Jonsy123

" 1*c faster than the train " ?, are you implying that for the observer beside the railway, the lights from the train travels at 2x the speed of light ?

Thanks for your answer Janus. And what if the train switched on its lights 1 hour before it was in front of me (assuming a very large planet ), and the light sensor was 1 light hours to my right ?, then these distances will not drop to nearly zero in length, right ? (and if they will, then just pick large enough distances). My question then will be, when will the light from the train hit the sensor ?, for me it will be together with the train, but for the people on the train it should happen before, no ?

Can anyone give me the outcome of the following situation ? Let's say that I'm standing in front of a railroad, and a train travels at the speed of light, in a direction from my left to my right (but at this point the train is still several kilometers to my left). Now, on the railroad, 100 meters from where I'm standing (to the *right* from where I'm standing), I put a sensor, that can detect light. Now, 1 second before I can see the front end of the train exactly in front of me, the train switches its flash lights on (the lights are on the front end of the train). My question is, after this second passes, and the front end of train is exactly in front of me, will the sensor (located 100 meter to my right on the railroad) at this moment will show that it already registered the light from the train's flash lights ? I can see two answers for this question: 1. from my point of view, because the speed of light is constant, the sensor will not register the lights at that moment. It will register the lights exactly when the train itself will hit it, becuse the train's lights and the train are moving at the same speed. 2. From the people on the train point of view, when the train is in front of me, the lights from the train already passed 300,000 km from where the train was a second before, so the sensor will of course registered the lights from the train. So, how could the sensor both register and not register the light ?, the registration of light can't be relative, either it registered or not, no ?, I mean, exactly when the front end of the train is in front of me, let's say I push a switch and the sensor is moved away from the railroad (so can't register the light anymore), then I go and take the sensor, and look if it registered the light or not, so the answer must be one, either it did, or it didn't.

CharonY, Thank you very much for your answer. Yes, I understand that Centrifugation and SDS-PAGE seperate proteins using somewhat different properties of the proteins (while SDS-PAGE seperates by molecular mass, Velocity Centrifugation separates by molecular mass and shape). The main thing I was confused about, is why in SDS-PAGE larger proteins move more slowly, while in centrifugation they move more quickly. So, I understand that you agree that this difference is because of the usage of SDS, which open the structure of the protein, and make friction a much larger issue (in comparison to when the protein is in its native fold).

Was my question not clear enough ?, or no one knows the answer ?.

I would like to know how come when we separate proteins in velocity sedimentation centrifugation (i.e using a sucrose gradient), larger proteins move faster than smaller proteins, but, if you separate proteins in SDS-PAGE, larger proteins move slower than smaller proteins. If we would have tried to separate proteins which were treated with SDS, by velocity sedimentation, would the larger protein move slower than the smaller, like in SDS-PAGE ?. My hunch is that if you talk about globular proteins, then the larger the protein, the faster it goes through the gel, but if you talk about filamentous proteins (SDS causes all proteins to become that way), then the larger the protein, the slower it goes through the gel, because friction now plays a much bigger part. I'm not sure if I'm right though...

So all of the light which is coming from a certain (x,y) point on the object's plane, will reconverge at a certain unique (x,y) point on the screen ?. But why doesn't it happen when the screen is a mirror ?.

Can anyone please explain to me why the following phenomena happens ?: When a video projector projects an image on a mirror screen, the image that we see on the mirror, seems to originate from inside the mirror, at a depth inside the mirror which equals the distance between the projector and mirror. On the other hand, when a video projector projects an image on a projection screen (which could be a white wall), we see the same image as with the mirror, only this time it does not seem to originate from within the wall, rather it seems to originate from the surface of the wall. Why is there a difference between these two situations ?. Are the microscopic events that lead to the image formation, different between the mirror and white wall ?. For the white wall, I know that each photon that hit a molecule of the white wall, gets absorbed, shifts an electron to a higher orbital, the electron is unstable at this higher orbital, and when it goes back to the lower orbital, it ejects a photon back. As far as I know, this is the explanation for why the white wall reflects light, and allow us to use it as a screen for the projector. Is this same explanation correct for what happens with the mirror ?. If so, why is there such a differenece between the image that is created by the mirror, and the one which is created by the white wall ?. Thank you very much for your help.

In Prokayotes like e.coli, there is a lactose/H+ symporter in the cytoplasmatic membarane: Such a symporter, can NOT exist for the cytoplasmatic membrane of Eukaryotes, because they do not have a proton gradient between this membrane. But, theoretically this transport can happen in the mitochondrial inner membrane. Does it happen in real life ?. If it does, what would the addition of valinomycin (which will transport K+ into the mitochondrial matrix, destroying the membrane electric potential) do for this symporter ?, will it make it slower ?, faster ?, why ?.

Can the sugar *lactose* enter the mitochondria ?, is there a symport of lactose with protons to the mitochondria ?.

Thanks Yggdrasil, that's what I thought so too. I was worried because one of the students said he thought the professor implied during the test, that only cholesterol affects the membrane fluidity, and so changing the van der waals interactions between the phospholipids won't have any meaningful effect. This seems to me like pure B.S. yes, the cholesterol can affect membrane fluidity, but it's not the only factor playing here, and no matter how much cholesterol you have in the membrane, if you interfere with the hydrophobic interactions between the phospholipids, you are going to increase the fluidity.

I had a test today in Biochemstry. In one of the questions the professor asked: Assuming you interfere with the van-der-waals intercations between phospholipids that create a biological plasma membrane, what will happen ?: 1) The fluidity of the membrane will go down. 2) The fluidity of the membrane will go up. 3) It will have no effect on the membrane fluidity. Is there a single *definitive* answer for this question ?.

mezarashi wrote: Why is it called "diffused reflection" ?, maybe because the direction of the momentum is lost in the process ?, so the photons emitted come out in all directions, and hence cause a "diffused" effect (I know you said the momentum is conserved in both diffused and specular reflection, but maybe in "diffused" reflection, the direction of the momentum isn't conserved... otherwise, what does the word "diffused" stands for ?.). mezarashi also said: (In this case I can understand how the direction of the momentum is conserved). DQW wrote: So, I guess DQW talked about diffused reflection ?...

But, must they be *infinitely* narrow in order for me to see some gap ?. Also, is there a simple explanation as to how come a metallic surface can reflect a photon, without the excitation/de-excitation mechanism ?. The picture I see is this: a photon is headed toward a metalic surface, at some point, it will hit it (otherwise, it won't change its course). It can either hit an atom's nucleus, or an atom's electron. I assume if it hits something, it will almost always be the electron. At the moment it hit the electron, I understand that the electron does NOT absorb the photon and excite to a higher orbital, but if not, then what happens instead ?, does the photon simply hit the electron, the photon's course is changed, at it continues its journey at that new course ?.

So, in metalic surface reflectance, the same photon which hit the surface, is the one which bounce back from it ?. Now at least I can understand how it comes out only at a certain direction (like a basketball hitting the floor, only without any energy loss, or maybe there is some energy loss as heat ?, afterall, there IS an imapct...). I don't think it would have been possible, if we were talking about excitation/de-excitation, since the direction information should have been lost in the process... (I guess my flashlight idea won't work afterall). Anyhow, the fact that a metalic surface can reflect *any* wavelengh between 450-451nm, still doesn't mean it will get the chance to do it in practice, since as DV8 2XL affirmed earlier, some wavelengths never happen in nature, due to the quantization limitation which is an inherent characteristic of any system which actually produce radiation.