Physicists Measure Gravity With Record Precision (gizmodo.com) 161
An anonymous reader quotes a report from Gizmodo: A team of scientists in China are reporting that they have now performed the most precise measurement of gravity's strength yet by measuring G, the Newtonian or universal gravitational constant. G relates the gravitational attraction between two objects to their masses and the distance between them. The new measurement is important both for high-powered atomic clocks as well as the study of the universe, earth science, or any kind of science that relies on gravity in some way. The values measured by the team "have the smallest uncertainties reported until now," according to the paper published in Nature.
In the new study, scientists performed two independent calculations of G using a pair of pendulums in a vacuum, one pendulum setup for each test. Each pendulum swings back and forth between a pair of massive objects whose positions can be adjusted. The pendulums measure the force of gravity in two ways. First, they measure the difference between how quickly the pendulum swings to the "near," or parallel position, versus the "far," or horizontal position. They also measure how the direction of the pendulum's swing changes based on the pull of the test masses. The researchers ended up measuring 6.674184 and 6.674484 hundred billionths (10-11) for the time-of-swinging and angular acceleration methods, respectively. These measures were both very precise, but are still different from one another for unknown reasons. This might have had something to do with the string used for the pendulum. The paper's reviewer, Stephan Schlamminger from the National Institute of Standards and Technology, wrote in a commentary: "Li et al. carried out their experiments with great care and gave a detailed description of their work. The study is an example of excellent craftsmanship in precision measurements. However, the true value of G remains unclear. Various determinations of G that have been made over the past 40 years have a wide spread of values. Although some of the individual relative uncertainties are of the order of 10 parts per million, the difference between the smallest and largest values is about 500 parts per million."
In the new study, scientists performed two independent calculations of G using a pair of pendulums in a vacuum, one pendulum setup for each test. Each pendulum swings back and forth between a pair of massive objects whose positions can be adjusted. The pendulums measure the force of gravity in two ways. First, they measure the difference between how quickly the pendulum swings to the "near," or parallel position, versus the "far," or horizontal position. They also measure how the direction of the pendulum's swing changes based on the pull of the test masses. The researchers ended up measuring 6.674184 and 6.674484 hundred billionths (10-11) for the time-of-swinging and angular acceleration methods, respectively. These measures were both very precise, but are still different from one another for unknown reasons. This might have had something to do with the string used for the pendulum. The paper's reviewer, Stephan Schlamminger from the National Institute of Standards and Technology, wrote in a commentary: "Li et al. carried out their experiments with great care and gave a detailed description of their work. The study is an example of excellent craftsmanship in precision measurements. However, the true value of G remains unclear. Various determinations of G that have been made over the past 40 years have a wide spread of values. Although some of the individual relative uncertainties are of the order of 10 parts per million, the difference between the smallest and largest values is about 500 parts per million."
Re:Gravity... (Score:4, Funny)
How do you explain a helium balloon's desire to move upward?
Well, it's got Helium inside instead of gravity. Duh.
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Einstein agreed that gravity was not a force. He noticed that it was directly proportional to mass just like fictitious forces were.
As for the being inside an object and desire to move downward, those don't seem like testable hypotheses.
Re:Gravity... (Score:4, Funny)
Correct, in the same sense that an object's desire to release phlogiston causes it to catch fire.
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Not quite right. There are a few different models of gravity, but the most accepted is Einstein's interpretation, that what perceive as gravity is the warping of space-time caused by mass. In essence, the greater mass an object has the more the straight line paths of the universe curve inward towards it. An equally valid mathematical model (but less generally accepted interpretation), is that there is a fundamental field, much like the electromagnetic field, that has directions and imparts momentum where t
Re: Gravity... (Score:5, Interesting)
The problem is that quantum gravity and relativistic gravity can't be reconciled. Only one of these can be correct, at most. The other must be false. They cannot be reconciled.
If QM gravity is false, QM is false.
If relativistic gravity is false, relativity is false.
Both underlying theories have stood up to every test thrown at them, but one of them is wrong.
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The problem is that quantum gravity and relativistic gravity can't be reconciled. Only one of these can be correct, at most. The other must be false. They cannot be reconciled.
If QM gravity is false, QM is false.
If relativistic gravity is false, relativity is false.
Both underlying theories have stood up to every test thrown at them, but one of them is wrong.
I don't think so. They may just be looking a different things and a "higher order" theory is needed to reconcile them. My example would be the wave vs. particle nature of light. They are both correct sort of, and both false, sort of.
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Solved!
