7 ms·
Einstein Was Right: Space-Time Is Smooth, Not Foamy
- deleted 14y ago[deleted]
- tocomment 14y agoDoes 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. [1] http://en.wikipedia.org/wiki/Hawking_radiation http://en.wikipedia.org/wiki/Hawking_radiation
- digeridoo 14y agoThe 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.
- ZoFreX 14y agoSurely over such a long distance, the staggeringly huge number of extremely small genuinely random changes would cancel out?
- Cogito 14y agoI'm interested why you would think that. 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.
- sageikosa 14y agoMy 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.
- Steuard 14y agoThe 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).
- Florin_Andrei 14y agoFor an analogy, look up brownian motion. http://en.wikipedia.org/wiki/Brownian_motion 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.
- misnome 14y agoSo, 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).
- skorgu 14y agoAlso the Casimir effect [0] is quantum-foam-y I think. [0] http://en.wikipedia.org/wiki/Casimir_effect http://en.wikipedia.org/wiki/Casimir_effect Edit: I don't know what I'm talking about.
- aroberge 14y agoNot at all. The Casimir effect has nothing to do with quantum foam; furthermore, it is something that has been measured experimentally.
- aroberge 14y agoNo, 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).
- tocomment 14y agoAny chance you can explain a bit more? It thought virtual pair creation and quantum foam were the same phenomenon.
- aroberge 14y agoSimplified, 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.
- tocomment 14y agoWhy the downvote? Isn't it a legitimate question?
- wheaties 14y agoThis 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.
- digeridoo 14y agoProof 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.
- Cogito 14y agoThey do not claim proof here. 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.
- yk 14y agoYes ( 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.
- CamperBob2 14y agoIf 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.
- Osmium 14y agoI 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.
- fdej 14y agoReally, 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. This seems to be the right abstract: http://adsabs.harvard.edu/abs/2013AAS...22115209N http://adsabs.harvard.edu/abs/2013AAS...22115209N
- Cogito 14y agoThey 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.
- CamperBob2 14y agoThis 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.
- jessriedel 14y agoArXiv: http://arxiv.org/abs/1109.5191 http://arxiv.org/abs/1109.5191 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.)
- yk 14y ago(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).
- jessriedel 14y agoThanks. > 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.
- Steuard 14y ago>> 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.
- methodin 14y agoIt 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.
- mtgx 14y agoDid 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?
- ghc 14y agoYes, 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.
- lmm 14y agoWith 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.
- jerf 14y agoI 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 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 http://en.wikipedia.org/wiki/Epimenides_paradox )
- chmike 14y agoSpace 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. [1] : http://www.meessen.net/AMeessen/STQ/STQ.pdf http://www.meessen.net/AMeessen/STQ/STQ.pdf [2] : http://www.meessen.net/AMeessen/STQ/STQ2.pdf http://www.meessen.net/AMeessen/STQ/STQ2.pdf
- brutuscat 14y agoInteresting! I've read in one Scientific American edition about this and about what the Prof. Craig Hogan is studying: http://arxiv.org/find/astro-ph/1/au:+Hogan_C/0/1/0/all/0/1 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. http://astro.fnal.gov/people/Hogan/ http://astro.fnal.gov/people/Hogan/
- skwosh 14y agoCan 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? [1] http://phys.org/news/2013-01-spacetime-smoother-brew-knew.html http://phys.org/news/2013-01-spacetime-smoother-brew-knew.ht...
- lobster_johnson 14y agoThis 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] [1] http://en.wikipedia.org/wiki/MAGIC_(telescope) http://en.wikipedia.org/wiki/MAGIC_(telescope) [2] http://www.news.ucdavis.edu/search/news_detail.lasso?id=8364 http://www.news.ucdavis.edu/search/news_detail.lasso?id=8364
- Benferhat 14y ago> potential evidence That's the most forceful language you can really use in this situation.
- huggah 14y agoIt'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?