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Thread: Gamma ray jets

  1. #1
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    Gamma ray jets

    What do they mean here that the jets traveling faster than light because of the medium thus itís not breaking Relativity?

    https://bigthink.com/surprising-scie...light.amp.html

    I mean the net effect is the jets are traveling faster than light, no?


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  2. #2
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    No article, only three lines of summary. What is the source journal?


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  3. #3
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    Quote Originally Posted by philippeb8 View Post
    What do they mean here that the jets traveling faster than light because of the medium thus it’s not breaking Relativity?
    Relativity issues would only occur if they went faster than the speed of light in a vacuum. Light typically is slowed down somewhat by going through a medium (gas, liquid, solid).

    Here’s the abstract of the article this is based on:

    https://iopscience.iop.org/article/1...38-4357/ab3bdf

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    Quote Originally Posted by Van Rijn View Post
    Relativity issues would only occur if they went faster than the speed of light in a vacuum. Light typically is slowed down somewhat by going through a medium (gas, liquid, solid).

    Hereís the abstract of the article this is based on:

    https://iopscience.iop.org/article/1...38-4357/ab3bdf
    It's not clear what they are saying but it would appear they imply time is reversed instead of simply implying the jets travel faster than light...

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    Quote Originally Posted by philippeb8 View Post
    It's not clear what they are saying but it would appear they imply time is reversed instead of simply implying the jets travel faster than light...
    No, not at all. They are saying that because the jets move faster than the speed of light in a medium (which is nothing new - it is the speed of light in a vacuum that is the limit) you get a series of signals. One of which is a mirror image of the other (in time, not space) due to geometric effects. Hence the 'time reversed signal'. If you look at figures two and three in the paper linked below you can see why. The contrasting sections of superluminal and subluminal propagation combine to create a signal (which is a mix of Cherenkov and emission) that is a composite of the propagating pulse and a mirror image of it in time because the areas stimulated to emit are seen in reverse order as the disturbance propagates superluminally but the signal we observe propagates at c.

    See: https://arxiv.org/pdf/1908.07306.pdf

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    Quote Originally Posted by Shaula View Post
    No, not at all. They are saying that because the jets move faster than the speed of light in a medium (which is nothing new - it is the speed of light in a vacuum that is the limit) you get a series of signals. One of which is a mirror image of the other (in time, not space) due to geometric effects. Hence the 'time reversed signal'. If you look at figures two and three in the paper linked below you can see why. The contrasting sections of superluminal and subluminal propagation combine to create a signal (which is a mix of Cherenkov and emission) that is a composite of the propagating pulse and a mirror image of it in time because the areas stimulated to emit are seen in reverse order as the disturbance propagates superluminally but the signal we observe propagates at c.

    See: https://arxiv.org/pdf/1908.07306.pdf
    Ah... they played with words in the title and I got caught!

    Thanks for the clarifications!


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  7. #7
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    I wonder how fast they could push a sail. And what the sail would transmute into over time

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    Quote Originally Posted by Shaula View Post
    No, not at all. They are saying that because the jets move faster than the speed of light in a medium (which is nothing new - it is the speed of light in a vacuum that is the limit) you get a series of signals. One of which is a mirror image of the other (in time, not space) due to geometric effects. Hence the 'time reversed signal'. If you look at figures two and three in the paper linked below you can see why. The contrasting sections of superluminal and subluminal propagation combine to create a signal (which is a mix of Cherenkov and emission) that is a composite of the propagating pulse and a mirror image of it in time because the areas stimulated to emit are seen in reverse order as the disturbance propagates superluminally but the signal we observe propagates at c.

    See: https://arxiv.org/pdf/1908.07306.pdf
    The authors' crude analogy of the skipping rock is quite effective.

    But, given the vast mileage the gamma rays travel through a non-vacuum, is there any chance we might detect a wavelength dependence on speed? I think extensive tests have not detected one color faster through a medium over another. And for that matter, Maxwell's determination for c includes no wavelength variable, unlike scattering equations for mediums. So would a GRB, assuming the emissions consists of a range of wavelengths, traveling trillions of miles through a non-vacuum be an opportunity for a more advanced test? [Determining the initial emission conditions might be the hardest part.]
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    Quote Originally Posted by George View Post
    But, given the vast mileage the gamma rays travel through a non-vacuum, is there any chance we might detect a wavelength dependence on speed?
    Its a prediction of some quantum gravity theories, so as you can imagine people have been looking for it. I think that one of the first papers on it was published in the 1980s but it took until a few years ago for experimental evidence to catch up. When it did there is no evidence for linear or quadratic dispersion relationships (which I believe were two of the more common predictions). There are now some modified quantum gravity theories which avoid that prediction but I am not that familiar with them.

    See: https://arxiv.org/abs/1305.3463 for an example of deriving constraints on dispersion from GRBs.

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    Thanks Shaula. That reads, for me at least, about as powerful as the GRBs themselves.
    We know time flies, we just can't see its wings.

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