|The formation of a snowflake or indeed of any crystal spontaneously breaks directional symmetry through the formation of specific special directions. Photo: Wilson Bentley|
A body in uniform motion tends to stay in uniform motion unless acted upon by outside forces, right? Right. And does it matter which direction the motion is in? Nope. There is no special direction in the universe. A Boeing 747 accelerating at 1.5 meters per second per second will be moving fast enough to take off in about a minute, and this will not depend on whether the acceleration is to the east or to the west or in the direction of a Caribbean isle.
At least, not as far as we know. There are certain speculative scenarios arising in string theory in which motion in one direction is slightly different from motion in other directions. The assumption that every direction in space-time is the same is called Lorentz invariance, and it is a fundamental assumption in Einstein’s theory of relativity. There is also a different possibility, that of something called spontaneous symmetry breaking in the nature of time and space. This could be an explanation for a previous peculiar DZero result, in which we found that the production of two positive muons differs from that for two negative muons.
Spontaneous symmetry breaking is common. Somewhere, right this minute, snowflakes are forming in some cloud. Every point in the cloud contains water and is pretty much the same as every other point. If the cloud cools to zero degrees Celsius or below, the water molecules will begin to stick together and start to form crystals. The snowflake in the above figure shows three special directions that have spontaneously appeared as a result of its formation. One such direction is left-to-right, and the other two are at 60-degree angles to that direction. Amusingly, the spontaneous breaking of the symmetry of the mist in the cloud is also the creation of a different six-fold symmetry in the shape of the snowflake.
But any difference in motion in one direction through space as compared to other directions has to be pretty small, or we would have noticed it by now. So we would need a very sensitive device to see it.
There is a widely used trick to measure an effect with great sensitivity called interferometry. We can use a certain category of mesons as interferometers. These mesons can decay into either positive or negatively charged muons. Detecting the difference between how often the two electrical charges appear is a sensitive way to look for the possibility of a special direction in the universe. If there is any special direction, the difference in the appearance of negative and positive muons will oscillate with a one-sidereal-day period as the Earth rotates relative to the fixed stars.
DZero has recently examined the asymmetry of the decays of the Bs for a variation with a period of a sidereal day. We do not see any such variation, indicating that there is not any special direction in (at least our part of) the universe. This is the first such search using the Bs meson and is about 10 times more sensitive than earlier indirect searches made about five years ago. Furthermore, the result is sensitive enough to rule out the idea that that unusual dimuon result is a result of Lorentz symmetry breaking.
|Iain Bertram (Lancaster University, England) and Rick Van Kooten (Indiana University) are the primary analysts for this measurement.|