One characteristic of quantum physics is being used to defeat another, with the aim of making more sensitive gravity wave detectors, in an international project with contributions from the University of Western Australia, the Australian National University, and the GEO600 Gravitational Wave Observatory in Germany. A problem with …
Good to see the reg talking about gravitational waves! I'm a PhD student at University of Birmingham working in the field, check out our page http://gwoptics.org. We're slowly building up a simple ebook on it and a bunch of applet for people to play with and get a better understanding of it all.
Are a scholar amongst gentlemen. I have a shaky grasp of quantum mechanics at the best of times (being no more than a turbo-nerd and armchair enthusiast of science), but I think I actually understood it the way you explained it.
I doubt I would have had so easy a time understanding it straight from the minds of the men working on it.
This article should be mandatory reading for science journalists-- it clearly explains a very complicated phenomenon in lay terms, while being honest and humble about the limitations of such an approach. I feel both informed about the research and about the degree of my understanding.
Tux, because there is no heart icon anymore. More of this, please, El Reg.
They just need some Heisenberg compensators
To paraphrase Star Trek IV
How do you know they haven't just invented them.
As Richard Feynman said...
"If you think you understand quantum mechanics, you don't"
"inability to perfectly measure velocity and direction at the same time"
Almost. Inability to perfectly measure both momentum and position is probably the version you meant. Direction is already a component of velocity, so the original sentence isn't so much wrong as actually makes no sense. Needless to say it is more complex than this, there are more pairs of measures beyond momentum and position, but that is the one people tend to hear about most.
re. pairs of measure
Could you give examples of these 'unceratinty' pairs (or give a link to a suitable article about them.)?
I assume that the link between momentum and position is because if a particle has momentum then it's position will change (and vice versa). I'm wondering which other pairs have this link and if they are similarly related.
"Could you give examples of these 'uncertainty' pairs"
All observables have associated special operators. In some cases, if you apply two operators one after the other, it doesn't matter what order you do it. In some cases, it does - you get different things. [like multiplying matrices together if that's any help]
Any pair of observables where the operators don't commute will have an uncertainty relationship between them.
So quantum mechanics would be a lot simpler if all the observables worked from home.
some more pairs
Energy and time (differences, actually)
Spin along orthogonal planes (3 such pairings)
Not that first one...
Actually, no. Energy and time is not an example of such a pair, this is just a frequent misconception. The reason is that time is not an observable, it is a parameter, and uncertainty relations are only between observables. (Although some physics instructors, particularly in undergraduate courses, like to say this anyway in a poor attempt to explain stuff like vacuum fluctuations.)
Re: Not that first one
Actually, it is true.
By your definition, position is also a parameter, so position & momentum should have no uncertainty.
Energy & momentum make up the 4momentum, just like time & position make up 4position. It is 4position and 4momentum that are uncertain pairings, so necessarily energy and time are.
(I am not a physics instructor, but do have a PhD in high energy physics, and this is not an attempt to explain vacuum fluctuations, but comes from the noncommutativity of the Dirac operators of position and momentum.).
Re:Not that first one
(I actually had to go look this up, it's been a while)
I guess that, in a sense, we are both right. You can heuristically (as you suggest) write down an uncertainty relation for energy and time. But it is not always clear which time interval is precisely meant, and historically this has caused a lot of headaches. In most cases, the time (interval) is the time when a system exists in the same state (or is perturbed only slightly from it) with respect to some observable.
In the end, Mandelstam and Tamm came up with a relation which has a clear meaning, where the standard deviation of the energy and the lifetime of the state with respect to a given observable are involved. In this case, the "time" is clearly different from the time variable in the Dirac equation.
Does this mean I can have an antigravity car? Or a star trek space ship with none of that silly floating around?
since these are gravitational waves,
... no antigravity /car/- just an antigravity surfboard.
So what you are saying...
Is that they can become certain of how uncertain they are (I think, maybe, perhaps, cat)
These inferometric gravity wave detectors rather smack of using a ruler to measure itself.
That's the part I don't get either. They suggest that the gravity waves minutely change the length of tunnel the laser beam has to travel. What I don't get is how come the photons are not also affected by the gravity wave?
It's remarkable how close you can get to observed relativistic effects if you assume photos have a mass given by E=mC2, and bung that into Newtonian Physics.
But they're using two rulers to measure each other, hoping that a gravity wave will shorten or lengthen them one at a time ever so infinitesimally and that they're able to catch it happening, and recognize it when it happens.
(Meh face, because that's what I look like with the headache I get when I think about this kind of stuff!)
Gravity waves are expected to make thing slightly longer and shorter as the wave travels at the speed of light.
