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Old 28-12-2010, 06:17 PM
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SR and GR once again passes with flying colours.

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In the famous twin paradox, a sibling who journeys in a fast-moving spacecraft will return home younger than the sibling who remained on Earth. While this apparent slowing of time occurs whenever a body is set in motion, it had been much too small to be detected for movement on a human scale.
But now physicists in the US have used two of the world's most accurate optical clocks to see this and other relativistic effects at speeds and distances on a human scale. The team has seen time slow down in a clock moving less than about 35 km/h relative to its twin. It has also showed that time speeds up in a clock that is hoisted a mere 33 cm above the other.
James Chin-Wen Chou and colleagues at the National Institute of Standards and Technology (NIST) in Boulder, Colorado used two clocks – each based on just one aluminium ion – to do their time dilation experiments. The first such clock was unveiled by the team earlier this year and has the ability to remain accurate to within one second in 3.7 billion years and the second has a similar accuracy.
Quantum logic readout

In both clocks the ion is trapped and cooled using electric fields and laser light. The frequency of the clock is given by a specific optical transition of the ion, which is measured by firing a laser at the ion and locking the laser onto that frequency at which the light is absorbed. This is done with the help of a single magnesium ion (beryllium in the second clock) that is entangled with the aluminium in a process called quantum logic spectroscopy (QLS).
To observe the time dilation at the heart of the twin paradox, the team set one of the aluminium ions into a slow oscillatory motion by adjusting the electric fields used to trap it. The ion in the other clock remained more or less stationary and when the team compared the frequency of the clocks it found that time on the moving ion slowed by a factor of about 10–16 when its average speed was about 10 m/s (35 km/h). The team repeated its measurements at different speeds between 0 and 40 m/s and found that the time dilation occurred exactly as predicted by special relativity.
The team then did a second experiment to try to see a consequence of Einstein's general theory of relativity called "gravitational time dilation". This occurs when one clock is elevated with respect to another and is therefore at a different value of Earth's gravitational potential energy.
Jacking up a clock

This effect was measured by first running the clocks at a vertical difference of 17 cm and then jacking one of the clocks up by 33 cm and running them again. This revealed a shift of about 4 Χ 10–17 in the frequencies of the clocks – in agreement with general relativity. In human terms, this time difference adds up to about 90 billionths of a second over an 80-year life span.
To make these measurements, the team must run its clocks for tens of hours to get the required accuracy. Chou told physicsworld.com that the team is now trying to reduce this time. If successful, the clocks could be used to detect tiny variations in Earth's gravitational potential. A network of such clocks placed around the world could, for example provide valuable information to geophysicists.
Gerald Gwinner of the University of Manitoba believes that such a network would be very useful. "The ability to connect such clocks via long-distance fibre links would indeed allow us to create a real-time network of gravitation monitors," he explained. "Geophysics and environmental sciences could benefit enormously from such tools."
Gwinner added that the NIST demonstration could help physicists explain time dilation to the public. "I will be able to tell the audience that now we can even see time dilation at the speed of waving an arm. Just like I waved my arm, they waved their ions back and forth."
The work is reposted in Science 329 1630.
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Steven
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Old 28-12-2010, 06:26 PM
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What's mind blowing is the fact that someone actually thought out and manufactured a measuring device to that degree of accuracy.
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Old 28-12-2010, 06:58 PM
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OMG !!

Look at the precision of these measurements!!!

… 10–16 in the 'Quantum Logic Readout' … and 4 Χ 10–17 in the 'Jacking up the Clock' experiment !!

Unbelievable !! .. And there's no other cited, possible other sources or errors in these measurements (??)

Incredible !! Fantastic stuff ! More to read up on with this one !!

Thanks for that Steven ! Great post.

Cheers
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Old 28-12-2010, 07:26 PM
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Absalutly incredible stuff
I am amazed that they can measure such small time scales
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Old 28-12-2010, 10:03 PM
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How these clocks work.

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There are three main elements to an optical clock. The first is a highly stable reference frequency provided by a narrow optical absorption in an atom or ion. This "clock transition" will typically have a natural line width of a few hertz or less. The second element of the clock is a laser, known as a "local oscillator", which should also have a very narrow line width so that it does not broaden the atomic transition. The third component is some way of counting the extremely rapid oscillations of the local oscillator; these oscillations are the "ticks" of the clock. A device called a femtosecond comb is used for this part of the device (see "femtosecond comb").

