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Is the Shapiro time delay effect accepted in science ?

  1. Feb 16, 2012 #1
    If it is : I would love someone to explain to me how it affect the redshift of galaxies.

    If it is not : I will toss it in the pseudoscience box and not think about it again

    Thank you

  2. jcsd
  3. Feb 16, 2012 #2


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    guacamolewar, Welcome to Physics Forums!

    I am not sure if the Shapiro time delay applies to the redshift of galaxies. But, it sure does apply to radar signals here in our solar system. Here is an excerpt from the Wikipedia page describing tests (proofs) of General Relativity. His prediction has proven to be correct; do not throw it into the "pseudoscience box".

    “Irwin I. Shapiro proposed another test, beyond the classical tests, which could be performed within the solar system. It is sometimes called the fourth "classical" test of general relativity. He predicted a relativistic time delay (Shapiro delay) in the round-trip travel time for radar signals reflecting off other planets.[26] The mere curvature of the path of a photon passing near the Sun is too small to have an observable delaying effect (when the round-trip time is compared to the time taken if the photon had followed a straight path), but general relativity predicts a time delay which becomes progressively larger when the photon passes nearer to the Sun due to the time dilation in the gravitational potential of the sun. Observing radar reflections from Mercury and Venus just before and after it will be eclipsed by the Sun gives agreement with general relativity theory at the 5% level.[27] More recently, the Cassini probe has undertaken a similar experiment which gave agreement with general relativity at the 0.002% level. Very Long Baseline Interferometry has measured velocity-dependent (gravitomagnetic) corrections to the Shapiro time delay in the field of moving Jupiter [28][29] and Saturn.”

  4. Feb 16, 2012 #3
    Thank you.

    Could this effect be responsible for the redshift observed in galaxies?
    Last edited: Feb 16, 2012
  5. Feb 16, 2012 #4


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    I could find no reference that says the Shapiro Time Delay effect is responsible for the redshift of galaxies. However, your question is interesting because of, from the above reference, this: "general relativity predicts a time delay which becomes progressively larger when the photon passes nearer to the Sun due to the time dilation in the gravitational potential of the sun."

    So, when photons emitted by a galaxy climb out of the gravitational potential well on their way toward us, wouldn't they experience this time dilation? If so, would it be equivalent to a redshift? Or is that simply "gravitational redshift"? I defer to others more educated in this area.
  6. Feb 17, 2012 #5
    Er, wouldn't a photon experiencing time dilation undermine the entire point of time dilation? Or does it actually explain why a photon can't exceed it's measured speed??
  7. Feb 17, 2012 #6
    Don't see how. Shapiro delay changes the travel time, but it doesn't change wavelengths.
  8. Feb 17, 2012 #7


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    I don't quite understand this. It kinda has to, at least during the transition from a lower time delay to a higher time delay, and vice versa. Though I would definitely accept that it doesn't cause any change in wavelength along a path that has no change in the time delay.

    But in response to the OP, no, the Shapiro time delay can't realistically have anything at all to do with the redshift of galaxies, because if it were then you'd end up with dramatically different redshifts just because one galaxy happened to be behind another, and since as many galaxies are going to be transitioning into such an alignment as transitioning out of that alignment, there would be exactly as much redshift as blueshift. None of this remotely matches what we observe.

    However, the Shapiro time delay is useful in cosmology, and attempts have been made to use this time delay to, for example, measure the Hubble expansion rate. The basic setup is that they observe a lensed object which has two or more images, an object which has some sort of variation (quasars, for example, vary a lot, and a lot of lensed objects are quasars). In general, we expect that different paths that the photons take will have different travel times. So we might see one image brighten just a few moments before another image, and the difference in travel times gives us information about the curvature of space-time around that lens.
  9. Feb 17, 2012 #8
    The shapiro delay is basically a consequence of gravitational redshift applied to radar signaling. Accordingly it has little to do with cosmological redshift that is due to expansion.
    It certainly involves deviation from expected radar frequencies in the absence of gravitational time dilation.
    It is totally accepted by mainstream and considered the 4th of the classical test of GR.
  10. Feb 17, 2012 #9


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    Staff: Mentor

    Guacamolewar is a sockpuppet of a banned crackpot.
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