Severian

Hmm... maybe I shouldn't post that article on how to build a nuclear device from old washing-up bottles....

Yes, this is a big problem for us at the moment, since we have quite a few 17 year olds in our first year intake. I suspect that many universities will now have to refuse to take 17 yeaar olds. In our case, I suspect the lecturing staff will now have to do all first year tutorials since it will be too difficult to get the postgrads and postdocs covered. This isn't decided yet though - we are still in panic mode trying to figure out a way round it. Thankfully the university is picking up the tab. I for one will refuse to do the classes for the 13 year olds if they make me do a background check, purely out of principle.

In the UK we have recently passed a new law which says that anyone who has contact with a person under 18 needs to have had a background check done by the police who then issue a certificate of disclosure. This needs to be updated every 6 month, is quite expensive and take some time to do. At my university I have been organising events for school children at the weekend. 40 kids or so come to the university and we have demonstrations and hands on fun with physics. It is supposed to encourage them to choose to take physics in their subsequent years. Now we need to have disclosure for everyone involved, even thought there are about 5 adults with the 40 kids in one big room. This is a huge hassle to organise because it means that we need to know who is going to help a long time in advance. We are currently discussing whether we should continue to hold the classes, or now regard them as too much work and just call the whole thing off. If we call them off it will be an just another example of labour policy disdvantaging our education system.

Isn't his axiom 'FIN' in contradiction with the statement "making a measurement in one part of a quantum system can have an instantaneous effect on measurements made elsewhere in the system"? So 'FIN' and 'TWIN' are incompatible in the sense that he uses them. In other words, he is not allowing for the non-locality of Quantum mechanics. In fact, I would have said that his 'proof' is in fact a proof of non-locality, via reductio ad absurdum.

What does the fact that the beam is not turning as it is raised tell you?

I completely agree with this, and this is what I was getting at. Of course, these things can(?) be discussed in the philospohy and religion forum, but since they have absolutely nothing to do with science, they shouldn't be tolerated anywhere else on the boards. The other option would be to just move anything about creationism/ID into the religion forum (or maybe have a crackpot ID forum like we have for pseudoscience?).

No it doesn't. Science uses the assumption that the laws of physics don't change with time (or' date=' at least, only change in a predictable way). There is absolutely no reason why the universe couldn't have been created by an omnipotent (or very powerful) being 6,000 years ago, with every particle in exactly the same position it would have been if it had evolved from a big bang 13.7 billion years ago. If this was true, you would [b']never[/b] be able to distinguish it from a big bang originated universe - all the results of all experiments would be the same. Of course, this is very much against the principle of Ockham's razor, and I don't believe it for a second. Even worse, the creation statement itself is non-predictive. One cannot derive any testable result from it, so it is not scientific. This means that science can make no statement about ID whatsoever. However, being non-scientific does not mean it can't be true. I do argue with them, but I don't do in the context of my job. I am a professional scientist and it would be wrong to use my qualification as a claim that I have a 'superior' viewpoint. I am trained in science, but as I pointed out, science says nothing about ID. In this light, I also don't think it is right for scienceforums.net to capitalise on its perceived science expertise to comment on an area which science can say nothing about.

I still use IE. I used Firefox for a while at work but switched back again after a while.

In my bored internet wanderings I found myself at the Death to Creationism site ( http://www.deathtocreationism.com ) and I noticed the statement "DTC is a joint project between Sayonara.info and www.ScienceForums.net". I must confess, I am a little uncomfortable with this. I realise that it is not 'Death to Creationists' but I still think the title is too strong to be associated with a science site. Science makes no statement about creationism at all. While I am not a creationist, and I disagree with the creationist philosophy, it is up to them to believe what they want to believe, and, as a scientist, I don't feel it is my place to argue with them. (I can argue with them all I want of course, but not in my professional capacity.) What was the intention behind this?

How sophisticated is your maths? The most complete way to do this equation is the rigourous way. Define [math]\vec{i}[/math] and [math]\vec{j}[/math] as unit vectors in the horizontal and vertical directions respectively. Then the initial velocity is [math]\vec{v}_0 = (30 \vec{i}+20\vec{j}) {\rm ms}^{-1}[/math], while the stone's acceleration is [math]\vec{a} = -g\vec{j} = - 9.81 \vec{j} {\rm ms}^{-2}[/math]. Now solve [math]\vec{a} = \frac{d\vec{v}}{dt}[/math] to find the velocity [math]\vec{v}[/math] as a funtion of time (using your initial condition on [math]\vec{v}_0[/math]). Now write the position of the stone as [math]\vec{r}[/math] with an initial condition [math]\vec{r}_0 = 0[/math] and solve [math]\vec{v}(t)=\frac{d\vec{r}}{dt}[/math] for [math]\vec{r}[/math] as a function of time (using your initial condition on [math]\vec{r}_0[/math]). Finally, your stone hits the ground when [math]\vec{r} \cdot \vec{j}=0[/math]. Solve this for [math]t[/math] (neglecting the trivial [math]t=0[/math] solution, and you have your answer.