QM + R = G
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In QM, gravity is transmitted by gravitons. It is key to the theory that all forces are handled by particle exchange.
There is no quantum gravity theory, per second, only the requirement of gravitons as a particle grouped with other leptons. This is part of the standard model. You will find gravitons in the particle zoo.
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Re: Gravity... (Score:2)
That is true. If and only if QM is false. QM specifically prohibits any such interpretation, which causes a severe problem.
Re: Gravity... (Score:5, Funny)
Your mama's so fat, she emits Hawking radiation.
Re: Gravity... (Score:5, Funny)
I will admit though, she already had :
"Your mamma's so fat that her spiritual guide refers to her aura as an event horizon"
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Yo Mama' so fat, they have to use the Large Hadron Collider to measure the mass of her atoms.
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Perhaps we should replace all Yo Mama' so fat with Trump is so fat or stupid jokes? /me runs and hides
Oh, I hit a nerve on some people I guess!
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They are, but c'mon, it's lifted from a Gizmodo article. Do you read Gizmodo for anything even remotely scientific?
In Nature it says...
We obtain G values of 6.674184×10^-11 and 6.674484×10^-11 cubic metres per kilogram per second squared, with relative standard uncertainties of 11.64 and 11.61 parts per million, respectively.
Re:Well what is it? (Score:5, Funny)
6.674184×10^-11 and 6.674484×10^-11
This kind of cryptic number stuff isn't suitable for a mainstream news site like this.
Re: Well what is it? (Score:2)
I dunno, Slashdot does a lot with cryptographic numbers. There are articles on them regularly. It's just some are ROT13ed into articles about something else.
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6.674184×10^-11 and 6.674484×10^-11
This kind of cryptic number stuff isn't suitable for a mainstream news site like this.
Those are not as accurate as they are precise. If they were, they would be identical.
Re: Well what is it? (Score:1)
I've been through the data and the answer is obvious: someone wrote a 1 instead of a 4 or possibly (following Heisenberg) vice versa.
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Einstein noticed the coincidence a long time ago.
Photons do have mass and both cause gravity and are affected by gravity.
Photons don't come from matter unless antimatter is involved. Usually photons are converted to or from kinetic energy (which also increases mass).
Most mass is already known to be kinetic energy or binding energy. Allegedly for a hydrogen atom only around 2% of the mass is rest mass and the remainder is due to kinetic energy or gluons.
Physics (Score:3)
Photons do have mass and both cause gravity and are affected by gravity.
Photons do not have a mass. If they did then the EM force would be short range like the weak force (whose bosons do have mass).
Photons don't come from matter unless antimatter is involved.
Have a look up in the sky (provided you are not in the UK or Seattle). See that bright shiny thing there? That's the sun. Notice how it is emitting photons? There is no antimatter involved. It emits photons because it is really hot, just like old incandescent light bulbs (they did not use antimatter either!).
Re: Physics (Score:1)
Only photons at rest have no mass. Unfortunately we haven't found any. All photons we know of have energy and thus mass.
Mass is Invariant (Score:2)
Only photons at rest have no mass....All photons we know of have energy and thus mass.
You cannot have both! Mass is a Lorentz invariant i.e. it is exactly the same in all inertial reference frames. If a photon at rest has no mass then it must have zero mass at any energy. Just because a particle has energy does not mean it has any mass because the energy can be kinetic energy instead of mass-energy. Mass is literally the energy of the particle when it is a rest and, as you point out, a photon at rest would have no energy and hence no mass.
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Only rest mass is Lorentz invariant. Mass in general is not.
The only mass used in physics is rest mass because it is the only one which has any physical meaning. Relativistic mass has absolutely no physical validity and comes from a fundamental misunderstanding of how relativity works. Einstein warned against using it. The gamma factor in relativistic momentum comes from the relativistic definition of velocity being different to what classically we think of as velocity. This is because relativity mixes space and time between reference frames and velocity depends on
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The reactions in the sun do release antimatter.
Yes, but the photons of visible light that you can see coming from the matter in the sun have absolutely nothing to do with the production of antimatter.
Matter does not become photons
The original quote was "Photons don't come from matter unless antimatter is involved."...and photons most definitely DO come from matter without any anti-matter being involved. If you heat up atomic matter sufficiently the collisions will promote bound electrons into higher orbitals and cause them to emit photons when they transition back. Hence photons c
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Trying one more time, the energy in the photons did not used to be matter, but rather used to be kinetic energy.
