Skip to comments.Scientists manage to study the physics that connect the classical [to] the quantum world
Posted on 09/09/2013 4:10:37 PM PDT by LibWhacker
How does a classical temperature form in the quantum world? An experiment at the Vienna University of Technology has directly observed the emergence and the spreading of a temperature in a quantum system. Remarkably, the quantum properties are lost, even though the quantum system is completely isolated and not connected to the outside world. The experimental results are being published in this week's issue of Nature Physics.
Quantum and classical physics: From the microscopic to the macroscopic world
The connection between the microscopic world of quantum physics and our everyday experience, which is concerned with much larger objects, still remains puzzling. When a quantum system is measured, it is inevitably disturbed and some of its quantum properties are lost.
A cloud of atoms, for example, can be prepared in such away that each atom is simultaneously located at two different places, forming a perfect quantum superposition. As soon as the location of the atoms is measured, however, this superposition is destroyed. All that is left are atoms sitting at some well-defined places. They behave just as classical objects would.
In this case, the transition from quantum behavior to classical behavior is initiated by the measurement a contact with the outside world. But what happens, if a quantum system is not influenced from the outside at all? Can classical properties still emerge?
Disorder in the quantum world
"We are studying clouds consisting of several thousand atoms", explains Tim Langen, lead author of the study from Professor Jörg Schmiedmayer's research team at Vienna University of Technology. "Such a cloud is small enough to effectively isolate it from the rest of the world, but it is large enough to study how quantum properties are lost".
In the experiment, the atom clouds are split into two halves. After a certain time the two halves are compared to each other. In that way, the scientists can measure the amount of quantum mechanical connection between the clouds. Initially, this connection is perfect; all atoms are in a highly ordered quantum state. But as the cloud is a large object consisting of thousands of particles, this order does not remain for long.
Loss of quantum properties without influence from outside
As the atoms interact with each other, disorder begins to spread with a certain velocity. Atoms in the already disordered regions lose their quantum properties. A temperature can be assigned to them just as in a classical gas. "The velocity with which the disorder spreads depends on the number of atoms", says Tim Langen. This defines a clear border between the regions which can be described by a classical temperature and regions where quantum properties remain unchanged.
After a certain time the disorder has spread over the whole cloud. The remarkable observation is that this loss of quantum properties happens just because of quantum effects inside the atom cloud, without any influence from the outside world. "So far, such a behavior had only been conjectured, but our experiments demonstrate that nature really behaves like this", Jörg Schmiedmayer points out.
Atomic clouds: A world on its own
In a way, the atomic cloud behaves like its own miniature universe. It is isolated from the environment, so its behavior is solely determined by its internal properties. Starting with a completely quantum mechanical state, the cloud looks "classical" after some time, even though it evolves according to the laws of quantum physics. That is why the experiment could not just help us to understand the behavior of large atom clouds, it could also help to explain, why the world that we experience every day looks so classical, even though it is governed by quantum laws.
What is a “completely quantum mechanical state”?
Bookmark for later.
Don’t know. I’m still amazed at the claim that it doesn’t have an initial temperature???
The article is so completely imprecise and oversimplified to be virtually meaningless, so I wouldn’t waste a lot of time on it.
The gas in its final form is still quantum mechanical.
Sounds like the 3rd law of thermo to me - tending toward disorder and all
bump for later reading
Danggit that’s like twice in just a few days where I got an obvious one wrong by .... That much
The First Law, for example, was originally stated as "There is no engine which can run perpetually without the input of heat from an external source." Over time physicists came to realize it was just a restatement of the conservation of energy.
The Second Law was stated by Sadi Carnot in several forms in the mid 1850's, but the one I learned was "There can exist no engine whose sole effect is to transfer heat from a cold place to a warm one." There are others, for example, "There can be no engine with a greater efficiency that a Carnot Engine."
This lead one wag to comment "Science owes far more to the steam engine than the steam engine owes to science."
The most useful form of the Third Law is probably this one, Where "X" is any thermodynamic state variable other than temperature (pressure, volume, magnetization, ... for examples):
Which does not come up often in casual conversation.