Critical to the performance of an optical clock is the first element - the clock transition. This needs to be as narrow as possible to make the clock stable. Its frequency should also be unaffected by external perturbations such as electric and magnetic fields so that the clock is as accurate as possible. The ideal frequency reference would be a single, motionless atom, unperturbed by any interactions with other atoms or the environment. We can come fairly close to this utopia by trapping a single ion in the tiny gap between the electrodes of an electromagnetic trap. This trapping allows the ion to be laser-cooled to a temperature of about 1 mK and be confined to a region of space just a few tens of nanometres across. The clock transition is therefore not broadened by the effects of temperature or motion.
To probe the clock transition one needs a highly monochromatic laser, which can be achieved by stabilizing the laser frequency to a mode of an environmentally isolated low-drift optical reference cavity. Lasers with line widths that are less than 1 Hz have been achieved with this approach by groups at the National Institute of Standards and Technology (NIST) in Boulder, Colorado, the JILA lab at the University of Colorado and NPL.
Unfortunately, it is not easy to monitor the light being absorbed because narrow transitions are intrinsically very weak. The solution lies with a technique developed by the Nobel laureate Hans Dehmelt, which enables the absorption to be detected with almost 100% efficiency. Known as "electron shelving", the technique is based on the fact that when the ion absorbs the probe light, it jumps to a long-lived excited state, where it remains for about a second. During this time, the ion cannot be laser-cooled - the process where the ion repeatedly jumps between its ground state and a short-lived excited state, absorbing and re-emitting photons at the cooling wavelength.
The upshot is that when the ion is "shelved" in the long-lived excited state, no fluorescence photons from the cooling transition are emitted. The absence or presence of this fluorescence tells us whether the probe light has driven the ion to the long-lived state or not. By measuring the probability of the ion jumping to the long-lived state as a function of the frequency of the probe laser, we can observe the narrow spectral profile of the clock transition. The frequency of the laser light can then be stabilized to the centre of this profile, where the transition probability is at a maximum.
A number of labs around the world are investigating optical clocks using various species of ion. The choice of ion depends on several factors, such as how narrow the clock transition is, the wavelengths of the lasers that are needed to cool and probe the trapped ion, and the predicted sensitivity to external perturbations, which can shift the transition frequencies.
In 2000 Jim Bergquist and co-workers at NIST observed a cold-ion clock-transition line width of just 7 Hz at 282 nm in a single mercury-199 ion - the narrowest of any such clock transition to date. This is only a factor of four greater than the intrinsic natural line width of 1.7 Hz. Meanwhile, researchers at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany, are studying a clock based on the 435 nm transition in ytterbium-171 ions, while groups at NPL and the Canadian National Research Council are investigating clocks based on the 674 nm transition in strontium-88 ions. All these experiments currently demonstrate line widths within a factor of 10 of the mercury line width.
Researchers in all of these groups have already stabilized the clock-laser frequency to the centre frequency of these narrow clock-transition profiles. In this way, frequency stabilities of better than one part in 1015 have been demonstrated for the mercury-199 clock and the ytterbium-171 clock when averaged for about 100 s and 1000 s, respectively. Ensuring that these clocks are affected as little as possible by external perturbations will be a key challenge.
Other ions being studied include some where the upper state of the clock transition is extremely long-lived, thereby allowing full benefit to be gained from the narrowest 0.1 Hz probe-laser line widths available. For example, researchers at NIST are studying a clock based on an aluminium-27 ion in which the upper state lasts for 30 s, while scientists at NPL are experimenting with a clock based on an ytterbium-171 ion in which the upper state lasts for an incredible six years! The only snag with the NIST clock is that the aluminium ion would have to be cooled using an extreme-ultraviolet laser, which is a formidable challenge.
Instead, Dave Wineland, Bergquist and co-workers at NIST are using an interesting experimental arrangement whereby the aluminium-27 ion is trapped with another species of ion - beryllium-9 - that can be cooled more easily. Laser-cooling the beryllium ion "sympathetically" cools the aluminium ion. The researchers cannot, however, probe the weak aluminium clock transition using the electron-shelving technique because there is no strong cooling fluorescence from the aluminium ion. Instead, Wineland and colleagues call upon techniques they developed for quantum-information-processing experiments, which enable information about the clock profile to be "mapped back" to the beryllium ion for read-out.

