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Regular empirical evidence of curved space or massless photons?

  1. Mar 19, 2013 #1
    NOT including the prediction capabilities of the particular math equations of GR or SR.

    In particular, hard evidence such as, or close to; here's an electron, because we measured it directly, or saw it in an electron microscope.
    Or heres a cell under a microscope.
    Or this is a brain scan/MRI etc of persons on LSD-25, these regions of the brain are active during lsd.
    Or heres x-chemical in this plant, because we analysed it in a GS-MS.
    Or this star emits x-form of electromagnetism.

    On this basis,

    *Is there any basically direct measurements/hard evidence of the curvature of spacetime?

    *Is there any basically direct measurements/hard evidence of the lack of mass in a proton?

    And if so, what is it?
    Last edited: Mar 19, 2013
  2. jcsd
  3. Mar 19, 2013 #2


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    How can evidence of the correctness of a theory not be matched to the predictions of the theory? What measurements are allowed and what aren't? Your request appears self-contradictory to me.

    Have you googled "measure mass photon"?
  4. Mar 19, 2013 #3
    Gravitational lensng is the most direct test of gravitational curvature I can think of. Because the photon is massless, it the fact that it's path can be altered by gravity naturally implies curvature (because then the photon would be moving "unnaffected by gravity", but not in a straight line). Also gravitational redshift also implies a curvature.

    For the lack of mass of a PHOTON, one way you could see this is that the photon never changes speed, and you never see photons travelling at different speeds. The only kind of path in spacetime that doesn't change speeds (including reference frame changes) is also associated with zero mass (the two imply each other).

    Of course, both of these are hard to measure. You can't just stick a "curvature meter" into space and read off the curvature. You also can't put a photon on a scale. But what we do have is a complicated network of experimental tests that work together to paint a theoretical picture that makes the photon mass less and space time curve. Some of these tests, as I have described above, are pretty direct.
  5. Mar 19, 2013 #4
    Yeah I found something which said it has not been measured but we can mathematically infer its maximum mass, and that if did have a mass it would have lots of implications basically. Wikipedia says something similar.
    Last edited: Mar 19, 2013
  6. Mar 19, 2013 #5


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    Sure. Spacetime curvature is the same thing as tidal gravity; there are lots of direct measurements of tidal gravity.

    I suspect what you really want to ask is what evidence do we have that GR is valid? A good reference for that is here:

    http://relativity.livingreviews.org/open?pubNo=lrr-2006-3 [Broken]

    You mean "photon", I assume. Try these:


    http://www.aip.org/pnu/2003/split/625-2.html [Broken]
    Last edited by a moderator: May 6, 2017
  7. Mar 19, 2013 #6


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    What exactly do you measure here? What exactly did you see? What is "directly"?

    If you look at the evidence for the existence of the electron, you'll see that the concept to show what it is is no different than our assertion of the massless photon and the field equation for GR.

    You have an a priori prejudice against these two, based on your previous posts. You should re-examine your acceptance of other concepts but not these.

  8. Mar 19, 2013 #7
    I like your answer. Thanks.

    But does a neutrino vary in speed, or slow down that we can detect?

    Seems like we are clocking them at either at the speed of light, or so damn close we cant pin it.

    They are thought to have a mass I think.
  9. Mar 19, 2013 #8
    Edit: you posted right before I posted this.

    (That's basically just how measurements work in science. We have measurement of the mass, and it has some error in it. If the error was not big enough to include zero, then it would be evidence that it has something other than zero mass. But so far, experiments are consistent with it being zero mass, and we have a lot of theoretical support for that idea.

    Absolutely nothing is 100% certain in science, though for all practical purposes they are. There is still a very small chance that everything we have measured has been a statistical fluctuation, but the odds are unfathomably small.

    Consider every example of a "direct measurement" you gave. Many of them involve the physics of a photon, the knowledge of which you just called into question. So those "direct" measurements all involve using something that is only known "indirectly".

    Edit: (I thought you didn't understand this, but it seems you did. Sorry to repeat the same point)
    As I tried to explain, you probably want to use some feature of massless particles other than it's mass to measure whether or not it is massless. Since it's mass has to be exactly zero in order to be massless, you will never successfully measure that property because you'll always have an error on the mass measurement. Instead you have to look for properties that distinguish it from a massive (but very small mass) particle, such as it's speed being independent of reference frame.)
  10. Mar 19, 2013 #9
    I don't actually know if neutrinos have been measured to have varying speeds, but they have been measured to have mass. However, we have strong bounds on photons having the same speeds.
    Last edited: Mar 19, 2013
  11. Mar 19, 2013 #10


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    That's true. For example, neutrinos from Supernova 1987A were observed basically concurrently with the light from the same supernova; if neutrinos traveled significantly slower than light they would have been seen quite some time later since the supernova was 168,000 light years away, so even a difference in speed of 1 part in 100,000 would have resulted in the neutrinos being seen a year after the light was.


    Yes, but only a very small mass, much much less than that of any other particle.

  12. Mar 19, 2013 #11
    Its just these two specific things, gravity and light, I'm curious about really. Thanks for the link. It may broaden my understanding alot.
    Last edited by a moderator: May 6, 2017
  13. Mar 19, 2013 #12
    Thats a cool example thanks.

    And yeah, thats what i thought regarding the speed.

    Thanks, this gives much more detailed reasoning for the neutrino having mass than where I was reading from.
  14. Mar 19, 2013 #13
    Okay that seems hmmm, fairly reasonable. I am out of my depth a bit, ill have to read some of these links and more in general, but I can understand this is more than a mere "nice fit" scenario, and so approaching the harder side of things.