To see this, take two rulers and place them at 90 degrees to each other.
If a gravity wave comes along that is parallel to Ruler A, it will make A get shorter then longer while Ruler B will get longer then shorter.
So you should be able to see a gravity wave by continuously comparing the lengths of these two rulers.
Unfortunately these changes in length are predicted to be very, very small, so you need really, really long rulers to make the total length difference be a detectable fraction of the wavelength of light.
(This is a lies-to-children explanation. Everything I just said is more or less wrong, however it's sort of close.)
GEO600 is using rulers 600m long, and LIGO in the USA has rulers 4000m long.
I count four rulers
In addition to your Ruler A and Ruler B you also have the mechanism to compare their lengths. My understanding is that this is done by shining lasers along the rulers. This would be all well and good if light was not affected by gravity, but there are numerous examples where light certainly appears to be affected such as bending around stars, or even the event horizon of a black hole. It's possible these are secondary effects, but the point remains that there is some interaction between light and gravity.
If light *is* affected by gravity, by whatever mechanism, then the laser beam measuring your Ruler A and Ruler B will also be affected, and affected in the same direction and magnitude as the effects on the ruler. So whilst your ruler may get slightly longer the tape measure you are using to measure the ruler will also get slightly longer.
serious scientists doing cool stuff
excellent article on something somewhat out of every day experience.
Kudos to researchers doing real science and making some interesting hardware.
Pleasant change from the hot air usually announced these benighted days.
Keep at it Richard. More stories like this cheer many of us up, especially when I hear ANU and UWA in the same sentence.
"If any piece of vacuum were truly a vacuum all the time, it would be amenable to an absolute measurement, since there’s nothing there."
This has to be one of the clearest sentences I've ever read to express that idea.
Well done on the writing.
bah geo 600 sucks
LIGO much better. USA USA (waves flag)
But gravity is only a theory!
Seriously though, great science article.
Great article! I think I almost understand how little I know, but I really enjoyed it anyway.
Paris, because, well, you know.........................or don't know.
Does this get us closer to the Infinite Improbability Drive?
Shoud I change my research to gravy from hot cups of tea?
Excellent article. Quantum physics is pretty much the most horribly counter-intuitive field of knowledge. It does not lend itself well to being simplified for laymen. Yet, this article explains its bit clearly and without making any gross mistake. Well done.
I sort of get it....
Like in radio waves light has an amplitude (brightness) and a phase (hmmm cant think what this would be,. frequency would be the colour of the light but there you go)
what they seem to be saying is they can push the vacuum anomalies into either amplitude or phase, and when they do, the interference at the quantum level reduces in the other part.
Meaning that if they shove all the interference into amplitude then read the phase, they get less noise and so can properly see the gravity waves without the distortion,,
Sort of makes sense to me.
Phase vs. Frequency
Effectively the same thing. Changes in one will by definition cause changes in the other.
In radio, FM (Frequency modulation) is functionally identical to PM (Phase Modulation). An FM radio, for example, will happily receive and demodulate PM signals.
Indeed, some FM transmitters actually employ phase modulation directly but are still regarded as and refereed to as FM transmitters.
Yeah, I think about it as AM to FM for radio. Most distortions change amplitude, so using frequency for data transmission lets you discard those distortions.
Either way, cool stuff!
Brilliant article. Very well written.
*waves his pipe agreeably*
Getting rid of quantum noise
<Beats fist on Universe next door>
KEEP THAT BLOODY ROW DOWN!!
A Flux capacitor across the Heisenberg compensators will allow you to be more certain about your uncertainty..
Add in the predictability of the ALP reaction to the latest polls along with a nice strong cup of tea and you may get somewhere.
Ahh . . .memories
I used to be a Ph.D. student up at Glasgow working on this stuff more years ago that I want to admit. Back then we had a 10m prototype to play with, Argon ion lasers and and all the liquid nitrogen we could drink (don't try this at home!). Even then the sources of noise that would affect the detector were surprising. I finished my Ph.D. before Geo600 got properly started.
Quantum squeezing had been suggested back then but it obviously turned out to be a bit more difficult than expected.
(Sits back in armchair with slightly nostalgic look on face)
How's the theory?
As far as I can understand, no one has yet measured / observed gravity waves ( or particles??), and the closest understanding we have of gravity is Newtonian - we can calculated G effects very precisely and know the final outcomes without understanding the mechanism, and these experiments are designed to find out that mechanism.
