By Ron Cowen
Space is not smooth: physicists think that on the quantum scale, it is composed of indivisible subunits, like the dots that make up a pointillist painting. This pixellated landscape is thought to seethe with black holes smaller than one trillionth of one trillionth of the diameter of a hydrogen atom, continuously popping in and out of existence.
That tumultuous vista was proposed decades ago by theorists struggling to marry quantum theory with Einstein's theory of gravity -- the only one of nature's four fundamental forces not to have been incorporated into the standard model of particle physics. If it is true, the idea could provide a deeper understanding of space-time and the birth of the Universe.
Scientists have attempted to use the Large Hadron Collider, gravitational wave detectors and observations of distant cosmic explosions to determine whether space is truly grainy, but results have so far been inconclusive. Now, Jacob Bekenstein, a theoretical physicist at the Hebrew University of Jerusalem, has proposed a simple tabletop experiment to find out, using readily available equipment.
As in previous experiments, Bekenstein's set-up is designed to examine the problem on the scale of 1.6 × 10−35 metres. This 'Planck length' is thought to mark the scale at which the macroscopic concept of distance ceases to have meaning and quantum fluctuations begin to cause space-time to resemble a foamy sea.
No instrument can directly measure a displacement as small as 10−35 metres. Instead, Bekenstein proposes firing a single particle of light, or photon, through a transparent block, and indirectly measuring the minuscule distance that the block moves as a result of the photon's momentum.
Light and mass
The wavelength of the photon and the mass and size of the block are carefully chosen so that the momentum is just large enough to move the block's centre of mass by one Planck length. If space-time is not grainy on this scale, then each photon will pass through the block and be recorded by a detector on the other side. However, if space-time is grainy, the photon is significantly less likely to make it all the way through the block. "I argue that the consequence of that crossing -- the translation of the block by a Planck length or so -- is something nature would not like," says Bekenstein.
If quantum fluctuations in length are important on the Planck scale, a sea of black holes, each with a Planck-scale radius, will readily form. Anything that falls into one of those black holes will be unable to escape until the hole disappears. So if the centre of mass of the moving block falls into one of the holes, the block's movement will be impeded (the photons are much larger than the Plank length, and so are not bothered by the miniature black holes).
Conservation of momentum in the experimental set-up requires that the photon cannot make it through the block if the block fails to move by a Planck length. So if fewer photons than expected are seen by the detector, it would indicate that the block's movement has been impeded by black holes, and that space-time exhibits quantum features at the Planck scale.
Bekenstein's design is simple, so the experiment could easily be put into practice using established methods of generating and detecting single photons, says Igor Pikovski, a quantum physicist at the Vienna Center for Quantum Science and Technology. Nevertheless, he adds, "distinguishing possible quantum gravitational effects from other effects will be very challenging".
Earlier this year, Pikovski and his colleagues published another scheme for probing the graininess of space-time in the laboratory, using optical pulses and and the principles of quantum theory to drive a system from an initial configuration to the desired final state. "The truth is that we do not know at what exact scale quantum gravity will play a significant role," says Pikovski. "There is plenty of room for granularity at much larger lengths [than the Planck length] and we do not have a full theory that could tell us the answer." Experiments such as Bekenstein's or his own may provide some of the first evidence for an answer, he adds.
This story is reprinted with permission from Nature magazine. It was first published on November 22, 2012.
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15 Comments
Add CommentWake me when the detection of quantum scale black holes is underway...
Reply | Report Abuse | Link to thisWhat is space? It is the orders of arrangement showing the interval between objects or different parts of an object. It also acts like a base or ground for the objects to exist. If a person is standing on the ground and we shove him, does his displacement show the nature of the ground? Certainly not. Thus, the experiment is wrongly conceived.
Reply | Report Abuse | Link to this"This pixellated landscape is thought to seethe with black holes smaller than one trillionth of one trillionth of the diameter of a hydrogen atom, continuously popping in and out of existence". This means virtual black holes! This must be an altogether different theory!
