Robert Bluhm: The death of common sense

Robert Bluhm: The death of common sense
 

Prior to the development of special relativity, the laws of physics and the laws of common sense were practically one and the same. Measurements of space and time were absolute. There were no limits in principle on how fast a person could travel. A meter was a meter and a second was a second no matter what.

The birth of Albert Einstein’s theory 100 years ago marked the death of these common-sense notions of space, time, and travel. According to Einstein, measurements of time and length intervals differ when made by observers who are moving relative to each other. There is no universal time. Nothing can travel faster than the speed of light. Einstein also deduced that mass is a form of energy, expressed by the famous equation E=mc².

As is often the case with revolutionary new theories, the theory of relativity emerged from a crisis in the physics community: How does light travel? The prevailing view before Einstein was that light waves traveled through an all-pervasive medium called the ether. The speed of light was defined with respect to the rest frame of the ether.

Albert Michelson’s experiments, however, failed to detect Earth’s motion through the ether. Without this medium, what could serve as the reference frame for light rays traveling through empty space?

Einstein proposed the radical idea that light in a vacuum always travels at the same constant speed, c (roughly 300,000 km/s). No matter how fast an observer travels relative to a light source, the emitted light always travels at the same speed, c. There was no need for an ether.

Einstein believed that any observer moving at constant velocity (in a so-called inertial frame) experiences the same laws of physics. If nothing distinguishes one inertial frame from another, then the speed of light would naturally be the same in all such frames.

Einstein’s radical theory ultimately gained acceptance, and now pervades all modern physics. Special relativity is an essential component in the Standard Model (SM) of particle interactions. The lifetimes of fast-moving unstable particles vary with their relative speed precisely as predicted by relativity. E=mc² is confirmed every time a particle and antiparticle annihilate to produce light.

But the Standard Model completely ignores the gravitational interaction. The SM is a quantum theory, and there is no known completely viable quantum theory of gravity (there are candidate models, such as string theory). Most physicists believe that, ultimately, a unified fundamental theory will merge a quantum theory of gravity with the SM.

Whether Einstein’s theory of relativity would then remain intact is unclear. Some researchers are looking for violations of relativity as a signature of quantum-gravity effects. A general theory (called the Standard Model Extension or SME) developed by Alan Kosteleck´y and co-workers at Indiana University has been used to search for relativity violations in particle, atomic, and astrophysical experiments.

One of the best tests of relativity theory — sensitive to an extremely delicate particle-antiparticle balancing act in kaons — has been conducted by the KTeV collaboration at the Fermilab Tevatron. KTeV tested interactions in the SME that would cause relativity violations to the level of parts in 1021. The BaBar experiment at SLAC conducts similar searches using B mesons.

So far, Einstein can rest easy — no violations of relativity have been found. However, increasingly-precise experiments will probe further into the realm where quantum-gravity effects are expected to appear. If they find violations of relativity, they would signal the beginning a new revolutionary period in physics as great as the one Einstein began 100 years ago.


Robert Bluhm is the Sunrise Professor of Physics at Colby College. His research in theoretical physics focuses on how low-energy atomic physics can be used to test fundamental symmetries and interactions in particle physics.