# How can you measure the speed of light, when it is not possible to define an absolute point in space?

Asked by Jbor (649) March 29th, 2008

Since placement in space has to be relative to other objects, how does one measure the speed of light? If one photon is used as the base of the measurement, another photon going in the opposite direction would travel at twice the speed of light, which is of course impossible. What am I missing here?

Observing members: 0 Composing members: 0

Congratulations, you’ve discovered the special theory of relativity.

Special Relativity

As you say, there is no problem in observing relative velocities which are greater than the speed of light. However, you will never observe something moving faster than the speed of light relative to yourself (i.e. in an ‘inertial reference frame’). In order to make these two statements compatible, Einstein had to develop the full machinery of his special theory of relativity.

chris (409)

Because time slows down as speed increases?
And thanks :-)

Jbor (649)

A photon by definition will travel with the speed of light (assuming vacuum here; air slows things down).
To measure its speed, and thus the speed of light, have two people with synchronised watches together. They then move (slowly, or at least well below the speed of light) in opposite directions. At some predefined time, one of fires of a photon (or simply uses a flashlight). The other one times when s/he detects the light, then calculates the difference in time. Take the distance between the observers, divide by the time difference et voilĂ , the speed of light.
As long as the two people don’t move between themselves, it’s easy to measure.
(Another way of measuring just let’s the photon bounce of a mirror, back to its origin. Even simpler.)

Time slowing down is also relative: it only slows down for things moving relative to you. Everything that moves with you (train, space shuttle, next-generation space ships, anything that you feel goes fast) has the same time. So travelling in a space ship near the speed of light will not prevent you from aging in the natural way; ie, you don’t get extra years so that you can read that long list of books.

Evert (167)

Before you start to read Einstein; you might want to begin in 1887 with the most famous failed experiment in history, so far- that of MIchaelson & Morley

Then follow the trail of breadcrumbs. It is fun to watch it all unfold.

gailcalled (54639)

Yes, gailcalled, I agree one hundred percent! It’s often easy to take the modern perspective in science for granted, but it is very interesting and rewarding to go back and consider what the ‘modern perspective’ was at the time when these great discoveries were made. The fall of aether theory is certainly an interesting piece of the puzzle.

chris (409)

@chris: are you a physicist? The history is truly fascinating. I am sitting here and thinking about Rutherford and his work with gold foil- voila: http://www.chemsoc.org/timeline/pages/1911.html

And one of the greatest mistakes of my life was staying on campus to study for an exam and passing up a day w. Neils Bohr, his wife and my in-laws-to-be at their cottage on Cape Cod.

gailcalled (54639)

@gailcalled: yes, or rather a physicist in training. I’ve still got a couple years of graduate school remaining. Coincidentally, I recently spent some time discussing the Rutherford experiment and its theoretical underpinnings in an oral exam.

It certainly would have been exciting to meet Neils Bohr! What was the pretense for the meeting in Cape Cod? Is yours a family of physicists and/or other scientists?

chris (409)

estimation?

or