MigL

The force IS the differential pressure. It is caused by differences in temperature, currents or winds in the liquid or gas, movement and rotation of the vessel, etc. Stop trying to goad and drive the discussion towards your own ends and speculative ideas.

Gases and liquids move in response to differential pressure. Pressure 'pushes' from high pressure to lower pressure, and the resultant spiral direction of tornadoes or toilet bowls, is dependant on northern or southern hemisphere location. As studiot has already pointed out, using actual forces and Newtons laws of motion, the acceleration and force are inwards,into the centre of the vortex, and so centripetal in nature.

Mike, your students were not feeling the strain of a force acting on them, but the strain of having to exert a force on the spinning mass. Just semantics, but it makes a big difference.

I suppose you're right D.H., mixing GR and Newtonian gravity can lead to confusion, but I've had numerous conversations with Mike and he picks up things pretty quick. I just thought I'd cover all bases and use both models to describe the same effect. The main point is that gravity, whether real ( Newtonian ) or fictitious ( space-time curvature in GR ) is the only active force and is centripetal in nature ( force vector is always pointing in ).

Mike, you've got to understand that an orbit is no different from a fall, a very extended one which never quite ends. And if you are falling you don't feel the force of gravity. There is no need for 'balancing' with a so-called centrifugial force. A body's natural state of motion is along a geodesic, which in the absence of curved space-time, is a straight line. Once a body enters an area of curved space-time, ie feels the force of a gravitating mass, that body is accelerated towards the centre of that mass, ie it falls or followes a 'curved' geodesic. if its original speed is optimized to the force ( gravity is centripetal ) or acceleration, the result is a stable orbit. There is no need for any other ( fictitious ) forces to account for this effect.

I happen to like what LQG attempts to do, although it doesn't get nearly as much exposure as SString/M theory. It preserves the uniqueness of GR in having no need for an absolute frame, or a background stage, if you will, on which things happen, Superstring theory on the other hand, still requires an absolute frame as far as I know. Carlo Rovelli wrote an excellent article for Scientific American ( I think, not sure ) which explains in lay terms the essentials of LQG. A google search may bring it up; I saw it several yrs ago.

Wether finite or infinite we have to assume the universe is unbounded. This is simple to see in the case of an infinite universe as no boundary or 'edge' would ever be reached. A finite universe without a boundary or 'edge' on the other hand, would have to close in on itself. This is easy to picture in the case of a 2D surface embedded in 3 dimensions, such as a sphere, but I assure you it is possible in higher dimensions without embedding. The reason for the unbounded nature of the universe is, of course, to avoid explaning what's on the other side of the boundary, and if you can keep going, is it an actual boundary or just another part of the universe ?

The units are important to the question. What John is trying to get at is called dimensional analysis. You cannot have different units on either side of an 'equal' sign. You had force (units)=mass(units), which is nonsensical. This is an easy way to check your results.

Is that so, Efanton ? Consider the set of real numbers extending to infinity. Now consider a second set of only even numbers also extending to infinity. Each number from the first set can be placed in a one-to-one correspondence with a number from the second set, ie second set members are first set members doubled. Both sets are by definition infinite, yet the second set has doubled the spacing between members, ie it has expanded by two. Your notion of infinity and its consequences is flawed.

To add further to Enthalpy's point, ALL particles follow Newton's laws of motion , except at high energies and speeds and extreme space-time curvature. At a certain scale however, quantum mechanics imposes a sort of 'smearing-out' effect on certain observables due to its probabilistic nature.

Sorry J.C., my original ( too quick ) analysis considered a very large sail only 'catching' the fan's flow. If it redirects it in the opposite direction, and we consider final momentums, then yes there is a net thrust and I'm wrong. Not the first time and I'm sure it won't be the last. Incidentally, this is the way thrust vectoring and reversers work on aircraft.

You are correct in that momentum is always conserved J.C., and that is exactly why a fan located on the boat will not generate any net thrust onto the sail at all. As for tacking, it will not work with a flat bottom boat, since even if the sail is used as an airfoil, it is impossible to generate a net thrust at greater than a perpendicular angle. Tacking only works when you have a 'sail' on the bottom of the boat also. Without this keel/sail interaction tacking is impossible.

Understand that physics is about model building, Robin and Kramer. We build models so as to make valid predictions of effects and reality. If we say an elementary particle like an electron sometimes behaves like a wave, that leads to certain useful predictions, but is it really a wave ? If we say sometimes it acts as a classical particle, does that mean it is a tiny billiard ball ? We can definitely say it is a quantum particle, but we have no common ground with everyday items and things. I have, in previous posts, given examples of some models. I'll give you another. If general relativity is an accurate description of reality except at certain boundaries like extremely small separations and extreme energies, then two gentlemen in the 1920s, named Kaluza and Klein decided to incorporate a fifth dimension to the space-time dimensions of GR. This 5th dimension would be compacted at the Planck scale and in effect, be invisible. To everyone's amazement all of Maxwell's equations describing electromagnetism 'popped out'. So does that mean that the charge of an electron could be an effect of the compacted 5th dimension. And if so how would you describe it, then ? It is certainly not a physical difference such as size, shape or composition. This approach of adding 'extra' dimensions is still used today, but to account for forces which were unknown in the 1920s ( the weak and strong nuclear ), the number of dimensions has risen to 11 in modern string theory ( see swansont's caveat in a previous post regarding string theory ).

