Gamma-Ray Photon Observations Indicate Space-Time Is Smooth 81
eldavojohn writes "Seven billion light years away (seven billion years ago), a gamma-ray burst occurred. The observation of four Fermi-detected gamma-ray bursts (GRBs) has led physicists to speculate that space-time is indeed smooth (abstract and a pre-publication PDF both available). A trio of photons were observed to arrive very close together, and the observers believe that these are from the same burst, which means there was nothing diffracting their paths from the gamma-ray burst to Earth. This observation doesn't prove that space-time is infinitesimally smooth like Einstein predicted, but does indicate it's smooth for a range of parameters. Before we can totally discount the theory that space-time is comprised of Planck-scale pixels, we must now establish that the proposed pixels don't disrupt the photons in ways independent of their wavelengths. For example, this observation did not disprove the possibility that the pixels exert a subtler 'quadratic' influence over the photons, nor could it determine the presence of birefringence — an effect that depends on the polarization of the light particles."
I guess we're not a huge-scale game of Minecraft (Score:5, Funny)
At least we know for sure that we don't need to deal with Creepers.
Re:I guess we're not a huge-scale game of Minecraf (Score:4, Funny)
I don't know....here in the U.S. they seem to be campaigning every day...
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Size matters... (Score:2)
" theories suggest the pixels should measure the size of the "Planck length," or about a billionth of a billionth of the diameter of an electron"
I thought electrons and all truly elementary particles had no size whatsoever, they were ideal points
Re:Size matters... (Score:4, Funny)
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The electromagnetic field of a point particle is infinitely large. In the collisions, the particles don't really collide; they just get so close that the force between them gets quite large.
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The post I answered to obviously interpreted it literally.
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I thought they collided Protons which are much bigger than electrons
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Re:Size matters... (Score:4, Informative)
Re:Size matters... (Score:5, Informative)
Probably they refer to the electron's Compton length, which in some sense can be viewed as effective size of the electron. If you try to resolve the electron beyond that size, you inevitably get particle creation.
However if I'm not mistaken, a billionth of a billionth of the electron's Compton wavelength is still about five orders of magnitude larger than the Planck length.
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Yes, I think you are right (and that they were sloppy about their orders of magnitude).
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" theories suggest the pixels should measure the size of the "Planck length," or about a billionth of a billionth of the diameter of an electron"
I thought electrons and all truly elementary particles had no size whatsoever, they were ideal points
Not on the Planck scale (or ,at least, it was assumed, not on the Planck scale). Nothing should be ideal (i.e., a point) on the Planck scale.
Re:Size matters... (Score:5, Funny)
I thought electrons and all truly elementary particles had no size whatsoever, they were ideal points
Don't worry, there's always more to learn. Before I came to Slashdot, I thought there were gnomes in my computer, riding gnus and drinking wine.
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Before I came to Slashdot, I thought there were gnomes in my computer, riding gnus and drinking wine.
And now you've been to Slashdot? "Now I know it for certain".
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Do we even know anything for sure? As far as I know, people ruled out anything bigger than a very small size, but saying that it has no size is a big jump from there.
Re:Size matters... (Score:4, Informative)
For a number of scientific considerations, one can treat elementary particles (like the electron) as point-like objects, and legitimately so.
But the Planck length is a unit that is about 18 magnitudes (i.e., 18 powers of 10) smaller than anything one might define as the "size" of an electron.
If you imagine a ruler with a dozen Planck lenghts as units printed on it instead of inches, then in comparison an electron would be an enormous object, much bigger than the size of a planet.
Re:Size matters... (Score:4, Informative)
It is not so much that elementary particles are mathematical points (zero dimensional objects) as that they have no internal "structure" like the 'non-elementary' particles (protons, for example) and no even more 'elementary' constituents. At the deepest level in the Standard Model all 'particles' are described as excitations of quantum fields and have positive probability density over a region in space which is not a point. Electrons are conventionally referred to as "point particles" but that is slang for the deeper description.
