Date: February 16, 2012
Title: The Electric Universe
Podcaster: W.T. Bridgman
Description: The How, Where, and Why of electric fields in space – a short introduction.
Bio: W.T. Bridgman is a support scientist and blogger with an interest in making debunking of “Creation Science” and similar pseudo-science accessible to more students and to the general public. Years ago, he discovered just how poor the science was in many of these pseudo-sciences and realized that many of the errors and misinformation were accessible to students at a more introductory level, especially when they could be linked to the science behind specific technologies. He obtained his Masters and Ph.D. in Physics at Clemson University working with the nuclear astrophysics group. His specific fields of study included x-ray processes in supernova remnants and high-time resolution variability of the black-hole candidate, Cygnus X-1.
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Hello. My name is Tom Bridgman and I operate the website and blog, “Dealing with Creationism in Astronomy”. The website dedicated to the idea that understanding WHY a theory doesn’t work can be just as important as understanding the theory that does work.
I’m here to talk about a cosmic process that receives a very mixed amount of attention – electric fields in space. Astrophysicists have known about the presence and effects of electric fields in space since the early 1900s, yet electric fields get nowhere near as much attention as their magnetic field counterparts.
So why is that?
Most astrophysicists have taken a full course-load of physics as well as astronomy. This course load includes learning about electromagnetism and plasma physics. Many of us have studied electromagnetism from the giant tome “Classical Electrodynamics” by J.D. Jackson, now in its third edition.
The roles of electric fields and currents are discussed in many subfields of the professional astrophysical literature. Unfortunately, astronomy has become so specialized that there are still a lot of professional astronomers working in other subfields who don’t know about many of the mechanisms where electric fields are important. Maybe they need a little refresher course in this topic to prevent them from making grossly inaccurate statements about the known role of electric fields in cosmic processes.
First, let’s consider the case of a plasma bound by gravity where the negatively-charged electrons and positively-charged ions have the same temperature, so their average kinetic energy is the same. Ions are at least 1800 times heavier than the electrons, so at the same energy, the ions must move much more slowly than the electrons. As any child knows from playing ball, in a gravitational field, a faster moving particle can travel much higher than a slower moving particle. This means the faster moving electrons can form a thin ‘atmosphere’ around the ions. This charge separation generates an electric field, forming a structure sometimes called a ‘double layer’ by laboratory plasma researchers. This process was recognized independently by Rosseland and Pannekoek in the early 1920s and is sometimes called the Pannekoek-Rosseland field. This discovery actually predates Langmuir defining the term ‘plasma’ for ionized gases. While Pannekoek and Rosseland examined the simple case of electron and ion gases at the same temperature, this process operates in far more general cases. The presence of dust can significantly strengthen the electric field since the charges adhere to the dust particles and dramatically increases the effective mass of the charge.
There are a number of astrophysical environments where astronomers think this process is operating:
- The atmosphere of the Sun and out into the solar wind
- Planetary ionospheres. The electric field contributes to the Birkeland currents that have been identified in planetary magnetospheres.
- Galaxies are surrounded by very thin ionized gas that transitions to the even less dense intergalactic medium, essentially a galaxy ‘atmosphere’.
- Accretion disks around black holes and neutron stars.
These types of configurations generate a very small electric field, but can do so over a very large region. This makes it possible to accelerate charged particles to very high energy. These configurations only operate a very low current. This is because a large amount of charge transfer would eventually reduce the charge difference, reducing the electric field, and eventually canceling the electric field.
Basically, this process operates anywhere that plasmas can be confined under gravity. This process may actually be the source of the ‘seed fields’ which are needed to start processes such as the magnetic dynamo in the Sun and other stars.
Next, consider a simple magnetic dipole, much like the classic demonstration of a bar magnet surrounded by iron filings which many people have seen. A dipole magnet at rest just produces a static magnetic field. But if the dipole is rotating and the magnetic axis of the dipole is tilted from the rotation axis, then the magnetic field measured at any point near the magnet changes with time. According to Maxwell’s equations, a changing magnetic field produces an electric field, a process called induction. This is also a consequence of the relativistic invariance of Maxwell’s equations. In fact, the easiest way to find the values of this electric field in this configuration is to transform the magnetic dipole into a rotating coordinate system. This offset rotating magnetic dipole was a popular demonstration of how to treat relativity in rotating coordinate systems and dates back to the 1930s. Here’s some areas in the astrophysical literature where this process is important:
- Pulsars: The strong magnetic fields of fast-rotating neutron stars (about a billion gauss) can generate very strong electric fields in the charged plasma environment around them, an excellent mechanism for particle acceleration.
