There’s an interesting parallel between electromagnetism and gravity. If a positive and negative charge are near each other they are attracted by an electric force. The strength of this force depends on the amount of each charge and their distance of separation. The electric force between the two charges follows what is known as an inverse square relation. That is, the strength of attraction depends on the square of their distance. So if two charges are attracted by a certain force, if you double their distance of separation that force will become a fourth as large as it was.
Magnets follow the same relation. A north and south pole are attracted by an inverse square relation. The force between magnets is particularly strong, and unlike charge it doesn’t tend to dissipate when you put two magnets together. This is part of the reason why there was so much interest in magnets in the 1600s.
We now know that gravity follows the same inverse square relation. Two masses experience a mutual gravitational attraction, and the strength of their attraction depends on the square of their distance, just like charges and magnets. The difference is that gravity is an extraordinarily weak force. The gravitational attraction between people-sized masses and smaller is very difficult to measure, and in fact wasn’t done until Cavendish performed his famous experiment in the late 1700s.
So how do we know that the planets aren’t held in their orbits by magnetism instead of gravity? That might seem like an absurd idea, but in fact it was an idea proposed by Johanne Kepler when he introduced what is now known as Kepler’s laws of planetary motion. When Kepler proposed that planets moved around the Sun in ellipses, one of the big questions was what would hold planets to their orbits. Kepler knew that the Earth could be treated as a giant magnet (that’s a simplistic view, but close enough), and it was reasonable to assume that the Sun and other planets also acted as magnets. Thus, a magnetic interaction between planets and Sun might explain their motion.
Kepler formed his idea from a few simple facts: Planets are attracted to each other through an inverse square force. Magnetic poles experience an inverse square force. The Sun and planets possess magnetic fields. All of this is true, so why do scientists insist on saying gravity holds planets in their orbits?
Because gravity matches experimental observation, while the idea of magnetic attraction does not. For example, Venus has no intrinsic magnetic field as Earth does, and has a mass about 80% that of Earth. If planetary motion was due to magnetic interaction, Venus should have left the solar system long ago. While magnetic interactions do play a role in astrophysics, and certainly played a role in the formation of planets in the early solar system, they do not play a significant role in the motion of planets today.
Although Kepler had a few facts about magnetism and a vague idea, Newton used his laws of motion to derive Kepler’s laws. For more than three centuries Newton’s laws matched observation again and again. This is the key point. Scientific models live or die based on experimental evidence, and if a model fails to make accurate predictions it will be pushed aside in favor of more accurate models.
Recently there’s been some commentary on my posts about something called the Electric Universe model. The term “electric universe” seems to have gotten popular after a book of the same name was published in 2007. The model stems from the work of Hannes Alfvén, who with Oskar Klein proposed the Alfvén-Klein cosmological model, which makes some very specific claims about the origin and history of the universe.
Hannes Alfvén was no slouch. He won the Nobel prize in physics for his work on magnetohydrodynamics (the behavior of plasmas interacting with magnetic fields), or MHD for short. His work on MHD plays a central role in astrophysics, because interstellar space is filled with both diffuse plasmas and magnetic fields.
Alfvén was also a bit of a kindred spirit. He believed in the supremacy of experimental evidence, and that one should be cautious of reading too much into a mathematical model in the absence of experimental evidence. Because of this, he felt extrapolations of cosmic expansion to a “big bang” were unfounded, bordering on scientific creationism. He also wasn’t a fan of the steady-state model advocated by Fred Hoyle, feeling it was also too speculative. As an alternative he proposed the Alfvén-Klein model.
Experimentally we know that when particles are created from energy they appear in matter-antimatter pairs. This raises a problem in cosmology known baryon-asymmetry. We and all the stars and planets around us are made of matter. From what we can tell, all the galaxies in the universe are made of matter. So if matter and antimatter form in pairs, where is all the antimatter?
The Alfvén-Klein model addresses this issue by assuming the early universe contained equal parts matter and antimatter in a plasma known as an ambiplasma. Over time, the matter and antimatter would naturally clump into pockets of matter and antimatter, which then form into the stars and galaxies we see today. So all the stuff we see around us is matter because we live in a matter pocket. Other regions of the universe would be antimatter because they are in an antimatter pocket.
This is an elegant solution to the baryon asymmetry problem which plagues us still today, so why do we discount the Alfvén-Klein model, as well as the electric universe model which is based upon it? It doesn’t match experimental evidence.
The Alfvén-Klein model is an alternative to the big bang, so it doesn’t predict the cosmic microwave background. The CMB not only exists, but agrees with the big bang model. If matter and antimatter existed in pockets, the interaction of their edges would produce tremendous x-rays. This doesn’t agree with the x-ray observations we have. Then there is the fact that the Alfvén-Klein model doesn’t make any predictions about the abundances of hydrogen and helium in the universe, where the big bang model does. Over and over, the Alfvén-Klein model fails to agree with observation. So it goes to the recycle bin.
Supporters of the electric universe model will argue that these discrepancies can be overcome by modifying the theory. If you tweak this or that in the model then it can be made to agree with experimental observation. But tweaking a model to make it fit is a weak argument. Using a model to make a clear prediction is a stronger one. The big bang model predicted the cosmic microwave background, which was then observed. The model predicted the abundances of light elements correctly. The distribution of galaxies in the universe matches the big bang. The big bang model works naturally, while the electric universe model must be tweaked and modified every time new cosmological evidence comes to light.
Scientific models live or die based on experimental evidence.