Although single trapped ions could lead to highly accurate atomic clocks, they are not perfect. In particular, the signal-to-noise ratio of the atomic absorption signal - and thus the stability of the clock - is compromised because there is only one ion in the trap. One possible alternative is to use weak transitions in clouds of a million or so cold atoms. Clocks that use such transitions should be very stable because the stability is directly proportional to the square root of the number of atoms contributing to the signal.
The bulk of the work in this area has been carried out at the PTB and at NIST. Researchers there have focused on neutral calcium atoms, which have a weak transition at 657 nm with a natural line width of about 400 Hz between the 1S0 ground state and the 3P1 metastable state. In these experiments, the calcium atoms are laser-cooled to a few millikelvin in a "magneto-optical trap" similar to that used for cooling caesium atoms before they are launched in a caesium fountain. The problem is that the cooling light strongly perturbs the atoms, which means that it has to be turned off before the 657 nm clock transition can be probed. As a result, the atom cloud expands under gravity during the probe pulse and so has to be recaptured and recooled before it can be probed again. The rapid expansion means that the atoms can only be probed for a very short period of time, significantly broadening the atomic transition. Nevertheless, various tricks - including extra cooling of the atoms to 12 μK and applying two probe pulses about a millisecond apart - have led to accuracies of one part in 1014.
So how can these atomic optical clocks be improved? One option is to use a narrower clock transition, such as the 698 nm 1S0-3P0 transition in neutral strontium, which offers a line width of just 10-3 Hz. However, to use these narrower clock transitions you need a way of lengthening the interrogation time. One solution was presented in 2001 by Hidetoshi Katori from the University of Tokyo, who suggested confining the cooled atoms in what is known as an optical lattice.
An optical lattice is a region of space where standing light waves overlap to create a 3D potential that rises and falls periodically with position. The lattice has regular sites that are less than a wavelength apart, in which atoms can be trapped - a bit like eggs in an egg box. By holding the atoms in the lattice sites, Katori reasoned, they could be probed for as long as one likes. One possible pitfall is that the trapping light beams will perturb the atoms and change the frequency of the clock transition. However, Katori proposed that the optical trap should be created with light at a "magic" wavelength of about 800 nm, where the shifts of the upper and lower levels of the strontium clock transition are exactly equal. The transition frequency will therefore be insensitive to intensity.
A number of groups are working on this lattice idea using strontium and ytterbium. The combination of high stability and low systematic frequency shifts provided by these atoms may offer the best of both worlds in the future.

One of the key challenges in building an optical clock is to count the "ticks" - the oscillation of the light source. However, light oscillates so fast - roughly once every femtosecond (10-15 s) - that it would be impossible to count individual oscillations using any conventional electronic device. The solution is to use a device called a "femtosecond comb". First demonstrated in 1999 by Ted Hδnsch and his group at the Max Planck Institute for Quantum Optics in Garching, Germany, this device bridges the gap between the microwave and optical regions of the spectrum in a single step.
The comb consists of a "mode-locked" femtosecond laser, which emits a train of pulses at a typical repetition rate, frep, of a few hundred megahertz. In the frequency domain, the sequence of pulses appears as a series of equally spaced frequencies - rather like the teeth of a comb. The frequency of any line in the comb is an integer multiple of the comb spacing (nfrep) plus an offset frequency (f0), which depends on the difference in the group velocity and phase velocity within the laser cavity. The all-important tick rate, fopt, is related to frep and f0, both of which can be determined experimentally.
The comb spacing, or repetition rate frep, can be measured from the beat signal between adjacent comb modes. The simplest way of determining f0 is to have a comb that spans a complete optical octave, i.e. a factor of two in frequency. Some femtosecond lasers can now produce an octave-spanning comb directly. Alternatively, a short piece of microstructured fibre, in which an array of air holes surrounds the fibre core, can be used to broaden the spectrum by means of nonlinear frequency-mixing effects in the fibre.
With frep and f0 stabilized to a microwave atomic clock - and thereby compared with the caesium primary frequency standard - the comb can be used to measure the frequency of an optical standard fopt. This is done by determining the beat frequency between the optical frequency and the precisely known frequency of the nearest comb mode. However, as first demonstrated in 2001 by Scott Diddams and colleagues at NIST, it is also possible to turn this process on its head and to stabilize the comb to an optical standard rather than to a microwave standard. The comb then acts as the "clockwork" of the optical clock, dividing the optical frequency to produce a countable microwave output frequency frep.
Regards

Steven
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Old 29-12-2010, 06:05 AM
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Cool! What I don't get is why some are so seen keen for SR and GR to be wrong?
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Old 29-12-2010, 07:30 AM
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There are those 'out there' who refute anything to do with with SR & GR because they see the theory as being purely 'mind experiments' not founded in reality.

This experimental test demonstrates that SR's and GR's time dilation and gravitational time dilation predictions have been measured, and are in accordance with the predictions.

This would seemingly eliminate the view that SR and GR are man-made, 'maths-based' conjectured models. Unfortunately, this view is unlikely to ever be yielded by those who make the accusations against GR & SR, as they are led by 'other' principles other than empiricism.

Other more historical GR supporting observations would be the
precession of Mercury (in arc-seconds per century):

Observed: 5600.7 +/- 0.4 arc-sec/century

Predicted by Newton: 5557.6 +/- 0.2 arc-sec/century
Predicted by Einstein: 5600.6 +/- 0.2 arc-sec/century

Source: http://books.google.com/books?id=fp9wrkMYHvMC&hl=en

I believe GPS satellite clock correction factors are provided by GR theory as well, (although I don't know much about how this works).

