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Physics, Astronomy & Cosmology / Does anti-matter curve space in the opposite direction?

« on: 13/04/2009 19:20:20 »

Any movment of mutualy gravitating bodies results in the generation of gravity waves.
I have read that Jupiter radiates 20 Watts somewhere.

It's the acceleration that causes it, AFAIK.  Not just movement.

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Physics, Astronomy & Cosmology / Does anti-matter curve space in the opposite direction?

« on: 13/04/2009 19:19:09 »

Just thought I'd point out that the mutual annihilation of matter/antimatter should produce a gravity wave due to the matter disappearing.

Why?  Gravity in GR has an energy density dependence, not a mass dependence.  You should get a reduction because the energy leaves, but not from the annihilation itself (i.e. the change in mass).  IOW, photons contribute to gravity.

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Physics, Astronomy & Cosmology / Is there such a thing as an electron reaction?

« on: 10/04/2009 17:40:02 »

In some sense an "electron reaction" is atomic physics/chemistry, since there are interactions involving orbital electrons. 

The energy scale for atomic-scale electromagnetic interactions is generally smaller than for nuclear interactions, often by 5 or 6 orders of magnitude.

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Physics, Astronomy & Cosmology / Why is the moon moving further away from the Earth?

« on: 08/04/2009 23:00:00 »

The reason that tides / rotation transfer energy between Earth and Moon is that they are big enough and near enough to each other that they can't be treated as 'point masses'.
We tend to think of orbits as depending just on the masses and separations but that is an over simplification if you want to get accurate predictions. The 'centre of mass' is a very useful idea but doesn't tell the whole story.

It's a little more subtle than that — it's not that they are big, nor that they are close that directly causes this.  If they were perfectly spherical with a uniform mass distribution then they could be treated as point masses (from Gauss's law).  But because of the tides causing a deformation, and the lag in the earth's tidal bulge (it's not pointed directly toward the moon), there is a torque between them.

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Technology / How does a thermal camera works?

« on: 08/04/2009 20:30:21 »

Much the same as a normal camera I believe, except working with infra-red light instead of visible light. In fact you can make a cheap one from an old webcam - http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/make-an-infra-red-camera/

That's near-IR, though, cutting off at around 1 micron.  Thermal imaging uses different detectors, because the peak at body temperature is at around 9 microns.

——

And I should note that IR and heat are not synonymous.  Things near room/body temperature radiate strongly in the IR, but you can transfer energy with radiation of any wavelength.  That's why a microwave works, and why you can burn things with visible light (of sufficient intensity) and a magnifying glass.

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General Science / Do we lose weight when showering?

« on: 08/04/2009 20:19:33 »

Your scale's resolution shouldn't allow you to consistently pick this up, but you do lose weight anytime you aren't eating or drinking.  You perspire, as mentioned by jpetruccelli, and will do so slightly more in the hot water of a shower.  You also breathe out water vapor and CO2 in excess of what you inhale.  But that shouldn't be more than a few grams unless your showers are really long.

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Physics, Astronomy & Cosmology / Why is the moon moving further away from the Earth?

« on: 08/04/2009 20:03:11 »

If the moon is moving away from the earth then would not the same be happening with the earth around the sun? Is that why we have to add a second to time every so often?

The moon moving away robs the earth or angular momentum.  That's the reason for leap seconds; more precisely, it's why leap second additions will become more commonplace, on average, if we continue to implement them.

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Physics, Astronomy & Cosmology / Why is the moon moving further away from the Earth?

« on: 08/04/2009 19:59:28 »

Current tidal force theory cannot calculate the 3.8cm correctly. I think moon’s moving away from earth is due to the effect of earth’s spinity. Because of earth’s spinity on moon, moon is accelerating in its orbiting and is moving away finally. We can use the following values:( S=2G/c^2=1.48*10^-27, Earth mass=5.9736*10^24, Earth radius=6378km, Earth spinning angular velocity=1/43200(1/sec), Moon orbiting period=27day, Angle thelta=20(cosTheta= 0.94), Moon’s distance from Earth is 384399km). After we get the acceleration a, we can calculate the moving distance by using S’=1/2at^2(t=31536000sec=1 year). Because circumference and radius has a relation (S’=2pi*r’), so r’=S’/2pi. Finally, we get the result r’=3.75149cm which is very close to the laser measurement 3.8cm. Thus, the spinity formula is correct.

