# Laws, principles and facts

1. Dec 18, 2013

### iantresman

Although scientists once thought that radioactivity violated the law of conservation of energy, then new understanding of nuclear decay helped demonstrate that the law was fact.

So why do we continue to call the Conservation of Energy a law, and not a fact. Why not a principle? Is there a difference?

2. Dec 18, 2013

### Drakkith

Staff Emeritus
From Wiki:

You could say a fact is saying, "Current in this circuit is 10 amps and the voltage is 100 volts". I can observe this by measuring the current and voltage.

In contrast, a law would be saying, "The current through a conductor between two points is directly proportional to the potential difference across the two points". (Which is Ohm's law) There is a relationship between current and voltage that this law describes. The law doesn't describe WHY current is proportional to the potential difference across two points, it only states that it is.

3. Dec 19, 2013

### Philip Wood

I don't think there's a contradiction between calling the law of conservation of energy a law and calling it a fact. I suppose that calling it a fact is a way of saying that we are sure it is true. My personal preference would be not to use the word fact in this way, but to reserve it to describe individual states of affairs in the universe, such as my living in England, or my cat being tabby.

4. Dec 20, 2013

### Claude Bile

But if radioactivity can do work then it doesn't violate the law of conservation of energy at all. All it means is that the avenues through with which work can be done were originally incomplete.

Claude.

5. Dec 20, 2013

### sophiecentaur

Once you recognise the equivalence of mass and energy, the conservation law works. But you have to remember that it's only in case of nuclear (very high energy) interactions that the classical law is violated.
Ohm's Law also has a certain range over which it applies and so does Newton's Law of gravitation. No one should loose any sleep over the changing meanings and uses of these names. The big step was taken a long time ago when the word Law ceased to mean a Law, laid down by (a) God and became a description of behaviour of a system. Words are constantly changing their meaning, aren't they? (Wicked!)

6. Dec 20, 2013

### vanhees71

The energy-conservation law holds within the realm of special relativity (and also of Newtonian mechanics), because it directly follows from the symmetry of physics under time translations (via Noether's theorem).

It does not hold within general relativity, where energy conservation is violated for non-stationary a non-stationary metrics. E.g., in the Cosmological Standard model space-time on the large scale is described as a Friedmann-Lemaitre-Robertson-Walker metric with an expanding scale factor, which leads to the redshift of photons that travel in free space. This means the photon's energy is not conserved due to the expansion of the scale factor in the metric.