|WikiProject Physics||(Rated Start-class, Mid-importance)|
|WikiProject Measurement||(Rated Start-class, High-importance)|
Is the metre a fundamental unit?
What a strange article. Seems to be an important subject but there's barely any references, there is a claim the metre isn't a fundamental unit while the article Metre claims it is and it even uses words like "dimensionful"... what's going on here? 188.8.131.52 (talk) 18:09, 28 December 2007 (UTC)
- The meaning of the word "fundamental" is different. The metre has been defined by defining the speed of light to be 299792458 metres per second (the second is defined by defining the frequency of some transition between two energy levels of some atom to be of some frequency). So, there can be nothing fundamental (in a physical sense) about the metre defined in this way.
- However, the reason the units are defined in the way it is defined is because you want to have the most accurate standards available. Any official length standard is ultimately derived from the metre as defined according to the SI standard. So, in this sense, one can talk about the metre being a "fundamental unit".
- The metre is certainaly not a fundamental unit in physics, as Nature does not care about what units we use. In fact, many physicist believe that all dimensionful quantities are human constructs and only dimensionless quantities have any meaning independent of our conventions. Count Iblis (talk) 23:44, 28 December 2007 (UTC)
This article seems to be erroneous, lacks references, and is too short for such an important subject. Furthermore SI_base_unit article should be sufficient to describe units that are thought to be mutualy independant. Ref: http://physics.nist.gov/cuu/Units/units.html Merge or redirect "Fundamental units" to SI_base_unit please. Abaharaki 05:15, 08 June 2008 (UTC)
- I don't agree. There is noting erroneous about this article. The SI system has more to do with having accurate standards for physical quantities. Most, if not all SI units are dependent on each other as far as theoretical physics is concerned. So, SI units are not relevant when we talk about fundamental units. Count Iblis (talk) 17:30, 8 June 2008 (UTC)
- I agree with merging, however the difference between fundamental units (a set of 6 units that form a basis to construct any physical quantity) and SI units (the set of 7 base units defined by ) should be made very clear! Usually the unit candela is the SI system is seen as the unit that's not really necessary, mainly because it is the unit which is farthest from our physical instinct. --Jaapkroe (talk) 10:56, 13 June 2008 (UTC)
- Again this erroneous idea of saving space at the expense of clarity. Those who lack in capability to segregate linguistic terms should stand off such proposals for mergers. Nobody would propose to merge physics and philosophy. I prefer those proposals which directly contribute input but simply criticise what has been the effort of others.Wireless friend (talk) 14:20, 11 August 2008 (UTC)
I have rewritten the article to develop the thoughts of the author to their logical conclusion. In this form it is NOT appropraite to merge it with the SI base unit article, as the SI base unit article is factual and the fundamental unit article is somewhat philosophical. Ehrenkater (talk) 20:32, 4 September 2008 (UTC)
Please feel free to change whatever words you consider are inappropriate, as long as the principles are retained. The article as it was was inappropriate, see eg Abaharaki's comments above.Ehrenkater (talk) 20:58, 4 September 2008 (UTC)
- I think that the ref to the article by Okun et. al should be restored, it is very relevant to the discussion about whether there exists any fundamental units or not. I agree that this article should not be merged with the article on the SI units. I disagree that this is because this article is less factual than the SI article. The SI article is written from another angle. This article is written from the POV of fundamental physics and then the view of theoretical physicists is more important. Count Iblis (talk) 21:31, 4 September 2008 (UTC)
Isn't the "bit" also a fundamental unit, important for information theory, thermodynamics, etc... I don't see how it can be made out of the other units mentioned here. 184.108.40.206 (talk) 10:04, 15 August 2009 (UTC)
Is the information on eliminating units correct?
I am unsure that the information on eliminating the SI fundamental units by fixing the universal constants as dimensionless numbers is correct.
Particularly, the segment, starting at, "In theory, a system of fundamental quantities (or sometimes fundamental dimensions) would be such that every other physical quantity (or dimension of physical quantity) can be generated from them. One could eliminate any two of the metre, kilogram and second by setting c and h to unity or to a fixed dimensionless number......" The segment continues to list the ways to eliminate units and finally references Plank's Units.
But, the way it is explained, will not c after assuming it equal to 1, still have a dimension, may be called "cunit"? So an object travelling at half the speed of light will have speed 0.5 cunit and not simply 0.5? There is no meaning of c becoming dimensionless in the this context.
In reference to Plank Units, although the constants are normalized to 1, there still are units like Plank Length or Plank Time and so on and a c normalized to 1 still has the unit LT^-1. The article seems to give the impression that there will not remain any units left and we will be able to describe the world in numbers.[not a bad idea if you ask me :)]
I am not a suitable person but should someone suitable not correct this information? Or is my understanding incomplete/over-interpreted and this is valid information?
- This is actualy somewhat controversial in the physics community, but it is also more of a philosophical thing (it doesn't actually affect the results of actual computations). See e.g. this article where the 3 authors gove their different views, this articles takes the view of Michael Duff.
- Thing is, the dimensions you assign to physical quantities is arbitrary. You can make a choice (usually based on historic conventions) like to measure time and distances in incompatible units and to enforce the incompatibility, you assign them different dimensions. Before we knew about special relativity this was well motivated, because there was no way you could compare a time interval to a spatial distance in a universal way, any comparison would have to be based on some ad hoc physical artifact. But within the context of special relativity, it makes little sense to measure time and distances in different units. However, if we stubbornly stick to the old units, you will force the equations to undo the assignment of different units for distances and time intervals, i.e. the constant c will appear in the equations and that c will have the right dimensions for that job.
- One can also say that you obtain classical physics from relativistic physics by rescaling all the velocities; you want to examine the limit where these go to zero. You have to do this rescaling in a careful way I explain that here. Then you can start with natural units (so c = 1 and dimensionless), but then because you want to examine to what theory relativity reduces to in the scaling limit, you introduce a dimensionless scaling constant. You can call that constant c, but the way you put this in in the equations is purely motivated by your desire to be able to see what is going on in the scaling limit.
- Then what you find is that in the limit of c to infinity, you will get incompatible physical quantities if they are related to each other by powers of c. E.g. to describe the physics at the classical scaling limit, you need to use the mass and the kinetic energy, and you lose the concept of the total energy but this means that you cannot measure mass end (kinetic) energy in the same units anymore. Count Iblis (talk) 13:44, 14 August 2013 (UTC)
- J.D. Jackson claims that the number of dimensionless units is arbitrary. He references Birge, and I believe the U.C. Berkeley physics department is located in a building named after him. To avoid POV, I just included references to that side of the argument.---guyvan52 (talk) 02:35, 13 January 2014 (UTC)