• Physics Discussion
    973 replies, posted
Well, I mean, you've gotta learn it sometime. It does kind of sound like a profound question if you don't know the answer. :v:
[QUOTE=JohnnyMo1;44627416]Well, I mean, you've gotta learn it sometime. It does kind of sound like a profound question if you don't know the answer. :v:[/QUOTE] I agree. I'm just a freshman and am mostly confused at every talk, but I at least look up what the titles mean (In this case the talk was literally called "Majorana Fermions and Neutrino Mass"). One interesting question was how is it possible for supernovae to produce a neutrino flux if neutrinos are Majorana particles. If they are all annihilating, then we should only see photons. But right handed neutrinos are at a much higher energy than left handed neutrinos, so at thermal equilibrium the amount of right-handed neutrinos is minuscule.
taking two physics courses next semester, one's about electric fields and circuits and the other is about acoustics. should be a good time. still not 100% sure if i want to major in physics or economics. might do a double major but then i probably won't have enough physics courses to head to grad school if that's what i decide i want to do...
[QUOTE=Falubii;44627508]I agree. I'm just a freshman and am mostly confused at every talk, but I at least look up what the title mean (In this case the talk was literally called "Majorana Fermions and Neutrino Mass"). One interesting question was how is it possible for supernovae to produce a neutrino flux if neutrinos are Majorana particles. If they are all annihilating, then we should only see photons. But right handed neutrinos are at a much higher energy than left handed neutrinos, so at thermal equilibrium the amount of right-handed neutrinos is minuscule.[/QUOTE] Oh I see. I didn't even know what a majorana fermion is. I just know photons are their own antiparticles. :v:
[QUOTE=postmanX3;44627770]taking two physics courses next semester, one's about electric fields and circuits and the other is about acoustics. should be a good time. still not 100% sure if i want to major in physics or economics. might do a double major but then i probably won't have enough physics courses to head to grad school if that's what i decide i want to do...[/QUOTE] I've actually heard a surprising number of stories about physicists by training going into the financial sector.
So why is it an open question if neutrinos are Majorana particles? If they were, wouldn't they be rather apparent that they're annihilating all over the place?
[QUOTE=Krinkels;44629112]So why is it an open question if neutrinos are Majorana particles? If they were, wouldn't they be rather apparent that they're annihilating all over the place?[/QUOTE] I'm definitely not an expert, so take this with a grain of salt. Being electrically neutral, neutrinos hardly interact with anything. I don't think neutrinos and antineutrinos can be created with the densities required for them to start colliding. Right now the primary method of inferring whether or not neutrinos are Majorana particles is by trying to observe a process called neutrinoless double beta decay. In order for this to occur, the neutrino emitted in the first beta decay must be the same as the one absorbed in the second beta decay. I don't want to try to explain any more because I don't know much more. Basically if they detect a double beta decay and no neutrinos then they are Majorana particles. Right so even if neutrinos are Majorana, there is a distinction between right-handed and left-handed neutrinos that would annihilate. For starters right-handed are far more rare.
Been on a lecture binge while practising my procrastination. This one was particularly awesome, about inflation and its implications: [media]http://www.youtube.com/watch?v=dhGRV8cD_tY[/media] As always, don't read the comments.
I found an interesting article on one of the applications of measuring antineutrino flux. [url]http://phys.org/news/2014-04-antineutrinos-nuclear-reactors.html[/url]
[QUOTE=Ziks;44635111]Been on a lecture binge while practising my procrastination. This one was particularly awesome, about inflation and its implications: As always, don't read the comments.[/QUOTE] lol @ 120 order of magnitude discrepancy
That about the expected vacuum catastrophe? I remember when my GR professor (a string theorist) bought that up in lecture. He was like, "And we find that there's a discrepancy of about 120 orders of magnitude between these expected values for vacuum energy. Not bad! I've seen worse."
[QUOTE=JohnnyMo1;44638206]That about the expected vacuum catastrophe? I remember when my GR professor (a string theorist) bought that up in lecture. He was like, "And we find that there's a discrepancy of about 120 orders of magnitude between these expected values for vacuum energy. Not bad! I've seen worse."[/QUOTE] I can't even begin to imagine a prediction being much worse. To QFT's credit, the prediction was only off by 107 orders of magnitude according to Wikipedia.
