Science questions not worth a thread I: I'm a moron!

Status
Not open for further replies.
Eating too many beans
 
What is the main scattering mechanism in modulation-doped shallow two-dimensional electron gases at 77 K? I know it's not optical phonons because they freeze out at 100 K.
 
What is the main scattering mechanism in modulation-doped shallow two-dimensional electron gases at 77 K? I know it's not optical phonons because they freeze out at 100 K.


I guess you are talking about a GaAs/AlGaAs heterostructure.

In that case scattering at impurities should not play a role because of the modulation doping. So the only thing that's left are acoustical phonons. I cannot think of another relevant scattering mechanism.
 
Do .bmp files have an rgb?

I'm doing my matlab homework w/ imaging, and when I imread in a bmp file, the size returns 1 layer

ie when I do
image = imread('stuff.bmp');
[row column layers] = size(image) it returns layers as 1

So they've been telling us how every image is a 3d array with three layers (RGB obviously) but .bmp files appear to be coloured and be a 2D array/matrix?
 
I suspect that matlab is treating each pixel as a 32 bit int. So each element in that array will have RGB component, but to get the separate components you would have to isolate each of the 3 bytes. The logic to do should look something like this, where >> represents right shift, and & represents bitwise AND:
red = color >> 16 & 0xFF
green = color >> 8 & 0xFF
blue = color & 0xFF

Though there are also indexed and monochrome bitmap formats that do not specify RGB for each pixel.
 
Special Relativity question:

Given:
KE=1/2 m*v^2
A0 stars have a T = 10,000K
T is directly proportional to KE
Thermal motions due to temperature in this case is non-relativistic. (the numbers can be fiddled with to make it non-relativistic, but I just chose these temperatures to illustrate a point)

An A0 star has been accelerated to a velocity relative to the observer so that it is time dilated to 1% its original value.
Therefore, its thermal velocities of its atoms are 1% its normal velocity when observed.
Therefore, by KE=1/2 mv^2, its Kinetic Energy is 1/10,000 times its value.
Therefore, the temperature of the A0 star will appear as a 1K determined by its thermal motion.
Therefore, the blackbody emission of the A0 star will peak be far in the radio, rather than in the UV pre-doppler shift. (the spectral line positions will be preserved regardless)
Likewise, the observer on the star will observe the rest of the universe to be in a similar situation, pre-doppler shifted temperatures of 1/10,000 of their measured values in their own frame of reference.


This does not seem right. Where am I making a poor assumption or deduction?
 
This does not seem right. Where am I making a poor assumption or deduction?

You make several of those:

KE=1/2 m*v^2
This is only true for the non-relativistic case.

T is directly proportional to KE
This assumes an isotropic distribution of velocities, which is certainly not the case if you view a star moving away at high speed.

Therefore, its thermal velocities of its atoms are 1% its normal velocity when observed.

And this is the big one: Time is dilated, but also length is contracted. So you cannot convert velocities like that. And the definition of thermal velocity is always relative to the system and makes no sense if you view a moving system from the outside.

Therefore, by KE=1/2 mv^2, its Kinetic Energy is 1/10,000 times its value.
Therefore, the temperature of the A0 star will appear as a 1K determined by its thermal motion.

You can only do that if you're in the inertial system of the star. And then there are no relativistic effects because nothing is moving fast and thus time dilation plays no role.

Therefore, the blackbody emission of the A0 star will peak be far in the radio, rather than in the UV pre-doppler shift. (the spectral line positions will be preserved regardless)
Likewise, the observer on the star will observe the rest of the universe to be in a similar situation, pre-doppler shifted temperatures of 1/10,000 of their measured values in their own frame of reference.

This is obviously wrong, as the effects have to be the same no matter which inertial system I am observing them from, save for the appropriate Lorentz transforms.

