What is a perfect physical science constant?

When I examine the NIST list of physical science constants, the numeric values are often dependent upon units that are strictly local.

http://physics.nist.gov/cgi-bin/cuu/Category?view=html&Universal.x=82&Universal.y=7

Mathematics has defined a number of perfect constants, they are dimensionless and are truly universal. In the physical sciences it is rare to find a dimensionless constant, at least there are none defined in the SI list of constants. The next best thing to a perfect physical science constant is one that would contain only one defining unit, but that unit should be something that is an actual universal value.

Are there any physical science constants that are truly universal values?

The fine structure constant (http://physics.nist.gov/cuu/Constants/alpha.html) is the only dimensionless physical constant I know of.

It may or may not have been different in the early universe.

there are a whole list of dimensionless constants in the standard model. 26, if i recall correctly. they include the coupling constants (like the fine structure constant) and mass of the fundamental particles. plus 1 more from GR, the cosmological constant.

the constants on that list aren't really constants, since they depend on your units, and can be made to take any value. as you rightly point out.

but even the dimensionless constants in physics are somehow less "perfect" than the mathematical constants. both mathematical and (dimensionless) physical constants seem to be fundamental properties of the fabric of our universe, but there is a chance that the physical constants may simply be emergent properties of a more fundamental theory, like the resistivity of metals was once thought to be a fundamental intrinsic property, but with the discovery of atoms and quantum mechanics, it turned out those could be calculated.

so maybe the 26 physical constants can one day be calculated from a smaller set of constants. but right now, the only way to find the values of these constants is to measure them. then you can only get as many digits as your measuring apparatus allows. in stark comparison to the mathematical constants, where you can calculate as many digits as you want.

perhaps there will be an ultimate theory which can calculate all the physical constants in terms of only mathematical constants. in other words, perhaps there are only mathematical constants, no physical constants.

Perhaps the 2 in various inverse square laws is a physical constant with a universal value independent of measurement units.

Perhaps the 2 in various inverse square laws is a physical constant with a universal value independent of measurement units.
that 2 can be derived and so is not what i would call a fundamental constant.

mass of the fundamental particles
How can a mass be dimensionless? :bugeye:
http://physics.nist.gov/cuu/Constants/Table/allascii.txt
This gives the masses of fundamental particles as having units of kilograms. Maybe you meant ratios of masses, which are massless.

How can a mass be dimensionless? :bugeye:
http://physics.nist.gov/cuu/Constants/Table/allascii.txt
This gives the masses of fundamental particles as having units of kilograms. Maybe you meant ratios of masses, which are massless.
yes.

Originally post by Lethe] " perhaps there will be an ultimate theory which can calculate all the physical constants in terms of only mathematical constants. "

That statement may have more truth than we think. I found the following URL years ago when it was on the site page index.

http://superstringtheory.com/unitsa.html

As I look at what they are doing, it seems they are artificially identifying a physical constant, one that is identified by two types of units, as a true mathematical constant, "1". It would be better to find a pure mathematical relationship where a physical constant can be substituted naturally.

One of the difficulties in finding a perfect physical constant is that the values in the SI list are dependent upon their "local" unit definitions. A Kg weight is only a kilogram at its stored prototype location. It serves its purpose to establish a reference mass, but that mass is peculiar to its location, definitely not universal.
http://physics.nist.gov/cuu/Units/kilogram.html

Oddly enough there is a universal constant that is invariable throughout the universe, very well known here on Earth, but not recognized as a constant. Take an iridium bar and cut it exactly to the length of the wavelength of the Larmor precession emission of neutral hydrogen and it will represent 1 wavelength. I can carry that bar anywhere in the universe and any intelligent ET that has advanced to our level of science will know what the bar length represents.

If we measure that bar using one of our systems of measurement, say metric, and it will come out to 21.106.....+ cm. Thus we can say about 21 cm represents 1 wavelength of a universal natural physical process. If a species coincidentally used the English system they would note it as 8.3097....+ inches. We cannot physicaly see the wavelength but we can measure it and represent it in a form that is visible and touchable.

Is that close to a universal constant?

