ars: Dropping an Einstein thought experiment down a shaft

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ars: Dropping an Einstein thought experiment down a shaft

Post by bystander » Thu Jun 17, 2010 11:25 pm

Dropping an Einstein thought experiment down an elevator shaft
ars technica | Nobel Intent | 17 June 2010
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    A Drop of Quantum Matter Bose-Einstein Condensation in Microgravity
Einstein was famous for performing what are termed "thought experiments"—hypothetical situations that illustrate the consequences of a theory—that allowed him to gain insights into the natural world without bothering to leave the confines of his own brain. One of these experiments involved placing a subject in an elevator that is then allowed to free fall. As far as Einstein could tell, there would be no way for the subject to tell if she was falling into the local gravity well, or simply out in space, free from gravity's influence—this insight supposedly helped him formulate his theory of relativity.

But it turns out that this is the sort of thought experiment that might be useful to translate to reality, since building a laboratory equivalent of an elevator shaft is a whole lot easier than sending something into space. But the sorts of physics experiments we'd like to do tend to involve large, complex, and delicate equipment that wouldn't take well to being dropped down an elevator shaft. In an impressive bit of engineering, researchers in Germany have created a device that produces and monitors a Bose-Einstein condensate while being dropped down a 146 meter high shaft.

The drop shaft, located at the Center of Applied Space Technology and Microgravity in Bremen, is pictured at right in all its phallic glory. The sample area is magnetically shielded and can have the air evacuated. Samples dropped from the top will experience nearly five seconds at 10-6g before experiencing a cushy landing in an eight meter deep pool of loose polystyrene packing foam. There's also a catapult on the bottom for launching samples that can handle acceleration. During their trip to the top of the shaft and back, these samples will get about nine seconds of microgravity.

But the facility is only half of the story. Bose-Einstein condensates involve getting a large collection of bosons—in this case, rubidium atoms—into an identical energy state. Once this is achieved, the collection of atoms acts as a single entity, described by a single wave function—in effect, a quantum mechanical system on macroscopic scales. Unfortunately, getting a collection of atoms in a single state is only practical by putting them in a very low energy state, which means cooling them to near absolute zero. That, in turn, requires what's typically a large collection of lasers to cool them, and then an additional collection of hardware to monitor the Bose-Einstein condensate's behavior.

The stunning bit of engineering here is that the research team crammed all of the requisite hardware into a 215cm high cylinder with a diameter of 82cm. They were aided immensely by the development of something called an "atom chip" that has nothing to do with Intel—instead, it's a solid state device that can spit out Bose-Einstein condensates with a minimum of fuss. In the authors' device, these were held to a very chilly nine nanoKelvin.

Once the condensates were in place, the device was set loose and, after a second or so, the system equilibrated to its microgravity state, after which the evolution of the quantum system was followed. It turned out to be exquisitely sensitive to local magnetic fields generated by the apparatus itself and the vacuum pump used by the facility. Once those factors were accounted for, the behavior of the Bose-Einstein condensate was identical to that predicted by theory within the limits of the monitoring equipment.

Although nothing of the sort appeared in this initial description of the system, one of the goals of this work is to establish a system in which we can test the relativistic behavior of a quantum system, and possibly start to identify where the boundaries between these two domains reside. One of the more interesting effects that could be amenable to study is frame dragging, in which a massive rotating object gives the space-time in its vicinity a bit of a swirl. The work involved in creating a compact Bose-Einstein system may also lay the foundation for sending similar hardware into space, where tests can be performed in a stable microgravity environment.
Physics in free fall
Science News | 17 June 2010
In an experiment that puts the good old-fashioned egg drop to shame, European physicists dropped a small blob of ultracold atoms down a 146-meter-tall shaft. The result: no yolk on their face.

In the new study, researchers created a cloud of about 10,000 ultracold rubidium atoms, so still and chilly that the atoms fused into a quirky quantum object called a Bose-Einstein condensate. Then they dropped the stuff off a lofty needle-shaped tower in Bremen, Germany, that stands just 23 meters shorter than the Washington Monument.

