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Happy 95th Anniversary, Relativity 120

Posted by samzenpus
from the it's-all-relative dept.
StartsWithABang (3485481) writes "It's hard to believe, but there are people alive today who remember a world where Newtonian gravity was the accepted theory of gravitation governing our Universe. 95 years ago today, the 1919 solar eclipse provided the data that would provide the test of the three key options for how light would respond to the presence of a gravitational field: would it not bend at all? Would it bend according to Newton's predictions if you took the "mass" of a photon to be E/c^2? Or would it bend according to the predictions of Einstein's wacky new idea? Celebrate the 95th anniversary of relativity's confirmation by reliving the story."
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Happy 95th Anniversary, Relativity

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  • 95 years but (Score:4, Interesting)

    by rossdee (243626) on Friday May 30, 2014 @03:08AM (#47126931)

    its less than that time if it was travelling at significant speed

    • by Anonymous Coward

      Eh, it's all relative.

      • Eh, it's all relative.

        Except for the speed of light. That is absolute.

        • by Ol Olsoc (1175323)

          Eh, it's all relative.

          Except for the speed of light. That is absolute.

          Nope. Depends on the medium and it's velocity factor. http://en.wikipedia.org/wiki/V... [wikipedia.org]

          And no, creationists, that does not prove your whack-a-doodle variable speed of light conjecture.

          • by barlevg (2111272)
            The photons themselves are still traveling at c. What's "slowing them down" is that they're being absorbed and re-emitted by the atoms in the medium. The speed of light is absolute.
            • by Ol Olsoc (1175323)

              The photons themselves are still traveling at c. What's "slowing them down" is that they're being absorbed and re-emitted by the atoms in the medium. The speed of light is absolute.

              Cherenkov radiation

              • by barlevg (2111272)
                Exactly my point. The light in Cherenkov radiation isn't travelling faster than c, it's just going faster than the "c" for that medium.
                • Exactly my point. The light in Cherenkov radiation isn't travelling faster than c, it's just going faster than the "c" for that medium.

                  The particle triggering the radiation is, the Cherenkov photons themselves are not -
                  that's what makes those nice Mach-like Cherenkov cones.

  • by mpoulton (689851) on Friday May 30, 2014 @03:17AM (#47126959)
    95 years of confusing the heck out of second-semester physics students! You didn't think you signed up for a calculus-based philosophy class with numerical answers to epistemological questions...
    • by barlevg (2111272) on Friday May 30, 2014 @08:25AM (#47127753)

      The truth is, relativity doesn't have to be as confusing as it's usually made out to be. The most accessible explanation I've found for time dilation came from Brian Greene's The Elegant Universe [wikipedia.org]:

      Suppose you have a race car that can only go 100 m/s, no faster, no slower. Suppose it's racing down a very wide track that's 1km in length . Depending on the angle at which the car travels, it may cross the finish line in 10 seconds, 20, 50 or however long, just no less than 10 seconds. So similarly, we can think of our journey through the universe as happening along a "time" direction as well as three "space" directions: the faster we travel through space, necessarily the slower we travel through time, but no matter what, we're travelling at c.

      The math even works out, in terms of c=sqrt(v_x^2+v_t^2) where "v_t" (your velocity through time) is c*dtau/dt.

      This analogy obviously only gets you so far, and the real "wow" of relativity comes from the concepts of simultaneity (I wish more SF authors realized that FTL and time travel are the same friggin' thing), but especially for non-majors this is a great way to get one's foot in the door and begin to understand what is a pretty alien concept.

      • by TopherC (412335)

        Funny, I read that book (which is excellent) but don't remember that analogy. But I think you're talking about special relativity, not general relativity. The best GR explanation I've seen is an article Lost in Hyperbolia [coffeeshopphysics.com]. For me that explanation worked perfectly.

        Now I remember reading in various places that the solar eclipse data on GR was not actually conclusive. Bad science. The earlier work Einstein did that explained the precession of mercury's orbit was actually the first confirmation of GR. Also, of

        • by barlevg (2111272)

          Indeed. Parent was talking about Special (simultaneity).

          Re: Mercury's precession, I'm still a believer in Vulcan [wikipedia.org].

