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Physics/Permit me to suggest a third idea


My RN husband was making a joke about the speed of light and then asked me what was the speed of dark. He argues that the speed of dark is zero.   I (math and science teacher) tried to explain to him that there is no speed of dark because dark doesn't really exist as it is the absence of light and is therefore undefined. He doesn't get the concept of 'undefined' I also tried to use other examples like slopes of vertical and horizontal lines and the fact that cold does not exist but rather is the absence of heat which is a measure of kinetic energy. How can I explain to him that there is no 'speed of dark' because you can't quantify something that doesn't exist.

First, let's define what we mean by the "speed" of something.

When we say, "The speed of light in a vacuum is about 300,000 km per second," we mean the following: "If light is created at time zero at a point in space that is surrounded by a vacuum, then one second later a light detector about 300,000 km away could detect light coming from that point."  We make it clear that a light detector 500,000 km away from that point could NOT detect light coming from that point till AFTER one second.

Likewise, when we say, "The speed of the top of Mount Everest, relative to the center of our Earth, is much less than 1 cm per second," we mean the following: "If we measure the distance between the top to the center at time zero, and then measure it again one second later, the two measurements will differ by much less than one centimeter." For all practical purposes, the "speed" of the top of Mount Everest is zero.*

Before we can say, "The speed of dark IS zero," we have to define WHAT THAT MEANS. Does it mean, "If there is dark at a point in space, then no dark detector will ever detect that dark, no matter how close to that point the detector may be." Or does it means, "If we measure the distance between one point and a point of dark, and then measure that distance a second later, there will be no measurable change in the distance between the two."

I hope you can see the the problem with either definition. Just what is "dark," what is a "dark detector," and what does one mean by the distance between one point and a point of dark?

Perhaps your husband has still a different definition of "the speed of dark" and why he concludes that the speed of dark is zero. He might claims, "If the distant between one point and a point of darkness is some distance, then that distance never changes -- meaning the speed is zero." Then point out that a point of darkness vanishes if light comes to that point -- ergo the distance changes quick dramatically.

Let me pose this question to you both, however: "If a light source GOES DARK at time zero, how long would it take for a point 300,000 km from that source to ALSO go dark?" That, to me, would be the best definition of "speed of dark." Clearly, such a point would go dark -- ie, it would experience absence of light -- meaning the speed of dark could NOT be zero.

If you both disagree with my suggestion for defining the "speed of dark," then there's little more I can add to this discussion.

* For the purposes of this discussion, we'll ignore that the top of Mount Everest does, indeed, move a few centimeters away from the center of the Earth every year -- we'll examine speeds during time periods of seconds.


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I can help with understanding physics that does not involve eggs. I will NOT help with academic or professional questions, which are NOT limited only to homework. Please do not waste your time by asking a question that comes out of ANY kind of academic, professional, or business matters.


Have been fascinated by physical laws ever since I learned, at age seven, that magnets work under water. My study continued through college and has not ceased even after I retired.

B.A. in Physics (with honors) from University of California at Berkeley.M.A. in Physics (with honors) from University of Texas Austin.

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