Freely falling objects are essentially weightless. So the successful drop shows that researchers now have the ability to monitor quantum objects in near-zero gravity — which may lead to a deeper understanding of heavy topics such as general relativity, ...

In an experiment that puts the good old-fashioned egg drop to shame, European physicists dropped a small blob of ultracold atoms down a 146-meter-tall shaft. The result: no yolk on their face.

In the new study, researchers created a cloud of about 10,000 ultracold rubidium atoms, so still and chilly that the atoms fused into a quirky quantum object called a Bose-Einstein condensate. Then they dropped the stuff off a lofty needle-shaped tower in Bremen, Germany, that stands just 23 meters shorter than the Washington Monument.

Freely falling objects are essentially weightless. So the successful drop shows that researchers now have the ability to monitor quantum objects in near-zero gravity — which may lead to a deeper understanding of heavy topics such as general relativity

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Re: ars: Dropping an Einstein thought experiment down a shaf

Post by The Code » Fri Jun 18, 2010 1:06 am

Interesting topic, Gravity.

Here's a thought. You would think: A billion gallons of cold water, is effected by gravity, differently than a billion gallons of boiling hot steam. Why? You would think from this difference, gravity was easy to work out Huh? No cigar.

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Re: ars: Dropping an Einstein thought experiment down a shaf

Post by bystander » Sat Jun 19, 2010 8:58 pm

How (and Why) to Chuck a Quantum Physics Experiment Down a Drop Shaft
Discover Blogs | 80beats | 19 June 2010
It’s a physics cliche: quantum mechanics looks at the really small, and general relativity looks at the really big, and never the twain shall meet.

In a study published yesterday in Science, physicists describe their attempts to study the overlap between these two theories–by dropping really cold rubidium (only billionths of a degree warmer than absolute zero) from a great height (480 feet). The cold rubidium behaves as an observable, quantum mechanical system and since gravity is a main driver in general relativity, watching gravity’s pull on that system might give researchers glimpses into how to tie the two theories together.
“Both theories cannot be combined,” said researcher [and coauthor of the paper] Ernst Rasel of the University of Hannover in Germany. “In that sense we are looking for a new theory to bring both together.” [Live Science]
Here’s what they did:

Step 1 — Cool it

Physicists first made super-cold Bose-Einstein condensates of rubidium. Since heat is really the random jostling of molecules, to cool things down, experimenters had to make those molecules sit still. They used an elaborate system of lasers to hold the molecules steady.

When rubidium atoms get that cold, they exhibit quantum mechanical behaviors that researchers can observe, acting like one giant particle-wave.
The idea is to chill a cluster of atoms to a temperature that is within a fraction of absolute zero. At that extreme, the atoms all assume the same quantum-mechanical state and begin to behave collectively as a sort of super-atom, known as a Bose-Einstein condensate (BEC). [Nature News]
In this study, researchers contained that complicated system in a two-foot diameter and seven-foot tall cylinder.

Step 2 — Drop it

To test the effects of gravity on that cold glob of atoms, researchers wanted to watch them as they experienced the weightlessness of free fall. That’s why they dropped the experiment in a tower at the Center of Applied Space Technology and Microgravity in Bremen, Germany.
The drop shaft, located at the Center of Applied Space Technology and Microgravity in Bremen, is pictured . . . in all its phallic glory. The sample area is magnetically shielded and can have the air evacuated. Samples dropped from the top will experience nearly five seconds at 10-6g before experiencing a cushy landing in an eight meter deep pool of loose polystyrene packing foam. [Ars Technica]
Because the fall time is fairly short, researchers repeated the drop 180 times. During the tests they systematically eliminated other effects on the cold atoms, like magnetic fields in the laboratory, to make sure the atoms only felt gravity’s sway.

