The inner workings of my hippocampal neurons

cosmicvastness:

NASA Astronomy Picture of the Day 2014 August 3
Dark Shuttle Approaching 
What’s that approaching? Astronauts on board the International Space Station first saw it in early 2010 far in the distance. Soon it enlarged to become a dark silhouette. As it came even closer, the silhouette appeared to be a spaceship. Finally, the object revealed itself to be the Space Shuttle Endeavour, and it soon docked as expected with the Earth-orbiting space station. Pictured above, Endeavour was imaged near Earth’s horizon as it approached, where several layers of the Earth’s atmosphere were visible. Directly behind the shuttle is the mesosphere, which appears blue. The atmospheric layer that appears white is the stratosphere, while the orange layer is Earth’s Troposphere. 
This shuttle mission began with a dramatic night launch. Tasks completed during this shuttle’s visit to the ISS included the delivery of the Tranquility Module which contained a cupola bay window complex that allows even better views of spaceships approaching and leaving the space station. View Larger

cosmicvastness:

NASA Astronomy Picture of the Day 2014 August 3

Dark Shuttle Approaching 

What’s that approaching? Astronauts on board the International Space Station first saw it in early 2010 far in the distance. Soon it enlarged to become a dark silhouette. As it came even closer, the silhouette appeared to be a spaceship. Finally, the object revealed itself to be the Space Shuttle Endeavour, and it soon docked as expected with the Earth-orbiting space station. Pictured above, Endeavour was imaged near Earth’s horizon as it approached, where several layers of the Earth’s atmosphere were visible. Directly behind the shuttle is the mesosphere, which appears blue. The atmospheric layer that appears white is the stratosphere, while the orange layer is Earth’s Troposphere.

This shuttle mission began with a dramatic night launch. Tasks completed during this shuttle’s visit to the ISS included the delivery of the Tranquility Module which contained a cupola bay window complex that allows even better views of spaceships approaching and leaving the space station.


scinote:

Question: 
In QED (Quantum Electrodynamics), when the repulsion of two negatively charged particles is described as the exchange of virtual photons, is a wavelength/energy imputed to those virtual photons?
Asked by tomcatpurrs

Answer: 
Excellent question.  Before we address it though, let’s reverse a bit and provide some background— just to make sure we’re all on the same page, so we can all participate in the discussion.
Photons are the quantum of light— we can think of them as particles that carry light over space. The history of “real” photons began in 1900 with Planck’s proposal of how the electromagnetic radiation emitted by an object (black body radiation) is related to its temperature.  Planck’s findings indicated that these energies must be quantized, meaning that they can only have specific values.
Building on Planck’s ideas, Lenard (1902) determined that the photoelectric effect depends on wavelength and not the intensity of the light, and Einstein (1905) concluded that Lenard’s discovery indicated that light itself must be localized in specific packets, rather than distributed in space uniformly.
While the idea of a particle of light might be ascribed to Einstein, conclusive evidence was not available until 1923, when Compton conducted his scattering experiments, and the term photon was supposedly first used by G.N. Lewis in a paper in 1926. (Check out the Wikipedia Page: “History of Quantum Mechanics” for more info and helpful links to some of the terms we’ve been referencing.)
Also during the 1920s, the fields of Quantum Electrodynamics (QED) and Quantum Field Theory (QFT) were developed in an attempt to unify relativity, electromagnetism, and quantum mechanics – primarily emerging from the ideas and mathematical proposals from scientists like Dirac.
One of the goals of these new fields was to explain how forces, like the electromagnetic repulsion between electrons mentioned in the posted question, can act over distances.  Eventually, scientists were able to come up with an explanation that these forces are exerted by the transfer of virtual particles.
 A useful analogy to envision how the transfer of a virtual particle would explain force at a distance is to imagine two people standing opposite of each other on a sheet of ice.  If one of them throws a ball to the other, the person receiving the ball will be pushed and slide backwards— in other words, he experiences repulsion.  In the case of electron-electron repulsion, the “ball” is a virtual photon.
Virtual particles are actually predicted by several important equations and concepts in quantum mechanics (including Feynman’s famous diagrams).  They are considered virtual because they only exist for a very brief period of time – too brief to ever be observed directly.

Above: example of Feynman diagrams. Click on the diagram to read more about what Feynman diagrams are. 
While virtual particles only exist for extremely small amounts of energy, there are many experiments and observations that indirectly support their existence.  So even though virtual photons aren’t directly observable and permanent like “real” photons, there is ample support for the belief that virtual photons are indeed actual entities.  The Heisenberg Uncertainty Principle, which you may have heard about before, predicts their existence.

Above: Heisenberg Uncertainty Principle
 Virtual photos (and other virtual particles) follow the uncertainty principle, which leads us back to the original question, finally!.  Since Heisenberg’s principle can be arranged to accommodate for uncertainty of time:
ΔEΔt = h/2π,
where ΔE is the uncertainty in energy, Δt equals the uncertainty of time, and h is Planck’s constant,
we can see that the right side of the equation is always a constant number. As such, it follows that the smaller the uncertainty in time, the greater the uncertainty in energy has to be in order to maintain the equality.
Since the fleeting existences of the virtual photons make the uncertainty in time extremely small, the uncertainty in their energies is necessarily large. There is no one set value for energy or wavelength, and there are a wide range of possible values. Even when we do the math, the uncertainty level is high, but that’s just the nature of the quantum world!
Lastly, we’d like to add that these ideas about virtual particles are theories. With the knowledge that we currently have, there seems to be ample evidence supporting these theories, but that doesn’t mean they’re unequivocally true. In fact, these ideas are still a matter of heated discussion and debate among many scientists, and as we discover more information about our universe, we may come to expand on these theories or perhaps prove them wrong. In the meantime, we encourage you to contribute your own thoughts to the ongoing discussion about the world of virtual particles!
Further Reading:
If you’re interested in learning more about virtual particles, try reading the blog post “Virtual Particles:  What Are They” posted by Harvard University Theoretical Physicist Matt Strassler. Professor Strassler makes a valiant effort to explain the complicated physics in not-so-complicated terms, and we definitely think it’s worth a read!
 Also:
Jones, Goronwy Tudor. “The Uncertainty Principle, Virtual Particles and Real Forces.” Physics Education 37.3 (2002): 223-33.
 Feynman, Richard P. QED: The Strange Theory of Light and Matter. Princeton, NJ: Princeton UP, 1985. Print.

