Translatrix,

Ouch!

>If I understand you rightly, you are saying that the term "energy" is used on a lot of different phenomena - VERY different, as heat, light, movement, a position from which you can gain speed by falling, the ability to release heat when being burned, even mass - just because they answer the physical formula and because they can (sometimes with trouble) be turned into each other, but there is no underlying similarity further than that, nothing you can say all these different forms of energy are "made of"?<

That is very pertinently and pointedly put!

About my only reservation on what you said is that I am not certain that when you say "used on a lot of different phenomena", that is strictly correct. I should be happier saying that we can get energy out of those phenomena.

I happily accepted that the joule, as a definitive measure of energy is the amount of work done when a force of one newton moves a mass through a distance of one metre. This would amount to saying that one watt-second of work had been done. I would remain happy to say that a falling stone, by virtue of its potential energy, had delivered 1 N of energy. Nor would I complain if you said that the dynamo had moved an electric charge of one coulomb through a potential difference of 1 volt, performing one coulomb volt of work, which remarkably, amounts to one watt-second.

Conversely I should feel unhappy saying that a falling stone is a dynamo, or even that the potential energy in the stone is the electromagnetic energy in the dynamo. There is nothing about the behaviour of the stone that says anything about electricity for example. That there is an underlying commonality is another matter. We can express all those things in terms of joules, and with due allowance for inefficiency, we can store or apply equal amounts of energy (ideally measured in joules) into or from any of those media.

Am I putting this more clearly or less? Is it becoming clearer why at some level we have great difficulty defining energy other than mathematically?

>Energy is defined not by what it IS but by what it CAN DO, is that right?<

For my money yes.

>And it can BE very different things?<

Now, there I fall off the bus. If I understand what you said correctly, that sounds to me like saying something like what I said a few lines back: "...that a falling stone is a dynamo, or even that the potential energy in the stone is the electromagnetic energy in the dynamo..."

I do accept that some abstract entity, or quality of the two systems may be the same, but the systems containing that quality emphatically are not the same. We could abstract their shared attribute in the way that an engineer might, when he does not care whether his power is to come from hydro electricity or solar power, as long as it is adequate and in other respects suitable for his purposes.

But he would have had to be a very, very unworldly engineer to imagine that he could drown in a solar cell. Conversely, having just struggled out of his hydroelectric dam, I would quickly be disabused of my opinion that it had nothing in common with his bank of solar cells, if I got identical electric shocks from both systems, or found that my electric lift could be powered identically from either system. Fair enough so far?

>Like, for example, calling something a "trigger" for what it does, not for what it is?<

Yes indeed, bearing in mind that you might well be alert enough to tell whether the trigger that I show you is the one made of bronze, rather than steel.

>But THEN what was so horrifyingly new and spectacular about saying that mass is also a form of energy?<

Remember that before Einstein first said that, there were two laws of conservation: conservation of mass, and conservation of energy. Mass was not energy and never could be. Energy was not mass, and never could be.

And the people who said so were no fools! They had established the reasons for saying such things in the face of great puzzles and mysticism. By that time the study of thermodynamics, as developed by some towering geniuses, had already achieved great sophistication. And so had a lot of other extremely recent branches of physics, such as chemistry. People were beginning to UNDERSTAND many things that had been so totally obscure; things by which in modern terms we could see that our undeniably intelligent forefathers simply had not understood the essence of the world they lived in.

And here comes Einstein and says... WHAT? Energy interconvertible with mass??

And the line of reasoning by which he came to that conclusion??

I am by no means certain that I would have liked Einstein had I known him, (much less, that he would have liked me) but I would dearly love to have had the kind of mind that enabled to him to conceive anything so stunningly, deeply, original; and to pursue that insight by logic, in the face of commonsense reality and without carrying on like a crank.

I don't know about you, but that impressed me out of my tiny mind! Remember: at that time no one had been civilised enough to explode any nukes to demonstrate the equivalence. Frankly I don't know why they go on doing it; I am already convinced, but some more highly civilised persons in power in various places still seem set on convincing themselves.

Oh well, who am I to countermand the Great and the Good?

Go well,

Jon