Relativistic gravitation has been measured like never before to slow down time on Earth

It is often said that the general relativity theory was discovered by Albert Einstein with the aim of being able to deal with the effects of the existence ofspace-time in repositories accelerated while its special theory of relativity accounted only for phenomena in reference frames in translation straight and uniform, therefore without acceleration.

This is not the case and it is quite possible to calculate, for example, the behavior of clocks in a rocket which is accelerating with respect to the initial reference frame where it started its acceleration. The theory of Relativity predicts that the flow of time will slow down relative to that measured by a clock at rest in this initial frame of reference.

Now, as Albert Einstein had understood, the fact that an accelerated reference frame is locally indistinguishable from a reference frame at rest in a field of gravity implies that the flow of time must slow down the more the gravitational field is intense. A clock on the surface of the Earth should therefore undergo a slower passage of time than a clock in a satellite initially synchronized with the first on the ground before its departure for space.

The effect is small but clearly measurable, in particular with the atomic clocks inherited from the work of the Nobel Prize for physical Norman Ramsey. In fact, we check it every day because the GPS must take into account this difference in the flow of time between the surface of the Earth and the network of satellites which makes it possible to be effective (the effect of dilation from durations because these satellites are also in movement with respect to the ground must also be taken into account, which means that it is necessary to use both special relativity and general relativity to operate a GPS).

Advances in atomic clocks are such that just over a decade ago, it was possible to demonstrate a slowing effect related to the field of gravitation terrestrial by comparing the flow of time between two points vertically spaced about 30 centimeters apart! The performance was performed by physicists from National Institute of Standards and Technology (NIST) in Boulder, Colorado (USA), as explained by Futura in the previous article below.

Atomic clocks to explore quantum gravity?

They are still physicists from the same NIST, in Boulder, more precisely from Joint Institute for Laboratory Astrophysics (JILA) who have just published an article in Nature (available on arXiv) announcing that he had again pushed the limits of the precision of atomic clockswhich enabled them to measure the effect of time dilation between two vertical points, this time only one millimeter apart!

JILA physicists have designed a atomic clock ultra-precise by cooling and trapping approximately 100,000 atoms of strontium with some lasers. The ultracold atoms obtained were then in sites of a sort of crystal lattice artificial where the lasers trapped them.

Described by the laws of Quantum mechanicsthese atoms were in a superposition of two states ofenergy between which they oscillated perfectly for 37 seconds before the effects of decoherence, all kinds of disturbance of the environment, destroyed the superposition and the oscillations (remember that decoherence is the main obstacle to the realization ofquantum computers powerful universals.

The ultracold atoms therefore became in a state more consistent with our classical intuition of reality where an object cannot be in two states at the same time, for example dead and alive in the case of the famous Schrödinger’s cat paradox whose enigma has been solved by the theory of decoherence exactly.

The ultra-precise time measurements that the JILA clocks allow have all sorts of potential applications, as the video above explains.

One of the most fascinating is undoubtedly related to a complete theory of decoherence which would explain in all detail why the world appears classical to us for macroscopic objects while reality is fundamentally quantum.

The JILA physicists think they can still improve precision with their clocks to the point of being able to test some of the ideas put forward in particular by the Nobel Prize in Physics Roger Penrose. Fine effects of quantum gravity would become perceptible on the matter waves of ultracold atoms, highlighting a decoherence effect produced by gravitation itself on sufficiently large quantum objects.

Relativity: time dilation observed directly in the laboratory

Article of Laurent Sacco published on 02/10/2010

An inevitable consequence of the theories of restricted and general relativity, the Time dilation could only be directly observed for a long time on board fast vehicles (planes or satellites). Thanks to a state-of-the-art atomic clock, it can now be demonstrated for movements of only a few tens of kilometers per hour, or at altitudes differing… by 30 centimeters!

Some time ago, physicists from National Institute of Standards and Technology (NIST) have announced that they have completed theatomic clock most accurate in the world. It consists of a single ion ofaluminum, trapped by electric fields, and which can be excited using a laser beam. We can thus “vibrate” the ion by making one of its electrons from one energy level to another in about a millionth of a billionth of a second.

Complemented by techniques borrowed from experiments on quantum computers (see the complete dossier on quantum computers), this atomic clock is superior to those based on the cesium and could one day lead to time standards a hundred times more precise than those existing today.

For now, physicists have just used it to measure a well-known phenomenon predicted by equations of the Relativity and even of general relativity : time dilation.

A century-old prediction

As early as his 1905 article, Einstein had in fact demonstrated that the constancy of the speed of light for all observers (regardless of experience in mechanics orelectromagnetism in reference frames in uniform rectilinear movements) required that the clocks do not measure identical flows of time. This astonishing prediction, well illustrated by the famous paradox of Langevin’s twins, has however received indisputable confirmations.

The same phenomenon of non-uniformity in the speed of the flow of time for observers placed in different gravitational fields is found in the theory of general relativity. It is also taken into account in the system GPS. This is not surprising since the non-uniformity of the speed of the passage of time was discovered by Einstein in order to make the theory of gravitation compatible with his theory of special relativity. In the case of the theory of Newtonwe can indeed show that the gravitation must make its influence felt millions of times faster than light.

In both cases, the effects are all the same so weak, taking into account the speeds that humans can reach or the smallness of the gravitational fields in the Solar systemthat it took the creation of the first atomic clocks to directly measure these surprising phenomena.

Atomic clocks with unrivaled precision

Today, as explained in an article by Scienceby employing the new atomic clocks which delay only one second every 3.7 billion years, it is possible to observe and measure the relativistic effects of time dilation on a human scale.

Indeed, during experiments with two clocks requiring only a few tens of hours, it was enough to move one of the clocks at a speed of the order of 35 km/h or to raise another by about thirty centimeters, to exhibit differences in the flow of time as small as 90 billionths of a second over 79 years.

For the researchers, this type of clock could one day equip a planetary network measuring very small variations in the Earth’s gravitational field and providing important information for geophysics.