Work done for a constant force is [math] W= \vec{F} \cdot \vec{s} [/math] where [math]\vec{F}[/math] is the force and [math]\vec{s}[/math] is the displacement. On the way up, [math]\vec{F} = -mg \vec{j}[/math] and [math]\vec{s} = h \vec{j}[/math] where 'h' is the height it reaches and [math]\vec{j}[/math] is the unit vector in the 'up' direction, so [math]W_{\rm up} = -mgh[/math] On the way down, [math]\vec{F} = -mg \vec{j}[/math] as before but [math]\vec{s} = -h \vec{j}[/math], so [math]W_{\rm down} = mgh[/math] The total work done is [math]W_{\rm up}+W_{\rm down} = 0.[/math] Alternatively, the work done (for a varying force) is: [math] W = \int \vec{F} \cdot d\vec{s} = \int_0^h (-mg) dy + \int_h^0 (-mg) dy = mgh-mgh=0[/math].

A new site has recently come online at http://www.particlephysics.ac.uk all about particle physics. In particular, there are some quite nice (basic) presentations of the fundamental principles under the 'explore' heading. It hink it is worth a look for anyone who is interested in this sort of thing.

I would agree with that assertion 100%.

That is the inperpretation that I would made.

The question 'what is mass' is actually rather deep, and it is a question we are currently trying to answer at particle colliders. We think mass is caused by the Higgs boson, but this has not yet been confirmed experimentally. for a 'quasi-political' explanation of the Higgs boson, suitable for the general publiuc, see http://www.hep.ucl.ac.uk/~djm/higgsa.html

The article specifically said that they were not visible, but maybe they meant in terms of luminosity rather than wavelength. But even then, I would still expect more than this - any spectrum has a long tail.

Since gravity is a conservative force, there is no way in principle to get energy out of it. You always have to put the energy in to begin with. So I don't think this is what the teachers were meaning. (This is even true for petrol - you need to create the plant/animal matter via photosynthesis, then put it under huge pressure for a few million years, before digging it up to refine and use.)

I was alos a bit confused about his energy levels. He is claiming that there is a lower energy orbit for the hydrogen attom's electron which no-one has noticed before, but if this is true, why would the electrons in normal hydrongen not fall down to this lower level naturally?

It is interesting, especially if it has been demonstrated in the laboratory. Irrespective of his dodgy maths, the energy must be coming from somewhere, and it may well be that that 'somewhere' is a viable energy source. I suspect his calculations (and model) are wrong, but it is possible that the energy is coming from somewhere else. Could there be a topological solution to the QM equations which we have missed, for example? On the other hand, his experimental set-up might be rubbish and the energy is really being input into the system from elsewhere....

Not necessarily. He could have a store of boulders high up, and after releasing the first one, some mechanism which resets the trebuchet (using the energy from the first boulder) and releases the next boulder. Then, after the initial setup, not work need be done by the user. (Obviously you need to do work to set it up.)

Sorry - I had a typo. I got the sign wrong in the denominator. It should be correct now.

Yes, that is the equation for the addition of velocities in special relativity. You would only need general relativity if the train (or the bullet) was accelerating.

No - not at all. That is the whole point of special relativity - it alters the transformation connecting the measurements of different observers. The velocities of the train and the bullet no longer just add. If the bullet has speed [math]v_b[/math] in the train's rest frame (the frame the gun is stationary in) and the train travels at speed [math]v_t[/math] relative to the ground, then to someone standing next to the tracks (stationary with respect to the ground) the bullet will appear to go at a speed. [math] \frac{v_b+v_t}{1+v_bv_t/c^2}[/math] You can convince yourself that this is always <c.

Moving is its natural state. Remember, even Newton knew this - we only need a force if we want to accelerate it, but the speed of light is constant, so no force required. We can't really slow light down - often it is claimed that we can, because light travels through media at speeds <c, but this is a bit of a fake. What is really happening is the particles in the media are absorbing the light and re-emitting it again, and this takes time delaying the light. But between emission and absorption it is travelling at c. Also, it is a particle (the photon) and a wave at the same time. The speed of light itself is not a limiting velocity. There is a limiting velocity called 'c' and it just so happens that if something has zero mass then it will travel at this limiting velocity. The photon has zero mass, but then so does the gluon and the graviton. We could have called c 'the speed of gravity'. I am not sure what you are asking here... This should have been explained by my earlier answers.