swansont

Same concept, but somewhat different application. In the photoelectric effect you ionize an atom. Photovoltaics use semiconductors that have a conduction band, and the photon can excite an electron into it. Same concept, but for a more complex system.

I would guess that the top trays freeze faster, because they are uncovered, and this may affect how the iced adheres to the tray. Merged post follows: Consecutive posts merged The protruding part is already frozen, as is the top of the cube, so they aren't prone to bend. It's not the same as an icicle that forms from water being added to the end.

Remember that we tend to idealize conditions in these problems, ignoring many real-world complications. Spacetime is flat, clocks are perfect, etc. asprung is not focusing on those issues, AFAICT. It appears to be an issue of imposing an absolute reference frame, which does not exist.

Because the traveler doesn't think his clock is running slow. In his frame it's fine — he's at rest, and the whole rest of the universe is moving at high speed. But they both have to be able to say that d = vt — they both have to agree that the physics works, and also that the traveler arrived at the destination just as he died. Merged post follows: Consecutive posts merged You've written down the general equations, and need to apply the problem conditions to them. [math]x=\gamma (x' + vt')[/math] That's a general transform between the two frames, i.e. how to find coordinate x in terms of x', with the frames moving at a relative speed of v, as a function of time in the transformed frame. But if I don't care how it evolves in time, I can take a "snapshot" of the two coordinate systems at t=0, which tells me that [math]x=\gamma x'[/math] if we share a common origin. So you know that at the end of the trip of length L, [math]L=\gamma L'[/math], which is the general length contraction fomula Or, I can see how one particular location evolves in time. Let's choose this to be the origin. Why? Because we can — choose convenient coordinate systems to make the math easier. The traveler is at the origin of his reference frame. How does that match up at the end of the trip? i.e. where is that point in the other coordinate system? [math]x=\gamma (x' + vt')[/math] so x'=0, and we get [math]x=\gamma vt'[/math] , with x=100ly and t' = 20 years, which (rearranged) is the formula I used in post 2 [math]\gamma v = 5c[/math] From there on it's algebra

In DC, the "opposing" current in the ring decreases with time, since it's proportional to the rate of change of the field. But it's still there with the AC, since the field is always changing. It's sinusoidal for AC, vs an exponential buildup curve for the DC, whose slope is decreasing with time. The direction of the current doesn't matter — the force will always be repulsive, so the AC case has a continual force that looks like a rectified cosine (it lags the current in phase), so it's only going to zero for brief instants. This may also be the limit on a DC actuator

My guess would be yes, from a simple economics analysis. The cost of the product is going to be a combination of labor, parts and energy. A CFL costs a few dollars more than an incandescent bulb, and even if that's all due to energy costs, the savings over the life of the bulb is much more. If electricity is $0.10 per kWh, you can save the cost difference of the bulb in about 1000 hours of use, i.e. in a year at 3 hours per day. Also consider that CFLs can last 3 times as long (if you deploy them in the right areas, e.g. where they are normally left on for a period of time) so you really need to compare the cost of 2 or 3 incandescent bulbs.

Ice expands when it freezes, and does so from the top down, because it's less dense than the liquid water. Which means that during the freezing process, an ice cube will be frozen on the outside but liquid in the middle, and the liquid wants to expand as it freezes. So once in a while, you'll have the conditions where it expands in a narrow spike rather than more isotropically. You might notice that while the liquid water gives you a flat surface, ice will be uneven, and occasionally very bumpy. The ice tray makes it "buckle" somewhat when it tries to expand, but can't because of the ice tray walls. That can give you a weak spot where the spike is forced out. There are probably other conditions that can cause this as well.

