Severian

Let's try and be a little more scientific with our references shall we? http://arxiv.org/abs/hep-ph/9910207 Notice that even the authors admit that two time dimensions is very unconventional and point out that it is really hard to accommodate. Also notice that the extra time diemsion is introduced by having a metric (1,1,-1,-1,-1): the '1's denote the time dimensions, as it is the signature of the metric which is the only thing that differentiates 'space' from 'time' (the usual metric being (1,-1,-1,-1)).

Has it been proven (genetically) that it is genuinly a different species? I had always thought that there were tribes of pygmies in South America. Why is this not just a slightly more extreme example?

The equation [math]\Delta E \Delta t > \hbar[/math] is saying that the system state does not simultaneously have a definite energy and definite time (ie. is not an eigenstate of both momentum and position operators). This has nothing to to with multiple time dimensions. If you make a measurement of a particle's momentum, then it will have an undefined position, and if you make a measurement of its position, then it will have an undefined momentum. But there is still only one time coordinate - you just don't know what it is for the particle. This is Everett's many worlds interpretation of quantum mechanics, which is by no means standard. It is also untestible since you cannot in principle return to a 'time line'. In essence, it claims that when you make a quantum measurement, rather than having one 'random' outcome (as per the Copenhagen interpretation) you have all outcomes because the universe splits into different 'time lines' each of which contains a different outcome. Then it is only which one of these you choose to follow which is random. But this should not be interpreted as an extra time dimension since it has none of the properties of 'time' in the traditional space-time sense. For example, you cannot make a Lorentz rotation from one time variable to the other. Indeed, you could could equally (un)justifiably call it a space dimension. Feynman's path integral formalism also includes a 'sum over histories' where all possible things happen, but are weighted by the exponential of i times the 'action' for the path. But again, even though it contains multiple paths, it still only has one time dimension. I have no objection to people not knowing what they are talking about (after all, one of the main goals of this site is to teach people about science), but you should not pretend to know about things you do not. You are just confusing people by presenting ill thought out and misunderstood ideas as fact.

Hassle is one problem of course - it takes ages to get one, so you can't just go to the US for a quickly organised business trip - it has to be organised months in advance. Secondly there is cost. To get a US visa, you have to call a visa line at the US embassy (you can't go in person). This line costs something like $2 a minute and they keep you on hold for half an hour, before cutting you off so that you have to call back. And then they charge you for the visa itself! True American capitalism in action. Finally there is the issue that they don't give them out to everyone. As I said earlier, I have a friend who lives in France but has an Algerian passport. He is a distinguished physics professor in Paris, and is often invited to give plenary talks at big conferences in the US. Unfortunately he cannot go, because the US won't give him a visa. They give no explanation why.

I think we should round up all Libertarians and put them on an uninhabited island somewhere in the pacific and see how they get on with no government.

In that case, if standards are increasing why do each new year's intake of physics students know less than the year before? This is becoming a serious problem for Universities: more basic courses now have to be offered in order to boost the knowledge of the students up to the standard of previous years. It is the Univeristies who have to go to extra effort (for no extra income) to teach things which should have been taught at school and it is the Univeristies who are blamed if the standards are not met.

Really? I didn't know that. That makes it even worse - to know that even the moderates in your country don't want me to visit. To be honest, it is not so much that I need a new passport - I could get one faily easily and at a cost of only £120. However, I have colleagues who are well respected scientists but cannot get a visa to the US and therefore cannot attend any conferences held there. So, as a matter of principle, I won't go either. LCWS05 will be held in Stanford next year; unfortunately it doesn't look like I will be going.

This will make it very hard to be sympathetic to Americans in the future. I will be cancelling all my planned trips to the US for the immediate future....

That's not what they are reporting here. The exit poll forcasts are claiming that Ohio and Penn will go to Kerry, most probably giving him the win.

What do you mean 'how long we have aged'? The 8 minutes or 10 billion years is how long it has taken the light to reach us. Why not? (Other than the fact that the Earth probably wouldn't be there anymore...) You are correct that the universe is 14 billion years old or so, but I don't get why you say the photon has to go back to where it started? What imaginary particles?

Yes of course. Just like you have a redshift from a gravitational field. I meant that it is not the acceleration in the twin paradox which is causing the age difference since the transition from v to -v is supposed to be intantaneous...

Ah, but maybe its a double bluff. Osama knows you will think that, so he says this in order to make you vote for Kerry. And OMG it is working!

The acceleration itself has no effect on the clocks, it is only the velocity which matters. But the twin who goes away and comes back again must change speed at some point. Even while in a closed spaceship with no windows he would know that he was the one who moved because he was the one who felt the acceleration. Edit: I just noticed I had a typo in post 7. I said acceleration at one point when I meant velocity. Maybe this is the source of confusion. Apologies, I have corrected it now.

