# Questions tagged [definition]

The definition tag is used in situations where the question is either about how some term or concept is defined or where the validity of an answer depends on a subtle definition of some term or concept used in the question.

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### Why are they called “cyclic” coordinates?

In Lagrangian formalism, when $\frac{\partial L}{\partial q} = 0$, the coordinate $q$ is called cyclic and a corresponding conserved quantity exists. But why is it called cyclic?
3answers
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### Axioms behind entropy!

The concept of entropy is very ubiquitous, we learn about its uses starting from Information Theory (Shannon entropy) up to its basic definition in statistical mechanics in terms of number of micro-...
1answer
541 views

### Introduction to Gauge Symmetries: Good, Bad or Ugly?

I'm trying to come up with a good (as in intuitive and not 'too wrong') definition of a gauge symmetry. This is what I have right now: A dynamical symmetry is a (differentiable) group of ...
4answers
37k views

### Differences between wavefunction, probability and probability density?

I am trying to understand the differences between wavefunction, probability and probability density. There are different definitions on the internet. For example: http://inside.mines.edu/~fsarazin/...
5answers
5k views

### In what sense can a complex number be a scalar?

A definition of a scalar like A scalar is a one-component quantity that is invariant under rotations of the coordinate system (see http://mathworld.wolfram.com/Scalar.html) seems to exclude ...
3answers
2k views

### What is a photon? [duplicate]

I'm trying to get a definitive and clear answer to the question of what a photon actually is. Textbooks seem to give quite vague explanations, all starting with Einstein's idea that a quanta is a form ...
3answers
3k views

3answers
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### Is a theory the same as a hypothesis? [duplicate]

“Any physical theory is always provisional, in the sense that it is only a hypothesis: you can never prove it.” Excerpt from Stephen Hawking, A Brief History of Time, iBooks. So does that mean ...