Unit overlap (Score:4, Interesting)
Shouldn't all of the points of space have their own frames of references and not be synced anyway? The planck length units would overlap practically infinitely.
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Sensational (Score:4, Insightful)
Is it the artifact of the social media/e-news and the ever growing need for public attention to science (which translates into the elusive funding dollars), that lately a lot of discoveries are being touted as "physics defying", "life altering" etc before they are scrutinized thoroughly? We've already had a faster-than-light and a second-law-of-thermodynamics-broken debacle, and who knows how many more (scour the arXivs and you shall find!). A lot of the stories of scientific discoveries diffuse out of public interest fast, especially now that people are cynical about groundbreaking claims. I wonder if we need to make a conscious effort to not make a big deal out of every discovery, at least not before the data is converted to valuable information. Although, I see the catch-22 here, as the scientific community is trying to break the stereotype of "hard, cold truths presented in a bleak technical manner" or "how does that even remotely affect me", to appease their indirect, impatient employers: the public.
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I am not qualified to comment on the accuracy of the findings and their subsequent interpretation of the data. However, as the senior scientist Giovanni Amelino-Camelia suggested, "But the claim that their analysis is proving that space-time is 'smooth with Planck-scale accuracy' is rather naive." (He was the first one to theoretically suggest methods with which one could test for the "discreteness" of space-time)
Is it the artifact of the social media/e-news and the ever growing need for public attention to science (which translates into the elusive funding dollars), that lately a lot of discoveries are being touted as "physics defying", "life altering" etc before they are scrutinized thoroughly?
Maybe, but I don't think so in this case. Note that Amelino-Camelia is not saying that they can't be right, just that there is some more due diligence that needs to be done. That is rather different from, e.g., the superluminal neutrino case (which actually, note, disagreed with neutrino speed estimates from the 1987-A supernova). This particular case has actually been building for a while (this is not the first look at the dispersion of spacetime over cosmic distances).
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Please stop trying to bring rational thought into science journalism. If you succeed, I'll lose my main source of income [xkcd.com].
Smooth? (Score:2)
I guess I don't really understand physics well enough. I thought at the quantum level space was all knobbly and twitchy.
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I thought at the quantum level space was all knobbly and twitchy.
It depends on how you touch it.
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Supposed to be, on purely theoretical grounds. This is evidence against that notion.
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I guess I don't really understand physics well enough. I thought at the quantum level space was all knobbly and twitchy.
I thought from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly, timey wimey stuff.
A blow against Quantum Gravity? (Score:5, Interesting)
If you ask, at what scale do virtual particles (the stuff continually popping in and out of existence) get so massive that they have gravitational effects (i.e., form little mini black holes), you get the Planck mass, and the Planck length and time come from that. It is, however, very hard to see how you can reconcile these experimental results with the notion that mini-black holes really are popping in and out of existence at the Planck scale. That may mean no space-time foam (what is supposed to result from this violent behavior at the Planck scale).
This is not a problem for General Relativity, but it is a problem IMHO for quantum gravity. The old question, at the Planck scale does General Relativity become more like quantum mechanics, or does quantum mechanics become more like General Relativity, may get an answer that the quantum mechanicians do not like.
Re:A blow against Quantum Gravity? (Score:4, Interesting)
I'm no expert in quantum gravity, but I have sometimes the impression that the pictures of spacetime quantization are often a bit naive; basically the pictures of quantum spacetime look to me more like a classical discrete spacetime. I can't of course exclude the possibility that it's just the presentation.
Think for example of the quantization of the electron spin: It has only two states, up and down. Does that mean that the electron has a certain preferred direction, because, after all, it can only be up and down? Definitely not! You can choose an arbitrary direction, and for each direction you'll find that it is either up or down, and nothing else. But that isn't a contradiction, because the electron isn't just a classical particle whose spin points in a certain direction, and when you measure it, you find out which spin it had. Instead, it's the measurement itself which determines the direction in which you get up or down, and it is the measurement which forces the electron into one of the states. Before it might have been in a superposition. And if you choose another direction, you'll find that the very same state corresponds to another superposition of the up and down states corresponding to that direction. Indeed, for the electron all directions are equal (the current state may be associated with a specific direction, but every direction has an associated state, making no direction fundamentally different than the others).
