just $$), and you just can't admit that all our troops' lives were in vain by pulling out, now can you? :tongue2:
     
  12. Garth

    Garth 3,513
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    Yes - my point is not really fiscal but by using the cash $ value I was trying to ascertain views on the scientific value of the experiment.
    For me the value of GPB as against all other measurements to date is in the fact that all other tests of GR have essentially tested the whether the path of freely-falling particles and photons are geodesics of the vacuum GR field equation.
     
  13. Garth

    Garth 3,513
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    A report in today's New Scientist "Neutron stars steal space probe's glory" (11 Sep p10) would seem to indicate that GPB has been upstaged and therefore its funding wasted.

    In their paper "Measurement of gravitational spin-orbit coupling in a binary pulsar system",(http://arxiv.org/abs/astro-ph/0408457) Stairs, Thorsett and Arzoumanian confirm that the geodetic and gravitomagnetic precessions of the axes of the neutron stars in the binary pulsar system PSR B1534+12 are as described by GR.

    However, as I have said before, deductions made from raw astronomical data are theory dependent, change the theory and those deductions may change too, so this does not necessarily imply that the precessions of the GPB gyroscopes must also be described by GR.

    I believe Francis Everitt (GPB project director) was correct to insist that these observations of binary pulsars do not render the GPB experiment obsolete.

    Gravitation may be best described by a non-metric, or semi-metric theory, such as that of SCC, in which highly relativistic particles, the degenerate material of these neutron stars, behave differently to ordinary matter. (see for example, "Experimental tests of the New Self Creation Cosmology and a heterodox prediction for Gravity Probe B", http://arxiv.org/abs/gr-qc/0302026 ) In this case the precessions of non-relativistic gyroscopes will differ from those of neutron stars.

    This may yet be demonstrated by GPB.

    - Garth
     
    Last edited: Sep 9, 2004
  14. turbo

    turbo 7,366
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    If GPB shows the the precession effect predicted by SCC (5/6th of that in GR), I'll bet you stand a pretty good chance of getting that orbiting interferometer. :cool: It would be a WHOLE lot faster and cheaper to build than GPB was, too.

    Regarding the Casimir Force experiment for (relatively) flat space-time, do you have a (non-technical please for this math-challenged guy :smile:) mechanism by which ZPE interacts with the gravitational field? Is this interesting wrinkle more properly seen as an artifact of doing the math in the JF(E) coordinate system with as-yet unknown mechanism? Certainly, it makes SCC falsifiable (although a trans-Jupiter probe=$$$$$$$$)!
     
  15. Nereid

    Nereid 4,014
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    I think this is *much* better question! :approve:

    So, leaving aside filthy lucre, it seems to me important aspects of an evaluation might include:
    - to what extent would a previously untested aspect of GR be tested?
    - how important is GR within physics, cosmology, etc?
    - looking ahead, how likely is it that any new aspects might be tested in other ways?
    - are there any competing theories in this domain? If so, to what extent would {GPB} be able to discriminate among them?
     
  16. Nereid

    Nereid 4,014
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    When you previously wrote things like this I let it slide; however, having been alerted by zforgetaboutit - wrt the CMBR (she included a link to a paper by Tegmark) - I started to think more on this.

    Would you please say more about what you mean here? For example, to what extent to you feel that exploration of the consistency of observational data with different (physical, cosmological) theories is limited by the data?
     
  17. Nereid

    Nereid 4,014
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    Perhaps I missed something; isn't LISA just such an orbiting interferometer?
     
  18. turbo

    turbo 7,366
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    Yes it is - I don't think it would detect the effect predicted by SCC (photons fall faster than particles), though. Garth's proposal was for an interferometer that would invert the light path every time the instrument orbited the Earth - an experiment specifically designed to test this effect. LISA might be able to test SCC, because the LISA array will tilted 30 degrees off perpendicular, making the beams pass through some gradient in the Sun's gravitational field almost all the time. The fact that the craft's orbits cause the triangular array to appear to rotate once every year would mean that the effect could not be "baselined out" at start-up and ignored. I just don't know if the effect (periodicity = one year) would rise above the noise of the system, though. LISA is designed to look for transient events, and the crafts' positioning relative to one another will not be rigidly fixed.
     
  19. Garth

    Garth 3,513
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    The first step would be to modify a LIGO interferometer by truncating one of its beams and sending it straight back, the Sun will do the rest. – And that would be even cheaper!

    Actually the experiment need not be too expensive, it could be miniaturized and hitch a lift on another deep space probe such as one to Saturn or the outer solar system (Pluto Express?)

