# Definition of a Quantum

1. Jan 5, 2010

### trogan

Is there a standard, agreed on definition of a Quantum in physics ?

Also, I have picked up this information about a quantum on the internet (http://www.peterrussell.com/SG/Ch5.php" [Broken]). Does this correctly describe Plank's constant and its relation to a quantum ?

"Although the amount of energy in a photon varies enormously, there is one aspect of the quantum that is fixed. Each and every quantum has a constant amount of action.

Mathematicians define action as an object’s momentum multiplied by the distance it travels; or the object’s energy multiplied by the time it is traveling–the two are equivalent. The amount of "action" in a ball thrown across a football field, for example, would be greater than the same ball thrown half the distance. Double the ball’s mass, and you double the action. Or imagine yourself running at a constant rate of energy output. If you run for twice as long, there will be twice the action–which makes intuitive sense.

The actual amount of action in a quantum is exceedingly small, about 0.00000000000000000000000000662618 erg.secs (or 6.62618x10-27 erg.secs in mathematical shorthand)–but it is always exactly the same amount.

This is called Planck’s constant (after its discoverer). It is the second universal constant to emerge from modern physics. Like the first–the speed of light–it is a constant of light. Light always comes in identical units of action.
"

Last edited by a moderator: May 4, 2017
2. Jan 5, 2010

Staff Emeritus
Quantum is an adjective, not a noun. So you won't find a universal definition of "a quantum" any more than you will find a universal definition of "a blue" or "a slow".

3. Jan 5, 2010

### trogan

Not according to the dictionaries I have visited.

For example, from the Free Dictionary:

quan·tum (kwntm)
n. pl. quan·ta (-t)
1. A quantity or amount.
2. A specified portion.
3. Something that can be counted or measured.
4. Physics
a. The smallest amount of a physical quantity that can exist independently, especially a discrete quantity of electromagnetic radiation.
b. This amount of energy regarded as a unit.
Relating to or based upon quantum mechanics.

4. Jan 5, 2010

### DaveC426913

So don't use the Free Dictionary....

Quantum means, generally: discrete. A quantum leap is a discrete leap.

5. Jan 5, 2010

### SimonA

For what it's worth, my definition of quantum would include the word "step". It's all about a step change, it's about being on this step and not the other. When we understand quantum properly, I reckon step will be a usefull word to use.

6. Jan 5, 2010

### trogan

Then there is "quantum of light" or "light quantum" where it is definitely used as a noun. It is the definition of the noun that interests me. I have books on physics where neither it nor Planck's constant are defined and I thought to myself I wonder why and whether physicists work with standard definitions of these two entities at all ?

7. Jan 5, 2010

### SimonA

Excellent point. I would contend that a photon is the result of an atoms energy level reaching the top step(ie no longer on the steps and free to travel elsewhere)

8. Jan 6, 2010

### Born2bwire

The "top step" as you put it of an atom would be the ionization energy, when the bounded electron is freed from the atom's orbitals. Photon emission does not require any such phenomenon, photons are emitted from atoms when an electron drops down from a given excited state to a lower state.

I would also throw my hat into the "quantum = discrete" ring. In general, one of the defining properties of quantum mechanics is the quantization of modes as opposed to the continuous spectrum of modes that exist for classical physics. This meaning can also be applied to the specific examples where we use quantum/quanta as well. For example, photons are the quanta of the electromagnetic wave. Thus, when physicists talk about the light quanta, they are talking about the discrete packets of light, the photons.

9. Jan 6, 2010

Staff Emeritus
If you are trying to learn physics from a dictionary, you have a difficult - probably impossible - job ahead of you.

If you are trying to learn anything at all without discarding your preconceptions, you have an even more difficult job ahead of you.

10. Jan 6, 2010

Staff Emeritus
There is no top step. The inverse square law has an infinite number of bound states.

11. Jan 6, 2010

### trogan

I will settle for a definition of the adjective. Does Physics have a standards body that oversees that sort of thing ?

It is weird. The description of Quantum Mechanics on Wiki does not say what "Quantum" means. And it seems wherever I look the Quantum seems to rate little or no mention when dealing with Quantum Theory. Has it been misnamed ?

12. Jan 6, 2010

### Mentz114

Not so. A photon is emitted when the electron drops to a lower energy level. This can happen spontaneously even if the electron is only 1 step above its ground state.

See any elementary text on quantum mechanics.

