fando

http://www.space.com/scienceastronom...ts_030402.html

To expand on the article, Astronomers are not seeing expected blurring effects due to light from distant sources being scattered ever so slightly by the hypothesized quantum foam. It is thought that around the very miniscule scale of the Plank Length, spacetime warps and contorts chaotically, forming countless infinitesimal singularities, twists, wormholes and ripples that then vanish like virtual particles. The underlying cause of the fluctuations is thought to be quantum in nature and the texture of these fluctuations can be described as foamy, hence the descriptive name quantum foam.

If spacetime really is like this at ultra small scales, it was predicted that the fluctuations could scatter light in very small ways, sort of like how turbulent air scatters light passing through it. To detect this, it makes sense to take a look at images of the very distant universe. Light passing through megaparsecs of quantum foam should deviate enough that the images will have a characteristic blurriness. That the Hubble pictures are ultra sharp no matter the distance of the object is a clue that this prediction is off in some way. This has implications for any theory of quantum gravity, which is a physics that can potentially describe reality at scales of the Plank Length and Plank Time.

Answerer

Are you suggesting spacetime might be quantized? I thought it is another unproved theory.

fando

Indirectly, yes. Spacetime quantization is required for quantum foam to make sense in that the uncertainty principle as applied to a quantized spacetime allows weird fluctuations below the threshold of direct observation. While the idea that spacetime can be quantized might be new and exotic to some, it is fairly old as far as theorists are concerned, although it is still poorly understood. Really, that only scratches the surface of the topic here. The subject is more about the natural fluctuations around the 'zero point' of systems, such as the vacuum fluctuations of the electromagnetic field that manifests as a sea of virtual particles constantly appearing and vanishing out of nothing and back into nothing. In this case, we're discussing fluctuations under a paradigm of quantized spacetime.

Please be careful about using the term 'unproved theory'. It makes more sense for an established theory to be disproven in some way, but usually not entirely. Physics is not math, where it is easy to disprove entire theorems. Rather, theories of physics are organic and tend to evolve or adapt to new data, such as the findings that is the thread topic, but rarely do they die completely.

I can say, though, that your initial thoughts are on the money: Much more data is needed before things like quantum foam and quantized spacetime become universally accepted. It can be said that this is one of the goals of the thread, to discuss the relevance of the findings towards acceptability.

Oh crud, I misspelt Planck.

theyeti

My limited understanding is that the quantum foam is a problem not just for the observation seen here, but for a more fundamental reason: It should prevent the transmission of gravity. All attempts to reconcile general relativity with the quantum foam end up with nonsensical answers like infinity. So attempts to answer this problem will invariably result either in a conception of gravity that is different than our current conception, and thus not affected by the foam (and I guess maybe light would fit in here too, but quantum mechanics and electromagnetism have been merged), or they'll result in the foam being done away with. String theory, for example, gets rid of point particles and replaces them with strings of minimal length, such that nothing esists that is small enough for the foam to act upon.

theyeti

AdamWho

I never really liked the quatum foam theory:
Just because uncertainty doesn't forbid these fuctuation doesn't mean that they exist. Maybe this theory is all washed up.

Beer God

I think many are getting confused between quantum foam theory, and the 'foam' that this article is taking about.

Quote from the article, "Conventional thinking is that space and time can be thought of together as a sort of foam." Quantum foam theory is anything but conventional. The belief in the "multiverse" as quantum foam theory predicts is not what the article is referring to.

fando

I've been casting about for the paper behind this report, but turns out it isn't published yet. (April 10 in Astrophysical Journal Letters). It could be that the purported cause of blurring is simply the quantization of spacetime. But I do know of attempts to detect the quantum foam, and some of them rely on the same techniques covered in this study of Hubble data. So it could be my own confusion. I hope to clear it up when the study is published.

By the by, I don't think there is a "quantum foam theory". As far as I know, it's mostly conjecture and hypothesis based on extrapolations from possible theories of quantum gravity. Very frontier stuff. Quantization of spacetime however....

Dominus Paradoxum

The non-existence of quantum foam would be good evidence that Superstring theory is the correct approach to quantum gravity.