Now when we come to the Planck length, I can imagine that the very same happens: The spacetime itself is not discrete, just as the directions of the electron spin are not discrete. But if we try to measure it, we can only get discrete values. But those discrete values are not a property of the spacetime itself, because we can make another measurement, and then maybe our discrete values are half a Planck length shifted, just as we can make a measurement of the electron's spin in z direction, and then in x direction, and we will find that the electron's spin after the second measurement is rotated by a right angle, despite the fact that for each measurement individually the only possible values are in opposite directions.
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Except for when it is both
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That's not how the electron spin works. When you're talking about the spin state of a single, non-entangled electron, the state "both up and down" is exactly the same as the state "up" in a direction perpendicular the original "up/down".
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If t
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The two main contenders are string theory (ST) and loop quantum gravity (LQG). ST doesn't quantize spacetime at all. LQG gives quantization of area and volume, but not lengths. It is definitely wrong to p
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Do all quantum gravity models require spacetime to be non-smooth at the plank scale?
The data only looks like 3sigma so it could be the mini-bursts of gammas were coincidence, but more data should really nail it down.
If true this is a really important bit of research.
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What you're presenting is similar to a point of view that was being pushed ca. 2006 by some folks at the Perimeter Institute doing loop quantum gravity (LQG). They were talking like they had a theory that really made predictions about vacuum dispersion that would be testable by the GLAST gamma-ray telescope. That was exciting, because if your theory doesn't expose itself to the possibility of falsification, then you're not really doing science. Unfortunately it then became clear that LQG doesn't actually ma
Doctor Who Premieres tomorrow (Score:2)
So it's not a wibbley-wobbley timey-wimey stuff?
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I would not go against the good Dr. [youtube.com].
I assumed, however, he was talking about the macro scale, not the micro.
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I would not go against the good Dr. [youtube.com].
I wouldn't call him "Dr." unless you want to chased out of town by rabid fans :) He's the Doctor [youtube.com].
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I didn't see any spelling... And, 53 seconds in, he calls himself "a Doctor," not "the" Doctor.
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The video was only 15 seconds.
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Mine was, but his was 16:45 [youtube.com].
Feinman (Score:1)
I don't remember exactly but Feynman said something to the effect that a sign of a bogus theory is that it makes predictions just beyond what is observed. An when observations improve, the theory is modified to predicted an even smaller effect.
Space/time duration/distance (Score:5, Interesting)
Seven billion light years away (seven billion years ago)
I may not have this right, but due to the expansion of space, wouldn't it have been closer than seven billion light years away at the time of the kaboom? And if the light's taken seven billion light years to get here, space will have expanded further, so the remnants would now be further than seven billion light years away. Right?
Or is this the sort of thing where you can be specific about the distance, or the time, but not both?
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Seven billion light years away (seven billion years ago)
I may not have this right, but due to the expansion of space, wouldn't it have been closer than seven billion light years away at the time of the kaboom? And if the light's taken seven billion light years to get here, space will have expanded further, so the remnants would now be further than seven billion light years away. Right?
Or is this the sort of thing where you can be specific about the distance, or the time, but not both?
Wikipedia [wikipedia.org] has an answer, but I think the above is just meant to give the layman some rough understanding of what's going on.
Beware that it is extremely difficult to measure these kinds of distances exactly. The figure may be a few orders of magnitude wrong, so whether you take into account the expanding universe or not may not be that important...
Cosmologists measure everything in gigaparsec. 7b light years is only 0.3 GPc so it may not be that important.
