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Space Science

Intergalactic Race Shows That Einstein Still Rules 227

Posted by samzenpus
from the the-speed-of-right dept.
Ponca City, We love you writes "The NY Times reports that after a journey of 7.3 billion light-years, a race between gamma rays ranging from 31 billion electron volts to 10,000 electron volts, a factor of more than a million, in a burst from an exploding star, have arrived within nine-tenths of a second of each other. A detector on NASA’s Fermi Gamma-Ray Space Telescope confirmed Einstein’s proclamation in his 1905 theory of relativity that the speed of light is constant and independent of its color, energy, direction or how you yourself are moving. Some theorists had suggested that space on very small scales has a granular structure that would speed some light waves faster than others — in short, that relativity could break down on the smallest scales. Until now such quantum gravity theories have been untestable because ordinarily you would have to see details as small as the so-called Planck length, which is vastly smaller than an atom — to test these theories in order to discern the bumpiness of space."
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Intergalactic Race Shows That Einstein Still Rules

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  • i'm confused (Score:2, Insightful)

    they arrived within 9/10th of a second of each other

    which indicates the opposite of the story's summary

    • by selven (1556643)

      Probably gravitational lensing making some of the gamma rays more curved than the others. Or whatever event caused them is not instantaneous (like every other large-scale event in existence).

      • Re: (Score:3, Insightful)

        by schnikies79 (788746)

        If it wasn't instantaneous, or nearly, how can we even use this data? They could have been released minutes or more apart.

      • How do they know (Score:5, Insightful)

        by 2.7182 (819680) on Thursday October 29, 2009 @11:38AM (#29912179)
        that the photons all left at the same time?
        • Re:How do they know (Score:5, Informative)

          by John Hasler (414242) on Thursday October 29, 2009 @11:52AM (#29912429) Homepage

          The event was approximately 2.2 seconds long. Thus it is plausible that these two photons left .9 seconds apart.

          • Re:How do they know (Score:5, Interesting)

            by Richard_at_work (517087) <richardprice AT gmail DOT com> on Thursday October 29, 2009 @11:57AM (#29912519)
            Its also plausable that they left at the same time, and arrived 0.9 seconds apart. How do we tell though?
            • Re:How do they know (Score:5, Informative)

              by John Hasler (414242) on Thursday October 29, 2009 @11:59AM (#29912531) Homepage

              We don't, and it doesn't matter. .9 seconds is much smaller than would be predicted by the theories in question.

              • I think it is pretty much the only thing that matters - until you can say when the photons left in relation to each other, there is no way you can say the delay validates or invalidates any of the theories in question.
                • by jabuzz (182671) on Thursday October 29, 2009 @12:13PM (#29912763) Homepage

                  Not true, if the theory requires that they would be separated by say 900 seconds, they left within 2.2 seconds of one another maximum, and we observe them at 0.9 seconds apart, then the theory is proved wrong.

                • Re:How do they know (Score:5, Interesting)

                  by gnick (1211984) on Thursday October 29, 2009 @12:14PM (#29912785) Homepage

                  Not true. If we know that the event that generated the rays lasted only 2.2 seconds and we have a theory that would delay one of the rays by more than 3.1 seconds (2.2 + 0.9) relative to the other, we can invalidate that theory. From my understanding, that is exactly the case we're dealing with. You are correct though that this cannot completely validate any specific theory - All it can do is reinforce the assumption that our current theory is more accurate than some others proposed and eliminate some competing ideas.

                  • Your post makes sense except that "more accurate" implies that "proved wrong tomorrow" is somehow better than "proved wrong today". Either its right, and will never be proved wrong, or it is wrong, and may eventually be proved as such.
                    • by gnick (1211984) on Thursday October 29, 2009 @01:37PM (#29914051) Homepage

                      Your post makes sense except that "more accurate" implies that "proved wrong tomorrow" is somehow better than "proved wrong today".

                      "Proved wrong tomorrow" IS better than "proved wrong today". "Proved wrong tomorrow" means that we've got nothing contradicting it yet and it's the best we've got. I'm perfectly happy to accept that everything we know is wrong and go through life using our best-available models for how the world works. If you wait for a perfect model of everything before you start using the models at hand, you'll never get anything done. So what's wrong accepting what we've got as a possibly-flawed, but best-available model and refining it as we learn more?

