MigL

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.

Do all stars really ignite at the same temperature, or pressure, as has been suggested ? I would think the make-up of a star would be a big factor. A blue giant as opposed to a red giant, population I as opposed to population II, hydrogen rich as opposed to helium rich, or spiral arm location as opposed to galactic core location. A core with a high percentage of helium will require a much higher temp to ignite and I would assume, as a result, that red giants would be much more common than blue giants.

I don't recall being taught that in any of my quantum mechanics courses !

Do you know what Quantum Field Theory, QFT, is ? It accounts for your quark/gluon interactions and is known as Quantum Chromodynamics. Look it up.

Don't follow your reasoning, splitinfinity, to put it mildly. Both GR and QFT make do with four dimensions and do an adequate job of explaning reality except for areas where they overlap. I wouldn't say that any more dimensions are necessarily needed. Strimg/M-theory uses 7 compacted dimensions along with the usual four, but its along way from making any verifiable predictions and dark energy is another name for the cosmological constant as a result of vacuum energy

We actually know quite a bit about dark energy. We know, for example, that it acts very much like Einstein's cosmological constant and is related to vacuum energy ( what you refer to as space or nothingness has very distinct properties ). The universe is not expanding into anything, as a matter of fact universe means everything there is. It does not matter if finite ir infinite as long as it is unbounded, otherwise you'd have to explain what's on the other side of the boundary. A sphere is finite in size but you can travel forever in any direction, ie its unbounded. Alternatively a sheetof infinite size is also unbounded for the same reason. The separation between objects in the universe ( on a scale where gravitational interactions are overcome ) is growing. That is what is meant by universal expansion, and yes, this separation growth can be superluminal. It is by no means an explosion of 'stuff' into an empty 'void'. But back to the OP... The singularity is nothing more than an indication that GR's field equations become 'undefined' at that point, ie. they are no longer a valid or sufficient description of reality. A theory which takes quantum behaviour into account is needed. See string/M-theory or Loop Quantum Gravity for some current efforts on that front. So yes, the possibility of not having a singularity are probabily very good.

You guys need to look up the definition of an inertial frame. A frame where you speed up to near c and then decelerate to 'sync-up' with earth's frame is not only NOT an inertial frame, but is also a different frame. Rule #1 of relativity: Do not mix frames ( as Swansont has pointed out several times now ).

The last emitted light from the big bang has overtaken the expansion of the universe already ( universal expansion only outpaced light during the inflationary phase ). It did so about 13.4 billion years ago, but since the universe has been expanding, this light has been shifted into the microwave region of the spectrum. We now call it CMB radiation, and it is 'visible' anywhere in the observable universe.