Monday, February 26, 2007

It's all relative, but to what?

This weekend I received an email from a reader who asked:

We say that everything in this universe is relative, and there is no absolute frame in this universe with respect to which we can measure everything. But I have read on many websites that our Sun is moving with a speed of 140 miles per hour, and our Galaxy the Milky Way is also moving with a speed of 190 miles per hour. With what frame we are measuring these speeds?

This is a very good question regarding what can be a tricky subject. Before we leap out into the Universe, let's start close to home. Suppose two joggers, Mick and Mack, are jogging toward each other, and we are being lazy and sitting on the porch watching them. If we measure each jogger's speed, we get 7 miles per hour. Yet if Mick Jogger measures the speed of Mack Jogger, he measures 14 miles per hour. Likewise, Mack Jogger measures Mick Jogger moving at a speed of 14 miles per hour. And both Mick and Mack measure our speed as 7 miles per hour. What speed you measure all depends on your frame of reference. Now, which of these three people measured the "right" speeds?

Many people would say that WE did, because we were sitting still. But we aren't sitting still -- the Earth is rotating, it is orbiting the sun, the sun is orbiting the galaxy, and so on. But in our daily lives, we don't need to think about all of those things. Without thinking, we choose a "convenient" frame of reference -- in this case, the people sitting on the porch -- because it makes sense to us intuitively. But, it doesn't matter whether you are Mick or Mack Jogger, or sitting on a porch, or flying in an airplane overhead. If you are trying to do any physics (say Mick is tossing a baseball in the air as he jogs and you want to measure its motion), any of those frames give you the right answer.

In fact, one of the main ideas behind Einstein's general theory of relativity is that there is no universal "inertial" reference frame -- in other words, in the Universe, there is no way to define being at rest. (This is a greatly, greatly simplified explanation, by the way, but is what is important for this question.) So, just like when looking at joggers on the street, we come up with a "best" reference frame depending on the situation.

For example, when the space shuttle docks with the International Space Station, you sometimes hear about how you have two objects moving at 17,000 miles per hour trying to gently hook up in space. But that speed is relative to us on the ground. During the final stages of docking, the relative velocity of the space station to the space shuttle is measured in fractions of a foot per minute, not tens of thousands of miles per hour!

For another example, we know that the planets orbit the sun. So when talking about our Solar System, we make the reference frame the "Solar System Barycenter," the true center of the orbits of everything in the Solar System. (It's pretty darn close to the sun, roughly 60,000 miles outside of the sun). Planetary orbits, spacecraft motions, and so on are often quoted with respect to the barycenter.

When we expand our study to include nearby stars, astronomers often change to talk about the "Local Standard of Rest," which is just the average of the motion of a lot of nearby stars, including the Sun. This Local Standard of Rest is useful for studying how stars are moving in the galaxy with respect to one another. The sun moves at about 10 miles per second in relation to this reference frame. The most widely-used reference frame is the Cosmic Microwave Background. This is the "echo" of the Big Bang, light that has been travelling unhindered through the Universe since the first hydrogen atoms formed. When this light was created, gravity had not yet had time to start creating big structures in the Universe, so most things were pretty close to at rest relative to each other. It is also pretty easy to measure our velocity relative to the cosmic microwave background using the Doppler shift -- almost exactly like a police officer uses radar to measure your car's speed. From this, we see that Local Group of Galaxies is moving at about 350 miles per second relative to this background. We think it is due to the gravitational pull of a largesupercluster of galaxies called the "Great Attractor."

So, in short, when you see mentions of how fast things in space are moving, you should ask yourself, "In respect to what?" You can get very different answers that can tell you very different things.

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