Re:Space/time duration/distance (Score:4, Interesting)
Re:Space/time duration/distance (Score:5, Informative)
There are multiple distance measures in cosmology - they are all in principle exact (at least, if you know all your cosmological parameters), but they differ significantly once you start getting above about 1 billion light years. Much above that, and they can differ incredibly much. Some of these measures are based on idealized measurements, others on the physics directly.
Some measures used in cosmological work are,
- proper motion distance (the distance a parallax measurement would give you)
- luminosity distance (the distance you would infer from the apparent brightness of a standard candle)
- angular diameter distance (the distance you would infer from the apparent angular size of a standard sized object). The angular diameter distance is notorious for getting smaller if you get far enough away in many cosmologies (including, apparently, the one we live in).
- look back distance (if you imagine that everyone has a clock synchronized at the big band, the difference between your time and the time you would read on the remote clock, if you could read it). This is also called the light travel time.
- proper distance (what some long yardstick would read).
- comoving distance (the proper distance divided by the scale factor - 1 plus the redshift, z - for the remote observer, to get a distance that doesn't change with cosmological time).
And, finally, each cosmological model will have a coordinate distance (the difference between the coordinates of two different places), which need not have a simple relation to any of the above.
It is fair to say that one of the easiest ways to make a fool of yourself in cosmology is to mix up distance scales. (As an additional cause of mixups, only proper distances can be subtracted - for the rest, the distance between A and B is NOT the difference of the distance to A and the distance to B, even if A and B are on a straight line as seen from the Earth.)
In this case, the Gamma Ray Burst 090510A was at a red shift of 0.897. Go to the Cosmology Calculator [ucla.edu] and you find that that
For Ho = 71, OmegaM = 0.270, Omegavac = 0.730, z = 0.897
It is now 13.666 Gyr since the Big Bang.
The age at redshift z was 6.376 Gyr.
The light travel time was 7.290 Gyr.
The comoving radial distance, which goes into Hubble's law, is 3053.8 Mpc or 9.960 Gly.
The angular size distance DA is 1609.8 Mpc or 5.2505 Gly.
The luminosity distance DL is 5793.1 Mpc or 18.895 Gly.
The proper distance is (1+z) times the comoving distance, or 18.89 Gly.
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Argh - a typo - there is both a parallax distance and a proper motion distance, and they are not the same.
Neither are used much as we can't measure proper motions or parallaxes at cosmological distances... yet.
Before we can totally discount the theory that... (Score:2)
"Before we can totally discount the theory that space-time is comprised of Planck-scale pixels..."
And you lost me.
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"Before we can totally discount the theory that space-time is comprised of Planck-scale pixels..."
And you lost me.
The author is simply referring to the Ultra-Retina Display that was added to the New Universe (3rd gen).
Until you make it angry (Score:2)
OW! (Score:1)
OW! MY HEAD!
Fixed summary (Score:2)
"Seven billion light years away (seven billion years ago), a gamma-ray burst may have occurred. The observation of four Fermi-detected gamma-ray bursts (GRBs) has led physicists to speculate that space-time is indeed smooth (abstract and a pre-publication PDF both available). A trio of photons XwereX was observed to arrive very close together, and the observers believe that these are from the same burst, which XmeansX suggests that there was nothing diffracting their paths from the gamma-ray burst to Earth. This observation doesn't prove that space-time is infinitesimally smooth like Einstein predicted, but does indicate it's smooth for a range of parameters. Before we can totally discount the theory that space-time is XcomprisedX composed of Planck-scale XpixelsX voxels, we must now establish that the proposed XpixelsX voxels don't disrupt the photons in ways independent of their wavelengths. For example, this observation did not disprove the possibility that the XpixelsX voxelsexert a subtler 'quadratic' influence over the photons, nor could it determine the presence of birefringence — an effect that depends on the polarization of the light particles."
Writing is not entirely a lost art, but it's close.
doesn't make sense to me (Score:1)
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Ricky Gervais (Score:1)
What kind of maniac thinks like that?
http://www.youtube.com/watch?v=6ktBQ51iGWw#t=106s [youtube.com]