Well, technically that is still wrong. The kinetic energy is converted into mass-energy by exciting an atom in a collision, converting it into mass-energy. The photons are then emitted from the excited state of an atom transitioning to a less excited state. The mass of an excited state of an atom is slightly greater than that of the unexcited state by the energy of the photon emitted. Normally we do not worry about this mass difference because it is at the eV level while the atom's mass is at the GeV/c^2 l
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A photon must be the same stuff as the matter it came from
Why? You can't tell people to question things, then turn around and immediately try to dictate physics by fiat with a "must" premise.
Re: Gravity / mass (Score:2)
Think quantum gravity.
Gravity is the exchange of particles between two particles.
If you double the particles, you double the exchanges. Scaling.
Where's the shock?
If you prefer relativity, mass bends space. Space can be thought of as having elasticity, although that's not quite right.
Then it's just a matter of applying Hooke's Law. Double the mass, double the bending. Again, scaling.
I don't see coincidence, I see cause and effect.
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Gravitons are not affected by the Higgs field, and gravitons wouldn't emit gravitons so can't be affected by gravity themselves.
Re: What about the friction of ether? (Score:2)
There is no ether.
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The average friction has to be zero, because the quantum foam averages for any value over any scale to zero. The particles in it are a product of statistical noise.
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It's what the naughty kids use when they try to catch the Ether Bunny.
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Ether was decommissioned in 1897 due to budget cuts, don't worry.
Their precision isn't very precise... (Score:1, Insightful)
These measures were both very precise, but are still different from one another for unknown reasons.
Also, precision is not accuracy.
Re: Their precision isn't very precise... (Score:2)
Experimental error is perfectly acceptable. It just has to be stated. Which it is.
Re: Their precision isn't very precise... (Score:2)
Also, they state there may be hidden variables. That means their measurements are fine, but the model is incomplete and so not all necessary measurements were made.
Scientists do this all the time. It's how we discover unknowns, by comparing theory with observation and establishing how they deviate.
Wer mist mist Mist (Score:4, Interesting)
Rough English translation: "Those who measure measure crap". Doing good measurements is difficult and you learn a lot refining your methods. You may also find effects you did not expect. That is why Physicists actually highly respect those that seem to do nothing than refine some measurement. It is effort well spent.
Wer misst misst Mist (Score:1)
I know that proverb as well, but the first two "mist"s have to be written "misst":
http://canoo.net/inflection/messen:V:haben
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You are right, of course. That happens when I write German (I am a native speaker) while thinking in English...
...but this is a crap measurement (currently) (Score:2)
While the individual measurements may be very precise the clear systematic difference between the two means that nei
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Except that this is really a bit of a crap measurement so far. The large discrepancy between the two measured values means that neither can be trusted with much accuracy. If you take the difference between the two values as due to an unknown systematic error, which seems likely, then the uncertainty you get (500 ppm) is MUCH larger than the currently quoted uncertainty on G which is 46 ppm [wikipedia.org].
This aren't the only measurements that have done that. They are all over the place. There'a nice chart at the top of this page [phys.org]. As the Wikipedia page you linked to says:
Some measurements published in the 1980s to 2000s were, in fact, mutually exclusive. Establishing a standard value for G with a standard uncertainty better than 0.1% has therefore remained rather speculative.
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And that _is_ something interesting! Why are these measurements all over the place? Nobody knows at this time.
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A mystery (Score:5, Informative)
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Its always bothered me that the majority of constants, like big G, must be measured and not derived
There is no other way. Science is the description of what we see. A model is a systematic description of many events observed over time. Even if you could find a way to derive G, it would be ultimately derived from other things that were observed.
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The gravitational constant would only be constant relative to the part of the universe it is in. So it is likely to vary quite a lot in the space between galaxies, simply due to the absence of significant gravitational masses for quite an extensive relative distance. Then in terms of the multi verse, would also have to take into account surrounding universes and how they shift over time, all be it, extremely, extremely, slowly. Relative to us not moving at all, relative to the mutli-verse probably quite an
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To follow up on that, it is possible to derive G.
From Wikipedia "The gravitational constant is taken as the basis of the Planck units: it is equal to the cube of the Planck length divided by the product of the Planck mass and the square of Planck time"
In Planck units, by definition, they are all 1, so G=1. However, we still need precise measurements for everything else, I mean I can't tell your height in plank lengths without first knowing how long a plank length is.