                      Either its right, and will never be proved wrong, or it is wrong, and may eventually be proved as such.

                      You left out an important option. There's "right", "wrong", and "unproven but useful and not yet proven wrong". Very few things in science or life can really be "proven" right. A lot of science is made of reasonable (sometimes radical) guesses that haven't yet been discredited. Even the "law" of gravity is still just a theory, but that fact doesn't make me mistrust my scale because we may be able to refine our knowledge of heavy-body attraction in the future.

                    • by lgw (121541) on Thursday October 29, 2009 @05:17PM (#29917517) Journal

                      Science doesn't deal in "proven right" and rarely in "proven wrong". That would be math with the proofs. Science deals in "accurate over the available data", and looks for "more accurate" models over time.

                    • Re: (Score:3, Interesting)

                      by MaskedSlacker (911878)

                      I hope your science profs failed you, because you don't understand science.

                      Everything in science is WRONG, strictly construed. Newton's theory of gravity is WRONG (gravity is not proportional to 1/R^2), but it's reasonably close for many applications. Einstein's theory of general relativity is WRONG, strictly construed, but it is LESS wrong than anything else we've managed to come up with (for certain applications).

                      Science is the continual quest for explanations that are less wrong, not right ones. Even

                  • by Dare nMc (468959)

                    Makes me wonder how, and how long they were looking for this. It seams like a whole stream of photons could have bounced off a celestrial body 2000 light years away, and the faster light stream came past earth 2000 years ago as well (and the slower stream will reflect from the same source, and come by us in 2000 years.) If they were just looking for a stream of photons within 1 second of each other, eventually we were likely to find proof, even if Einsteins theory was wrong.

              • by 2.7182 (819680)
                But how do we know the length of such an event so accurately? Maybe it took 10 minutes.
                • Re: (Score:2, Interesting)

                  by Anonymous Coward
                  Because it is incredibly unlikely that:
                  1) the event lasted 10 minutes
                  2) produced gamma rays steadily decreasing in energy
                  3) such that the effects of the difference of the speed of light on the particles over 7.3 billion years would compress the burst to 2.2 seconds
                  4) that we would be located at the exact location to observe a burst of only 2.2 seconds (closer or farther away and the burst would still be spread out)

                  Since all gamma ray bursts are short and have different energy radiation, there are only

              • Wouldnt the theories be wrong, just on a different scale than predicted, if the more energetic signals had different travel times?
            • Re: (Score:3, Insightful)

              by csartanis (863147)

              Perhaps your theory is _not_ plausible, and the scientists know more than the average slashdot commenter?

              • Of course, most of what he knows isn't true...

              • If only that sort of rebuttal were how scientific theories were validated, imagine the progress we could make. "Of course my theory makes sense, I'm smarter than you!"
                • Re: (Score:3, Interesting)

                  by SleazyRidr (1563649)

                  We all know that isn't the way science is done, but I (and a lot of other people) get rather pissed off by the inevetable commenters who read the summary and then seem to think the researchers are retarded monkeys who didn't finish high-school. They've spent a long time on this, they've thought out a lot of possibilities, you aren't going to prove them wrong with your 30-second insight.

            • Re: (Score:3, Insightful)

              by bidule (173941)

              0.9 seconds over 7.3 billion years is... at most a 10^-16 speed difference. IOW, the speed of light is not affected by the wavelength.

              Suppose there was a negligible 1 in a million speed variation between those wavelengths, and this speed variation occurred in a magic part of space representing 1 millionth of the total distance. We'd still get 2-3 days of lag between our reception of both wavelengths.

              That should tell you why your question is meaningless.

          • by noundi (1044080)

            The event was approximately 2.2 seconds long. Thus it is plausible that these two photons left .9 seconds apart.

            Mod parent up, obviously this is a mere effect of the size of this sun. Different layers of the sun produced different wavelenghts in the bursts as the sun would consist of various elements in layers, but you already knew this.

          • by Tiger4 (840741) on Thursday October 29, 2009 @12:14PM (#29912777)
            Do we call in the MythBusters to stage an experiment? I need to see a Confirmed, Plausible, or Busted branding iron before I believe anything.
          • by mikael (484)

            What if there was only one super-high-energy photon that left the supernova, and collided with an proton creating two gamma rays later on?