The idea was to see whether quantum objects break the rule that says that gravity works on all objects in the same way:
It explains why a pebble and a piano fall at the same speed if dropped from the same roof, despite their different masses. It’s also a necessary first step toward describing the effects of gravity as curvature in spacetime. “It’s a very important cornerstone,” said physicist Ernst Rasel of the Leibniz University of Hannover in Germany. But, he added, the equivalence principle “is just a postulate — it’s not coming out of a law.” So of course, physicists have spent the past century trying to break it. [Wired]
Step 3 — Send it into Space?

The experiment didn’t find evidence that gravity acted differently on a quantum scale–but Rasel and his colleagues are justly proud of creating the experimental conditions that can test such a thing. Because this research created a robust little setup of these very special quantum mechanically behaving atoms, one possible next step would be to watch the atoms during an even longer amount of time in free fall, for example, in orbit around the Earth on the International Space Station.

Rasel is just happy that the experiment survived the first drop:
“I was very worried,” Rasel says of the moments before his team first dropped their experiment. “It was coming towards the end of a PhD thesis of a student,” he adds, explaining that it would have caused serious problems if anything went wrong. [Nature News]
Wired has a video of the experiment, here.
A Drop of Quantum Matter
Physics in the 20th century witnessed two major revolutions, relativity and quantum mechanics. General relativity relies on the equivalence principle. When an object in a gravitational field undergoes free fall, it is indistinguishable from the same object in an inertial reference frame—it acts as if it were weightless in outer space. A popular account of a free-fall environment was given by the thought experiment of "Einstein's elevator". General relativity is mainly formulated in terms of classical objects. On page 1540 of this issue, van Zoest et al. describe an intriguing experiment that brings together fundamentals of general relativity and quantum mechanics. They follow the evolution of a prototypical quantum object, a Bose-Einstein condensate (BEC), under free-fall conditions. The use of BECs in atom interferometers should allow for more sophisticated tests of general relativity.
Bose-Einstein Condensation in Microgravity
Albert Einstein’s insight that it is impossible to distinguish a local experiment in a "freely falling elevator" from one in free space led to the development of the theory of general relativity. The wave nature of matter manifests itself in a striking way in Bose-Einstein condensates, where millions of atoms lose their identity and can be described by a single macroscopic wave function. We combine these two topics and report the preparation and observation of a Bose-Einstein condensate during free fall in a 146-meter-tall evacuated drop tower. During the expansion over 1 second, the atoms form a giant coherent matter wave that is delocalized on a millimeter scale, which represents a promising source for matter-wave interferometry to test the universality of free fall with quantum matter.
  • Equivalence Principle
    • A little reflection will show that the law of the equality of the inertial and gravitational mass is equivalent to the assertion that the acceleration imparted to a body by a gravitational field is independent of the nature of the body. For Newton's equation of motion in a gravitational field, written out in full, it is:
      • (Inertial mass) * (Acceleration) = (Intensity of the gravitational field) * (Gravitational mass).

      It is only when there is numerical equality between the inertial and gravitational mass that the acceleration is independent of the nature of the body.

    – Albert Einstein

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MPQ: Quantum gas in free fall

Post by bystander » Tue Jun 22, 2010 5:53 pm

Quantum gas in free fall
Max Planck Institute of Quantum Optics | 22 June 2010
Physicists produce a Bose-Einstein condensate at zero gravity - a step towards extremely sensitive quantum sensors for gravitation

A sensitive measuring device must not be dropped - because this usually destroys the precision of the instrument. A team of researchers including scientists from the Max Planck Institute of Quantum Optics has done exactly this, however. And the researchers want to use this experience to make the measuring instrument even more sensitive.

The team, headed by physicists from the University of Hanover, dropped a piece of apparatus, in which they generated a weightless Bose-Einstein condensate (BEC), to the bottom of a drop tower at the University of Bremen. The particles in a BEC lose their individuality and can be considered to be a 'super-particle'. The researchers want to use such an ultra-cold quantum gas at zero gravity to construct a very sensitive measuring device for the Earth's gravitational field - in order to find deposits of minerals, and also to settle fundamental issues in physics. (Science, June 18, 2010)