    On this topic, has anyone tried to test neutrinos for their speed independance to reference frames/invariance like they have with light, or in general how neutrino speeds might vary or stay the same?
    Last edited: Mar 19, 2013
  15. Mar 19, 2013 #14


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    First, you need to understand what spacetime curvature is. Spacetime curvature is simply tidal gravity.

    From: http://en.wikipedia.org/wiki/Introduction_to_general_relativity#Tidal_effects
    "A deeper analogy relates tidal forces with a property of surfaces called curvature"

    And from: Thorne's "Black Holes and Time Warps"
    "space-time curvature and tidal gravity are the same thing expressed in different languages, the former in the language of relativity, the later in the language of Newtonian gravity."

    And in section 25.5.2 on Relativistic Description of Tidal Gravity: http://www.pma.caltech.edu/Courses/ph136/yr2012/1225.1.K.pdf
    "Spacetime curvature prevents them from commuting and thereby causes the relative acceleration."

    So the tides are direct evidence of spacetime curvature as is the fact that gravity points towards the north star at the south pole and away from the north star at the north pole.

    Note, because spacetime curvature is simply tidal gravity and Newtonian gravity includes tidal gravity, it turns out that Newtonian gravity can be expressed as spacetime curvature also. This is called Newton-Cartan gravity: http://en.wikipedia.org/wiki/Newton–Cartan_theory. So the difference between GR and Newtonian gravity is not spacetime curvature, but rather those "particular math equations" that you wanted to avoid.
    Last edited: Mar 19, 2013
  16. Mar 19, 2013 #15
    I just want answers relate directly from individual concept/postulate, to individual observation/test, not to whole models, or their predictive capabilities. They dont really have to be direct measurements, but, for me, there should be direct connection between the individual concept and its evidence.

    Models are useful, and their interpretations are commonly accepted.

    I just personally want something more conventional, in terms of basic scientific theory regarding hypothesis testing.

    Something that relates not to the whole model, theory, or math, and its accepted interpretation but instead to the individual specific hypotheses, in this case; of masslessness of light and gravity via spacetime curvature.

    As a pure analogy, I could propose that their are 11 dimensions, and that the whole universe is made of vibrating strings, and create math to describe it - if the math could very accurately predict stuff, that would be both very useful and very interesting but it would be more compelling, to me personally, if they could provide some form of evidence say, that there are 11 dimensions, or that things are made of vibrating strings.

    I am fully convinced of the invariance of c, because its been directly measured and calculated. This other particular stuff, I would like to be more convinced about, outside of the math's predictions/models.
    Last edited: Mar 19, 2013
  17. Mar 19, 2013 #16


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    The specific hypothesis itself relates to the whole theory and model, so I fail to see your objection as being reasonable. A hypothesis is the foundation of a theory, which is what a model is made from. The model enables us to make advanced and complicated predictions of the real world. And ALL of these predictions that can be tested can be used as evidence for the correctness of the underlying theory and hypothesis.

    You already run into a problem. String theory cannot predict NEW things which we have the capability of observing. That's exactly why it ISN'T compelling. Not yet. It may be promising, or interesting, but I wouldn't call it "compelling" in this sense. On the other hand, GR itself DOES make predictions which we are capable of testing, and when first developed it predicted and explained NEW things, not just the old.

    Considering the hypothesis, theory, and model are based off of math I don't think your request makes any sense. Observing anything at all follows known rules which have mathematical laws that govern how everything works. Things which aren't part of this math simply aren't explainable at this time.
  18. Mar 19, 2013 #17


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    But the invariance of c has never been measured or calculated, rather it is simply defined by Einstein's second postulate. Einstein's theory of Special Relativity turns out to be consistent with all measurements but so do other theories. We favor Einstein's theory over the others simply because it is simpler than the other theories. But there is no measurement that can decide between Einstein's postulate that assigns the speed of the propagation of light to be c in all Inertial Reference Frames and other theories that deny that postulate.

    By the way, where did you learn that the one-way speed of light has ever been measured as opposed to defined?
  19. Mar 19, 2013 #18
    curved space: Shapiro delay.


    When bouncing radio beams off Venus and the sun is in between, there's a slight delay compared to when the sun isn't in between but the Newtonian distance is equal. That's as direct as you're going to get.

    photon masslessness: this one is hard. The easiest way to think about it is that photon number isn't conserved in everyday life (turn off the lights at night. photon number rapidly diminishes from huge number to near 0) while numbers of particles with mass is conserved in everyday life.
  20. Mar 20, 2013 #19
    Well I thought there had been experiments that, ended up showing invariance, by seeing if motion effects the speed of light (such as the motion of the earth, or a distant star) - indirectly, but not terribly so. Thats what i read from wikipedia anyway.

    Ive also heard lots of people calculate/measured the speed of light, before it was fixed in 1975 to what is currently beleived to be the most accurate measure (by defining the meter based the distance travelled by light in vacuum in 1/299,792,458 of a second)

    Although I guess you make a point, c, which is thought of as a constant, and the speed of light, had not been defined as its current value, when einstein wrote that formula.

    Well, if true, that is interesting to learn. Although it does go against some of what I read.
  21. Mar 20, 2013 #20


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    Why don't you look up the wikipedia article on the Speed of Light and scroll down to the section called "Fundamental role in physics". Is there another wikipedia article that contradicts this one?
    As the article points out, it is possible to measure the two-way speed of light but not the one-way speed.
    It was thought of as a universal constant when Einstein wrote his 1905 paper. Look down at the end of the first section and you'll read:
    There's alot to read out there that isn't true.
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