Ask a physicist anything. (6)

[Chalnoth]

Why do the starsky appear diffrent in diffrent districts of my home town? the starsky I observed as young is not the same as now when i live in another district then before. The former sky include more stars, and diffrent set of star images!

Either you're looking at the stars at a different time of the year (the stars that we see rotate across the sky as the Earth moves around the Sun), or there is a different amount of light pollution. When there are a lot of lights around, it can be very difficult to see the stars.

 

[Chalnoth]

Loopy or stringy or neither? Which way do you lean?


eudaimonia,

Mark

Personally, I tend to lean towards string theory more than loop quantum gravity.

 

[chris4243]

Why do the starsky appear diffrent in diffrent districts of my home town? the starsky I observed as young is not the same as now when i live in another district then before. The former sky include more stars, and diffrent set of star images!

Brighter cities have more light pollution meaning some of the stars you cannot see. If you move north or south, you will see different stars, as (to your view) the stars are shifted to one direction exposing some stars previously hidden from you by the horizon. Similarly, different seasons show you different stars, in this case because you can't see some of the stars during the day but throughout the year they make a whole rotation (the constellations of the horoscopes are a good example of this). Furthermore, there is a slight wobble in the earth's axis that means the seasonal constellations slowly change seasons, but you won't notice that.

 

[Naraoia]

Now, theorists aren't wed to the idea that the laws of physics must be unitary, but so far all of the ones we have tested experimentally are, and so are all of our best candidate theories for what may lie beyond the physics we have tested.

Does that mean there are candidate theories that aren't unitary?

Also, I might have garbled your explanation, but is "unitary" more than a fancy way of saying "time-reversible"?

 

[Chalnoth]

Does that mean there are candidate theories that aren't unitary?

Not any well-developed ones.

Also, I might have garbled your explanation, but is "unitary" more than a fancy way of saying "time-reversible"?

Sort of. It doesn't necessarily require the laws be the same looking forward as backwards (though all of our more fundamental laws of physics are symmetric in time).

The term "unitary" comes from linear algebra. In linear algebra, unitary transformations are the generalization of rotations. When you rotate a system, you're just looking at the same system from a different angle. It may look different, but it's really the same system.

All unitary transformations, it turns out, have this property. If the laws of physics are unitary, then this means that when you move to a later point in time, it's basically just like the rotation: it's the same system, just a different perspective.

 

[Lillen]

Either you're looking at the stars at a different time of the year (the stars that we see rotate across the sky as the Earth moves around the Sun), or there is a different amount of light pollution. When there are a lot of lights around, it can be very difficult to see the stars.

Better answer then the former!! thanks

 

[Lillen]

Another question; Why is the breath colder when you blow out the air from the loungs then when you breath silently? Shouldn't the friction make it warmer?

 

[pgp_protector]

Another question; Why is the breath colder when you blow out the air from the loungs then when you breath silently? Shouldn't the friction make it warmer?

And you measuring this temp difference between blowing fast & silentlyconfused how?

 

[Wiccan_Child]

Another question; Why is the breath colder when you blow out the air from the loungs then when you breath silently? Shouldn't the friction make it warmer?

It's not. Your breath will be pretty much at the temperature of your lungs, a toasty 37°C I believe.

 

[Chalnoth]

Another question; Why is the breath colder when you blow out the air from the loungs then when you breath silently? Shouldn't the friction make it warmer?

As Wiccan_Child mentioned, it isn't. It will feel cooler, however, for the exact same reason why it feels cooler in the wind than it does out of the wind: the wind helps to evaporate sweat from the surface of your skin.

The main reason is that when sweat evaporates, it increases the humidity of the immediately-surrounding air, suppressing the evaporation of additional sweat. But when the air is moving, the evaporated sweat quickly moves away, allowing for more evaporation.

 

[Lillen]

I don't buy that!

Another question: How can we messure the distance to stars with trigonometry - we only know one angle?! I think i asked this before but didn't follow up the answer!!

 

[Chalnoth]

I don't buy that!

Why not?

Another question: How can we messure the distance to stars with trigonometry - we only know one angle?! I think i asked this before but didn't follow up the answer!!

Well, no, we know two angles and the length of a side of the triangle. The direction to the star at one time of year makes one angle. The direction six months later makes the other angle. And the length of the side is the diameter of the Earth's orbit. That is enough to get the distance to the star.

 

[Lillen]

Because you made that up!


I don't buy that either, because you made it up too or didn't you!?

 

[Wiccan_Child]

I don't buy that!

Why not?

Another question: How can we messure the distance to stars with trigonometry - we only know one angle?! I think i asked this before but didn't follow up the answer!!

As Chalnoth said, we have two angles and the length of one side, therefore we can calculate the other two side lengths, giving us the distance to the star.

 

[Mr. Pedantic]

I should point out, that at no point do you have to take us at our word; it is well within your ability to do a little Googling to corroborate what we say with the internet, your nearest university, and each other (since we all seem to have posted this at the same time).

Another question: How can we messure the distance to stars with trigonometry - we only know one angle!! I think i asked this before but didn't follow up the answer!!