If you push the person standing on the ground you can tell how smooth or rough the ground is. As with any experiment you measure what the experiment is intended to measure. To be honest, I also have my doubts that the experiment will work or show the Planck length nature of spacetime but it is such a simple and inexpensive experiment it would be a shame not to run it and publish the results. It is an interesting approach, and may lead to other areas of research.
Reply | Report Abuse | Link to thisPlanck scale uncertainty from space-time fluctuations doesn't forbid to move an object by less than the Planck length. It just makes it impossible to measure any shift by a distances smaller than the Planck length because the position uncertainty is always larger than that. That is to say, photons would have no problem passing through the block. And, sorry Ron, but the title is terribly misleading because whether or not there are Planck scale black holes doesn't enter the argument whatsoever.
Reply | Report Abuse | Link to thisVery good comments (excluding my own little quip)!
Reply | Report Abuse | Link to thisThe opening paragraph takes a definitive stand on a subject exhibiting increasing uncertainty - a brief preview of David Tong's article can be found at:
http://www.scientificamerican.com/article.cfm?id=is-quantum-reality-analog-after-all
As for the grand unified theory of everything, the forces that are included the standard model of particle physics are all properties of particles whose effects are mediated by the exchange of particles. In contrast, gravity's effects are most completely described strictly as an energetic property of spacetime that acts upon the material property of mass-energy.
While the physical mechanism that produces the effects of gravitation has not been fully described, it is clearly and fundamentally distinct from the three forces of matter, whose local influence is greater but whose effective range is limited by the physical exchange of mediating particles.
It has heretofore only been presumed that gravity and spacetime can be quantitatively defined. As pointed out by previous commentators, the proposed experiment only seems to test whether the momentum of a photon is a discrete quantity, not excluding the possibility that spacetime and its effect of gravity are continuous.
Still, the results would be interesting - why haven't some researchers already performed this simple experiment?
To state space as an arrangement or interval between objects or a ground on which matter and energy are based is a quite simplistic semantic approach to describe space. The most intriguing issues are following.
Reply | Report Abuse | Link to thisWhat is the nature of the space -- interval between material objects or ground?
Of what stuff- physical or non-physical that interval between objects i.e space is composed of?
From where that stuff which composes space emerges out?
What is the physical mechanism involved in the evolution of space?
Is there any interacting force between matter/energy and space? if Yes, what is the nature of that force?
No entity can exist unless it is supported by some tangible stuff -- physical or non-physical or whatever
This just in: While in the shower, several months ago, a scientist thought of an extreme simple experiment that can be done using a bottle of mustard and a cheese grater, which can falsify the existence of God. Several other teams of scientists are now reportedly working through the process of obtaining the necessary items. A progress report is promised in six months, but really the experiment is very simple and can be performed with household items.
Reply | Report Abuse | Link to thisIsn't science amazing?
"No entity can exist unless it is supported by some tangible stuff -- physical or non-physical or whatever", I am having a hard time finding the proof to that theorem. Could you please point me to it?
Reply | Report Abuse | Link to this@Bee, it seems you are discribing the effect while the article is describing the mechanism. I don't see how what you claim invalidates what the article claims. Perhaps you could provide more details.
Reply | Report Abuse | Link to thisLaroquod is trying to be snarky by suggesting that scientists can come up with ideas for experiments but then make the actual building and conducting of the experiment more lengthy and/or difficult than it needs to be.
Reply | Report Abuse | Link to thisI expect that while Bekenstein's idea may be simple for those who actually conduct experiments, it's not something that one just snaps together like Lego blocks, especially since it sounds like there's some careful calibration required for the photons to be used and the block at which it's aimed.
I don't understand why less photons passing would be the evidence for graininess. If the block is "pinned" to a space-time grid point, wouldn't conservation of momentum require that the photon pass through it unimpeded and unchanged? If it were absorbed, its entire momentum would go to the immovable block, a violation. So I would expect more photons to pass, not less.