If they both move away at the same speed, turn around and come back, they will be the same age.

Yes there is. They are indistinguishable. That was the whole point of GR' date=' that one cannot tell whether one is in an accelerating frame, or under the influence of a force. The travelling twin's velocity is not in the same direction all the time. If he goes round in a wide loop he will feel a force to the left or right. Also, he has to reneter the potential well of the planet (or where-ever the first twin is) and will be aware of this extra force. All that is required by the problem is to break the symmetry between the twins. As long as they experience something different, there is no paradox.

The weak force has no role in binding the nucleus together, but it does cause radioactive decay of the nuetron. This is discussed in another thread: http://www.scienceforums.net/forums/showthread.php?t=6972

Put simply, you can tell the difference between the twin's motion. The one who goes away and comes back again has to decelerate to turn round (and accelerate again afterwards). In his frame, that deceleration looks like a force (so he would feel like he has weight in the direction of motion). The other twin (who doesn't move) feels no force.

Yes they do, and that is what that is. The half life is defined by imagining you had a large number of protons, and counting how long it takes for half of them to have decayed. Since no-one has seen even one proton decay, the half-life must be veeery long. When you consider that the age of the universe so far 1.4x1010 years, the limit on the half life is pretty strong... Protons aren't fundamental particles by the way.

Before, obviously! The 10^34 years is only a limit. No-one has ever seen a proton decay, but that doesn't mean that it does not - just that it is incredibly unlikely.

I suggest that we shake up the physics forum categories a little. At the moment there is a lot of overlap, so the same questions tend to pop up in different fora. At the mo', we have: Classical Mechanics Vector forces, gravity, acceleration, and other facets of mechanics Quantum Mechanics Elementary particle physics and related topics Quantum Gravity Relativity black holes and GR Modern/Theoretical Physics Atomic structure, nuclear physics, etc neutrons Astronomy and Cosmology Topics related to observation of space I particluarly object to discussing particle physics under the heading quantum mechanics. I would suggest: Classical Physics Special and General Relativity Astrophysics and Cosmology Quantum Gravity (LQG and string theory) Elementary Particle Physics Atomic and nuclear physics Classical Physics is better than Classical Mechanics, because people may want to discuss topics in classical physics which they don't regard as mechanical. Quantum mech will naturally fall into particle physics, and particle physics will have a well defined place rather than always falling between 'Quantum mechanics' and 'Modern/theoretical physics'. You will also keep Martin happy with the new section on Quantum Gravity. If you think that is too many, you could consider having relativity in the Astro section as 'Astrophysics, Cosmology and Relativity'. This is maybe too much work of course, but I thought I would input my 2c....

To be honest I am probably not the person to ask, since I would not visit sites for the 'layman'. It is rather difficult to find anything which is suitable because once one goes beyond the simple '3 quarks in a proton' sort of thing, one really needs quite abstract maths. On a simple level, the particle adventure is a good place to start, but might be too basic. There are some links to other sites too. Perhaps you could also try CERN too? There is also an equiring minds site at Fermilab. For something more advanced you need to go to text books. 'Quarks and Leptons' by Halzen and Martin is a good place to start, but bear in mind that it is a textbook. This is the simplest textbook that I can think of which is reasonably rigourous. It doesn't really do justice to the beauty of the Standard Model though; for that you would need something much more advanced, like 'An Introduction to Quantum Field Theory' by Peskin and Schroeder. This latter is quite advanced mind - it is a book I would give to a starting PhD student.

You have to remember why the three quarks bind together in the first place. In electromagnetism (QED) the particles have an electric charge (which may be zero): they like to bind together into objects which are neutral. The 'color force' (QCD) has three colors, so it is not quite as simple as QED. An object is colour neutral if it has all three colors equally balanced. So a proton is a uud composite where each quark has a different color, and a neutron is likewise a udd composite with no net color. (The 3 colors of QCD is why baryons have 3 quarks.) Now, if they were bound together at a point, they would be color neutral and there would be no further effects from QCD. It would not contribute to binding nuclei together. But they are not bound at a point, so if you are standing(!) close enough to the proton, the color force from the closer quark will not be perfectly balanced by the others (since they are further away) and therefore you will feel a net color force. But since this is a 'residual' force, it will be much weaker than the raw QCD. This is what binds nuclei together. Since the neuton is, in terms of residual QCD, almost identical to the proton, it plays just as important a roll in holding together the nuclei. In fact, as someone pointed out, it is easier to bind together since you don't have to overcome the repulsion of too like charge protons....