# What are the fundamental physical units of measure ?

1. Jan 31, 2012

### Islam Hassan

I was wondering what were the fundamental physical units of measurement, ie those from which all other units can be derived.

To my mind, there are only two things which exist in the universe and from which all units of measurement can be derived : space and matter. The reasoning is as follows :

• From space we can define location, distance and volume in 3 dimensions either in linear or angular terms.
• All matter comes with its associated force field (weak, strong, electromagnetic) and a curvature of spacetime attached to it. From a measure of the amount and type of matter present (not the mass per se), we can therefore derive a measure of force in a given point in 3D space.
• With such measured force acting on matter, we get movement, ie velocity as well as acceleration and can define measurement units for them too.
• With acceleration and force, we can derive a measure of mass.
• For a given frame of reference and with two measurements of movement, one which we will define as being The Standard Movement (ie, a clock) and another as being The Observed Movement, we can define units of time which is just a pacing of 2 or more relative movements through space.

I believe that with measurement units thus defined/assigned for i) type/quantity of matter, ii) location, iii) distance, iii) volume, iv) force, v) velocity, vi) acceleration, vii) mass and viii) time, all other physical measurement units can be derived.

Is this line of reasoning correct/complete?

IH

2. Jan 31, 2012

### Pythagorean

I would say space, time, and mass can give you everything else. Some might even argue just space and time, mass being a ratio of spatial dimensions.

3. Jan 31, 2012

### Islam Hassan

To my mind, both mass and time are derived measures.

Time can be defined as relative movement with respect to a standard movement reference (ie, a clock) in a given frame of reference and mass is derived from force (which fields/spacetime curvature are a feature of matter) and acceleration which itself is derived from spatial observation of matter in movement.

Time in particular, is a derived measure for me, and not a fundamental given, or 'medium' through which events flow. The basic premise to this thinking is that one cannot measure time independently of movement; it is simply not conceivable.

IH

Last edited: Jan 31, 2012
4. Jan 31, 2012

### HallsofIvy

What are considered "fundamental" and what are "derived" is pretty much a decision made when setting up the system, not given by nature.

I have seen physics papers which used measures based on "natural constants". The speed of light is a natural constant so we use speed as a fundamental measure. Plank's constant is a natural constant with units of "action" so we take that as a fundamental measure. The gravitational constant (the G in G mM/r^2) is a natural constant so we take that as a fundamental measure. We then derive things like "distance" and "time" from those.

5. Jan 31, 2012

### Studiot

Hello Hassan, what has electric charge done to avoid mention?

6. Jan 31, 2012

### Pythagorean

Electrical charge has SI units: sA (seconds-Ampere). Ampere can also be expressed in terms of time/space/mass I believe. On iPhone currently so not digging.

7. Jan 31, 2012

### Pythagorean

According to http://www.qsl.net/g4cnn/units/units.htm

ampere has dimensions of mass^(½)length^(1½)time^(-2)

8. Feb 1, 2012

9. Feb 1, 2012

### 256bits

Islam Hassan
A thought provoking post.

10. Feb 1, 2012

### Pengwuino

With that logic, there exists no fundamental physics units of measure. Time can be expressed in terms of Coulombs and amps or volts or joules or some other odd combination of units. That does not make them fundamental.

I think the only fundamental constant is [itex]\pi[\itex]. and 2. Maybe 60 is fundamental as that seems to be a rather ubiquitous speed limit in miles per hour.

11. Feb 1, 2012

### Pythagorean

Except for electrical current has no prevalent conceptual relevance across all phenomena in the universe. Time and space do. It comes down to the qualitative interpretation in the end, not mathematical (dimensional) analysis.

12. Feb 1, 2012

### Pengwuino

What about photons that experience neither?

13. Feb 1, 2012

### Pythagorean

I think you're reaching...

14. Feb 1, 2012

### Pengwuino

Study GR, time and space are not so simple.

15. Feb 1, 2012

### Pythagorean

Pedantry is not really a valid argument.

16. Feb 1, 2012

### Pengwuino

I don't want to make a thread on the use of spacetime in GR. I personally don't believe it's accurate to consider something to be fundamental that has some seemingly non-fundamental properties. I would surely say electric charge has a fundamental origin as one can't really transform between reference frames and find something with a different electric charge or different color charge. There is something fundamental to those kinds of quantities in field theory (not all the charges, though) in my opinion. Something about spacetime just doesn't feel right.

Of course, I did my entire MS thesis bashing someone who claimed that spacetime wasn't fundamental, so maybe I'm not the right person to be claiming that spacetime might not be fundamental.

17. Feb 2, 2012

### Islam Hassan

Agreed. I should have added that my conception of what is fundamental is what can be most simply, ie directly, observed. A more naïve and problematic way to put it is that 'fundamental' would attach to observation of what 'exists'.

Matter and its motion (and motion's higher derivatives) can be observed. The space they occupy can be 'observed', well Ok perceived rather. Accelerated matter can be observed so that the force field associated with accelerated matter can be inferred even if it is -strictly speaking- 'invisible'.

That is why I deem that only:

i) matter (ie, type/number of objects/particles) + its associated and inseperable force field;

and

ii) space

yield the fundamental units of measurement.

Mass is not fundamental because it is derived as force/acceleration, both fundamentally measurable.

Time is not fundamental because it derives from a measure of relative motion (observed motion vs a standard motion reference we may call a clock). If one were to ask whether time exists as a directly observable phenomenon, the short, simple though controversial answer would to my mind be no.

Any thoughts?

IH

Last edited: Feb 2, 2012
18. Feb 2, 2012

### Islam Hassan

I included electric charge under the force field heading Studiot, which I also deem to be fundamental.

IH

19. Feb 2, 2012