The big flaw in your reasoning is that you try to look at thermal effects form the point of view of a moving observer. This cannot succeed, as the description of temperature makes the assumption, that the whole system does not move in the inertial system you look at it. What you have to do is to look at the system itself where no relativity is needed look at what comes out and then do the relativistic conversion on the output. In your case this is just the Doppler shift of the usual black-body radiation of the star.
 
Why do men have nipples?

Because the "genetic program" "applies" nipples before the gender is applied to the embryo.
It decides first about the nipples, before it decides what you get else.
No idea again why this :dunno:.
 
You make several of those:


This is only true for the non-relativistic case.
Well, it can be fiddled with so that it isn't (as mentioned). Drop it down to a 10 degree blackbody, and they will not be a bad assumption at all. It just simplifies calculations with a 10,000 degree star with 1/10,000 energy difference. (simple numbers)


This assumes an isotropic distribution of velocities, which is certainly not the case if you view a star moving away at high speed.
There is after you subtract out the star's motion. (My problem was Galilean Transforming the speed to do so, rather than Lorentz transforming; see below)


And this is the big one: Time is dilated, but also length is contracted. So you cannot convert velocities like that. And the definition of thermal velocity is always relative to the system and makes no sense if you view a moving system from the outside.
Only in the direction of motion. The thermal motions in all other directions are not affected. Since thermal motion is random, there will be plenty in the non-length contracted directions of motion to work with.



You can only do that if you're in the inertial system of the star. And then there are no relativistic effects because nothing is moving fast and thus time dilation plays no role.
Ok, I think this might be what I did wrong.

I think I just did a simple Galilean Transformation with the implied velocity conversions to subtract out the motion of the star from the observed motion of the atoms (observed velocity of atoms subtract velocity of star to get the thermal motion).

Or am I doing it wrong now, and did it right earlier? ( :crazyeye: , just making sure)


This is obviously wrong, as the effects have to be the same no matter which inertial system I am observing them from, save for the appropriate Lorentz transforms.

Hence my asking questions where I messed up. ;) I realized it was wrong, I just didn't know where.

The big flaw in your reasoning is that you try to look at thermal effects form the point of view of a moving observer. This cannot succeed, as the description of temperature makes the assumption, that the whole system does not move in the inertial system you look at it. What you have to do is to look at the system itself where no relativity is needed look at what comes out and then do the relativistic conversion on the output. In your case this is just the Doppler shift of the usual black-body radiation of the star.

Well... All observers are moving, as there is no stationary point. The view from the star that it is stationary and everything else is moving is just as valid as everything else is stationary and the star is moving. So to illustrate my point of the apparent inconsistency, we just assume the observer is stationary, and changed the location of the observers from away from the star to on the star.

Anyways, correct me if I'm wrong:
If an A0 star is moving such that it is 1% time dilated, after Lorentz addition (in this case, subtraction) of the appropriate velocities, you find the star's thermal motion is no different.
Thus the will emit the standard 10,000 K blackbody temperature, which will then be Doppler shifted to varying wavelengths depending on the motion of the star.

Likewise, an observer on the star will have to perform the correct Lorentz addition to derive the appropriate thermal velocities, and apparent paradox solved.
 
I also think you may have got confused between the thermal velocities, which transform according to Lorentz, and the thermal speeds, which is what determines T, and transforms differently because it could be moving either towards or away from you.
 
Well, that's one more command that has to be added on - to check to see if the embryo is male, and then to not add nipples. From a selection standpoint, that is probably costlier than devoting a small amount of resources to just adding the darned things. Less likely to go wrong, too.
 
What is an accurate equation for the force of friction that depends on velocity?
 
Do you mean air resistance or static friction from e.g. sliding across a floor?

Air resistance is proportional to the velocity squared I believe whilst static friction is proportional to velocity. Roughly (pun intended).
 
Do you mean air resistance or static friction from e.g. sliding across a floor?

Air resistance is proportional to the velocity squared I believe whilst static friction is proportional to velocity. Roughly (pun intended).

Air resistance. I want to construct a realistic, approximation-less equation for the force applied on a pendulum.
 
Status
Not open for further replies.
Top Bottom