Basically what we do is we take a picture of the night sky when the Earth is at one point, and then we do the same 183 days later - i.e. when it's on the other side of the sun. This distance is roughly 300 million kilometers, and the distance moved means that the stars move as well. How much they move depends on how far away they are from us - the closer they are, the more they move. Stars that do not move at all between pictures are too far away for the distance to be measured correctly.

Stellar parallax was known for hundreds of years, and was actually one of the scientific objections to heliocentrism - it was inconceivable that the stars were so far away that they did not move at all to the human eye when the Earth orbited. Of course, they do move, but they are so far away that most parallax calculations are done by satellites (probably the most famous one is the Hipparcos satellite (that produced the Hipparcos Catalogue of stars).


For more distant objects we can use the red-shift that Hubble found, and analyzing the emission spectrum of the object in question, and comparing it to what we know it is made of. The amount of red-shift is a rough indicator of distance.

Because you made that up!


I don't buy that either, because you made it up too or didn't you!?

Is this about the exhaled air feeling cold? Yeah, Wiccan is right. You can test this for yourself. Purse your lips, and exhale strongly onto the back of your hand (not someone else's ). Then, in mid-exhale, open your mouth wide, being careful not to change how fast you're breathing out. You'll notice that when you open your mouth wider, the air travels slower, and does not evaporate as much water off the skin, so it feels warmer than when you purse your lips and the air has to travel fast to get through a smaller opening.


Your response is common, though. The same principle is the origin of a myth in Korea and Japan; they think that leaving a fan on while you sleep will kill you because of hypothermia, because fans cool you down. In fact, they don't. They just make sure that the air in the room is more efficient at evaporating moisture off your skin.

 

[Chalnoth]

Because you made that up!

No, I didn't. I even Googled around a bit to make certain I was right. Feel free to look up the physical reason for the "wind chill factor" that goes into weather reports. Breathing into your hand fast vs. slow is the same effect.

I don't buy that either, because you made it up too or didn't you!?

Absolutely not.

Although I do admit that I was being a little bit fast and loose with regard to how it is actually done. The way it is actually done is that we get one angle from the difference in observing direction at different times of year. The difference in observing direction can be measured vastly more accurately than the absolute direction, and is equal to the angle labeled "parallax angle" in the picture that Wiccan_Child posted.

Then the other angle comes from the orbital inclination. However, if you only observe the star at times of year when the Sun-Earth-Star triangle makes a right triangle, which will happen twice a year for any star, then this simplifies things a bit.

 

[Lillen]

And another question: Why does ocean water evaporate under the degree of 100 celsius?

 

[Chalnoth]

And another question: Why does ocean water evaporate under the degree of 100 celsius?

Well, 100C is the boiling point. You still get some evaporation no matter what at any temperature. But the ocean won't boil away entirely unless it gets that hot.

The way this works is that at any given temperature, there is a rate at which molecules in the water evaporate and enter the atmosphere. At the same time, molecules in the atmosphere return to the ocean at some rate. And that rate is the same for every molecule in the atmosphere, so that if you have more molecules in the atmosphere, the faster they return to the ocean.

Thus, at any given temperature, there is an equilibrium point where just as many molecules are evaporating as are returning to the ocean (this would be 100% humidity). As long as the number of molecules of water in the air above the ocean is smaller than this point, the ocean will continue to evaporate.

So what happens is that wind carries the humid air from the surface of the ocean to higher altitudes, where it cools so that the water condenses and forms clouds. This process of circulation guarantees that the oceans are frequently evaporating, with the water being replenished by the rain.

 

[Mr. Pedantic]

And another question: Why does ocean water evaporate under the degree of 100 celsius?

To start off with, all matter is made of molecules. And molecules are made of atoms. Atoms are made of electrons, protons and neutrons, which are made of quarks...etc etc.



The forces that hold the molecules together is what determines whether something is solid, liquid, or gas:

You'll see that in a gas, there are virtually no intermolecular forces, whereas in a liquid, there are some, and in a solid, the forces prevent almost all movement of molecules relative to each other.


The salient point about this is that something turns from a liquid to a gas when you break the bonds that hold the molecules together. And you do that by making the molecules move faster (more kinetic energy makes more heat energy). That's what you do when you boil a pot of water. The energy from the element goes into the molecules, making them move fast enough to break free from the pull exerted by the other water molecules, and escape.


Now, the important thing is that not all particles have the same amount of energy. The distribution in particular is called a Boltzmann distribution. The particles with the most energy need a little bit of energy to escape; the particles with the least energy need a lot more. And that is what is happening when you heat water below boiling, or when you leave a pan of water out for a long time, or when you blow on hot coffee to cool it. The particles with the most energy are getting that extra bit they need, and they're breaking free from the liquid and leaving as gas. In the case of the coffee, this means that because the highest-energy particles have left, the average of the remaining particles is decreased, which means that it is cooler.

 

[Lillen]

I prefer short answers, because i cannot get a grasp over the enlightment when you lie!!