Answered by Brian C., Expert Leader
Edited by Peggy K.  View Larger

scinote:

Question: 

In QED (Quantum Electrodynamics), when the repulsion of two negatively charged particles is described as the exchange of virtual photons, is a wavelength/energy imputed to those virtual photons?

Asked by tomcatpurrs

Answer: 

Excellent question.  Before we address it though, let’s reverse a bit and provide some background— just to make sure we’re all on the same page, so we can all participate in the discussion.

Photons are the quantum of light— we can think of them as particles that carry light over space. The history of “real” photons began in 1900 with Planck’s proposal of how the electromagnetic radiation emitted by an object (black body radiation) is related to its temperature.  Planck’s findings indicated that these energies must be quantized, meaning that they can only have specific values.

Building on Planck’s ideas, Lenard (1902) determined that the photoelectric effect depends on wavelength and not the intensity of the light, and Einstein (1905) concluded that Lenard’s discovery indicated that light itself must be localized in specific packets, rather than distributed in space uniformly.

While the idea of a particle of light might be ascribed to Einstein, conclusive evidence was not available until 1923, when Compton conducted his scattering experiments, and the term photon was supposedly first used by G.N. Lewis in a paper in 1926. (Check out the Wikipedia Page: “History of Quantum Mechanics” for more info and helpful links to some of the terms we’ve been referencing.)

Also during the 1920s, the fields of Quantum Electrodynamics (QED) and Quantum Field Theory (QFT) were developed in an attempt to unify relativity, electromagnetism, and quantum mechanics – primarily emerging from the ideas and mathematical proposals from scientists like Dirac.

One of the goals of these new fields was to explain how forces, like the electromagnetic repulsion between electrons mentioned in the posted question, can act over distances.  Eventually, scientists were able to come up with an explanation that these forces are exerted by the transfer of virtual particles.

 A useful analogy to envision how the transfer of a virtual particle would explain force at a distance is to imagine two people standing opposite of each other on a sheet of ice.  If one of them throws a ball to the other, the person receiving the ball will be pushed and slide backwards— in other words, he experiences repulsion.  In the case of electron-electron repulsion, the “ball” is a virtual photon.

Virtual particles are actually predicted by several important equations and concepts in quantum mechanics (including Feynman’s famous diagrams).  They are considered virtual because they only exist for a very brief period of time – too brief to ever be observed directly.

image

Above: example of Feynman diagrams. Click on the diagram to read more about what Feynman diagrams are. 

While virtual particles only exist for extremely small amounts of energy, there are many experiments and observations that indirectly support their existence.  So even though virtual photons aren’t directly observable and permanent like “real” photons, there is ample support for the belief that virtual photons are indeed actual entities.  The Heisenberg Uncertainty Principle, which you may have heard about before, predicts their existence.

image

Above: Heisenberg Uncertainty Principle

 Virtual photos (and other virtual particles) follow the uncertainty principle, which leads us back to the original question, finally!.  Since Heisenberg’s principle can be arranged to accommodate for uncertainty of time:

ΔEΔt = h/2π,

where ΔE is the uncertainty in energy, Δt equals the uncertainty of time, and h is Planck’s constant,

we can see that the right side of the equation is always a constant number. As such, it follows that the smaller the uncertainty in time, the greater the uncertainty in energy has to be in order to maintain the equality.

Since the fleeting existences of the virtual photons make the uncertainty in time extremely small, the uncertainty in their energies is necessarily large. There is no one set value for energy or wavelength, and there are a wide range of possible values. Even when we do the math, the uncertainty level is high, but that’s just the nature of the quantum world!

Lastly, we’d like to add that these ideas about virtual particles are theories. With the knowledge that we currently have, there seems to be ample evidence supporting these theories, but that doesn’t mean they’re unequivocally true. In fact, these ideas are still a matter of heated discussion and debate among many scientists, and as we discover more information about our universe, we may come to expand on these theories or perhaps prove them wrong. In the meantime, we encourage you to contribute your own thoughts to the ongoing discussion about the world of virtual particles!

Further Reading:

If you’re interested in learning more about virtual particles, try reading the blog post “Virtual Particles:  What Are They” posted by Harvard University Theoretical Physicist Matt Strassler. Professor Strassler makes a valiant effort to explain the complicated physics in not-so-complicated terms, and we definitely think it’s worth a read!

 Also:

Jones, Goronwy Tudor. “The Uncertainty Principle, Virtual Particles and Real Forces.” Physics Education 37.3 (2002): 223-33.

 Feynman, Richard P. QED: The Strange Theory of Light and Matter. Princeton, NJ: Princeton UP, 1985. Print.

Answered by Brian C., Expert Leader

Edited by Peggy K.