# principle and law

Discussion in 'Physics' started by bradstormer, Aug 12, 2010.

Aug 6, 2010
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In physics what is the difference between a principle and a law?
For instance Newton has Laws of motion, where Heisenberg has the uncertainty Principle..

2. ### russ_hensel Distinguished Member

Jan 11, 2009
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There is probably a good answer to this, but fine distinctions like this are not a great use of time.

3. ### Wendy Moderator

Mar 24, 2008
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To me a natural Law is something that has no exceptions. Since quantum mechanics is a statistical process it can have a lot of exceptions, it is the average that is the norm.

Don't know if this is correct, it is just my take.

Jul 7, 2009
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Fundamentally, they're not well-defined terms that have the same meanings to everyone. Take a look at wikipedia's page for some background.

Unfortunately, most "laws" that are quoted break down somewhere, so one's thorough education needs to explore the limits of where these laws apply. Examples are where classical physical laws didn't give the correct answers, leading to things like quantum mechanics or relativity.

An interesting one is Ohm's "Law", which is really two different things. First, the original content of Ohm's work showed an empirical linear relationship between current and voltage for metallic conductors. While it's very useful, it's only approximately correct -- just keep doubling the current until it's no longer true . Another more general form of Ohm's Law is really the definition of resistance as the ratio of voltage to current. But it's a macroscopic approximation too -- just keep making things smaller until statistical fluctuations cause inaccuracies or anisotropies require tensorial relationships. Ignoring these border behaviors, it's one of the most useful empirical relationships in electrical work!

Aug 6, 2010
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so basically they are both the same thing. and in their most basic form they are a set of mathematical equations used to describe an event, but if the numbers get extremely small a new law is required to describe the slightly different situation due to the differing strength of the forces at sub atomic masses?
sorry to keep on with this but i just find the area very interesting

6. ### Wendy Moderator

Mar 24, 2008
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I'll disagree a little with someonesdad, in that first and foremost a law is repeatable. We used to be constantly bombarded with HHO threads, most being true believers in spite of major lack of repeatability. There is something deep in human psyche that is convinced if you try hard enough you can get something for nothing. With them it is energy. There is a Natural Law that covers this fairly well (Laws of Conservation of Energy), but they can't wrap their heads around it. One of the common failings is the believe that you can split water into hydrogen and oxygen with the same amount of energy (or less) than you get when you combine hydrogen and oxygen, the inconvenient losses are ignored. One of our mods got tired of the repeat arguments and the attempts to use this site to justify their beliefs in spite of the repeated conclusions that it was so much hot air. That and a lot of the people selling hardware were basically scammers spouting pseudo science to make money.

The belief that someone can flap their arms and fly is all well and good, but my money is on Newton's Laws of Gravity, exceptions in relativity aside.

Jul 7, 2009
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No, I didn't want to imply something like that is general. The heart of the thought is that laws and principles reflect an intense amount of experimentation and empirical evidence. A thoughtful person might look at the experimental evidence and either deduce or induce a pattern; in science these patterns are usually expressed mathematically.

Now, here's the key. This person then publishes his pattern in the scientific literature along with his arguments, experimental data, and his own best critique of his own work. This puts it in front of many other scientists. They use it to make predictions, check their work in their own measurements, and try to find a flaw in the reasoning or results. If it stands up to scrutiny, it eventually may get the term "law" associated with it -- or it may not. For example, you might hear "the conservation of energy law" or "the principle of energy conservation". The semantics aren't important; what is important is that it has been verified experimentally many, many times -- and, remember, one negative result can cause it to be tossed out, no longer regarded as a "law".

People use these laws for prediction and extrapolation. When exploring the boundaries of the known (i.e., new science), deviations from the laws are interesting and cause people to make up new world views, repeating the process alluded to in the previous paragraph.

A careful person would acknowledge that most of our scientific experience is incredibly local (both spatially and temporally) with regards to first-hand evidence. For example, how do we know what the physical characteristics would be on the Earth if we and the Sun were transported to an analogous place in the Andromeda galaxy? How about in another spiral galaxy 10 billion light years thataway? The deep answer is that we don't know. However, based on evidence from electromagnetic radiation received from those locations, astronomers think they can explain much of what they see and deduce that it appears that e.g. the hydrogen atom seems to have the same spectrum across vast gulfs of distance and time. But we have to rely on indirect evidence that a "law" here and now is the same as "there" and "then". It's interesting to speculate how much things could be different, yet still appear to be empirically the same to us.