Reply | Report Abuse | Link to thisI provide one crude illustration to highlight my point of view. Our mental thoughts especially emotional thoughts like anger, love, fear are hard existential realities. By existential realities, I imply some real existence which exist either in abstract or non-abstract form either by virtue of its existence or due to existence of some other realty in cause-effect relation.
Reply | Report Abuse | Link to thisThere is no denying the fact that emotions exist very much but we are not aware of the "stuff" or "real existence" of which they are composed of. Electric and chemical signals in brain are not the composition "stuff" of emotions but electro-chemical activity in brain arises due to arousal of emotions. Emotions are the cause and electro-chemical activity is the effect. Any thing which exist should have tangible character of existence. This is another matter that we may be unable to comprehend the "tangibility". Our inability to comprehend the tangibility of the stuff of which emotions are composed does not preclude the "tangibility" from such abstract existences.
Another example, we know that e.m radiations/photons are existential realities. But we lack tangible ideas about the "stuff" of which e.m radiations are composed of. Whatever ideas we have about the composition of photons are in very abstract form-- some ephemeral existence. But "abstractness" in our mind about any existence does not affect the very existence of such realities. In view of this, any entity which exist should be composed of " "something". To comprehend that "something" in tangible form is the real challenge for the faculty of our mental understanding
Getting back on the subject matter of the article, the concept behind Bekenstein's test is not that difficult. Shorter wavelengths of photons have more momentum because they have more energy. A transparent block of fixed mass will be moved more when a photon having higher energy passes through it. Refraction reduces the speed of light. By slowing down the photon, momentum is transferred from the photon to the transparent block. This change in momentum causes an immeasurably small change in the velocity of the transparent block. When the photon exits, it recovers its momentum but the block has changed position. This change in position is a function of the change in velocity.
Reply | Report Abuse | Link to thisFor most sizes of transparent blocks, the change in position will not be close to the Planck Length. It will typically be greater. So, the center of mass of those blocks is much less likely to fall into a quantum hole in Wheeler's "spacetime foam" because it will skip past. In this case, energy is conserved entirely in our normal space. This energy is necessary for the photon to continue existing inside and to emerge from the transparent block. We may measure the rate of accidental deflections of the photons away from the detector and the rate of failure to detect among other unknowns.
Once this baseline measurement error is known, we adjust the mass the transparent block or change the angle of the incoming photons so that the momentum transfer causes the deflection of the transparent block to be some multiple of the Planck Length. This would increase the probability that the center of mass of the transparent block would fall into a quantum hole causing the energy of the photon to be absorbed instead of being conserved in ordinary space. When that happens, there is no energy to bring the photon back into existence so that it can be detected exiting from the transparent block. Hence, fewer photons would be detected under this precise condition.
The apparatus would act rather like a Planck Length scaled interferometer for measuring the granularity of the hypothetical "spacetime foam". If we see fewer photons detected at specific combinations of the mass of the transparent block and momentum transfer of each photon, which must necessarily be fired one at a time, then space can no longer be reckoned smooth. The experiment would demonstrate measurable granularity.
A nice explanation, as near as I can determine, at least. The "Nature" article contains a helpful illustration and a link to the freely available preprint study:
Reply | Report Abuse | Link to thishttp://www.nature.com/news/single-photon-could-detect-quantum-scale-black-holes-1.11871
Jacob D. Bekenstein, (Nov 16 2012), "Is a tabletop search for Planck scale signals feasible?"
http://arxiv.org/abs/1211.3816v1
I haven't read it yet, but wouldn't discretely quantitative motions of the 'transparent block' be more a measure of its material's quantum nature than the granularity of spacetime?
"When the photon exits, it recovers its momentum but the block has changed position."
Reply | Report Abuse | Link to thisWill some momentum be not consumed in changing the position of block? If photon on exit recovers its original momentum, will block not restore back to its original position?