Einstein Was Right: Space-Time Is Smooth, Not Foamy
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I don't understand how photons with wavelength ~ 10^-17 meters are supposed to probe the planck scale, p_l ~ 10^-35 meter. I've skimmed the paper, but it seems most of that explanation is buried in references 11, 14, and 15. (Especially 15.) From their language, my hunch is that this is rather model dependent, i.e. the observations only bounds specific models of spacetime foam which happen to have the nice property of amplifying the dispersive effects.
(I'm a physicist, but this is way out of my area of expertise.)
c_gamma=c_0 + a ( E_gamma / m_p) + b ( E_gamma / m_p)^2 + ...,
with c_gamma the speed of a photon of energy E_gamma, c_0 the speed of light ( for low energy photons), m_p the Planck mass and a,b are some parameters determined by your model. This style of papers then sets limits on a,b. And the general expectation of some space time foam model is, that a,b should be roughly 1.
From this the way they can probe the Planck scale is then simply a huge baseline, in this case 7e9 light years, compared to a rather short duration of the event ( ~ days).
> And the general expectation of some space time foam model is, that a,b should be roughly 1.
OK. Any idea how generic this expectation really is?
> compared to a rather short duration of the event ( ~ days).
What's this timescale have to do with it? They must be assuming they can identify photons emitted at the same time since they're measuring the arrival time difference in milliseconds.
> OK. Any idea how generic this expectation really is?
I can't comment on this case specifically, but the entire notion of the "Planck energy" is based on that idea: that once you've found the combination of fundamental constants with the right units, the numerical coefficient that multiplies it will wind up being within one or two orders of magnitude of 1. By some miracle, the vast majority of systems in physics seem to match such expectations.
[Omitting a longish rant, that once we give up Lorence invaraiance we do no longer know anything.] AFAIK it is essentially the fine tuning argument. So not really generic but in the absence of a good argument a reasonable default value.
> What's this timescale have to do with it?
The timescale was essentially meant illustrative, so a short everyday timescale compared to gigayears. ( And it is conservative, in the sense that I do not need to remember the exact details of a GRB spectra to use this one ;)
Here are a few relevant publications from Fermi:
https://www.sciencemag.org/content/323/5922/1688.full
http://www.nature.com/nature/journal/v462/n7271/pdf/nature08...
There's a difference in method between the Fermi and Nemiroff methods. The Nemiroff method is novel, but with lower statistics. Nemiroff himself has stated the results are not yet statistically significant. More short, bright GRBs need to occur.
> A much more controversial observation is an energy dependence in the speed of light of cosmic rays coming from a short burst of the blazar Markarian 501 on July 9, 2005. Photons with energies between 1.2 and 10 TeV arrived 4 minutes after those in a band between .25 and .6 TeV. The average delay was .030±.012 seconds per GeV of energy of the photon. If the relation between the space velocity of a photon and its energy is linear, then this translates into the fractional difference in the speed of light being equal to minus the photon's energy divided by 2×1017 GeV. The researchers have suggested that the delay could be explained by the presence of quantum foam, the irregular structure of which might slow down photons by minuscule amounts only detectable at cosmic distances such as in the case of the blazar. [2]
[1] http://en.wikipedia.org/wiki/MAGIC_(telescope)
[2] http://www.news.ucdavis.edu/search/news_detail.lasso?id=8364
That's the most forceful language you can really use in this situation.
I mean is it more like with Edison "inventing the lightbulb" (many others did at the same time, or would've done it soon anyway), or would things be fundamentally different today?
Of course that's only the first and by no means the greatest of his ideas.
People who get too attached to the idea that the he must have created the theory from whole cloth might be offended by that idea, but... honestly, that really was a stroke a genius. A lot of very smart people had the mathematics all but yelling at them that the model of rigidly separate space and time wasn't working, and when you dig back into it, it was "yelling" for decades. (For instance, Maxwell's equations, first published 1861, have a speed of light that is simply a constant, not a function based on the velocity of the observer or anything else like that. In hindsight, this is obviously a Big Clue.) It really was a stroke of genius that he put it together.
Had he not, I guarantee somebody else would have. It may have been years later, or possibly even a decade later, but probably not much more than that. Eventually evidence would have started stacking up and the conclusion would have become inevitable. Einstein did it off of not a lot of concrete evidence. But we certainly would not be living in a world in which nobody had ever come up with the idea or anything stupid like that.
If you want to dig through it, I find http://mathpages.com/rr/rrtoc.htm Reflections on Relativity to be a dense, but fantastic book on the topic. (Ooo, this is the first time I've ever clicked through that link and seen the physical book finally exists, after years of me wishing for it. Nice.) RoR even digs back and shows how the Greeks had paradoxes that demonstrated the impossibility of a rigidly separated space and time, and had we taken those paradoxes seriously much earlier, who knows what we might have done.
(I do find it intriguing how much math from that era can be framed in terms of taking the ancient paradoxes seriously, really really seriously, and not just as wordplay. One of the biggest moments in 20th-century math history, Godel's Incompleteness Theorem, can be viewed as "simply" taking the ancient Epimenides paradox seriously. http://en.wikipedia.org/wiki/Epimenides_paradox )
Sorry? "Even" Edison? Especially Edison. The man was a charlatan and a crook.
No! The real charlatan and crook was Newton! (If you're German)
> This article might interest you: http://www.forbes.com/sites/alexknapp/2012/05/18/nikola-tesl...
general relativity is by far Einstein's most amazing idea/theory, and as far as i know it doesn't have any obvious inspirations. Feynman said as much. arguably if you cobble together special relativity + the idea of space and time as being one space + the equivalence principle, you might be able to come up with the idea for general relativity... but probably not.