    A quick explanation of this aspect of SCC, the details are in the published papers. Two of its principles, Mach and the Local Conservation of Energy yield two solutions of the gravitational field around a static, spherical mass. These converge when r tends to infinity, but slightly diverge in the presence of curvature. This is because the 'Casimir-force' virtual electro-magnetic field contains energy but is not coupled to the Machian scalar field. The harmonisation of these two solutions requires the vacuum to have a small density. In a ‘hand-waving’ explanation: curvature “tries to force the two solutions apart”, but the requirement for consistency between them “draws” energy from the false vacuum, which then becomes observable. This is made up of contributions of zero-point energy from every quantum matter field, which has a natural re-normalised ‘cut-off’ Emax determined, and therefore limited, by the harmonisation of these solutions.

    1. I think there are three possibilities: i. GPB behaves exactly as predicted by GR, ii. there is a slight deviation at the one part in 10^(3 or4) level that will open the way to modify GR to allow integration with quantum gravity, or iii. it behaves unexpectedly. As I have said before all tests to date have essentially tested the GR vacuum field equation, and asked, ‘do particles/photons travel on geodesics/null geodesics’? GPB does not – although now the double pulsar PSR B1534+12 also provides an alternative test.
    2. I think you know how important GR is within physics and astronomy, the fact that it cannot be reconciled with quantum gravity speaks of both being as yet incomplete, but any replacement must reproduce GR’s successes and therefore reduce to GR in some respect, e.g. GR being its first approximation etc.
    3. Looking ahead from SCC’s point of view the crucial question will be to test the equivalence principle by comparing “how particles and photons fall”. (See my thread on the subject). [The experiment would measure how far a horizontal beam of light bends towards a gravitating body as suggested above –and below]

    “I let it slide” – I am not sure what you mean, is my statement “deductions made from raw astronomical data are theory dependent, change the theory and those deductions may change too “ not self-evident?

    For example if gravitation is adequately described by GR then the observation that space-time is flat means the total density parameter is unity. But in BD part of that density is scalar field energy and in SCC space-time flatness means a total density parameter of one third. Thus the conclusion about how much Dark Matter and Energy is out there depends on which gravitational theory you use to analyse the data with.

    As far as the binary pulsars are concerned in SCC; as they are neutron stars their internal matter is relativistic and decoupled from the SCC scalar field force. They therefore behave exactly as in GR.

    In order to answer your query about the LISA interferometer, which will not detect the difference between GR & SCC, I need first to repeat one property of SCC. In the theory test particles and photons travel on the geodesics of GR. [the presence of the BD type scalar field perturbs space-time but the SCC scalar field force on particles exactly compensates for this] Therefore in SCC the LISA beams behave exactly as in GR. However in my modified LIGO apparatus where the beam is perturbed by the Sun’s gravitational field, or my ‘space interferometer’, in which one half of a split beam is sent around a circular race-track of mirrors for 2km and re-combined with the other split-beam that has traversed just a metre or two, the beam is being compared with physical mass of the apparatus. The SCC deflection of the beam relative to the apparatus towards the gravitating body is tiny, about 1Angstrom, but detectable by the interferometer, of course the GR deflection is null.
     
    Last edited: Sep 10, 2004
  20. turbo

    turbo 7,366
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    Thank you for the very illuminating explanation! I have been wondering for some time about how the ZPE fields can be affected by curvature, and what contributions of these fields can make to the properties of matter embedded in them.

    https://www.physicsforums.com/showthread.php?t=37724
    https://www.physicsforums.com/showthread.php?t=28868

    I am hampered by inadequate math, however and have been mining Citebase for papers that I can understand well enough to connect the dots. Your explanation has been more valuable than months of digging.

    If SCC is correct, and ZPE is proportional to the difference between the Mach and LCE solutions (which diverge with increasing curvature), then ZPE should be a very strong player in galaxies and galactic clusters, and should be a bear in the vicinity of a black hole. If that is so, black holes should evaporate much more quickly in SCC than as predicted under GR. The real particles created by the capture of their antiparticles by the black hole will be promoted to their "real" states at extremely high energies.

    Since both members of the virtual particle-antiparticle pair have mass, the black hole will swallow either with no preference. The area outside the event horizon should therefor consist of a mix of real particles and antiparticles at very high energy states, producing some very "interesting" interactions. It seems to me that no black hole can ever appear black under these conditions. Could this be the source of quasar luminosity?

    As I said above, my math is inadequate to model this. Have you done so, Garth?

    Thank you again for your explanation!
     
    Last edited: Sep 10, 2004
  21. Phobos

    Phobos 2,020
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    Don't let it be said that chroot doesn't have a sense of humor!

    astrobiology...search for water...ah, never mind
     
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