studiot

I think you misunderstood my point.

Yes I agree, some good stuff in there. Though I would sound the same warning about the ancient greeks (esp Plato) that swansont has already made. I am suspicious that this is too easy. It is easy to be seduced by thoughts of "how things ought to be" to the detriment of actually looking to see how things really are. So I am rather worried about this idea. Anyway, back to gravity as a force or not. Thank you for the excellent discussion recently +1 I have been thinking further and I have to say that now occurs to me that I have already presented a test of sorts. Is there, even theoretically, a situation where gravity occurs, but no force is exerted ie no force occurs ? If it is the case that gravity can occur without a force then gravity cannot itself be a force. Gravity may give rise to a force and seems to do so in some (most) situations, but that is not the same thing. One further comment. The comment by your hero that gravity depends upon scale is surely in direct conflict with The Principle of Relativity due to Einstein ? That is about the proposition that the universe is isotropic and homogeous. I do agree with his statement about fuzzyness however

This I cannot understand, how can one approach at a minus velocity, they both see each other approaching at 0.8c? Maybe this is the big hiccup but... if I run towards you at 5 km per hour, then you are approaching me at 5km per hour, from your perspective you, I would be approaching you at 5km per hour. Ignoring that in this example I am obviously the one running, but let's say we are in space and neither of us is doing anything, we are just in motion. Why would one of us think the other person is going backwards (this is what negative velocity implies, or at least how I interpret it, but that is probably wrong then). You call it the relative velocity, but that relative velocity must remain a positive number right, otherwise one would think someone is receding while the other thinks they are approaching? Since you are listed as still online, let's try and deal withthis one quickly. Let us say that T is to the west of S and travelling towards S. That is T is travelling east. Then travel to the east is positive and travel to the west is negative. So the velocity of T is positive. However T may also himself as standing still and S to be travelling towards himself as he stands still. Then S must be travelling west in this view. So the velocity of S, as seen by T is then negative. Now for yourself work out the same situation with regard to the point of view of S.

Oh dear and Oh good, at least you understand something. Every object has a frame in which it is 'at rest'. We call this its own frame. So S is at rest in the S frame T is at rest in the T frame E is at rest in the E frame and so on. S sees other objects moving 'relative to itself' in the S frame T sees objects moving relative to itself in the T frame E sees objects moving relative to itself in the E frame and so on. If the velocity of such movements constant the frame is an inertial frame If the velocity is not constant then there is acceleration (or deceleration) and the frame is not an inertial frame. Now taking just two objects, T and S, I said "The relative velocity between them" and you said "not moving at the same speed relative to each other" The point of relativity is that whatever T sees in the T frame, S sees in the S frame, but in reverse. So if T sees S approaching at velocity v (in this case 0.8c), then S sees T appoaching at -v (or -0.8c) (Note that we are saying T sees S approaching, it does not see itself as travelling it sees itself as at rest) Which is what I said, but not what you said. It is important to get this straight, before moving on to proper distance and proper length. v is the velocity you enter into the formulae for relativity and is called the relative velocity between T and S. (Note that observers in other frames will observe a different value for v) If you look back at my maths you will see two things. 1) The calculations come to the same figure whether you use v or -v since it is squared. If v has a value other than zero T and S are moving relative to each other but calculations in either frame will give the same result ie S measures the same distance to T as T measures to S. 2) If v = 0 then T and S are not moving relative to each other, and the measured distance between T and S is a maximum. This distance between them is not changing with time as the movement occurs. This maximum distance is called the proper distance. I will state that the proper distance has the same numerical value measured by all observers, not just T and S. Since T and S are not moving then they are in the same frame. That is we have reduced or transformed the motion of T to the S frame or S to the T frame. The proper distance is the distance measured between any two points in the same frame, ie at rest with rspect to each other.

I don't know if you understand what is meant by a traditional line vector force ? Gravity is a distributed force, also called a body force. This means it acts on each an every particle of a body individually. The combined effect of all these individual actions can often be modelled as a single line vector, acting therough the centre of gravity. So we have that bane of the schoolboy, 'a brick or block resting on a table', behaves (to use swansont's word) like this sort of line vector. But the dam I offered way back could well topple over if it has been designed as though the dam behaviour under the influence of gravity was the same as that of the block. It is necessary to consider the distribution of forces in such a case. So where does that bring us to ?