          • How can they know the length of the event? The particles were RECEIVED over a space of 2.2 seconds. The issue is that the only information we have (that i could see in the article) is based on EMS signals, and that any inferences we draw from that data will RELY on our theories on how EMS signals move and behave. If Einstein is completely wrong, and if gravity works in vastly different ways at larger scales, we have no way of validating or refuting any of those based on THIS data. What if the higher ene
        • Re: (Score:2, Funny)

          by pilgrim23 (716938)
          they didn't. the slower one was waiting for its wife to get ready.
        • by krlynch (158571)

          They didn't, but that doesn't matter to the argument :-)

          We know from more nearby GRBs that photons at all energies are emitted throughout the "explosion". That is, high energy photons aren't emitted earlier than lower energy photons. In other words, if you look at the mean emission time for all photons from energy, say, 10keV to 11 keV, and for those from energy 10MeV to 10.1MeV, (and every other energy band), those means are all the same.

          Now, if HIGHER energy photons covered the distance faster than lowe

      • > Probably gravitational lensing making some of the gamma rays more curved than
        > the others.

        No. Gravity has exactly the same effect on all photons. However, the photons were emitted over a period of several seconds.

        • Re: (Score:3, Insightful)

          by LordLimecat (1103839)

          No. Gravity has exactly the same effect on all photons

          How do we know this is true over long distances? Seems to me a lot of this is conjecture.

    • Re:i'm confused (Score:4, Informative)

      by Nevynxxx (932175) on Thursday October 29, 2009 @11:36AM (#29912137)
      I'm guessing that the error bounds on the readings were great enough that 0.9 seconds over 7.3billion years, was within them....
    • by oodaloop (1229816) on Thursday October 29, 2009 @11:37AM (#29912149)
      Probably due to the reflex time when they started and stopped their stopwatches.
    • Re:i'm confused (Score:5, Informative)

      by eldavojohn (898314) * <eldavojohn AT gmail DOT com> on Thursday October 29, 2009 @11:38AM (#29912185) Journal
      The importance is that that puts the effect at smaller than a planck length (which is the assumed smallest possible distance that something measurable can happen in classical physics). From the first article:

      The spread in travel time of 0.9 second between the highest- and lowest-energy gamma rays, if attributed to quantum effects rather than the dynamics of the explosion itself, suggested that any quantum effects in which the slowing of light is proportional to its energy do not show up until you get down to sizes about eight-tenths of the Planck length, according to the Nature paper, whose lead author was Sylvain Guiriec of the University of Alabama.

      Granted they say it would have to be proven much smaller than a planck length for most people to accept this as empirical proof, it is empirical data backing Einstein. The 9/10s could be due to the explosion or a physical effect but the latter is now more unlikely given the many light year distance.

      • by Tiger4 (840741) on Thursday October 29, 2009 @12:19PM (#29912877)
        You read the article. Obviously, injecting evidence and facts into the discussion is not helpful with the wild speculation, conjecture and skepticism of the uninformed.
      • by steelfood (895457)

        Granted they say it would have to be proven much smaller than a planck length for most people to accept this as empirical proof

        You mean most physicists. Most people couldn't tell you what qualifies as empirical proof, much less what planck's length is, if it hit them on the head.

    • by Anonymous Coward on Thursday October 29, 2009 @11:41AM (#29912247)

      The light propagated through space for 7.3 billion years. A delta of +/-0.000000000000000039% is not even close to statistically significant. It could have been from local variations in the density of a nebula or something.

    • Re:i'm confused (Score:5, Insightful)

      by zerosomething (1353609) on Thursday October 29, 2009 @11:43AM (#29912279) Homepage
      9/10th of a second is only about 3/4th the distance from earth to the moon. I don't know for sure but I think that's a rather small difference and could be accounted for just by the size of the star that exploded. Our own sun is about 4 seconds across isn't it?
      • by Anonymous Coward on Thursday October 29, 2009 @12:08PM (#29912689)

        9/10th of a second is only about 3/4th the distance from earth to the moon. I don't know for sure but I think that's a rather small difference and could be accounted for just by the size of the star that exploded. Our own sun is about 4 seconds across isn't it?

