Slashdot Mirror
Physicists Measure Gravity With Record Precision (gizmodo.com)
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."
Your mama's so fat, she emits Hawking radiation.
Correct, in the same sense that an object's desire to release phlogiston causes it to catch fire.
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.
Most ACs are not even worth the keystrokes to insult them. Be generically insulted by this and ignored otherwise.
Thanks for this... my daughter has officially added this to her arsenal of mamma jokes that only a handful of people understand.
I will admit though, she already had :
"Your mamma's so fat that her spiritual guide refers to her aura as an event horizon"
It's actually quite interesting how the force required to accelerate one kilogram at 9.8m/s^2 is the same force required to keep it stationary under standard earth gravity whose free fall acceleration is (9.8m/s^2). The idea that they could be identical and indistinguishable is a significant part of what led Einstein to develop relativity. It seems simple, on one hand due the same force being applied, but to realize a theory relating accelerating reference frames would then also be a working theory of gravity was revolutionary. Its always bothered me that the majority of constants, like big G, must be measured and not derived, perhaps with enough precision measurement and enough eyes on it someone or some group will find a way to derive them with a unified theory. Until then, minor discrepancies can be quite interesting and provide valuable insight.
How do you explain a helium balloon's desire to move upward?
Well, it's got Helium inside instead of gravity. Duh.
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.
It's a small world and it smells funny; I'd buy another if it wasn't for the money; Take back what I paid (SoM)
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.
CLI paste? paste.pr0.tips!
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.
It's a small world and it smells funny; I'd buy another if it wasn't for the money; Take back what I paid (SoM)
|burtosis said: Its always bothered me that the majority of constants, like big G, must be measured and not derived...
Hypotheses non fingo
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 particular propositions are inferred from the phenomena, and afterwards rendered general by induction.
His critics went so far as to suggest his calculation for G was based on occult numerology because Newton was likely a savant and inexhaustibly explored patterns of all types and kinds.
Would were! Should is! Could be! And live a hundred times three.
So Newton managed to get his name used for the unit of force, Joule for energy, etc. They at most one unit each. Until Planck comes along and somehow gets his name stamped on units for length, mass, time and according wikipedia pretty much all the others too. The man's a branding genius!
Electrons exist. So do positions. So do all other leptons.
Remember a world without autocorrect? Peppermint Farm remembers.
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.
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 working, testable theory yet.
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!).
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.
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. 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.