No your history is correct, but incomplete. No there is no universal definition of force, it needs qualifiers ( basically selective filters of interactions) An electromotive force creates current in an electric circuit. Does gravity do this ? Are there other differences between an EMF and gravity ? In an isolated universe containing one single solitary massive body ( a massive body is a body with mass, not necessarily a large mass) there is expectd to be gravity, but is there any mchanical force ? So the contact boys have a point. It takes two to tango. A force is applied between at least two bodies. Gravity can do this when our test universe contains at least two bodies. But how does it do this ? Gravity is a distributed force, so if the region of operation is large can it be considered the same as a traditional line vector force ? You see I am being 'scientific' in trying to narrow things down, using my preferred method of considering interactions. What about action at a distance? Well consider a small ring (small means small enough for our considerations) How does gravity act on it ? Gravity acts through the 'centre of gravity' of the ring, which is actually in the middle of the hole, and not a material part of the ring at all.

So we come right back to my original question How do we say what we mean by a force ? Possibly our recent discussion leads to the suggestion that one good way to a fruitful discussion is to consider how the force interacts with other better known (defined) quantities. Then we can limit our consideration to those quantities of interest, and state that "for our purposes", whatever those purposes may be, the quantity under consideration (gravity) is or is not a force.

Another problem with 'real ', which can be overcome in English but less well in Philosophy or Science is the fact that nouns come in many guises. Some nouns are simple and occur all at once, at the same time and in the same place. For example a rich tea biscuit. Others develop over time. This leads to the moral argument about when is an embryo / a foetus/ a human. Yet others change over time but are always recognisable, for example the answer to the question What is the gravitational field of an object, varies depending upon what place and time you are asking it about.

But some (Stokes) never believed in the aether. And yet others held an open mind. Yes they wrote mathematical works about it. Stoke's maths is interesting because he discusses the application of negative mass to gravity in that work. There is a summary in Berkson, that I referenced earlier. Let me play philosopher, or at least what I call the philosopher's favourite game; that of combining at least two incompatible statements to make a paradox. I offer that there is no such object as a four sided triangle.

Has that concept not affected many people for several centuries in their search for it, like your holy grail or perhaps the Physicist's TOE? I hope you will agree this is a more difficult question.

I'm glad you said reasoning ie rational thought. Yes Harry Potter was merely an example to show why the terms 'real' and 'exist' are not as tightly set at might first appear. This is also why I hold that English is more versatile than either Science or Philosophy because it can allow the existence of Harry Potter as an abstract concept. Science, of course, has branches that study the effects of HP on the 'real world'. As a matter of interest can you not think of a better offering , whose unreality is undisputed, to point 1,

Sounds good. 5. Many people have been affected in other ways, whatever they believe about HP. For instance many people have gained $millions on account of HP. Some people, who have never heard of HP and have no beliefs about him, have had their lives saved by charities funded through HP related donations. As a matter of interest, related to earlier discussion on belief in this thread, which definition of believe are you using ?

From a thermodynamic point of view per Caratheodory's version of the Second Law is appropriate. "Not all states are accessible, from a given state." (the shortened version) From a QM point of view there are electron orbital (bound) state transitions that are inaccessible. These are known as 'forbidden transitions'

The terms real and reality ; exist and existence can be thought of as ill defined. Instead of asking if something is 'real' can you offer me something that is not 'real'. How would I test this ? Can something which is not 'real' affect some things that are 'real' in any way ? Is Harry Potter Real ? Harry Potter has certainly affected millions of 'real' people. This, I would suggest, is the real reason behind why Science shies away from such terms (pun intended)

Let us do this a little bit at a time. Then it might 'click' for you. Did you understand lines 1, 2, and 3 above ? In particular did you understand how you get from line 2 to line 3 ?

If the entire cell was at absolute zero then every electron would be in its lowest or ground state, by the definition of absolute zero. So any change to even the state of a single electron would involve raising the temperature above absolute zero, something you have forbidden.

That's a cheap trick. I didn't say 'all' and I didn't imply they would say exactly the same thing, I stated that the circumstances would have to be exactly the same The aims of scientists at all times and in all places is to too wide ranging and multipartite to be a useful measure of anything. All to often I find Philosophical discussion lacking because it tries to tar too many monkeys with one brush and ends up with a paradox.

Yes different scientists often express apparently different views, but ask them both (all) to comment on exactly the same circumstances and they will most likely say the same thing. That 'thing' may be later found to be incorrect and Science as a whole will incorporate the new knowledge and no one would then support a contradictory view. For example there was general concensus that life could not exist and develop around the deep ocean geothermal vents, yet no one would deny that possibility today.