Type 1A supernovae have the same absolute brightness, ie. the infalling mass that converts a white dwarf star to a neutron star occurrs at the same total mass every time, and we can deduce the distance from the apparent brightness. This is the only distance since the 'actual' distance you refer to is meaningless. Space-time is a four dimensional manifold, and as such it can have positive or negative curvature, but it doesn't have to be embedded in another dimension, so I don't know what you mean by the 'void' at the centre of the Big Bang. There is no centre to a positively curved universe because that is not part of the manifold.

Why do you assume the difference has to be physical Robin ? The abstract definition of charge is the generator of the U(1) symmetry of electromagnetism, such that, as per Noether's theorem, the electric current is conserved. The associated ( radiated ) gauge field, when quantised, is the gauge boson known as a photon. Does this sound at all like an observable, physical difference ?

Below the lowest energy level, ie below the ground state, of every atom, there exists a sea of electrons, also known as the Dirac sea, and wherever an electro is absent in this sea, the 'hole' is termed a positron. In effect, a positron is the absence of an electron. This is how P.A.M. Dirac first postulated the positron in the 30s. The general idea has somewhat changed since then. Or it could be as swansont has suggested, both are dimensionless points but one is red and the other blue, corresponding to their different charges. But as far as the rest of us are concerned, the only difference between the two is their respective charges. If that still doesn't answer your question maybe you should be asking 'What is charge ?'

You're right Dekan, your interpretation of the Big Bang sounds extremely unlikely. The scientifically accepted version, however, not so much. It is however a theory, which although not proven, has substantial observational backing and certainly deserves further development, unlike the 'dead' Steady-State theory. For the correct interpretation of the Big Bang theory, either consult an elementary cosmology text, or take the information given to you by some of the members of this board. You'll soon find out which are the educated and knowledgeable ones and which are the cranks with no scientific background, just trying to peddle their speculative ( but wrong ) theories.

Even after many people have just finished correcting all your misconceptions about the Big Bang, you keep right on believing that its a flawed theory which will be replaced by a modified Steady State. Really ??? Did you even bother to read their posts Dekan ?

Georgi, stop writing gibberish. Even elementary texts in cosmology and particle physics will set you straight, if you bother to read them.

Make all the river analogies you want as no-one ever said time WAS a river. Just like an electron is not a small aphere or a wave, but it can at times act as either depending on the type of experiment, so will time act like a river under certain considerations. However what 'flows' is actually the 'now moment', at one second per second usually, since all events in space-time are fixed, past, present and future. This 'now moment' is like a vertical slit moving across a window pane as it moves foreward in time and the view changes ( we remember the past but have no recollection of future events ), however it moves foreward at differing speeds along the length of the slit due to differing conditions and the finite speed of light ( there is no universal now ), just like river currents are different due to differing conditions in the cross-section of the flow. This is just an opinion of course, but you do see how difficult it is to describe complex phenomena like time using simple, everyday analogies ?

As simply as possible... Draw a space-time diagram where the X-axis denotes position and has units of 300000 km, and the Y-axis represents time with units of seconds. A vertical line such as X=constant, indicates stationary position. A horizontal line, Y=constant, indicates stationary in time. Note that a light cone has, in this instance, a V-shaped slope of +/-1 ( 45 degree ), and while a vertical line is perfectly acceptable, a horizontal one is NOT since it falls outside the light cone and implies FTL motion. As swansont and elfmotat have hinted at, this applies to events and frames where v<c, not to massless particles such as photons where v=c.

Are you sure about that Michel123456 ??? Its possible ( in some specific cases ) to be stationary in space. How would you go about being 'stationary' in time, then ???

So how do you assume stars form Dekan ????? And for that matter, given sufficient gravity, the electrons of an atom, even hydrogen, are forced into the nucleus ( or proton ) to form neutronium. Or how do you assume neutron stars form ??? And have you never heard of the strong nuclear force ???

Really, John C., how dare you inject a modicum of common sense and knowledge into this fairy tale that these ' gravity pushes' adherents are peddling, Don't you know that their unsupported statements are more valid than your statements supported by accepted scientific facts ? Really, when I need a good laugh, I read the speculations forum. Its better than reading Lewis Carroll, nothing makes sense. And they cannot be convinced.

Oh, come on !! A little thinking before posting goes a long way.