But the tidal forces are there, so if the moon recedes at 3.8 cm/year and if that's what your theory predicts, it means your theory is almost certainly wrong.

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Physics, Astronomy & Cosmology / Why is the moon moving further away from the Earth?

« on: 08/04/2009 19:56:20 »

I can prove that the Standard Model argument which states that the Moon is induced with angular momentum because of the Earth's spin is INCORRECT:

                    WHY IS THE MOON NOT SPINNING THEN??  []

Cheers,

AL

The moon does spin.  Once per orbit.

Otherwise we'd see the other side of it.

(oops.  I see now that this was already addressed.  Missed that in all the … stuff)

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General Science / Why Does A Windscreen Crack In A Wobbly Line & Not Straight ?

« on: 27/03/2009 18:35:23 »

Why would it crack in a straight line? Would that not be much weirder? Is there much in nature that happens in a straight line or precise angles (other than crystal structures)?

That's the crux of the issue, I think.  Glass isn't a crystal.  I've cleaved Silicon and gotten a nice straight line, because the crystal axis is the weak point.  There is no corresponding structure in glass.

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General Science / Is it safe to take a shower during a thunderstorm?

« on: 27/03/2009 18:31:30 »

You need to define "dangerous."  The risk is small, but not zero.  However, from what I've read you are more likely to be injured using a corded phone or other appliance.
http://www.lightningsafety.noaa.gov/indoors.htm


"10 to 20 people in the United States are shocked annually while bathing, using faucets or handling appliances during storms."
http://www.seattlepi.com/health/281449_hrail17.html


"In 2008, 27 Fatalities: All were outside"
"302 Injuries: There were 176 adults, 62 teens and 15 children injured, 201 males and 60 females. 184 were outside and 13 were inside."  (which doesn't add to 302)
http://www.struckbylightning.org/

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Physics, Astronomy & Cosmology / Can we detect changes in the

« on: 26/03/2009 17:54:00 »

But how could we notice a change in frequency of the cesium atom

We would notice that our solar days no longer match our clocks.

That would take a while.  But rubidium, mercury and hydrogen are used as well (and other elements), so certain changes would show up as difference in these clocks — this is one way of checking for changes in the fine structure constant.  If time itself were changing, they would all show the same result — this is exactly what happens in relativity, where the clock type is not important.

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Physics, Astronomy & Cosmology / How does a laser work?

« on: 26/03/2009 17:48:14 »

The stimulated emission concept is quite interesting.

Now we know that inherently the energy "gap" in the excited-state atom is perfectly matched to the rest of the laser-radiation in the cavity.
We also know that the excited state has to be relatively "long lived" (metastable or whatever) to as to "hang around" long enough to be taken to the ground state predominantly by stimulated emission rather than random emission (sorry I can't think of the proper word).
The fact that it "hangs around" could be interpreted/considered as some kind of energy potential-barrier.
So if the excited atom momentarily "borrows" some energy from a passing "photon" then it can return to the ground-state.

Buuuut... I said before the energy gap is "perfectly matched", which might imply a concept a bit like a high-Q resonant system has some relevance? ... something that would couple to an EM wave of the precisely correct frequency...? And if this was the case you might expect the stimulated "photon" to be phase-matched and polarisation-aligned to the stimulating "photon"?


A bit handwavy I'm afraid - tis late at night, plus a decade or more since I studies undergrad photonics.  []

What do you think? sophiecentaur?