8. ### jonnylazer New Member

Jan 12, 2010
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My take here is...that laws emerge from principles, the area of emergence is a very interesting area of modern physics

there is no difference between these terms, they are used interchangebley all of the time. The only difference really is a semantic one i.e what do people mean when they use these words.

the two examples given are interesting though because Newtons laws were thought to be universal but Einstein showed us that one of their underlying assumtions was incorrect, namely that, mass is a constant. Einstein was able to make a correction and showed that mass increases with velocity.

The uncertainty principle is different as far as I know in that it seems to be built in to the fabric of reality so that no matter how good our test equipment becomes and how powerful our predictive abilities become, we will always find that it is true.

I would like to disagree with whoever said that questions like this are not a good use of time, I think that is profoundly short sighted.

Just a word on quantum mechanics...I think it is wrong to say that there are exceptions to quantum mechanics and therefore it is the average that is the norm. All quantum mechanics shows us is that to the best of our current understanding random behaviour is the norm.

9. ### retched AAC Fanatic!

Dec 5, 2009
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I think they are spelled differently too.

L-A-W
and
P-R-I-N-C...

Yup. DEFINITELY spelled differently!

Aug 6, 2010
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thanks for the replies everyone its very interesting to read through them.

11. ### retched AAC Fanatic!

Dec 5, 2009
5,201
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No problem.. I know spelling was not my strong suit either.

12. ### muralinaik New Member

Aug 16, 2010
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law is a cause principle is that's application

13. ### steveb Senior Member

Jul 3, 2008
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I think that terminology can sometimes be more confusing than the physics it tries to describe. Many terms don't have clear definitions, or can have multiple definitions. It's better to look at something and try to understand it's nature, rather than worry too much about the terminology. However, this is a very fair and good question.

I won't try to answer this question because it is too difficult for me to answer. However, I can offer a look at some "data points" to get some ideas to sort this out to one's own satisfaction. Principles seem to be either very isolated ideas for a very specific application, or they can be very general.

For a specific isolated example, consider the "Pauli Exclusion Principle" http://en.wikipedia.org/wiki/Pauli_exclusion_principle . This is a nice rule that was developed before a full theoretical understanding was at hand. The principle is still useful practically, but the underlying theory trumps the principle.

For a general example, consider the "Principle of Least Action" http://en.wikipedia.org/wiki/Principle_of_least_action , which describes a very simple idea. The math (calculus of variations) is a little tricky until you get used to it, but the basic mathematical ideas are quite simple once you work it out. This principle turns out to be so fundamental that it shows up in EVERY single physics theory that has been found useful, such as classical mechanics, classical electromagnetics, general relativity, quantum mechanics, quantum field theory and even string theory. I have little doubt that if a "theory of everything" is ever discovered it will obey this principle.

So, this latter case is an example where a "principle", which is sometimes regarded as below a Law in hierarchy, seems to trump all Laws and Theories. Yet, it is very rare to hear someone refer to a "Law of Least Action", but I can't think of a better example of a "law" since it is one of the most pervasive ideas in physics. Newton's Laws of mechanics and gravity has exceptions (for example General Relativity), yet the principle of least action, which underlies Newton's laws, has continued to hold up without exception.

What this tells me is that there are two flavors of "a principle" and one flavor is too simplistic to be a Law, since it is a rule without basis. The other flavor is a type of general idea or statement that applies to many different situations, perhaps wearing different clothing depending on the situation under consideration. Things like laws (or theories) are very specific, and reveal a deeper understanding, or at least are generally perceived that way.

Last edited: Aug 16, 2010
14. ### studiot AAC Fanatic!

Nov 9, 2007
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For what it's worth my take on the difference is that a law is a statement, very often mathematical, which science holds to be true, but does not guarantee that we can solve the mathematical expression.

for example Ampere's Law states

$\oint {H.dL = I}$

But it does not mean that we can actually perform the integration.

By contrast a principle is a method of attack on a particular issue, which was usually introduced to simplify the mathematics.

Huygen's principle is a case in point.