        The speed of light = 299 792 458 m / s (commonly 3.0 * 10^8 m/s)
        The average centre-to-centre distance from the Earth to the Moon is 384,403 kilometres
        (384 403 kilometers) / the speed of light = 1.28223039 seconds

        The diameter of the sun = 1391000 kilometers
        (1392000 kilometers) / the speed of light = 4.64321221 seconds

        Either you do good math in your head, or I respect your physics teacher for teaching you such interesting facts.

      • by AndrewNeo (979708)

        4.64 seconds [wolframalpha.com]

      • Re:i'm confused (Score:5, Insightful)

        by radtea (464814) on Thursday October 29, 2009 @12:57PM (#29913425)

        I don't know for sure but I think that's a rather small difference and could be accounted for just by the size of the star that exploded.

        You're confused because the summary, and the press release on which it is based, are misleading and wrong.

        This is a gamma ray burst (GRB), which originate from neutron stars, not a super-nova (which is the only reasonable meaning one can give "exploding star".) Neutron stars are small, resulting in much finer burst timescales.

        The paper discusses the time-structure of GRB's, which has been extensively studied. The fundamental result they get is from a single high-energy gamma ray at the end of the last spike in the burst, which comes 0.9 s after the onset of that spike (seen in the lower-energy photon flux). They do a lot of analysis to argue that the most plausible explanation of that single photon is that it is a member of that spike rather than a random cosmic ray. Anyone familiar with modern statistical techniques will see that this is straightforward, albeit non-trivial.

        This is the way science works: we squeeze limited and imperfect experimental evidence as hard as we can using established theory and other, supporting, observations. All the "yeah, well, it could be something else" kind of commentary we see so much of on /. is irrelevant to the scientific process, because it is doing nothing but repeating what everyone already knows: sometimes the most plausible explanation turns out to be wrong.

        The exciting thing about this measurement is that they have shown it is possible to put quantum gravity to a rather good test using entirely conventional gamma-ray spectroscopy techniques, and repeating this kind of measurement over the next few years or decades on different bursts will rapidly push down the limits on potential planck-scale effects, because eventually we'll see bursts where there are a few high-energy photons closer to the onset, or we will see bursts from objects at larger (known) distances.

        The present authors argue, rightly, that their observation makes theories that have a linear dependence of light velocity on wavelength less plausible. At some point in the next few years it is likely that those theories will be dead, and there's really nothing so beautiful as a theory killed by a fact.

    • Re:i'm confused (Score:5, Insightful)

      by natehoy (1608657) on Thursday October 29, 2009 @11:58AM (#29912527) Journal

      Actually, it really indicates nothing, except that any "bumpiness" of space doesn't have a profound effect on the speed of light within the wavelength range tested. It's good data. However, this neither proves nor disprove there was an effect, just proves that the effect (if it exists) is very insignificant at the tested wavelengths.

      Insignificant != Nonexistent
      Tested Wavelengths != All Wavelengths

      In order to prove or disprove the theory that light changes speed based on wavelength or other factors, you'd need to be sure that both pulses started the race at the exact same moment, that the two pulses travel through the same space without interfering with each other,and that they complete the race at the exact same moment (ie, within the margin of error of your testing equipment). The margin was almost one second, which is terribly insignificant when compared to 7 billion years, of course, but demonstrates clearly one of the following three things:

      1. The pulses left about a second from each other, which we can neither prove nor disprove.
      2. The test equipment was flawed and they really did arrive at the exact same time (which leads to #1, maybe they left at different times and just happened to arrive at the exact same moment).
      3. The speed of the various wavelengths WAS affected by "space potholes", but it took 7 billion years to accumulate less than one second of variance.

      If #3 is possible, which it still is even after this test, then the theory of bumpiness of space has not been disproven, it just appears that evidence points toward the bumps being really, really small or somehow only marginally effective at affecting the speed of light.

      Plus, the original article goes on to explain that the tested wavelengths were relatively large, and that much smaller wavelengths might be more susceptible to the "bumpiness" of space depending on the size of the bumps. If the bumps are really tiny, then they might have just tested wavelengths that were too large to be affected by them. If we can measure some really high-frequency (low-wavelength) pulses against the ones we think are nearly identical, that would be much more compelling data.

      • by JamesP (688957)

        You sir, are a true scientist.

        I'm really tired of "skeptics" that in reality don't want to see anything.

        A true skeptic would not suddenly ignore every other event where there has been a different in arrival times (similar to these), with some lame excuse. But that's exactly how TFA sounds.

