Ask a physicist anything. (6)

Jazer · Jul 8, 2011

Chalnoth said:
The aliens just put a filter on the sky to make it look blue to us."

What would you say back to him?

That is correct, you get Blue because all the other colors are filtered out. If you have black all light is absorbed and nothing is reflected. If you have white nothing is absorbed and everything is reflected. In photography you photograph the light that bounces off the object. You can not photograph light that is absorbed. In theater we use gels on the lights that are a filter to only allow the color or the part of the light spectrum that we want to allow through to the stage, costumes, props or actors. So if we control the light, then we control what people see.

Chalnoth · Jul 8, 2011

Insane_Duck said:
Here is a silly question I have:

Helium balloons rise because they are lighter than the surrounding air. Since a helium balloon's contents is sealed off from the surrounding air, how does it know to rise? (if it can't interact with the surrounding air)

Could these sort of disturbances be used to transmit information instantaneously?

It works through the force of buoyancy. Basically, the pressure of a fluid is larger the further down you go, so that any object will have a larger pressure on its underside than its upper side. This leads to a net upward force.

Now, if the object in question is the exact same density as the surrounding fluid, then the upward force exactly cancels with gravity, and it doesn't have a tendency to move up or down. This is because when an object displaces some amount of fluid, there is always an upward force exactly equal to the weight of the displaced fluid. The helium balloon has a lower density than the surrounding air, so the force of gravity pulling the balloon down is lower than the force of buoyancy pushing it up, so it goes up.

pgp_protector · Jul 8, 2011

Insane_Duck said:
Mythbusters actually got a lead balloon to fly.

Yep (that's the only one I recall ever seeing though)

Chalnoth · Jul 8, 2011

Jazer said:
That is correct, you get Blue because all the other colors are filtered out.

Nope, that's actually not true. It's actually blue because blue light tends to bounce off of molecules in the air more readily. So it's not that other light is filtered, but the Sun's rays passing through various parts of the atmosphere sometimes bounce off various molecules instead of coming straight to us from the Sun. And the bluer the light is, the more likely it is to bounce off of a molecule, so that the light that comes to us after bouncing is most likely to be blue.

If the sunlight didn't bounce off of molecules in the atmosphere, then the sky would appear black.

This effect, it turns out, is also the same reason why sunsets appear to transition from blue to yellow to red: the lower the Sun gets in the sky, the more atmosphere the light travels through, and the more times the light bounces off of molecules before it reaches our eyes. The blue light bounces lots of times and never reaches us, while the red light bounces fewer times and is able to get to our eyes.

Tinker Grey · Jul 8, 2011

pgp_protector said:
Ya just need a larger balloon for the lead balloon to work

Precisely why I specified "of a traditional size".

Jazer · Jul 8, 2011

Chalnoth said:
It's actually blue because blue light tends to bounce off of molecules in the air more readily.

There are two color theorys, one is the additive and the other is the subtractive color theory.
So what is the cause of the northern lights?

Wiccan_Child · Jul 8, 2011

Jazer said:
So how does the moon tilt the earth?

The Moon's orbit is such that you get great waves moving across the oceans, following the Moon. This almighty redistribution of mass tilts the Earth somewhat, like sitting on a spinning chair and waving your arms about. The Earth's tilt is thought to be largely due to the impact with a proto-planet that caused the Moon to form.

Wiccan_Child · Jul 8, 2011

Insane_Duck said:
Here is a silly question I have:

Helium balloons rise because they are lighter than the surrounding air. Since a helium balloon's contents is sealed off from the surrounding air, how does it know to rise? (if it can't interact with the surrounding air)

Could these sort of disturbances be used to transmit information instantaneously?

Air molecules bounce off the rubber, and because there's more bouncing on the outside than the inside, the outside molecules get the advantage. So, in the war of gravity, they win, and they collect beneath the balloon with more fervour than the Helium does - meaning the balloon is pushed up.

Wiccan_Child · Jul 8, 2011

Jazer said:
There are two color theorys, one is the additive and the other is the subtractive color theory.
So what is the cause of the northern lights?

Charged particles hit the atmosphere, exciting the air molecules. Excited molecules eventually emit photons to de-excite themselves, so the glow of the aurora comes from excited molecules emitting photons, and they're excited due to charged particles. They occur at the poles because the Earth's magnetic field acts as a sort of funnel, moving them all towards the North (and South) poles.

Chalnoth is right about the colour of the sky, though.

Chalnoth · Jul 8, 2011

Jazer said:
There are two color theorys, one is the additive and the other is the subtractive color theory.

Well, no, the physical explanation I listed is the explanation for the color of the sky: it's all about how light from the Sun bounces off of atoms in the atmosphere. That is just the way it happens.

Jazer said:
So what is the cause of the northern lights?