          • by TopherC (412335)

            Re: Mercury's precession, I'm still a believer in Vulcan [wikipedia.org].

            Yeah, even the term "disproves" is not exactly correct. Newtonian gravity has a very hard time explaining Mercury's precession and is completely untenable with today's observational evidence. General relativity explains Mercury's orbit without having to invent new invisible planets & stuff. And today General relativity is still doing spectacularly well with many careful neutron star observations as well as experiments closer to home, like Gravity Probe B's measurements of frame dragging and more.

        • by TopherC (412335)

          Oh, what I do remember from Bairn Greene's The Elegant Universe was his analogy for Bell's Inequality. Looks like that has been put up on Wikipedia [wikipedia.org].

    • I never found relativity to be that hard or confusing. Now quantum mechanics on the other hand.... *shudders*

  • 95? (Score:1, Funny)

    by Tablizer (95088)

    Meh, anniversaries are relative.

  • "Would it bend according to Newton's predictions if you took the "mass" of a photon to be E/c^2?" Did Newton predict that the 'bend' of something in a gravitational field, was related to its mass?? I always thought it was speed that mattered. Doesn't matter if its an atom, or a hammer., both will bend the same way.
    • by Sockatume (732728)

      Newton's law of universal gravitation doesn't include velocity at all; it's just the product of the masses over the square of the distance between them.

      • by Sockatume (732728)

        Ha, actually looking at the physics shows that the mass just cancels out and velocity is all that matters. Newton didn't predict this though, lacking a good velocity for light.

    • Re:Does mass matter? (Score:5, Informative)

      by TapeCutter (624760) on Friday May 30, 2014 @05:43AM (#47127313) Journal
      Newton's primary insight is the gravitational field, ie: two bodies attract each other with a force proportional to the combined masses and the distance between them. That he invented calculus to prove it and wrote it all down in his "Principia" is why he is remembered. A photon is neither a hammer , nor an atom. Photons did not have mass so they were believed to be unaffected by gravity. Einstein came along and said mass and energy are two forms of the same thing and a photon would be affected by gravity. The experiment in TFA allowed the universe to make the final call.

      Trivia: Newton's Principa contains only two explicit assumptions, one of them was the assumption that "time is constant".
      • by Anonymous Coward

        Trivia: Newton's Principa contains only two explicit assumptions, one of them was the assumption that "time is constant".

        I was under the impression that Leibniz though disagreed:

        https://en.wikipedia.org/wiki/Philosophy_of_space_and_time#Leibniz_and_Newton

      • Re:Does mass matter? (Score:5, Informative)

        by AthanasiusKircher (1333179) on Friday May 30, 2014 @08:57AM (#47127903)

        Newton's primary insight is the gravitational field, ie: two bodies attract each other with a force proportional to the combined masses and the distance between them.

        It's worth noting that this insight was not at all unique to Newton. There was, in fact, a major dispute [wikipedia.org] in the scientific community about who came up with this idea at the time, since Robert Hooke had already published on this notion. Other scientists had basically also postulated similar ideas in the decades before the Principia.

        That he invented calculus to prove it and wrote it all down in his "Principia" is why he is remembered.

        Yes -- Newton may have been the first to explicitly identify the specific inverse square relationship (rather than a general form relationship mentioned in the first quotation above), and he had the mathematical apparatus to prove how it all worked.

        But it's also important to be clear that the idea of a "gravitational field" or an "unseen force acting at a distance" was a very spooky and strange notion to contemporary scientists in Newton's era. In fact, such ideas were commonly associated with occult ideas; they didn't fit in with the conception of a simple mechanistic universe. Thus, Newton's idea of some strange unseen "force" acting across vast distances would seem like invoking the power of God or angels or some mystical astrological "force" today.

        Because of that, many scientists were initially very suspicious of Newton's methodology. Newton therefore wrote a clarification [wikipedia.org] as an appendix to the second edition of the Principia explicitly saying he was NOT assuming the existence of unseen forces and fields. Instead, he claimed his model was valuable simply because the mathematics were an accurate model. (Some historians have argued that this was in fact the most important element of Newton's revolution in thought: he argued for the acceptance of a mathematical model as a scientific explanation, even if we can't explain the underlying causes of that model.) Of course, Newton was a pretty weird guy and believed in all sorts of things that modern science would think weird, so obviously he thought the unseen forces were real. But it's interesting that he worked so hard to distance himself from such ideas at the time -- to be in accord with science of the time, the "force" in his model was thus to be considered a mere mathematical contrivance, rather than how the universe actually worked.