      • Re:i'm confused (Score:5, Interesting)

        by Rary (566291) on Thursday October 29, 2009 @12:48PM (#29913299)

        We know that the pulses were caused by an event that lasted 2.2 seconds, therefore we know that they left anywhere from 0 to 2.2 seconds apart. However, the point isn't to determine a simple boolean result to the question "did they arrive at the same time", the point is to invalidate the predictions of theories. The existing theories predicted that the arrival times of these pulses, having left at most 2.2 seconds apart, would be at a minimum significantly more than 0.9 seconds. However, they were not, therefore the theories' predictions are wrong, and thus the theories are invalid. The one theory that predicted that they would arrive at most 2.2 seconds apart remains — not proven, but still not disproven. That's how science works.

        • by astar (203020) <max.stalnaker@gmail.com> on Thursday October 29, 2009 @12:56PM (#29913411) Homepage

          Yah, but think of the poor string theorists. Here they spend 30 years working on it, and they stack the department hiring processes so they will not be criticized for never have created a testable conjecture. Now some data comes in and half their theories crash.

        • Re: (Score:3, Informative)

          by Cyner (267154)

          Exactly.

          Competing theories stated the difference would be more than 3.1 seconds (The possible variance given a 2.2 seconds event and 0.9 seconds difference). Therefore they are invalid.

          Einstein stated the total variance would be equal or less than the total duration of the event. Total duration was 2.2, difference was 0.9, this fits. Einstein's theory is not proven invalid.

          Any other conculsions are logical falacies.

      • Re: (Score:3, Funny)

        by darsal (18194)

        An engineer, a mathematician, and a physicist are riding a train through Scotland.

        The engineer looks out the window, sees a black sheep, and exclaims, "Hey! They've got black sheep in Scotland!"

        The mathematician looks out the window and corrects the engineer, "Strictly speaking, all we know is that there's at least one black sheep in Scotland."

        The physicist looks out the window and corrects the mathematician, "On one side."

      • You possibly have the most relevant post in this entire conjecture filled thread. We have no way of PROVING anything with this data, because the only things it could prove or disprove are the theories that would be used to interpret the data in the first place (speed of light, distance, etc). It can further validate things, or show the insignificance of the effects mentioned on the speed of light. Its really obnoxious to hear people claiming that because there is discrepancy in the reading, but certain e
    • by JamesP (688957)

      Thank you!

      Not to mention other events where this effect has been much more exaggerated.

    • they arrived within 9/10th of a second of each other

      which indicates the opposite of the story's summary

      Notice the key word here, "within".

      One second in 9.3 billion years is a pretty good measurement. It indicates a difference in speed of no more than 0.0000000000000003 percent.

    • they arrived within 9/10th of a second of each other ... which indicates the opposite of the story's summary

      1. One leaves from near one pole. One leaves from near the other pole. Or any two antipodes depending on where the Earth is. Even from a star comparable to the size of the Earth-Moon radius exploding, 0.9 seconds is quite reasonable.
      2. Near the explosion there are also going to be elastic collisions between the gamma rays and the neutrons that are being created en masse. The likelihood of photo
    • In short, .9s is well within the uncertainty on the measurement, and from that uncertainty, you can decide which class of quantum gravity theories your experiment is sensitive to. If you had a theory that said the deviation should be .000001s at these energies, then this experiment would never be able to say anything about that theory. But we have otherwise good theories of quantum gravity that claim the deviation at these energies should be much larger than the experimental uncertainty. This experiment
  • how do we know (Score:3, Insightful)

    by Anonymous Coward on Thursday October 29, 2009 @11:38AM (#29912187)

    that these rays are all from a star which exploded 7.3 billion light years away? what was monitored to predict arrival?

  • So-Called? (Score:3, Interesting)

    by Afforess (1310263) <afforess@gmail.com> on Thursday October 29, 2009 @12:06PM (#29912651) Journal
    I'm curious, why is the Plank-length "So-called"? Hasn't it been firmly established as a unit of measurement?
    • by Compholio (770966)

      I'm curious, why is the Plank-length "So-called"? Hasn't it been firmly established as a unit of measurement?

      It's a unit of measurement derived from dimensional analysis. Some believe that the Planck length is the unit that describes the quantization of space, but this assertion has not successfully been tested.

