Calculate, not measure the value of c

• B
• Suppaman
In summary, the speed of light, denoted as "c," is a fundamental physical constant that cannot be derived from first principles and is instead established empirically. It is also a fixed number in SI units due to the definition of other fundamental constants such as \mu_0 and \epsilon_0. Changes in the value of c would also affect other dimensioned constants, such as the fine structure constant.
Suppaman
Recently I watched a youtube video about the speed of light and how it was not about light. I was wondering if the value of C can be determined instead of measured. Is C a constant in some formula that has well-known parameters and so the value of C can be calculated? I am just a science hobbyist so I will ask questions a degreed person might consider the answer to be self-evident.

I am taking your question to mean, "can c be derived from first principles, purely on paper, not relying on measured quantities?", in which case the answer is no. It is considered a fundamental physical constant that is established empirically.

FactChecker
Suppaman said:
Recently I watched a youtube video about the speed of light and how it was not about light. I was wondering if the value of C can be determined instead of measured. Is C a constant in some formula that has well-known parameters and so the value of C can be calculated? I am just a science hobbyist so I will ask questions a degreed person might consider the answer to be self-evident.
What is the value of c? Scientists usually set c=1 as it depends on length and time, both measured in arbitrary, man made units. The point is, that it does not change its value (in vacuum).

rumborak said:
I am taking your question to mean, "can c be derived from first principles, purely on paper, not relying on measured quantities?", in which case the answer is no. It is considered a fundamental physical constant that is established empirically.
That was true until 1983. Then the metre was redefined so that the speed of light is now exactly 299,792,458 m/s, by definition. Or, if you prefer, 1 light-second per second.

There is a constant that shows up in the electrostatic equations: ## \epsilon_o ## as well as Ampere's law. There is also a constant ## \mu_o ## that shows up in the magnetostatic equations as well as Ampere's law. The speed of light ## c=\frac{1}{\sqrt{\mu_o \epsilon_o}} ##. From the relationships, Faraday's equation ## \nabla \times E=-(\frac{\partial{B}}{\partial{t}}) ##, and Ampere's law in vacuum, ## \nabla \times B=\mu_o \epsilon_o (\frac{\partial{E}}{\partial{t}}) ## , the wave equation is derived which contains the speed of light as ## c=\frac{1}{\sqrt{\mu_o \epsilon_o}} ##. ## \\ ## I do think with measurements involving ## E ## and ## B ## and the effect that the change in one produces in the other, it is possible to generate the result of the a numerical answer for the speed of light without actually measuring the speed of light, but rather, generating the result from indirect measurements. ## \\ ## @vanhees71 might you give an input here please, because I'm not 100% sure that my previous statement is correct.

Last edited:
Jehannum
rumborak said:
I am taking your question to mean, "can c be derived from first principles, purely on paper, not relying on measured quantities?", in which case the answer is no. It is considered a fundamental physical constant that is established empirically.

Is this true for all "fundamental physical constants?" If that is a fact it seems we can never really know our universe. 1+1 must = 2, not ~2

Charles Link said:
There is a constant that shows up in the electrostatic equations: ϵo \epsilon_o as well as Ampere's law. There is also a constant μo \mu_o that shows up in the magnetostatic equations as well as Ampere's law.

And because $\mu_0$ is set by the definition of the Ampere, and c is set by the definition of the meter $\epsilon_0$ is a fixed number in SI.

Suppaman said:
Is this true for all "fundamental physical constants?"
No, there are some constants that really are fundamental, in the sense that we have to measure them to know what they are and once we do, they tell us something about how our universe behaves.

These constants will be dimensionless, so their value is independent of how we define our units. Everyone's favorite example is the fine structure constant which is equal to (approximately - it's something we measure and there's always some small uncertainty in our measurements) 1/137 no matter what units we choose. If we were to define the meter to be something other than the distance that light travels in 1/299792458 seconds then ##c## would have a different value - but the values of the other dimensioned constants that appear in the definition of the fine-structure constant would also change, and all the changes would cancel in such a way that the value of the fine structure constant stays the same.

Whenever you hear someone talking about whether the speed of light might change, or what would happen if it did change, they're really talking about the fine structure constant.

Suppaman
Suppaman said:
Recently I watched a youtube video about the speed of light and how it was not about light. I was wondering if the value of C can be determined instead of measured. Is C a constant in some formula that has well-known parameters and so the value of C can be calculated? I am just a science hobbyist so I will ask questions a degreed person might consider the answer to be self-evident.

The simplest answer would be “c” cannot be derived from any more fundamental parameters. Einstein was willing to throw away the ideas of space and time being fundamentals in order to hold onto only “c” being fundamental and building his theory on that postulate.

DrGreg said:
That was true until 1983. Then the metre was redefined so that the speed of light is now exactly 299,792,458 m/s, by definition. Or, if you prefer, 1 light-second per second.
I liked it when it used to be 186,000 miles per second (UK) ;)

What is the value of c?

The value of c is a constant that represents the speed of light in a vacuum, approximately 299,792,458 meters per second.

Why do scientists calculate the value of c instead of measuring it?

Measuring the exact value of c is a difficult and complex process, as it requires specialized equipment and techniques. By using mathematical equations and principles, scientists are able to calculate the value of c with a high degree of accuracy.

How is the value of c calculated?

The value of c is calculated using the equation c = λ * ν, where c represents the speed of light, λ represents the wavelength of light, and ν represents the frequency of light. This equation is based on the relationship between the speed, wavelength, and frequency of light.

Are there different values of c?

No, the value of c is a constant and is the same for all observers in all reference frames. This is a fundamental principle of Einstein's theory of relativity.

What is the significance of calculating the value of c?

The value of c is a fundamental constant in physics and has important implications in various fields, including astronomy, quantum mechanics, and electromagnetism. It also serves as a critical constant in many mathematical equations and theories, helping scientists to better understand the universe and its behavior.

Similar threads

• Other Physics Topics
Replies
2
Views
3K
• Other Physics Topics
Replies
1
Views
602
• Other Physics Topics
Replies
4
Views
2K
• Other Physics Topics
Replies
1
Views
879
• Other Physics Topics
Replies
8
Views
3K
• Introductory Physics Homework Help
Replies
4
Views
517
• Quantum Interpretations and Foundations
Replies
57
Views
2K
• Special and General Relativity
Replies
45
Views
3K
• Other Physics Topics
Replies
25
Views
3K
• Quantum Physics
Replies
2
Views
637