Are there any sound theories as to how that mechanism works? In any case very cool and interesting stuff. Antigravity here we come :)
No, the «closest understanding we have of gravity»
is definitely *not* Newtonian ; that General Relativity provides a better analysis of gravitation phenomena has been known since observations taken during the solar eclipse of 29 May 1919 were published. Experimental errors in these observations are said to have cancelled out each other, but later observations have confirmed the predictions of Einsteins equations and no observations hitherto have conclusively invalidated them. One of the consequences of the theory is the existence of gravitation waves, which, however, have not yet been observed - Richard Chirgwin's article discusses the difficulties involved in measuring these in an exemplary fashion - kudos !...
Einstein understanding vs Newton understanding
The fundamental problem with Newtonian version of gravity is it's non-locality - ie action at a distance, faster than light can travel.
Einstein's interpretation is that it is the warping of space-time, and cannot propagate faster than the speed of light. This is a huge huge difference in understanding.
In Newton's version, if the sun were to get suddenly yanked away into outer space, the Earth's gravity would change instantly, but for 8 minutes we wouldn't know why, as we would still be able to see the sun exactly where we thought it was.
In Einstein's version the gravity change arrives at the exact same time as we see the sun disappear - after 8 minutes.
There is already indirect evidence for gravitational waves in the measurements of fast moving pulsars.
In terms of mechanisms, well, yes there are different theories. Warping of space time. Fields. Exchange of gravitons.
That's not entirely correct - and I am dredging up a deep channel of memory here but:
Newtonian gravity doesn't predict gravitational waves; that is entirely a prediction of Einsteinian relativity. You also need to draw a distinction between direct measurements of gravitational radiation, as these detectors are designed to do, and indirect detection such as the changing orbits of neutron star or black hole binaries. The energy radiated away alters the orbits of the bodies involved. Pulsars in binaries are particularly useful in this case. It's a while since I looked it up but I think there was some good indirect evidence for gravitational radiation from these studies.
Gravitons have been proposed as the particle 'carrier' for the gravitational force but I've lost track of where they currently lie in the morass of Supersymmetry, string theory, Quantuim Loop Gravity et al.
@henri, AC, John
Thanks for the explanations, guys, a lot clearer now :)
Just what Douglas Adams wanted
"That's right!" shouted Vroomfondel, "we demand rigidly defined areas of doubt and uncertainty!"
Lies for children
"lies for Children" - Great expression which nicely sums this article up.
The Heisenberg uncertainty principle simply says we can't know the exact position and the exact momentun of a particle - and this is the important bit - at the same time. The reason is simply because measuring one affects the other. This does not, as suggested, lead to vacumn fluctuation since it is possible to know exactly the position of a particle, there fore it is possible to know, with absolute certainty, that it is not there.
If you want to play with energy-time uncertainties, then you need to be able to play with Schrödinger's cat without getting scratched.
At the risk of exposing my own ignorance I'm not sure that is right. If you measured a particle's position with 'absolute' precision then you wouldn't be able to put any bounds on the momentum at all. If it were just down to measuring one disturbing the other then you couls place bounds on it but the uncertainty principal places a fixed bound on the relative uncertainty in the two measurements.
Denying quantum mechaicx
@Paul Smith: Sorry, Paul, but you're playing the same tune as many previous deniers of quantum physics - trying to frame uncertainty as a practical issue "if you measure it, you change it", rather than as the fundamental law of nature that it is.
confused, i am
Is this the same "noise" that Craig Hogan (Fermilab) claimed as possible evidence of a holographic universe? That is noise from Plank length "granularity", which should only be detectable at resolutions of 10^-35 meters, becoming detectable by GEO600 despite its 10^-15 meters resolution limit. The original 10^-35 Plank length "pixels" expand to become detectable on the 2D event horizon.
"If any piece of vacuum were truly a vacuum all the time, it would be amenable to an absolute measurement, since there’s nothing there."
Some say that it is through the act of measurement that the uncertainty arises. So even if it were to be possible to locate a region of absolute vacuum it would not be possible to measure it without the introduction of the same quantum uncertainty that occurs everywhere else.
This was the early interpretation of the work of Planck and Heisenburg
but it turns out to be wrong. It is an inherent property of Energy and/or matter.
when you've got a tricky problem phase sensitive detection often seems to be the answer
Worked for RV Jones in 1937.
Still working in 2011.
Mines the one with the oversize pockets for my copy of Horowitz & Hill.
the advantage of squeezed light
is not really the reduced uncertainty, but one of reduced power - which then minimises error, as (crudely) less photons bounce of the mirrors, so the mirrors wobble less.
This is because to create squeezed light requires a high-power beam to be converted into a lower-power squeezed beam. And the phase uncertainty in the raw unsqueezed beam (required to create a phase-squeezed beam) is about the same as that in the lower-power squeezed beam.
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