That's a rather different phenomenon. This stems from charged particles from the Solar wind striking the atmosphere. These charged particles get redirected by the Earth's magnetic field toward the poles. When they strike atoms in the atmosphere, they tend to excite the electrons in those atoms, sometimes even kicking the electrons away from their atoms entirely. When the atoms in the atmosphere return to their ground state, they emit light. It is that light that we see in the auroras.

You can read more up on it on Wikipedia, as usual.

Jazer · Jul 8, 2011

Wiccan_Child said:
The Moon's orbit is such that you get great waves moving across the oceans, following the Moon. This almighty redistribution of mass tilts the Earth somewhat, like sitting on a spinning chair and waving your arms about. The Earth's tilt is thought to be largely due to the impact with a proto-planet that caused the Moon to form.

So if the moon did not break the earths crust what caused plate tectonics, just pressure from below the surface?

razeontherock · Jul 8, 2011

Wiccan_Child said:
the closet the Moon has been

It's just amazing all the details we have on the moon. When was it in a closet?

Jazer · Jul 8, 2011

Wiccan_Child said:
Chalnoth is right about the colour of the sky, though.

Yep, I am just a techie, your the physicist.

razeontherock · Jul 8, 2011

pgp_protector said:
Ya just need a larger balloon for the lead balloon to work

Eh, it'd go over like a Led Zeppelin

pgp_protector · Jul 8, 2011

razeontherock said:
Eh, it'd go over like a Led Zeppelin

This one?

?

razeontherock · Jul 8, 2011

pgp_protector said:
This one?

?

No, this one:

YouTube - ‪Black dog--led zeppelin‬&rlm;

Naraoia · Jul 8, 2011

Chalnoth said:
Well, sure, but we're talking less than a percent change over the history of the universe, not something that can explain a million-fold error. And besides, we have geometrical measurements that show the universe must be much older tan 6,000 years: SN1987A and the Age of the Universe

I meant that if YECs want to change the speed of light so it got here from billions of light years away in under 6000 years, they have to explain why no one ever saw the consequences in the spectra of distant objects.

Have to admit that that small problem is quite dwarfed by all the others your link discusses.

Chalnoth said:
A little while ago, just for kicks, I made my own version of this plot using the FIRAS data at LAMBDA - Legacy Archive for Microwave Background Data in order to show the errors in the measurement:

Those error bars that, for most of the plot, you can barely see at all? Those are 50 standard deviations. To show how absurdly accurate this measurement is, at 50 standard deviations, the error is less than one part in 10^500. That means that there is less than one part in 10^500 chance that the true value of the CMB spectrum lies outside the error bars on the plot. And yet, for the most part, those error bars are about as close together as the thickness of the line.

Wow. I knew the measurements were a ridiculously good match to theory, but I hadn't known how good.

How's that even possible?

Holy cow. Holy cow.

Insane_Duck said:
Mythbusters actually got a lead balloon to fly.

Is that on Youtube?

Chalnoth · Jul 8, 2011

Naraoia said:
Wow. I knew the measurements were a ridiculously good match to theory, but I hadn't known how good.

How's that even possible?

Holy cow. Holy cow.

Well, basically, it's just due to how amazingly bright the CMB is. Something like 99% of all of the radiation ever emitted is in the CMB. In fact, the CMB is so incredibly bright, that in the frequency range where the CMB is near its peak, the small deviations in temperature, which are fluctuations of around one part in one hundred thousand in temperature, are much brighter than our galaxy across most of the sky.

Naraoia said:
Is that on Youtube?

Yup!
YouTube - ‪Mythbusters - Lead Balloon Clip - Jamie and Adam‬&rlm;
http://www.youtube.com/watch?v=eGdHRupe7Zc

I'm not entirely sure if there is a video with the whole thing put in one clip, but there are two clips of it

razeontherock · Jul 8, 2011

Fascinating stuff:

Chalnoth said:
Something like 99% of all of the radiation ever emitted is in the CMB.

Sounds a lot like G-d is Light, combined with "The heavens declare the glory of God; and the firmament sheweth his handywork. Day unto day uttereth speech, and night unto night sheweth knowledge. [There is] no speech nor language, [where] their voice is not heard. (Psalm 19)

FrenchyBearpaw · Jul 8, 2011

razeontherock said:
Fascinating stuff:

Sounds a lot like G-d is Light, combined with "The heavens declare the glory of God; and the firmament sheweth his handywork. Day unto day uttereth speech, and night unto night sheweth knowledge. [There is] no speech nor language, [where] their voice is not heard. (Psalm 19)

That, or:

Sounds like FSM is Light, combined with "the heavens declare the glory of FSM; and the firmament sheweth his handywork. Day unto day uttereth speech, and night unto night sheweth knowledge. [There is] no speech nor language, [where] their voice is not heard. (FrenchyBearpaw 1:1)

See what I did there? Did ya'? Just replaced "God" with "FSM," and still explained just as much.

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