- Ionospheric & Magnetospheric physics, driven by planetary magnetic fields: Because the induced electric fields are roughly parallel to the magnetic field, this configuration is another contributor to the Birkeland currents identified in planetary magnetospheres.
Another type of electric field configuration can form because photons interact with electrons much more strongly than protons. The shininess of metals is actually due to the weakly bound outer electrons of metallic atoms as they are hit by photons. When electrons are in free space, collisions with photons can transfer energy to the electrons and increase their velocity. This process is called Thomson scattering at low energies, and Compton scattering at high energies. This process also enables electrons to separate from ions to the point that the average force from the photons balances with the electric field produced by the charge separation. There are a couple of environments where astronomers think this process can act:
- As a driver of stellar winds. In the stellar wind outflow, especially for high-temperature O and B-type stars that emit strongly in the ultraviolet, the electrons can get an extra boost outward due to momentum transfer by scattering with the outflowing photons.
- Gamma-ray bursts: High-energy X- and gamma-ray photons can create a charge separation in the ionized gas of the interstellar medium.
Finally, we’ll consider the case of a plasma accreting onto a black hole. Due to the gravitational time dilation, to an external observer, the infalling material appears to get closer and closer to the event horizon without ever actually crossing. To the external observer, this material forms a thin layer of hot plasma just outside the event horizon. This layer acts like a conducting metal shell around the object. As I noted earlier, it is the free electrons in a metal that makes it shiny. If someone asks you what is the color of a black hole, the best answer might be ‘shiny’. Imagine that!
If this conducting shell is within a magnetic field, such as that which could be generated by an accretion disk, this configuration can generate powerful electric fields, forming a unipolar inductor. Much of this research was summarized in the book “Black Holes: The Membrane Paradigm” by Thorne, Price & MacDonald, published in 1986. We see the currents generated by this process in the radio jets observed from some active galaxies.
So with all these examples, why don’t astronomers talk about cosmic electric fields more? There are several key reasons.
- Electric fields are very difficult to measure with remote sensing technologies. George Ellery Hale tried to measure electric fields on the Sun as early as 1915 but was only able to place an upper limit on the strength of such a field. Measurement techniques have improved since then, but still, it’s difficult to talk about it if you can’t reliably measure it in the first place.
- Since matter is normally electrically neutral in bulk quantities, it takes at least as much energy to separate the positive and negative charges in neutral matter as you obtain when the charges recombine. This is because energy is conserved and the fact that moving charges radiate photons, resulting in an additional energy loss in both the separation and recombination process.
- If an electric field is created purely by charge separation with no additional forces keeping the charges apart, it can’t last very long in free space. Opposite charges attract each other and eventually they will move to cancel the electric field.
- The flip side of this cancellation process is that for a time, the moving charges create an electric current. Electric currents create magnetic fields, and since the current changes with time, so does the magnetic field. And the process can repeat, maintaining this field for quite some time after the original current is gone. Magnetic fields are much easier to detect with remote sensing techniques. Since the magnetic field is easy to tie back to actual observations, astronomers principally talk about the magnetic field, and use Maxwell’s equations and plasma physics to infer the electric fields behind them.
This is just an introduction to the long history of how astronomers have recognized the action of electric fields in space. All of these mechanisms create the charge separations and resulting currents using energy from other processes, which can usually be traced back to processes driven by gravity. The charge-separation itself is not the original energy process, but can provide a mechanism to convert energy in thermal processes to non-thermal energy distributions.
Now if you do an online search for information on electrical processes in space, you will no doubt eventually come across the pseudo-science of the “Electric Universe” whose central creed is that astronomers ignore the contributions of electric fields in space. Electric Universe ‘theorists’ go so far as use discoveries in mainstream astronomy’s understanding of electric fields described above, to justify their own more bizarre claims such as that the Sun is powered not by internal nuclear fusion, but by cosmic-scale external electric current streams. Their cosmology essentially depends on the Universe being filled with gigantic, invisible, electric currents powered by similar generators. Of course, we don’t detect the currents and they provide no information on the origin, nature, or even location of those generators.
So with this gentle introduction, perhaps you have a new appreciation for the real role of electric fields in the cosmos. And you have a way to counter those who claim that astronomers ignore cosmic electric fields.
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365 Days of Astronomy
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