    • ...It's so small they we don't have anything that can even come close to measuring anything even close to this length, so anything said about conditions at this length is untested (and possibly untestable?), and what we have that explains what happens at these distances (mostly quantum gravity) are highly speculative and many of the hypotheses are contradictory ...

      This is an actual case of science being "only an hypothesis" that actually equates to the reporters "Only a theory" ..But this experiment probabl

  • by mbone (558574) on Thursday October 29, 2009 @12:07PM (#29912677)

    This was proposed by G. Amelino-Camelia et al. back in 1998 [nature.com]; here [arxiv.org] is a review from 2004. Even though the wavelengths of even the most energetic gamma rays are much, much, longer than the Planck length, roughness in space time at the Planck length adds up over cosmological distances, and could be in principle detectable. (The Planck length can be thought of heuristically as the length at which the gravitational effects of virtual particles should be strong enough to create virtual black holes; general relativity cannot be ignored in quantum mechanics at that scale, and vice versa.) What this current test is ruling out is a particular violation of Lorentz invariance - a variation of photon speed with energy. There were similarly negative results using radiation from the Crab nebula in 2003 [nature.com].

    It should be noted that this does not rule out quantum gravity - it seems pretty clear that General Relativity and Quantum Mechanics cannot both apply at the Planck scale. What this work is doing is beginning to constrain models of quantum gravity (there is as yet no general theory that makes precise predictions). What would be really cool is to detect some effects, which would maybe help nudge the theorists along.

  • by sexconker (1179573) on Thursday October 29, 2009 @12:08PM (#29912691)

    So the $500 high-energy gamma-output cables I bought actually DON'T improve my ping?

    FUCK YOU MONSTER CABLE!

  • by PinkyDead (862370) on Thursday October 29, 2009 @12:09PM (#29912697) Journal

    Aaarrggghh!!!

  • This Einstein guy is starting to really piss me off. Someone gotta take him down a notch or two.
  • by wowbagger (69688) on Thursday October 29, 2009 @12:44PM (#29913231) Homepage Journal

    OK, here's one for the physicists in the audience (and pardon the simplification of terms here, but...)

    1) Being deeper in a gravity well slows time relative to being further out.
    2) All things which have mass have gravity wells.
    3) Photons have mass (NOTE TO THE CLUELESS: "mass" and "rest mass" are two different things - photons have no rest mass, but they most certainly have relativistic mass).
    4) By 2 and 3 photons should have a (small) gravity well. More massive photons (higher energy and thus shorter wavelength) have deeper wells.

    Thus, wouldn't 1 and 4 lead to higher energy photons "clocks running slower" (since they are deeper in a gravity well) and thus propagating as a lower speed as viewed by an observer outside their gravity well - and that effect would be negligible for all but the most massive photons.

    (for the physicists: feel free to expand and clarify on the oversimplifications I've made here. This is, after all, targeting a Slashdot audience which has rather a wide spread of backgrounds).

    • Re: (Score:3, Interesting)

      by mea37 (1201159)

      "Thus, wouldn't 1 and 4 lead to higher energy photons 'clocks running slower'"

      It's been too long since I've really looked at relativity for me to agree or disagree with this bit, but let's assume you're right...

      " thus propagating as a lower speed as viewed by an observer outside their gravity well "

      I think not. Your clock has nothing to do with how fast I see you moving. That's why it's your clock and not mine.

    • My armchair understanding of it would be: if their clocks are running slower, they would appear as a lower frequency photon. They would still propogate at the same speed.
    • by Tyler Durden (136036) on Thursday October 29, 2009 @02:44PM (#29915037)

      Anything that moves at the speed of light does not experience a passage of time and has no "internal clock" to speak of.

      This is how people figured out that neutrinos had a rest mass when at first the Standard Model assumed that they didn't. It was discovered that neutrinos could oscillate between different lepton flavors. But for that to happen they would have to experience the passage of time. And since particles without a rest mass always travel at the speed of light, they had to have mass.

  • more information (Score:5, Informative)

    by bcrowell (177657) on Thursday October 29, 2009 @12:49PM (#29913311) Homepage

    This is actually just the latest in a series of measurements of this type. Since the Nature paper isn't free online, people may want to look at this [arxiv.org] similar paper from earlier this year that is available.