Are there any others ?

Cheers & Rgds
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Old 29-12-2010, 10:19 AM
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Quote:
Originally Posted by CraigS View Post
There are those 'out there' who refute anything to do with with SR & GR because they see the theory as being purely 'mind experiments' not founded in reality.

This experimental test demonstrates that SR's and GR's time dilation and gravitational time dilation predictions have been measured, and are in accordance with the predictions.

This would seemingly eliminate the view that SR and GR are man-made, 'maths-based' conjectured models. Unfortunately, this view is unlikely to ever be yielded by those who make the accusations against GR & SR, as they are led by 'other' principles other than empiricism.

Other more historical GR supporting observations would be the
precession of Mercury (in arc-seconds per century):

Observed: 5600.7 +/- 0.4 arc-sec/century

Predicted by Newton: 5557.6 +/- 0.2 arc-sec/century
Predicted by Einstein: 5600.6 +/- 0.2 arc-sec/century

Source: http://books.google.com/books?id=fp9wrkMYHvMC&hl=en

I believe GPS satellite clock correction factors are provided by GR theory as well, (although I don't know much about how this works).

Are there any others ?

Cheers & Rgds
http://en.wikipedia.org/wiki/Tests_o...ral_relativity

Regards

Steven
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Old 29-12-2010, 12:26 PM
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Until I meet two twins of different ages I wont buy it.
Most interesting Steven and I thank you for taking the time to present this exciting information.
alex
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Old 29-12-2010, 02:46 PM
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Originally Posted by xelasnave View Post
Until I meet two twins of different ages I wont buy it.
Most interesting Steven and I thank you for taking the time to present this exciting information.
alex
That'd be an interesting experiment. Get the second one out asap then put them in bunk beds see if one catches up on an 80yr span.
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Old 29-12-2010, 03:35 PM
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Interesting .. I notice in Steven's original post the words:

Quote:
To observe the time dilation at the heart of the twin paradox, the team set one of the aluminium ions into a slow oscillatory motion by adjusting the electric fields used to trap it. The ion in the other clock remained more or less stationary and when the team compared the frequency of the clocks it found that time on the moving ion slowed by a factor of about 10–16 when its average speed was about 10 m/s (35 km/h). The team repeated its measurements at different speeds between 0 and 40 m/s and found that the time dilation occurred exactly as predicted by special relativity.
As a slight aside, I thought time dilation, as demonstrated by the 'twins paradox', was due to one of them undergoing acceleration (relative to the other).. I suppose they are talking about average velocities and different speeds, which I suppose implies relative accelerations/decelerations between them.
Then again, I'm reminded of this one: In Twin Paradox Twist, the Accelerated Twin is Older. In this mind experiment they are saying that:
Quote:
time dilation can cause the accelerated twin to be older if that twin is moving slower than the other twin; in this case, velocity is the deciding factor of age, and the twin with the greater velocity is younger.
It'd be great to use this technology to confirm/disprove this one … as we could learn lots more about whether we can define the absolute motion of the twins in terms of specific global properties of spacetime (aka the curvature).

A mind bender !! .. (again !! .. groan !!)

Cheers
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Old 29-12-2010, 03:47 PM
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What's with all the quotes in large font that runs into the right margin? Doesn't seem to be happening elsewhere on the site. Or is it just me?

Hmmm, maybe a GR-SR effect....

EDIT: AArrghhh, gone! Crackin' up methinks, sorry people...



Cheers -
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Old 29-12-2010, 04:03 PM
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As a slight aside, I thought time dilation, as demonstrated by the 'twins paradox', was due to one of them undergoing acceleration (relative to the other).. I suppose they are talking about average velocities and different speeds, which I suppose implies relative accelerations/decelerations between them.
One of the clocks has an oscillatory motion. The acceleration is obtained by changing the direction of motion. The twin paradox is explained in the same way. The travelling twin has to change direction to meet the stationary twin. This results in acceleration even though for most of the journey the twin is travelling at a constant velocity.

Regards

Steven

Last edited by sjastro; 29-12-2010 at 04:16 PM.
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Old 29-12-2010, 04:07 PM
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Originally Posted by Rob_K View Post
What's with all the quotes in large font that runs into the right margin? Doesn't seem to be happening elsewhere on the site. Or is it just me?

Hmmm, maybe a GR-SR effect....

EDIT: AArrghhh, gone! Crackin' up methinks, sorry people...



Cheers -
Pick up your monitor and shake it violently. This will induce length contradiction of the fonts and allow it to fit on the screen.

Regards

Steven
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