One of the difficulties in finding a perfect physical constant is that the values in the SI list are dependent upon their "local" unit definitions. A Kg weight is only a kilogram at its stored prototype location. It serves its purpose to establish a reference mass, but that mass is peculiar to its location, definitely not universal.
http://physics.nist.gov/cuu/Units/kilogram.html

Oddly enough there is a universal constant that is invariable throughout the universe, very well known here on Earth, but not recognized as a constant. Take an iridium bar and cut it exactly to the length of the wavelength of the Larmor precession emission of neutral hydrogen and it will represent 1 wavelength. I can carry that bar anywhere in the universe and any intelligent ET that has advanced to our level of science will know what the bar length represents.

If we measure that bar using one of our systems of measurement, say metric, it will come out to 21.106.....+ cm. Thus we can say about 21 cm represents 1 wavelength of a universal natural physical process. If a species coincidentally used the English system they would note it as 8.3097....+ inches. We cannot physicaly see the wavelength but we can measure it and represent it in a form that is visible and touchable.

Is that close to a universal constant?

Sure - that's why it was used on the Pioneer plaque.

The frequency of the hyperfine Hydrogen transition (1,420 MHz) is also a useful constant, in this case for time.

The speed of light is a constant in the same way, but a speed is harder to encapsulate than a length.

The gravitational constant and Planck's constant are also constant quantities, but are even harder to physically encapsulate because they have complex units.

This thread reminded me of the current state of the speed of light and the length of the meter. How many here are aware that the speed of light has been decreed to be some particular value and that it now defines the length of a meter?

Any future experiment which measures the speed of light in a vacuum more precisely will be viewed as a redefinition of the length of the meter.

Any future experiment which measures the speed of light in a vacuum more precisely will be viewed as a redefinition of the length of the meter.
what you mean to say is "any future experiment which measures the meter more precisely will be viewed as an experiment which measured the meter more precisely"

you're just as likely to measure the number of inches in a foot more precisely as you are to measure the speed of light more precisely. which is to say, not at all likely.

This thread reminded me of the current state of the speed of light and the length of the meter. How many here are aware that the speed of light has been decreed to be some particular value and that it now defines the length of a meter?

The NIST site uses what one might call a circular definition for the meter and the speed of light.

http://physics.nist.gov/cuu/Units/current.html

Check the following site: http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/ltrans.html, which includes the following statement.[list]Therefore the above speed of light has been adopted as a standard value and the length of the meter is redefined to be consistent with this value.[list]That site also provides Lorentz transformation formulae.

I read an article quite a few years ago about this decision by the scientific community, and I think they might also have defined a second of time in terms of the frequency of some atomic phenomenon.

Lethe: The speed of light was measured to be 299,792,458 meters per second at the time of the above decision. At that time the meter was defined to be c/299,792,458

Suppose some future experiment resulted in deciding that the above value should have been 299,792,458.3 meters per second. It does not seem impossible that some future technology might be considered more reliable than the technology which produced the currently accepted value.

It is my understanding that the meter would still be defined to be c/299,792,458 resulting in the conclusion that the meter is a bit longer than it was previously thought to be. Such a length difference is probably less than the errors resulting from attempts to calibrate to a standard meter stick, which was the calibration method used for decades.

Consider two physicists in different laboratories doing the same experiments requiring extremely precise measurements. When comparing measurements or trying to verify results, they cannot use a wooden meter stick to define the standard of length. There was a time when a "meter stick" in Paris was the standard. Laboratories doing precise experiments had a meter stick or some other device calibrated to the standard meter in Paris, which was made available on rare occasions to allow the production of length standards for use around the world.

Now local devices will be calibrated to the speed of light. I have no idea how this is accomplished.

Hi Dinosaur,
How precisely can one measure the separation between two marks on a platinum-iridium bar?
How precisely can that separation be duplicated for use in labs around the world?

A better standard would be one that can
a) be more precisely measured than the distance between two lines
b) be independently reproduced from the standard anywhere.

Such a standard was introduced in 1960, being a certain number of wavelengths of a particular transition of a particular isotope of Krypton.

This standard was chagned to the current speed of light standard in 1983, with the goal of improving the precision of the standard.


Suppose some future experiment resulted in deciding that the above value should have been 299,792,458.3 meters per second.
You are speaking as if the meter has some other objective definition that is more precisely known than the speed of light - that is not the case!

The NIST site uses what one might call a circular definition for the meter and the speed of light.

The speed of light is a particular quantity, independent of any definition. It is what it is. It is not defined by any value placed on it by humans, and such definition is not the intention of the standard. The standard defines a meter - it doesn't define the speed of light.