Tim Thompson

If you are looking for a paper in astronomy & astrophysics (or a lot of other things), look here first: arXiv.org e-Print archive.

Lack of observational evidence for quantum structure of space-time at Plank scales
R. Ragazzoni, M. Turatto & W. Gaessler
Astrophysical Journal 587: L1-L4, Part 2, April 10, 2003
Abstract: It has been noted (Lieu & Hillmann, 2002) that the cumulative affect of Planck-scale phenomenology, or the structure of space-time at extremely small scales, can be lead to the loss of phase of radiation emitted at large distances from the observer. We elaborate on such an approach and demonstrate that such an effect would lead to an apparent blurring of distant point-sources. Evidence of the diffraction pattern from the HST observations of SN 1994D and the unresolved appearance of a Hubble Deep Field galaxy at z=5.34 lead us to put stringent limits on the effects of Planck-scale phenomenology.

The phase coherence of light from extragalactic sources - direct evidence against first order Planck scale fluctuations in time and space
Richard Lieu, Lloyd W. Hillman
Astrophysical Journal 585(2): L77-L80, Part 2, March 10, 2003
Abstract:
We present a method of directly testing whether time continues to have its usual meaning on scales of <= t_P = sqrt(hbar G/c^5) ~ 5.4E-44 s, the Planck time. According to quantum gravity, the time t of an event cannot be determined more accurately than a standard deviation of the form sigma_t/t = a_o (t_P/t)^a, where a_o and a are positive constants ~1; likewise distances are subject to an ultimate uncertainty c \sigma_t, where c is the speed of light. As a consequence, the period and wavelength of light cannot be specified precisely; rather, they are independently subject to the same intrinsic limitations in our knowledge of time and space, so that even the most monochromatic plane wave must in reality be a superposition of waves with varying omega and {\bf k}, each having a different phase velcocity omega/k. For the entire accessible range of the electromagnetic spectrum this effect is extremely small, but can cumulatively lead to a complete loss of phase information if the emitted radiation propagated a sufficiently large distance. Since, at optical frequencies, the phase coherence of light from a distant point source is a necessary condition for the presence of diffraction patterns when the source is viewed through a telescope, such observations offer by far the most sensitive and uncontroversial test. We show that the HST detection of Airy rings from the active galaxy PKS1413+135, located at a distance of 1.2 Gpc, secures the exclusion of all first order (a=1) quantum gravity fluctuations with an amplitude a_o > 0.003. The same result may be used to deduce that the speed of light in vacuo is exact to a few parts in 10^32.

The paper by Ragazzoni et al. argues that the expected effect due to first order quantization is not there, but higher orders are not ruled out. So it's not a matter of arguing that space-time is not quantized, but that it is quantized in a manner other than anticipated. I saw no reference as to whether or not this observation favored string theory as a quantizing theory, though I rather like the string idea myself.

However, don't jump the gun just yet. According to D.H. Coule, the Lieu & Hillman paper (and by association, also the Ragazzoni et al. paper) is wrong. He claims that the long distance should cause the quantum effects to cancel out, eliminating the expected "fuzziness" in the HST images.

Planck scale still safe from stellar images
D.H. Coule
March 4, 2003
Abstract: The recent paper of Lieu and Hillman [1] that a possible (birefringence like) phase difference ambiguity coming from Planck effects would alter stellar images of distant sources is questioned. Instead for division of wavefront interference and diffraction phenomena, initial (lateral) coherence is developed simply by propagation of rays (cf. van Cittert-Zernike theorem). This case is strongly immune to quantum gravity influences. The phase ambiguity, if actually present, would instead reduce any underlying polarization of the light rays.
However, we argue that, as expected since any inherent quantum discreetness of space should become increasingly negligible over larger distances, such a phase ambiguity is rapidly cancelled if a more realistic constantly fluctuating quantum ``buffeting'' occurs.

So, the issue remains in doubt.

fando

Tim, thanks for digging that up! I had completely forgotten about arXiv.

I vaguely understand this. String theory is not my forte, but from skimming the papers and Tim's summary, is that because strings or membranes require "smooth" spacetime all the way down to the scale of these objects? That is, if spacetime were choppy around Planck scales, membranes would make no sense because they're supposed to be much smaller?