    The article talks about testing "some theories" of quantum gravity. AFAIK the only theory of quantum gravity that makes anything like a prediction that could be tested in this way is loop quantum gravity [wikipedia.org] (LQG). The two leading contenders for a theory of quantum gravity are LQG and string theory. String theory essentially assumes a background of flat spacetime (plus an xtra 6 rolled-up dimensions), so I don't think it's capable of addressing the issue of whether spacetime is frothy at the Planck scale. LQG doesn't assume a background of flat spacetime, and in fact one of the main research programs in LQG is focused on showing that flat spacetime can emerge as a solution to LQG in the appropriate limit. LQG unambiguously predicts that the vacuum is dispersive, i.e., that the speed of light depends on the energy of the photon. However, LQG does not unambiguously predict the exact form of the energy-dependence. The possible form that is usually assumed in order to evaluate observational tests is |v/c-1|~(E/E_P)^n, where v is the speed of the photon, c is the speed of cause and effect in relativity (often referred to as the speed of light), E is the energy of the photon, E_P is the Planck energy, and n=1 or 2. Previous observations, such as the one in the arxiv paper I linked to above, have pretty much ruled out n=1, so if LQG is right, we'd presumably have to have n=2. Some people have been saying that LQG is ruled out by these measurements, but I don't think that's really correct, it's just constrained by them. Here [arxiv.org] is a paper by LQG researchers discussing the empirical tests, and they don't seem to be saying "OK, we give up." It's actually very exciting for people in quantum gravity to have observations that even have some chance of disproving a theory (or some version of a theory); the whole field is a dead end if it can never be tested by experiment.

    In a broader sense, the holographic principle [wikipedia.org] gives strong, model-independent reasons for believing that spacetime is probably discrete, not continuous, at the Planck scale. Otherwise it's hard to imagine how there could be an upper bound on the information content of a given region of space. And any theory in which spacetime is discrete at the Planck scale will naturally give a dispersive vacuum. Therefore I'd say that either (a) we should eventually observe dispersion of the vacuum once the observations get sensitive enough, or (b) the holographic principle is telling us something that we don't yet understand.

    Two good popular-level books that get into this kind of thing are Three Roads to Quantum Gravity by Smolen, and The Black Hole War by Susskind. Because Smolen and Susskind represent very different points of view on quantum gravity, anything that both books agree on is probably correct.

  • Dispersion (Score:3, Interesting)

    by sugarmotor (621907) on Thursday October 29, 2009 @01:21PM (#29913799) Homepage

    I thought the speed of light does depend on the medium through which light travels.

      * http://en.wikipedia.org/wiki/Dispersion_(optics) [wikipedia.org]
      * http://en.wikipedia.org/wiki/Prism_(optics) [wikipedia.org]

    What they measured is a bit surprising that way.

    Stephan

    • > I thought the speed of light does depend on the medium through which light
      > travels.

      It does when the medium is not a vacuum. This observation established an upper limit on the dispersion of the integalactic vacuum.

      • "This observation established an upper limit on the dispersion of the integalactic vacuum." ->
        That is not how it is presented. It is presented as if it was known to be a vacuum. (Einstein would still have an easy way out otherwise)

        Stephan

  • Planck length (Score:5, Informative)

    by elrous0 (869638) * on Thursday October 29, 2009 @01:22PM (#29913821)

    I heard a good analogy once explaining just how small the Planck length really is--and why it's so out of reach of any conceivable measurement we can even dream of:

    If the nucleus of a single atom were expanded to the size of the known universe (15 billion light years across--itself an almost unimaginable distance), the Planck length would be about as long as a tall cedar tree.

    • by MikeURL (890801)
      I think we're going to be embarrassed to have to admit that there was a time when we thought we could see things that occurred 7 BILLION light years away. Someday it will be said "what the hell were they thinking".
    • Re: (Score:3, Insightful)

      size of the known universe (15 billion light years across--itself an almost unimaginable distance)

      Nitpicking: It's 93 billion light-years (radius 46.5 billion) [wikipedia.org], see Misconceptions.
      It doesn't really matter, as the distances involved are so mindbogglingly large that in most cases the only explanation you need will be: "So large that your mind can't cope with it". The same thing goes for the infinitesimal size of an atomic nucleus. An error of an order of magnitude still gets the point across :)

  • Man oh man.. I stepped out to use the bathroom and I missed it... I had been waiting 7.3 billion years for this.

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