Branding Genius (Score:3, Funny)
Re: A mystery (Score:4, Interesting)
Ultimately, G must be derivative, a consequence of some deeper theory. And it may well be that this accounts in part for the errors in measurement. If forces are to be unified, each force derives from a single common theory that can generate the somewhat bizarre strong nuclear force as well.
Another likely source of errors is relativity. Relative velocity changes relative mass, relative time and relative distance. How to avoid Newtonian assumptions?
Also, how to measure velocity accurately enough to not change things at the fifteenth or sixteenth decimal point? The act of measuring changes the system, as does the gear used to make the measurement.
Time measurements are a problem, as the more accurate the clock, the greater it impacts the gravitational field. So a clock good enough to measure time accurately enough to give us the precision needed is a clock that isn't an inert part of the experiment but a direct contaminant.
I'm sure some of this is explained in the article in Nature, but it does show the difficulty.
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"Also, how to measure velocity accurately enough to not change things at the fifteenth or sixteenth decimal point?"
Observing will change what you observe, but shouldn't it be possible to know 'how much' observing it will change and factor that in?
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Well, the more accurate the observation, the greater the energy you put in and so the greater the error you create.
Relativistically, the energy will depend on the actual relative velocity of the observer to the observed. So in order to calculate the energy and thus the error, you need the velocity. You can probably derive the value, since you know the range of possibilities and can get a computer to binary search that range for one where all the values match observation.
That's before factoring in the gravit
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Time measurements are a problem, as the more accurate the clock, the greater it impacts the gravitational field. So a clock good enough to measure time accurately enough to give us the precision needed is a clock that isn't an inert part of the experiment but a direct contaminant.
The Heisenberg uncertainty principle is between time and energy, not between time and gravity.
Newton Agrees (Score:3)
Hypotheses non fingo [wikipedia.org]
I have not as yet been able to discover the reason for these properties of gravity from phenomena, and I do not feign hypotheses. For whatever is not deduced from the phenomena must be called a hypothesis; and hypotheses, whether metaphysical or physical, or based on occult qualities, or mechanical, have no place in experimental philosophy. In this philosophy par
Atomic clocks (Score:2)
There seems to be some confusion over them. Since they're impacted by G and since you need them to measure G, they're an important part of the story.
https://www.newscientist.com/a... [newscientist.com]
https://www.sciencealert.com/p... [sciencealert.com]
Basically, they work off state changes. Caesium atoms that generated pulses of radiation as they changed energy level, the wobble of aluminium atoms, the motion in a quantum gas of strontium.
They do not, and never have, work(ed) from radioactive decay.
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I think secondary school/high school textbooks do teach it as decay, using very ambiguous language.
Indeed (Score:2)
They dropped a Golden Delicious, because the old variant Newton used is difficult to find.
Re: Indeed (Score:2)
I know people who own trees grown from cuttings from Newton's.
I've stayed in one of Newton's country houses. Whoever did the dendro date for the fireplace did a horrible job.
Not good for commercial apple growers (Score:2)
It is not difficult to find a cutting of the apple variety that inspired Newton.
The real problem for commercial growers, however, is that the darned thing keeps dropping its apples . . .
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Platinum is even better. Then black. No wait, that's credit cards...
Wouldn't it be interesting if the value varied? (Score:2)
Although some of the individual relative uncertainties are of the order of 10 parts per million, the difference between the smallest and largest values is about 500 parts per million."
Do any of these experiments keep running for a long period of time? Years for example, continually taking measurements.
I am not suggesting that G does change, but to just assume it is a constant could be a gap in our complete understanding of the concept.
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When I was looking up info on how to configure an atomic clock, because you can get one for your computer for the cost of a new computer these days, they said you need to take into consideration the strength of gravity in your location.
Where did you find a personal atomic clock? Everything marketed by that name is nothing but a GPS receiver at best, or a radio synch receiver that may end up useless after NIST shuts off the beacons. It might help if I knew which technology it uses. Is it a cesium clock? Aluminum? Strontium?
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I remember looking at "local" atomic clocks when I was following up on the whole question of "accurate local time" after the wife got me the radio-corrected watch. They're not cheap - a few thousand dollars - but comparable with a really chunky laptop or desktop gaming rig. But the structural work the house would have needed (basement room for thermal regulation, vibratio
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I remember looking at "local" atomic clocks when I was following up on the whole question of "accurate local time" after the wife got me the radio-corrected watch. They're not cheap - a few thousand dollars - but comparable with a really chunky laptop or desktop gaming rig.
I did find the CSAC product the anonymous coward posted. It's a thick chip in size, with a carrier board considerably smaller than modern video cards. $6000 for chip and board.
