Archives of Ask A Scientist!

About "Ask A Scientist!"

On September 17th, 1998 the Ithaca Journal ran its first "Ask A Scientist!" article in which Professor Neil Ashcroft , who was then the director of CCMR, answered the question "What is Jupiter made of?" Since then, we have received over 1,000 questions from students and adults from all over the world. Select questions are answered weekly and published in the Ithaca Journal and on our web site. "Ask A Scientist!" reaches more than 21,000 Central New York residents through the Ithaca Journal and countless others around the world throught the "Ask a Scientist!" web site.

Across disciplines and across the state, from Nobel Prize winning scientist David Lee to notable science education advocate Bill Nye, researchers and scientists have been called on to respond to these questions. For more than seven years, kids - and a few adults - have been submitting their queries to find out the answer to life's everyday questions.

Previous Week's Question Published: 27 September, 2006 Next Week's Question
'Principle of relativity' tells us there is no 'absolute speed'
Question
What happens at the absolute speed of zero? The earth is moving at some speed, the galaxy is moving at some speed. If you could stop all direction and speed, what would happen?

Question
Actually, it is impossible to measure an "absolute speed", since we can only measure speeds with respect to some reference frame. Speeds

are all "relative".

Consider the following experiment: let's say that you walk across our laboratory and we measure your speed. We must remember that this is only a relative speed - this is your speed with respect to the floor of the laboratory. For instance, if our laboratory is actually in a fast train and we ask another scientist standing on the ground outside of the train to measure your speed, he/she will say that you are moving with a speed much larger than what we had measured. The reason we measure different speeds is that we are in different reference frames. You and I are measuring speed with respect to the floor of the train, and the scientist that is outside is measuring speed with respect to the surface of the Earth. These techniques will give different answers.

Which is the "right" answer? In fact, they are both right. There is no one correct frame of reference, because the laws of physics are the same in all reference frames that have a constant speed and direction of motion. This idea is called the "principle of relativity", and it was first proposed by Galileo Galilei in 1632. Since then, this idea has been tested experimentally and it is now widely believed to be true.

So what is the speed of the Earth, then? Well, with respect to me, the speed of the Earth is zero, since I'm sitting at my desk! However, with respect to the Sun, the Earth is moving in an orbit at a speed of 18.5 miles per second, which is quite fast. The Earth has an even faster speed with respect to the center of our galaxy, since Solar System itself is moving relative to the center of our galaxy with a speed of 155 miles per second.

If the Earth's speed with respect to the Sun were to suddenly become zero, then we would be in big trouble because of the gravitational force that attracts these objects to each other. It is precisely that large relative speed of 18.5 miles per second that keeps the Earth from falling into the Sun and colliding with it. In the absence of this orbital speed, the Earth and the Sun would merge, and all life on Earth would cease to exist. Similarly, if our galaxy suddenly stopped rotating, it would collapse together for this same reason - the nonzero speed of orbits keeps our galaxy spread out like it is.