
#1
Jul507, 02:45 AM

P: 284

How did they experimentally verify that there is pair creation and annihilation in the vacuum? What kind of particles usually pops in and out of the vacuum?




#2
Jul507, 06:58 AM

Emeritus
PF Gold
P: 734

The wikipedia article is a nice starting point for this:
http://en.wikipedia.org/wiki/Casimir_effect There is also a beautiful picture related to it in http://antwrp.gsfc.nasa.gov/apod/ap061217.html 



#3
Jul507, 08:37 AM

P: 443

A strong electric field has to create actual observable pairs of charged particles out of otherwise empty space but that is not experimentally verified yet. The lasers we have are still not powerful enough.




#4
Jul607, 05:16 AM

P: 284

Pair creation and annihilationSo, this means that up till today, no one have directly see or verify that there is pair creation and annihilation ? But these virtual particles is just one interpretation of quantum field theory, much like Schroedinger wavefunction mechanics and Heisenberg matrix mechanics. Have we observe these particles for real yet? 



#5
Jul607, 09:50 AM

P: 443

Quote from V. F. Mukhanov and S. Winitzki "Introduction to Quantum Fields
in Classical Backgrounds", available for free (still) at http://www.theorie.physik.unimuench...ge/T6/book.pdf A static electric field in empty space can create electronpositron (e+e−) pairs. This effect, called the Schwinger effect, is currently on the verge of being experimentally verified. To understand the Schwinger effect qualitatively, we may imagine a virtual e+e− pair in a constant electric field of strength E. If the particles move apart from each other to a distance l, they will receive the energy leE from the electric field. If this energy exceeds the rest mass of the two particles, leE ≥ 2m_e, the pair will become real and the particles will continue to move apart. The typical separation of the virtual pair is of order of the Compton wavelength 2π/m_e. More precisely, the probability of separation by a distance l turns out to be P ~ exp (−π m_e l). Therefore the probability of creating an e+e− pair is P ~ exp(−m_e^2 /eE) The exact formula for the probability P can be obtained from a full (but rather lengthy) consideration using quantum electrodynamics. 



#6
Jul2307, 11:45 AM

P: 79

On the other hand, quantum field theory allows the creation/annihilation of virtual particles from vacuum. But, as they are not onshell they do not live long enough to be observed directly. So you observe their effect only indirectly (like forces). Now, to my point of view, the transition between "virtual" and "real" particles is not really clear in the case of very small energy fluctuations. Is a particle we observe really onshell I think we cannot proove ? So I would say the only thing we can tell is that we have particles in our models. 



#7
Jul2407, 09:27 PM

P: 7

Hawking radiation emited by black holes is also a manifestation of pairannihilation creation just near the horizon, one particle being trapped by the black hole and the other remaining in the Universe. It is seen then a particle created by the black hole.
This and other outreach articles about particle physics (in Romanian) can be found at our portal: http://fizicaparticulelor.ro. Clubul Fizica Particulelorportal de popularizare a fizicii particulelor: http://fizicaparticulelor.ro 



#9
Jul3007, 10:15 PM

Sci Advisor
P: 1,732

understood and experimentally observed for decades in relativistic heavyion collisions. The setup is that 2 heavy nuclei collide, temporarily creating a state whose Coulomb field is so immense that it can produce (onshell) pairs. I.e: the energy density of the field is greater than the combined masses of the electron & positron. This is explained in more detailed in some of Greiner's textbooks, e.g: "QED of Strong Fields". Regarding lasers, etc, I vaguely recall an announcement claiming such (onshell) pairproduction from free photons had been done recently (at SLAC?) but I don't have a reference, sorry. 



#10
Jul3107, 04:59 AM

Sci Advisor
P: 4,496





#11
Jul3107, 07:50 PM

Sci Advisor
P: 1,732

are quite extensive and detailed. They've been in the mainstream for quite a while. In the Schwinger effect, an implied consequence is that a sufficiently strong field could not persist very long  because pair production would carry away energymomentum. So creating a nucleus with Z >= 139 and observing the results seems like a reasonable way to investigate the phenomenon. 



#12
Jul3107, 08:17 PM

P: 1,746

Hi strangerep, I am not familiar with this experiment and its analysis. However, it seems logical to me that when we collide two heavy nuclei with suffient energy, there could be a number of different channels for producing electronpositron pairs. (For example, such pairs can be produced simply in collisions of two electrons if the centerofmass energy is high enough.) Was it possible to separate all these channels and say exactly which portion of electronpositron pairs was produced by the strong field, as opposed to any other reason? Thanks. Eugene. 



#13
Aug107, 03:22 AM

P: 83

Lemme ask this: Isn't there a more common example of this phenomenom?
When a gamma ray photon creates a positronelectron pair, isn't this equivalent to a virtual e+  e pair being torn apart and made real by the energy in the photon's electromagnetic field? (I understand that some posters have specified a *static* field, but others mentioned lasers. This is addressed to the latter.) 



#14
Aug107, 05:30 AM

P: 79





#15
Aug107, 07:35 AM

Mentor
P: 28,841

If you shoot a gamma photon into a crystal, and the gamma photon disappears, an electronpositron pair comes out, and the crystal remains the same, have we then shown an experimental verification of the question? I would say it has, and the physics that describes this process would also confirm that. And in case anyone doubts that this has been done, it has. This process is what will used to generate the positrons for the proposed International Linear Collider (ILC). When you are intending to spend $8 billion, you can't use untested methods. Zz. 



#16
Aug107, 11:10 AM

P: 79

He wanted to know if we already have been observing directly pair creation from vacuum which is quite different ! 



#17
Aug207, 07:50 PM

Sci Advisor
P: 1,732

Both the theoretical and experimental analysis are very difficult. The experiments involve "gentle" colliding of (say) a stripped Uranium nucleus with a target, such as Uranium or Curium or some other very heavy element. One needs Z > 173 (my previous recollection of 139 was wrong). At certain energies, this can produce "quasimolecules" where the binding energy become supercritical, allowing spontaneous pair creation to occur, manifesting as positron emission. Detailed theoretical investigation involves multipole analysis of a 2centre Dirac eqn, which is remains difficult, even numerically. I'm not familiar with the gory details. Slightly higher energies overcome the Coulomb repulsion further to allow formation of a superheavy nucleus, and of course different experimental effects. So I think the short, inadequate, answer to your question is that while some unwanted effects can be reduced by careful choice of the collision energy, there are still multiple effects occurring, which need to be teased apart. The textbooks from which I originally read about this subject were: Greiner & Reinhardt: "Quantum Electrodynamics" 1994 Greiner, Muller, Rafelski: "QED of Strong Fields" 1985 Both of these books are bit old now, so I'm sure the state of the art has advanced since then. Sorry I can't be more definitive. 



#18
Aug207, 08:02 PM

P: 1,746

Eugene. 


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