# Constant c and not its invariance.

• dpa
In summary, The value of c, the speed of light, is experimental and manmade, with dimensions LT-1. However, it is also theoretically Lorentz invariant. The value of c depends on the fabric of space and is defined by the permeability and permittivity of free space. These are conversion factors between manmade units and do not characterize nature. The concept of c can also be seen in relation to geometry and is affected by the fabric of the vacuum. Its value is different in different units, but ultimately depends on independent length and time standards and the fabric of space.
dpa
hi,
value of c is experimental
however c itself is theoretically lorentz invariant.
So value of c depends upon fabric of cosmos right?

Thanks.

The existence of c is of cosmic importance, but the value is entirely manmade. With dimensions LT-1, c is just a way of relating our distance units to our time units. The only fundamental values are the ones that are dimensionless, such as the fine structure constant.

dpa said:
hi,
value of c is experimental
however c itself is theoretically lorentz invariant.
So value of c depends upon fabric of cosmos right?

Thanks.

The value is historical, the unit of length was originally based on a king's arm, later a fraction of the circumference of the Earth, and the unit of time is roughly a heartbeat but also a fraction of the period of rotation of the Earth. The Babylonians liked counting based on 60 because you could divide it by 2, 3, 4, 5, 6, 10 and 12 easily.

Scientists just use c=1 (in units of light seconds per second).

Just think of "299792458 m = 1 second" in the same way as "25.4mm = 1 inch".

Bill_K said:
The existence of c is of cosmic importance, but the value is entirely manmade. With dimensions LT-1, c is just a way of relating our distance units to our time units. The only fundamental values are the ones that are dimensionless, such as the fine structure constant.

I'd say take it a step further & call c a concept of geometry. Specifically from measure & geometry (Euclid). Is that manmade or the nature of physical stuff?

Opps I just read you said value,

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Free space (vacuum) has two universal Lorentz invariant constants; the permeability of free space μo, and the permittivity of free space εo.

the speed of light is then defined as
$$c = \frac{1}{\sqrt{\epsilon_o \mu_o}}$$
So c depends on the fabric of the vacuum.

This is an unfortunate consequence of the SI system, the belief that ε0 and μ0 are somehow fundamental. Even more than c, these quantities are nothing more than conversion factors between manmade units. They do not in any way characterize nature.

Bill_K said:
This is an unfortunate consequence of the SI system, the belief that ε0 and μ0 are somehow fundamental. Even more than c, these quantities are nothing more than conversion factors between manmade units. They do not in any way characterize nature.

I don't know what the squiggly lines mean, did you say that length & time do not in any way characterize nature?

There are actually four quantities in the SI system that characterize the (fabric of the) vacuum; permeability (Henrys per meter), permittivity (Farads per meter), characteristic impedance (ratio of E/H of radio wave in ohms), and speed of light. All are Lorentz invariant. Only two are independent. Only two can be measured in dc circuits.

Last edited:
dpa said:
hi,
value of c is experimental
however c itself is theoretically lorentz invariant.
So value of c depends upon fabric of cosmos right?

Thanks.

Right. Of course the value of c is different in different units, but as long as* you express it as an independent length standard divided by an independent time standard, the value that you obtain depends on those standards and on the fabric of space.
For how this works, see for example the Shapiro effect:
http://en.wikipedia.org/wiki/Tests_of_General_Relativity#Light_travel_time_delay_testing

*for increased precision nowadays our length standard depends on c

## 1. What is the significance of the constant c in physics?

The constant c, also known as the speed of light, is a fundamental physical constant that represents the maximum speed at which all matter and information in the universe can travel. It plays a crucial role in many theories and equations in physics, including Einstein's theory of relativity.

## 2. Why is the constant c considered to be invariant?

The constant c is considered to be invariant because its value remains the same in all reference frames, regardless of the relative motion between the observer and the source. This is a fundamental principle of Einstein's theory of relativity.

## 3. How is the constant c related to the concept of space-time?

The constant c is closely related to the concept of space-time, as it is used to define the relationship between space and time in Einstein's theory of relativity. It represents the maximum speed at which an object can travel through space-time.

## 4. Can the constant c ever be exceeded?

No, according to our current understanding of physics, the constant c cannot be exceeded. It is considered to be a universal speed limit that cannot be surpassed by any form of matter or energy.

## 5. What are some real-world applications of the constant c?

The constant c has many real-world applications, including in the fields of telecommunications, astronomy, and particle physics. It is used in the development of technologies such as fiber optic communication and GPS systems, and it also helps us understand the behavior of objects in space and the properties of subatomic particles.

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