Also he got his Nobel prize for his work on the photoelectric effect. He wasn't the first to come up with the idea of photons, but his model was the first that could explain the effect. https://en.wikipedia.org/wiki/Photoelectric_effect#20th_cent...
My understanding is that one of the predictions of LQG is that photons of higher energy are likely to lag behind those of lower energy due to propagation through spin-foam (meaning velocity isn't c but dependent on energy or frequency).
It seems like this is what they're getting at in [1]...
Would something like this categorically invalidate something like LQG or merely some of its assumptions?
[1] http://phys.org/news/2013-01-spacetime-smoother-brew-knew.ht...
http://arxiv.org/find/astro-ph/1/au:+Hogan_C/0/1/0/all/0/1
In his current work he is developing the theory of a proposed new phenomenon, which he calls “holographic noise”, a fundamental, universal uncertainty in the fabric of spacetime, akin to pixelation in an imperfectly sampled digital audio file or video display. The theory may lead to the development of experiments that could allow a direct measurement of the minimum interval of time.
[1] : http://www.meessen.net/AMeessen/STQ/STQ.pdf [2] : http://www.meessen.net/AMeessen/STQ/STQ2.pdf
How in the world do you get from point A to point B in this line of reasoning? They received three photons of similar wavelength at a similar time. Everything else seems to be some stuff they made up.
This seems to be the right abstract: http://adsabs.harvard.edu/abs/2013AAS...22115209N
This bunch of photons, and two other bunches, seem to have not been dispersed. As the abstract linked on one of the other comments says,
...the limit on the dispersion strength is k1 < 1.61 x 10-5 sec Gpc-1 GeV-1 ... In the context of some theories of quantum gravity this conservative bound [suggests] that spacetime is smooth at energies perhaps a factor of 1000 below the Planck length.
The idea that we can make any inferences about photon disperson based on this particular observation seems about as credible as Noah's Ark. But, as has been pointed out elsewhere in the thread, the article is the primary source, not the pop-sci summary.
I thought the model was pretty simple to infer. Gamma rays are rare. GRBs dump a lot of energy out at the same time; enough that we can get a spectrum. Given the observation date, it's easy to find that this was GRB 090510, which was a short burst of 0.33 seconds. It was detected by Fermi-LAT, so that gives somewhere under a degree of angular resolution.
Few gamma rays normally, huge numbers at once, from the same direction, means that few of huge numbers are going to be background, and likely means that those gamma rays came from the same source.
Still, it might be a coincidence, which is why the summary we both read includes the phrase "There is a possibility of a statistical fluke, or that space-time foam interacts with light differently than we imagined," and mentions "If future gamma-ray bursts confirm this."
How did you not draw this same inference?
The problem I have isn't with the inference that the photons came from GRB 090510, but with the idea that anything can be inferred about the medium they passed through.
If quantum-foam fluctuations can be assumed to be uniformly distributed along the paths of all three photons, then the number of disturbances encountered by a photon is proportional only to the photon's path length. Here, the path lengths of all three photons are enormous and essentially identical, so it seems reasonable to assume that each one encounters a similar number of quantum "potholes," and that no difference in their arrival energies/times/whatever would be noticeable on a millisecond scale.
How in the world do you get from point A (a BS summary in the article) to point B (that they made the conclusion in the paper up) in this line of reasoning?
Read the freaking original article, not the news bite.
If the situation was chaotic (which I would presume a system with many random perturbations to be) the expectation is for the repeated small changes to have a significant impact.
That is, if the burst lasted 1 second (I made this number up!), you'd expect all photons to arrive within 1 second of each other. But if space-time foam had a strong effect, you would expect the same average time, but you might expect the spread of the data to be larger: maybe 5 seconds (also made up). (Essentially, randomness like this would be expected to increase the standard deviation of the arrival times.)
So by measuring the spread of the photon arrival times, you can put an upper bound on how big the space-time foam effects can be: the actual spread is (very roughly) the sum of the actual length of the event plus the spreading due to space-time foam. These folks are claiming that for certain models of space-time foam, the photons they observed arrived too close together to be consistent with the existence of that foam at the expected scale (assuming it's not a statistical fluke).
http://en.wikipedia.org/wiki/Brownian_motion
All those molecules bumping into a particle do cancel each other out over a long time. However, particles do drift noticeably due to this phenomenon.
[0] http://en.wikipedia.org/wiki/Casimir_effect
Edit: I don't know what I'm talking about.
However, for gravity, we do not have an experimentally verified quantum theory. It is thought that such a theory would contain the equivalent of virtual pair creation, namely fluctuations in the space-time. So, at the Planck scale, one would expect that space-time would not be smooth and thta you would have spontaneous appearance of quantum fluctuations that would give your space-time a foamy structure. However, these structures would be incredibly tiny and, for "macroscopic objects like a proton (!)", the space-time would still look smooth; fluctuations of the even horizon of a black hole would occur at much smaller distance than those involved in virtual pair creation.
I kind of liked quantum foam as a possible way of explaining gravity observation that are not explained by the amount of visible matter and thus attributed to 'dark matter', because 'matter' is the only thing we know that would cause gravity. This tells me I shouldn't get too excited about that possibility.
Further, they cite two other bunches of photons that "Two other short duration photon bunches bolster the statistical significance of this limit"
Every result, except for those whose environment is completely fabricated (such as in pure mathematics), is a statistical or probabilistic one. The question here is if their data is significant or not, and if they have made any mistakes analysing it - not that they only had N=1.