13. Jan 7, 2010

### trogan

Actually, there IS a very satisfying definition of a quantum on WIKI:

"In physics, a quantum (plural: quanta) is the minimum unit of any physical entity involved in an interaction. An example of an entity that is quantized is the energy transfer of elementary particles of matter (called fermions) and of photons and other bosons. The word comes from the Latin "quantus", for "how much." Behind this, one finds the fundamental notion that a physical property may be "quantized", referred to as "quantization". This means that the magnitude can take on only certain discrete numerical values, rather than any value, at least within a range. There is a related term of quantum number.

A photon, for example, is a single quantum of light, and may thus be referred to as a "light quantum". The energy of an electron bound to an atom (at rest) is said to be quantized, which results in the stability of atoms, and of matter in general.

As incorporated into the theory of quantum mechanics, this is regarded by physicists as part of the fundamental framework for understanding and describing nature at the infinitesimal level, for the very practical reason that it works. It is "in the nature of things", not a more or less arbitrary human preference.
"

14. Jan 7, 2010

### Feldoh

I have a question for the OP. Now that you have a supposed definition of "quantum" do you feel any smarter?

Have you learned anything that has built up your physical intuition?

Probably not. My point is is that learning definitions is not what you should be focused on it you want to understand physics. A rose by any other name would still smell as sweet, and all that...

15. Jan 8, 2010

### trogan

Define "smarter" !

Seriously, I am quite satisfied with what I have achieved here and thank you for your advice.

16. Jan 8, 2010

### trogan

It seems then that a quantum is a discrete amount of energy. Can someone tell me then how Quantum Physics came to be mainly about waves in various forms ? Would Wave Physics better describe the reality ?

17. Jan 8, 2010

### trogan

Also, it would seem that (for example) a photon and an electron are the same deal in one respect in that they both have the same "action" potential (as given by Planck's constant). The big difference between the two (as far as the quantum of energy each contains) seems to be the rate at which they vibrate. This is my extremely hard-won understanding. Is it correct ?

This seems to me to strongly point to elementary particles at least being composed of photons with some attributes changed.

18. Jan 8, 2010

Staff Emeritus
Trojan, you are flying towards crackpottery at a million miles an hour. I suggest you slow down and change direction. Yesterday you didn't know if the word "quantum" meant. Today you are saying that people who have studied quantum mechanics for decades have their models all wrong.

19. Jan 8, 2010

### DrChinese

The name is fine as is. As Vanadium 50 says, you might want to spend a additional time learning about some of the quantum concepts, reasoning and experimental support before trying to come up with improved names. Everyone starts out with lots of questions and hunchs, and these are mostly addressed in introductions to the subject. So you will learn about things like wave/particle duality. There have probably been 250,000+ papers/experiments published on QM since inception, so please be cognizant that the premises have been studied and examined in great detail by a lot of people.

20. Jan 8, 2010

### trogan

I am a long way from being a novice. I have many books on the subject (for example Quantum Reality by Nick Herbet and The Emporer's New Mind by Roger Penrose). I understand the concepts reasonably well. The maths I only seek to understand if I really need to. I am totally aware that some of the things I talk about will be naive. I try and limit my questions on Physics Forum to those I cannot find answers to in my books and searches.

My deeper understanding of the quantum as expounded here only came in recent days, yet I have studied Quantum Mechanics for years. Ditto for Planck's Constant. I hope other novices like myself will benefit from it as it is as rare as hen's teeth elsewhere.

I wonder why noone gave me a definition of a quantum. I am guessing the question seemed trivial.

21. Jan 8, 2010

### trogan

I had a fairly good idea what quantum meant. I was after a definition that would verify what I thought. I am not saying quantum mechanical models are all wrong. Heck I am not THAT naive.

What I would really like from you and other experts like yourself is answers to my questions. I thought I gave a good account of why an electron and a photon are similar quantum-wise. So what is wrong with my statement that: "This seems to me to strongly point to elementary particles at least being composed of photons with some attributes changed" ?

22. Jan 8, 2010

### DaveC426913

Because your statement is not a question?

23. Jan 8, 2010

### trogan

Hokay, I will rephrase it:

"Given that what I have just said is true, does this point to elementary particles (at least) being composed of photons with some attributes changed"

I don't like to seem ungrateful but this thread has been pretty tortuous form my point of view and none of my questions to date have been answered. Are you trying to discourage me (don't blame you ... being a software developer I can be annoyingly pedantic) ?

24. Jan 8, 2010

### DaveC426913

Two things sharing a property does not mean they are the same thing.

"Java and JavaScript share a similar name, it seems to me therefore that they are probably the same language with some syntactical differences."

I am a software developer as well. There is nothing wrong with being pedantic. The resistance you're encountering is due to the fact that you are putting forth unfounded hypotheses and then asking for validation of them. You will get nowhere fast.