        • by drinkypoo (153816)

          Newton therefore wrote a clarification as an appendix to the second edition of the Principia explicitly saying he was NOT assuming the existence of unseen forces and fields. Instead, he claimed his model was valuable simply because the mathematics were an accurate model.

          But that was some pretty weaselly bullshit, because that's precisely what he was describing mathematically. And per your link, the text which you claim distances him from unseen forces and fields in fact assumes the existence of unseen forces and fields, simply attributing the forces to God.

        • by mattack2 (1165421)

          Of course, Newton was a pretty weird guy and believed in all sorts of things that modern science would think weird

          Covered in a 2005 episode of Nova. See it here
          http://www.pbs.org/wgbh/nova/p... [pbs.org]

    • by fermion (181285)
      This is where not understanding science makes everything wonky. The major problems with Newtonian physics is domain and history. In the later, we observe nature and sometimes see energy moving through a medium and call that a wave. We see object that we can hold an call that mass. Our experience then tells us that some things are waves and some things are mass. It is like an ancient person seeing a piece of wood catch on fire and saying that the fire was in the wood. Current experiments do support suc
  • Of course, we're celebrating this now because its age will only asymptotically approach 100 years.

    • by maroberts (15852)

      Amusing, but on seeing this news article I can't help but wonder if Slashdot celebrated every 5 year anniversary with such enthusiasm. 95 isn't really a special marker, and this story is a bit of a non-event.

  • I make it 99 years since general relavity was written, and 110 years since special relavity was written, what is so special about the first confirming solar eclipse that marks it the beginning of relavity, the solar eclipse meanly showed that light could be bent by gravity, one of the prediction of general relaivity.
  • Newton's Law of Gravity showed that the force of attraction was proportional to the masses of the objects and inversely proportional to the distance squared: Fg=kM1M2/r^2

    Einstein demonstrated in his experiment, through gravitational lensing effect, that mass bends space-time and his famous equation showed mass and energy to be equivalent. This effect, not normally observable in our daily lives, shows that Newton's law is still correct. It's at relativistic speeds and at the quantum level that other terms

  • 95 years according to which frame of reference?

  • by wjcofkc (964165) on Friday May 30, 2014 @08:09AM (#47127679)
    When contemplating phenomena in this universe, I find that in a small number of situations, a rudimentary understanding can be more readily had by a humble and feeble intellect such as mine if I simply drop the speed of light squared from the equation. C squared is where things get strange. Consider the following: A star 100 million light years away ignites. From out relative position and motion, we measure the light as traveling at ~186,300 miles per second over a distance of 100 million light years. As far as we are concerned, it took a long time to get here. Now for the tricky part. As everyone here I am sure knows, time slows down the closer you get to the speed of light, coming to a standstill once the cosmic speed limit is reached. As a consequence, as soon as the light from that star was generated, it was instantly already here. From our perspective, it took 100 million light years to get here. For the perspective of the light itself (so to speak), the transit time was 0. Apply that to light that is older than the Earth and it becomes a real mind-fuck. In fact, kick back and expand on that concept in many different ways. At least this is according to Dr. Tyson. Despite the complexities, E = MC squared is elegant mathematical poetry.
    • Also, since electrons do not experience time (as you stated), then NOTHING ever happens to them. Ergo, it is boring to be an electron.

      That was the point, right? Mind-blowing = infinitely stultifying?
  • PBS program on Einstein mentioned images and calculations from 1919 eclipse was doubtful. Some claimed it did not show bending of light (star positions still where they should be) but others said, "not so fast." It was 1922 which William Campbell got beautiful images of stars around the eclipsed sun, observed 92 stars observed positions from where they should be. Campbell's first telegram was to Albert saying these images and calculations definitely support his theories. Of course it was not easy for Campbe
  • I thought the 1919 solar eclipse was the first confirmation of General Relativity [wikipedia.org] not special relativity (E = mc^2).
  • Newtonian gravitation was still being taught at my high school less than 20 years ago.

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