That speed is known so precisely, and it is so invariant, that it is the most precise tool that we have of measuring distances.


Imagine that a scientist wants to measure a particular distance.
They measure it a number of ways.
The distance is (these figures are based on fact, but primarily for illustrative purposes. Don't trust them as statements of fact):

1.000000 +/- 0.0001% times the distance between two marks on a platinum-iridium bar

1,650,763.73 +/- 0.000001% times the wavelength of a transition in an isotope of kryptonite

1579800.2987277 +/- 0.00000001% times the wavelength of an iodine stabilized Helium-Neon laser

1/299,792,458.00004 +-/ 0.0000000000001% times the distance light travels in a second


Which is the most precise statement of the distance?
Note that it's not until the last measurement that we can tell that the distance measured differs from the meter standard.

Lethe: The speed of light was measured to be 299,792,458 meters per second at the time of the above decision. At that time the meter was defined to be c/299,792,458

Suppose some future experiment resulted in deciding that the above value should have been 299,792,458.3 meters per second. It does not seem impossible that some future technology might be considered more reliable than the technology which produced the currently accepted value.

yeah, and if some future experiment using more advanced technology determines the number of inches in a foot to be more than 12, the foot will become longer too. right

Pete: Using the speed of light to define the length of a meter seemed like a good idea to me when they did it, and still seems like a good idea. I do not need to be convinced on this issue, although using wave lengths of the light emitted by some particular isotope also seems reasonable. I wonder which of these two definitions is easier to apply in practice. Might ease of use be responsible for the change in definition? From a philosophical point of view, the speed of light definition seems like the way to go.

BTW: I would not put down those who defined the meter in terms of marks on a piece of metal in Paris. It was good enough for the purposes of those times, and I doubt that the technology of that era was able to provide a much better way to define it.

I made an error when I posted the current definition of a meter.c / 299,792,458 is the value I posted.

The distance traveled by light in 1 / 299,792, 458 seconds is the value I should have posted. This definition was adopted in 1983 according to you, Pete.Consider the following.The speed of light was determined some time prior to 1983 using the then current definitions of the meter and the second.

I claim that the experimental design and/or the technology used to determine the speed of light prior to 1983 might be considered inferior to some future experimental design and/or technology. This could lead to some future experiment which determines that the pre-1983 experiment should have provided a slightly different value, perhaps 299,792,458.3 meters per second, using the pre-1983 definitions of the meter and the second.

If the above occurs, I assume that the speed of light will remain defined as 299,792,458 meters per second, resulting in the same definition of the meter.

The consequences are that a future physicist can validly state: The meter used today is slightly longer than the meter used in the late 20th century, but the speed of light has not changed.

I would not put down those who defined the meter in terms of marks on a piece of metal in Paris.
Neither would I. That wasn't my intention (although my phrasing was unnecessarily harsh).

The consequences are that a future physicist can validly state: The meter used today is slightly longer than the meter used in the late 20th century, but the speed of light has not changed.
The same physicist could also validly state:
The meter today is within the error margins at the time of the meter used in the late 20th century.

Is the speed of light a perfect physical constant? We know that speed of light is a constant, but the numeric value we use depends upon two man defined units, the meter and the second. The meter and the second are not physical constants themselves, which makes the speed-of-light numeric value a conditional constant.

As lethe pointed out in an earlier post (10-12-04, 11:50 PM), "constants on that list aren't really constants, since they depend on your units, and can be made to take any value."

The meter hasn't been around very long, less than the megalithic yard. It is conceivable that the scientific community might, in the future, decide upon a standard of length that has some relevance to a physical science characteristic, and leave the meter for commercial use. Whatever the new standard length might be called, if its end-to-end length is different from the meter the numeric value for the speed-of-light will change.

In the same post as above, lethe stated, "the dimensionless constants in physics are somehow less "perfect" than the mathematical constants."

They may be less perfect than mathematical constants only because they have to be measured, and they are not as well known. The neutron-electron mass ratio is a well establish "constant" , 1838.6836598, and is known to more significant figures than the speed-of-light.

Curiously, an engineer pointed out several years ago, that number is presented as a specific dimension in the Great Pyramid, but I don't think we want to go there.

The Pioneer 10 plaque did not include a "scribed" line 21.1061 cm long, even though the plaque was large enough, the value was presented in a cryptic form.