Contrary to Isaac Newton’s model for space and time, we now understand that there is no absolute standard of rest.  Einstein’s framework for a mathematical description of reality (“physics” we call it) showed that natural law doesn’t care how you are moving with respect to the other stuff in the Universe (the other stuff might care, though, if you crash into it!)  The physicist Ernst Mach contemplated this deeply, wondering if the origin of inertia might be related to the relative motion of a body with respect to the rest of the matter in the Universe.  These ideas greatly influenced Einstein’s development of his Theories of Relativity.  

So we need to establish standards of space and time from which to measure motion, since the Universe hasn’t made the choice for us.  The ultimate frame of reference might be that of the background radiation of the Universe itself.  When mapping the afterglow of the Big Bang, the COBE spacecraft found that there is a strong redshift of the light in one part of the sky, and the opposite side of the sky is strongly blueshifted.  Only after subtracting off this signal could cosmologists detect the “ripples” predicted by theory and validate the Big Bang model (you can faintly make out emission from the Milky Way galaxy as a horizontal feature in the center of this image.  The redshift and blueshift I’m talking about actually manifest in this image as “dark” and “bright”, respectively, so I think the color scale on this image shows it “backwards”).

This “dipole” signature in the cosmic microwave background represents our motion (mostly the Milky Way’s motion) through this omnipresent gas of photons left over from the explosion that created the Universe.  This means that the stuff in the Universe does have a preferred frame of reference, even if the laws of the Universe don’t.  Crazy.  Sometimes I wonder:  how did the Universe pick that frame?  Other times I wonder whether that question has any meaning at all.  Other times I wonder which of those wonderings is more profound.  Then I get back to changing my daughter’s diaper and my priorities are rightfully restored.

Anyway, here in the Milky Way all of the stars and gas and dust and other stuff is swirling around the Galactic Center in various orbits.  For the most part, stuff in the disk of the Galaxy, like the Sun, move together, but there is some “random” motion on top of that, sort of like how on a multilane freeway all the cars are moving more or less together, but there are some variations (some lanes move faster than others, some cars are anomalously slow or fast, some cars are passing others or even entering or leaving the freeway altogether).

When you are on the freeway you can tell if you are moving faster or slower than the rest of the cars by whether you are passing the rest of the traffic or it is passing you.  We can do the same thing with nearby stars to get a sense of whether the Sun itself is a slowpoke or speedster:  by looking at the apparent radial velocities of the local stars, we can see if we are, on average, moving towards or away from them and give a sense of our motion with respect to the average.

The “average” speed of stars and other stuff near the Sun is called the “Local Standard of Rest,” and determining it helps astronomers subtract off the peculiar velocity of the Sun from the overall average when trying to figure out how things are moving compared to the Milky Way.  

Carly Chubak and Geoff Marcy of UC Berkeley have completed a new study of the radial velocities of stars in the California Planet Search.  This updates a seminal paper by David Nidever, who did a similar project a while back.  These stars can be used as radial velocity standards for astronomers who want to measure how fast other stars are moving:  they can use the stars in Chubak’s catalog as standards to calibrate their instruments.

These are “absolute” velocities measured with respect to the Sun’s motion, not the usual “differential” radial velocities we use to find planets.  Generally, we can measure how much a star’s velocity has changed with a precision of almost 1 m/s, but we don’t know the actual radial velocity to much better than 1 km/s!  This lets us find planets, even if we don’t know the overall velocity very well.  Chubak’s work has found these overall velocities to 100 m/s, which is very, very good and pretty close to the practical limits of such work (set by our incomplete understanding of the motions of the star’s atmospheres).  

What’s really neat is if you plot the radial velocities, as co-author Howard Isaacson did in the image here, you see a clear pattern in them.  There is a strong “dipole” signal, just like the CMB dipole.  The stars in blue are the ones we’re racing towards as the Sun “changes lanes” in the great racetrack in the sky, and the red stars are the ones in our rear-view mirror.  It is studies like this that help establish the Local Standard of Rest, from which we take the measure of all things astronomical.