Wouldn't that be off topic anyway ? These are a particularly poignant example because they are endowed with mass. This 'type' of mass is called effective mass in this case and is a sort of inertial mass. And of course you can't really discuss gravitation without gravitational mass. It is one of those mysteries that Science has not managed to answer How does gravitational mass come to accord exactly with inertial mass.

Not sure this is relevant, but what about wave/particle duality. Your appeal to logic should be replaced by an appeal to rational thinking, which has much wider applicability. For example my red post box. The box is red because the enamel coat reflects incident red light and absorbs the rest. So when illuminated by a light source that includes red light the box appears red otherwise it does not reflect any light and so appears black. Normally light sources such as the Sun or car headlights include red light so showing the box as red. Sodium street lights do not contain red, they contain only a particular shade of yellow. So illumination by a sodium street light will show the box as black. Yes I agree that the two theories have underlying compatibilities, but so what if what you actually want to do is navigate to Australia? Navigators choose the geocentric theory as the most convenient for calculation, although they know it to contain a false premise. But there are other possibilities for two theories. The more complicated theory may actually reduce to the simpler one in certain circumstances, which is the case with GR and Newton. In the case of statics (my dam for instance) GR and Newton are identical.

A large number yes, but enough to cover the entire spectrum as claimed ? I agree there are many real world constraints on the process of conversion of the energy of photons to electron current and have nothing realy to add to your great job describing these.

What on Earth do you mean by that ? Surely you are not proposing a periodic table with infinitely many different elements ?

No I have not come across this one. Is it online and do you have a reference ? I do agree that studying the History and of Science and the Philosophy of Science can help understand the subject of Science itself, particularly looking at 'what was known at a particular time/stage and what thinking did it lead to' . Some Scientists (eg Millikan) , some Science Historians (eg Sarah Dry) and Some Philosophers (eg Berkson) and some Mathematicians (eg John Derbyshire) have been able to write briiliant expositions looking at these particular parameters. Were you going to respond to my previous post ? Edit, I just noticed that you provided a link to a rather expensive commercial site that seems more interested in making money than disseminating Science.

Can I just tell you something before I reply ? The use of the @ symbol on this site, in front of the member name, works to provide a notification to that member of a reply. But you have to do it correctly. Type the @symbol. Then start typing the member name. (case is important here) The site will offer a drop down box with suggested members that is adjusted as you type more letters. Select the appropriate member. The result will look like this @Davy_Jones But if you use the quote function from their post the member will also be notified. Hope this helps Now to your question. Thank you for the reply. That's cool and I have a better idea of where you are coming from. You also seem to be more au fairt with Philosophy authors or scientists discussing the philosophy of science. As such have you heard of Berkson ? He has written a philosophical book on this subject Berkson : Fields of Force : Routledge. My own answer is that that 'It depends upon the circumstances'. Consider a standard red post box such as you will find all over the UK. This one is situated under a street lamp with a sodium bulb. What colour is it ? Answer, it depends upon circumstances. In the day it is red, in the night it is black. For your question about gravity, there are many more circumstance. It depends upon scale and distance amongst other things. For instance if you wished to consider the structural mechanics of a dam, Newtonian force analysis is 100% accurate, whilst GR will not help you one bit. So in those circumstances gravity is a distributed force. In the circumstances controlling the motion of the planet Mercury, Newtonian force analysis is less than 100% accurate, whilst the GR view is nearly so, to the limits of our measurement capability. Does this move the discussion forward ?

I am going to take you up on this because, as ever, things are more complicated than this over exaggerated popsci ideal. Yes theories must match any observations in their domain of applicability but, There are situations (theories) where we have learned to make a transformation of a difficult problem to an easier one, solve the easier one and make a reverse transformation back to the original. These techniques (theories) can be be very successful and as accurate as required. It is often stated in popsci that relativity is the most tested theory or QM is the most tested theory. Again this is an exaggeration. The theories I am referring to have never been know to fail, yet they premise upon which they are founded is known to be imaginary. I refer to such techniques as Virtual Work, Maxwells Mesh Method, the theory of logarithms, and many more. Thank you for your response I did not ask you to answer this question You asked specifically and I responded specifically to this request with a request for clarification to avoid discussion at cross purposes, which most of this thread has been. Note I did not ask you to define a force or state what a force is. But I did offer reasons for my quandrary because you had already pointed out that different scientists, in different situations, use the term force in different ways. This is a fair and reasonable observation that deserves proper discussion. So I just wanted to get us on the same page as to the sort of force we were talking about. I was also trying to acceed to your request about using layman's terms, since laymen are even more hazy about forces than scientists.