Possibly. Low potassium is one cause of cramping. http://www-comdev.ag.ohio-state.edu/~news/story.php?id=2470

Welcome to relativity. Here's something that helps me keep my head from exploding. Everybody has to agree on what happened, even if they don't agree on issues of simultaneity. The fixed observer (A) sees the rocket go 100 LY at some speed. The rocket passenger (B) thinks it took 20 years. But they have to agree on the physics. They reconcile this by A claiming that B's clock ran slow, and B thinking the trip was a shorter distance, both by the same factor. So yes, L is the distance (the convention I'm used to for length contraction). All I've done is write down x'=v't' for the moving frame. We know the relationship between x and x' and that's useful because x is known. v and v' are the same (I was sloppy with notation here because of that, sorry), and t' is a given. The unknown terms are all related to speed. We figure out that gamma = 5.1, then we know that A's clock reads 102 years, and B thinks he went 19.6 LY. (And you also discover that rounding errors have huge effects in these problems) They both agree on the speed, since that's symmetric in the problem

There's always the layer of cheese nip dust Merged post follows: Consecutive posts merged Where does Phi find the time to do that, and all of the hair styling? Simply amazing.

There's a whole thread dedicated to Rovelli's paper IIRC he's advocating getting rid of time in a treatment of quantum gravity, not in all of physics. And by parameterizing it.

After lose/loose, I think the next one on my list is moot/mute, and then jibe/jive.

You enjoy embroidery?

Your idea of simultaneity, as you've previously expressed it, is not how it is defined in relativity. There really can be no intelligent discussion of the subject if you insist on using nonstandard definitions, because we will spend all of our effort untangling semantical issues. The equations to disprove are the relativistic equations of motion.

By inspection it's going to be a little over 5Gamma of dilation/contraction (100 LY in 20 years at c, means the distance has to be contracted to 20 LY, but he can't travel at c) The dilated distance the traveler sees is his speed multiplied by the time in his frame, which we know to be 20 years [math]\frac{L}{\gamma} = vt'[/math] [math]\gamma v = \frac{L}{t'} = 5c[/math] from there it's algebra (I get v = .9805 c, or gamma = 5.1)

"Speed of the force" is a bit of an awkward phrase. A force is a force, but the analysis using force alone may not be appropriate. Applications of conservation of momentum and conservation of energy may tell you more than a force analysis. i.e. the time during which a force acts (momentum), or the distance through which it acts (work/energy) may give you more information than simply looking at the force.

They could, but they generally don't.

If you are expanding it the intensity is going to decrease. But yes, I think you need to use mirrors for something on this scale. It should be cheaper than lenses, unless you can get Fresnel lenses that big (I've never run across any bigger than ~ a meter)

The laws are the same. How things behave as a result is different. Take diffraction as an example. Things diffract when going through a slit or around a corner, but you only notice it when the wavelength is about the same size as the slit. massive particles have wavelengths, given by h/p, meaning my wavelength is going to be on the order of 10^-37m, and I am not going to noticeably diffract when walking through a doorway. But an atom going at a few cm/s through a 100 nm slit will.

You appeared to claim that it would lead to instant communication in post #10.

Auugh! I've been outed!

Then you have to look at density, which is the point of the golf/ping-pong ball demo in moo's link. Things that are more dense than air will continue moving and push the air out of the way. This is why a helium-filled balloon will move backward when you hit the brakes — the air continues to move forward, since it's more dense. Merged post follows: Consecutive posts merged You shouldn't be using moment of inertia here — there's nothing rotating. Mass and density are sufficient to explain the phenomena.

No, it's often a pain the ass to find the error when you make a problem complex enough, and doubly so when it is described in vague terms. But the result — FTL communication — tells me that there is an error, because it violates relativity, which is self-consistent. Thought problems that have a contradiction contain an inconsistency, either in the setup or in the application of the physics. The only thing that can disprove a theory is an actual experimental result. Do you have one? You should be aware that argument from ignorance (i.e. "it's true until proven false") is a logical fallacy.

The Clausius statement, applied to radiation, is about net energy flow. The photon path is reversible, and the hotter object emits photons, too. And more photon energy, which traces back to the cooler object. There is a net heat transfer from hot to cold.

Quantum eraser experiments generally do not test for FTL phenomena. You can't simplistically assume that if you scaled them up (i.e. moved the detectors far apart), they would exhibit this behavior. If you can find an experiment that explicitly tests for this, then please cite it. Otherwise it's a thought experiment that assumes the answer you're looking for. The Wheeler experiment page doesn't claim anything happens FTL http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser Pay attention to the discussion section, where it is explained why this does not send information FTL or violate causality.