But the structural work the house would have needed (basement room for thermal regulation, vibration-absorbing pedestal) would have cost more.
The basement, I have. A pedestal large enough to support a board that size together with a fanless computer to host the CSAC carrier board should be much less than $6000, with the most expensive part being the elastomer sheeting. Miniature air spring cylinders are only $16 each.
I'm definitely tempted. Maybe some year.
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Where did you find a personal atomic clock? Everything marketed by that name is nothing but a GPS receiver at best, or a radio synch receiver that may end up useless after NIST shuts off the beacons. It might help if I knew which technology it uses. Is it a cesium clock? Aluminum? Strontium?
Used HP cesium clocks are available and can be refurbished by a dedicated user. Used rubidium atomic clocks can be had for 100s of dollars.
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The experiment used three primary sites that were on different continents. Once we saw the oscillation we also added two additional lower precision shorter-term sites. The equipment wasn't large or terribly expensive. Having multiple sites made removing local noise much easier. The short-term sites were moved a couple of times to both the northern and southern hemispheres, whereas the primary sites had two in the northern hemisphere and one in the southern hemisphere. The temporary sites were of lower
Scientific notation? (Score:2)
Five hundred parts per million. Hundred billionths. Ten parts per million.
In the following years, researchers would invent a clear and compact way of representing these numbers.
Is this Gizmodo's standard? (Score:2)
Well, "Doh!", it took me about 3 minutes once I'd downloaded the paper (using my OpenAthens login via my professional body) to establish that in one of the two experiments (time-of-swing) they used a fused silica "piece of string", and in the other (angular-acceleration-feedback) they used a tungsten "piece of string". There were other treatments (e.g. a conductive germanium-bismuth coating on the silica fibres to reduce noise from st
Re: Bullshit!!! (Score:5, Informative)
If any of these forces could explain gravity, it would be unified by now.
That's how we have the electroweak force. We do not have the electrogravitational force.
Gravity cannot be unified with the other forces with standard physics. We know this. It requires superstrings or other new physics. And even then nobody actually has a grand unified theory.
Anyone on Slashdot who claims otherwise had best produce the Nobel prize in physics and Fields medal in mathematics, because it's going to require just as many discoveries there.
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If any of these forces could explain gravity, it would be unified by now.
That is simply not true. Gravity becomes as strong as the other forces are energies of 10^18 GeV (or about 100 million million times higher in energy than the LHC). We have literally no idea what physics goes on at that energy scale and it might well be possible that there is a single, unified force of which gravity is one aspect.
The incredible weakness of gravity (it is MUCH weaker than the weak force) means that we know almost nothing about its fundamental nature at the moment. We don't even have a wo
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I have no problem with a grand unified theory. I have a problem with it being based on the electron, which exhibits none of the behaviour you describe.
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I have no problem with a grand unified theory. I have a problem with it being based on the electron, which exhibits none of the behaviour you describe.
I never said that a GUT should be based on an electron - although it had better include it along with all the other fundamental particles otherwise it is wrong. What I was pointing out is that your assertion that "If any of these forces could explain gravity, it would be unified by now." is wrong. It is entirely possible (even likely) that there is a unified force which could explain both these forces and gravity that we do yet know about.
Re: Bullshit!!! (Score:2)
Electrons exist. So do positions. So do all other leptons. This suggests gravitons also exist.
You cannot convert an electron into any other fundamental particle. They explain nothing other than themselves.
If a Grand Unified Theory exists, it will use quantum loops or membranes, not electrons.
Re: (Score:3, Funny)
Electrons exist. So do positions. So do all other leptons.
Remember a world without autocorrect? Peppermint Farm remembers.
Re: Bullshit!!! (Score:1)
Electrons exist, for some uncertain definition of existence.
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They have mass, energy, spin, charge.
Now, how we define "exist" is a bit more ambiguous. I rather like the idea that the physical universe is emergent and a product of mathematics. "Exist" in this sense still means physical, it just means we have to be cautious if we examine closer than that.
I also like M-Theory, where electrons are the protrusion of a single membrane into 4D spacetime. Again, the electrons still physically exist, but individual electrons don't exist in isolation.
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Re: high-powered atomic clocks? (Score:2)
It doesn't work off radioactive decay. It works off the spin of ultra-cooled atoms.
Re: Another LIE from China (Score:2, Funny)
Tea is from China and Tea is the drink of the gods, second only to mead.
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Is this rambling available in English?
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No, because that would make it's incoherence even more obvious.