You've got it right.  The concept of cavity Q and phase matching is very relevant to laser operation.  Though because of the high frequencies involved, and therefore really large Q's, the "free spectral range" is sometimes introduced
http://en.wikipedia.org/wiki/Free_spectral_range

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General Science / Re: Does a fan cool a hot room?

« on: 16/03/2009 17:08:05 »

It sounds like there hasn't been sufficient experiement recorded on this.

A new building fully insulated and cooler than outside - 2 rooms just Closed. nothing inside except one room has a fan and it is turned on.
That room can be 1-2 deg C cooler than the exact same room without the fan.

There may be air drawn in from outside but the air is hotter outside than inside anyway.

Yet the fanned room is actually cooler []

Do you have any empirical evidence to support this?  It contradicts known physics.  A room will naturally have a temperature gradient in it, and if you are measuring in a region that is hotter, the fan will mix the air and reduce the temperature where the sensor is, even if the overall temperature is unchanged.

If you think otherwise, you need to present the results of an actual demonstration where the temperature has decreased.  With thorough documentation.  Assertion is not enough.

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Physics, Astronomy & Cosmology / Bunting the Baseball

« on: 16/03/2009 16:58:58 »

If you hit the ball below its center of mass it will tend to pop up.  If you hit it above the center of mass, it will deflect down. 

I think it would be clearer if the discussion used proper terminology — the fat part of the bat is the barrel, and the skinny part of the bat, with the knob, is the handle.  If you hold it so the barrel is below the handle, I think you will have a tendency to hit the ball below its center of mass.

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Physics, Astronomy & Cosmology / Speed of light

« on: 14/03/2009 18:14:45 »

We quote the speed of light in a Vacuum.

But space is not a vacuum it is full of dust, gas etc etc.....

Or is it that our definition of a vacuum takes into account a non perfect vacuum.

How does this effect light travelling to us from distant galaxies?

Even with all of the dust, etc., space is still a pretty good vacuum.  The index of refraction of a complete physical vacuum is 1.  By comparison, the index of refraction of air at STP is 1.00029.   Space  has 18-19 orders of magnitude lower concentration of atoms.

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Physics, Astronomy & Cosmology / Difference between flux

« on: 13/03/2009 17:07:57 »

In radiation terms the flux is the number of particles passing through (or striking) a unit area per unit time.  Rad is an absorbed dose, not a flux.

One rad is equal to an absorbed dose of 100 ergs/gram or 0.01 joule/kilogram
http://www.nrc.gov/reading-rm/doc-collections/cfr/part020/part020-1004.html
(REM, another common radiation term, is a dose equivalent that takes into account how concentrated the energy deposition will be)

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Physics, Astronomy & Cosmology / What materials may be used as the core of an electromagnet?

« on: 12/03/2009 11:04:34 »

An electromagnet need not have a core.  You can use air, though this means the electromagnet won't be as strong.
 
As Soul Surfer has mentioned, one configuration of such a system is a Helmholtz coil pair.
 

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Physics, Astronomy & Cosmology / Did the meteorite that wiped out the dinosaurs alter Earth's rotation?

« on: 08/03/2009 14:50:28 »

The angular momentum of the meteor is going to be small compared to the earth's angular momentum.   Order of magnitude comparison:  The relevant quantity is mvr for the meteor, and ~(2/5)mvr for the earth.  r will be the contact point, the earth's radius, so that's the same for both.   

The earth's mass is 6 x 10^24 kg, with a surface rotation speed of order of .5 m/s, or ~10^24 kg-m/s of momentum

Even if the meteor's mass is 10^10 kg and moving at 100 km/s (10^5 m/s), that's 9 orders of magnitude smaller.   

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Physics, Astronomy & Cosmology / Why do objects appear larger when closer?

« on: 08/03/2009 14:31:16 »

Geometry.  The size of an object is proportional to the angle it subtends, and also to the distance.  So an object twice as far away but twice as tall will take up the same angle of view.  Without other information for comparison, they will appear to be the same size, e.g. the moon and the sun.