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.)
(I did not read the paper yet.)
It is not much model dependent, the general idea is that a energy dependent speed of photons should be something like
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.
[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 ;)
>> 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?
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.
No no, I get the basic idea of making estimates based on dimensional analysis. My question is: how generically do we expect the dispersion relation to be of this form? I could imagine, for example, that differences in speed are exponentially suppressed by the ratio E_gamma/M_p.
It is just a Taylor expansion around c_0. And since we do not observe any strange effects we know that it should kind of work. (That the speed of light should be well behaved.) In addition the planck mass is the only constant, that we know of, which fits there.
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.
This is interesting, since the MAGIC telescope [1], a gamma ray detector, had some potential evidence for quantum foam:
> 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]
Did other scientists have similar ideas at the time, that would lead us to the same conclusions today, but Einstein always got the spotlight because he was famous, or would 2013's technology and science look different today if it weren't for Einstein?
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?
Yes, other scientists had similar idea at the time, but the papers Einstein wrote were groundbreaking all the same. Those papers made Einstein famous, so it's not as if others' contributions are overlooked because of his prior fame. The only rumored stain on his character is that he "stole" ideas from Poincare's and Pauli's prior papers, but people who make those claims don't really understand how the scientific publishing process works. Even Edison [edit: I meant Einstein, not Edison] stood on the shoulders of giants.
With special relativity I can't help feeling Einstein's real insight was to treat the implications of the mathematics as really true - and wonder whether Lorentz had already done this, but not thought it worth mentioning (us mathematicians tend to have a slightly different conception of reality from physicists).
Of course that's only the first and by no means the greatest of his ideas.
I think that's a fairly accurate summary. The Lorentz transformation predates relativity. Einstein's genius was taking it seriously.
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 )
this is all true until you get to general relativity, which took Einstein some 10 years after special relativity to come up with.
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.
Einstein was the first (and it really was a breakthrough) to realize the exact nature of spacetime. He reformulated the Lorentz transformation to show that the passage of time actually changes in different reference frames. (Edit: apparently the reformulation was done by Poincaré, but he still had the idea of aether.)
Can anyone comment on the impact of this result on the theory of Loop Quantum Gravity (LQG)?
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?
It's really not acceptable that the author of this article can't even consistently use 'photon' instead of 'proton'. Can we stop advertising pop sci summaries that fail to handle summarizing papers?
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.
Space Time may look smooth at the Plank resolution, but space time quantification at a much lower resolution is still possible. Beside if the quantification is organized as a regular 3D lattice and not "foam bubbles" then the space time quantification theory [1][2] would not be invalidated by the experiment.
If this foam indeed exists, the three protons (sic) should have been knocked around a bit during their epic voyage. In such a scenario, the chances of all three reaching the Fermi telescope at virtually the same time are very low, researchers said.
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 is a summary of a presentation, not a paper, so it doesn't have all the details.
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."
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.
They would drift apart from each other, in a manner somewhat similar to the brownian motion. Essentially, small-scale foam would be similar to a slight inherent "blur" of the vacuum for gamma rays.
I missed your "sic" the first time I read your comment, so I feel like it's worth emphasising: yes, space.com did manage to mix up protons and photons in the same article. Not sure if that's evidence of bad writing or bad editing, but either way it's embarrassing from a science website of all places.
They expect space to disperse particles like this, if space is indeed foamy. This is because small deviations over a long space-time would lead to large variations.
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.
This is because small deviations over a long space-time would lead to large variations. And the reason the RMS deviation of those variations wouldn't be expected to decrease in proportion to the (immense) path length would be...?
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.
Really, most science sounds like it might have just been made up when you read popular accounts of it. The actual reasoning in the paper is surely based on math.
>Everything else seems to be some stuff they made up.
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.
Does that mean Hawking radiation [1] doesn't exist? Matter, anti-matter pairs being created on opposite sides of the event horizon of a black hole and thus not being able to pop out of existence.
No, Hawking radiation is based on well-known physics (virtual pair particle-anti-particle creation) which arises near an horizon (which you can treat classically: the so-called quantum foam play no role).
Simplified, hand-waving explanation: virtual pair creation can occur in any quantum field theory. We have quantum field theory describing three of the fundamental forces (strong, weak, electromagnetism ... ok, two forces as the last two are unified as electroweak). Pair creation can occur at a (length) scale that is much larger than the Planck scale. (the energies involved are much lower than that of the Planck scale). Treating a black hole classically, one particle from a virtual pair can cross the horizon and the other escape at infinity - thus you get Hawking radiation. At those scales, the event horizon would appear to be smooth - like a pure 2-d surface.
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.
The article is a bit dodgy in this regard. It just means that at a quantum level, the energy fluctuations (as observed in this experiment) are not big enough to cause significant space-time alterations.
So, it is foamy, but not as foamy as other people thought (the idea of virtual particles having no gravitational contribution sits uncomfortably in my mind).
The average would be unchanged, but the whole point of their measurement is to measure the resulting statistical broadening of the event.
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).
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.
My guess would be this would be true only if they were coherent, and the resonant frequency were some multiple of the Planck length. The many small random perturbations would largely cancel each other out and the remainder would be insignificant (not energetic enough) to meet the lowest energy requirements to nudge a photon by even a miniscule amount.
This is what I sometimes lament about astrophysics, N=1 is sufficient for proof. It's one observation of one event. You can't make any claims about that unless it's a direct violation of some which will occur 100% by theory. In this case there is no such guarantee.
Proof is a word that shouldn't be used lightly. I don't think anyone would see this as proof that quantum foam doesn't exist. The laws of physics allow for quantum foam to happen, which may lead us to start looking very differently at the structure of the universe. However, we now have some strong evidence from measurements over a reasonably large distance that quantum foam is not an immediately noticeable factor in the universe.
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.
Yes ( and I can assure you that anybody working on astrophysics is aware of the problem). In addition perfectly ordinary astrophysics can sometimes mimic the effects of such observations. On the other hand, given the current funding situation, there is simply no way to build a laboratory were you can use a 7 billion light year baseline in a controlled and repeatable way.
It seems to me the art of reasoning is one that is slowly dissipating as the quick access to information continues to infiltrate all aspects of our lives and replace the need to deduce. It is quite amazing that someone who lived quite a while ago, before the boom of science/technology, could be so accurate simply through the power of observation, logic and reasoning.
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.)