Anyone who's interested in the theoretical justification for the big bang (and has some working knowledge of GR) should read chapter 10 of Hawking and Ellis' [I]The Large Scale Structure of Space Time[/I]. Beautiful book, beautiful chapter and argument. The singularity theorems are some of my favorite parts of theoretical physics, they're so incredible and powerful. Also, been reading about tachyons and spontaneous symmetry breaking and they're pretty neat and I can't wait until I get to where they're discussed in Zwiebach so I can understand them more formally.
[IMG]http://i.gyazo.com/874c572e1cf601f2259f2d0aa16a7c6b.png[/IMG] This is a question I'm trying to do. It's really simple, and I know how to do questions of this type. The only thing that throws me off is the fact that theta and T are unknowns, and I can't work out the tension because of this. What am I supposed to actually do?
ugh i h8 those the two y components should cancel out the y component of the mass which means you can solve for the y component of T, from there you can solve for theta i think which should let you solve x and then T its been like a year and a half since ive done one of those
Write that the three vectors should add up to the zero vector, project them onto the x and y axes, and then you should end up with two linear equations and two unknowns and from there it's just solving a system. That is, if they're linearly independent... or some crap like that It's been a long time for me too and at the time I was really bad at them (probably still am)
It's been a while since I've done any mechanics and I may have made some mistakes in my working, but here's what I came up with: [code]T * sin(theta) + 69.6 * sin(25) = 49 T * cos(theta) - 69.6 * cos(25) = 0 T * sin(theta) = 49 - 69.6 * sin(25) T * cos(theta) = 69.6 * cos(25) (T * sin(theta)) 49 - 69.6 * sin(25) ---------------- = ------------------- (T * cos(theta)) 69.6 * cos(25) sin(theta) 49 - 69.6 * sin(25) ---------- = ------------------- cos(theta) 69.6 * cos(25) [since sin/cos = tan] theta = atan((49 - 69.6 * sin(25)) / (69.6 * cos(25))) theta = 0.301057903 radians or 17.2493472 deg Now use theta to find T; T * cos(theta) = 69.6 * cos(25) T = (69.6 * cos(25)) / cos(theta) T = 66.0497189 N[/code] [editline]my working[/editline] If it isn't obvious, the two equations I started with are based on the fact that the Y components of the two tensions must equal the gravitational force and the X components of those tensions must also be equal.
I got the same answer.
Why did I learn to use phasors if imaginary numbers are infinitely more useful?
[QUOTE=Falubii;44695367]Why did I learn to use phasors if imaginary numbers are infinitely more useful?[/QUOTE] I don't think I've ever had phasors explained well. I thought they [I]were[/I] complex vectors. Not that it really matters. I don't plan on ever using them again. Fuckin circuits.
[QUOTE=JohnnyMo1;44698539]I don't think I've ever had phasors explained well. [b]I thought they [I]were[/I] complex vectors.[/b] Not that it really matters. I don't plan on ever using them again. Fuckin circuits.[/QUOTE] They are, but we were just supposed to take them at face value. Using phasors and geometry, the most complicated AC circuit you can analyze is an inductor, resistor, and capacitor in series. Using complex numbers you can pretty much analyze anything. Analyzing circuits bores the hell out of me though, definitely would not want to be an engineer.
Took a whole electronics lab junior year. It was awful. Would not do again.
Can someone explain time dilation?
Sure. The principle of relativity states that the laws of physics should be the same everywhere, regardless of relative motion ( non-changing velocity, i.e. constant speed and direction). Maxwell's equations of electromagnetism can be manipulated to a form that satisfies the wave equation. The speed of this wave (which strangely happens to match the experimentally determined speed of light, hmm) is dependent on two physical constants. Interestingly, these constants are defined in empty space, so following the principle of relativity, one would expect that this law of physics should be true regardless of relative motion. Basically, the speed of light is the same for everyone. But how can that be? How does everyone see the speed of light as the same? There's a set of linear transformations, called the Lorentz transformations, that describe what strange effects must occur when traveling close to the speed of light. If it is truly constant for everyone, then the length of objects and time must change to accommodate. Namely, when you see someone flying past you, the watch on their wrist, their heartbeat, and anything else that indicates time are slowed down. In addition, they appear squished in the direction of their velocity.
Does surface area play a role in whether something sinks or not? I was thinking a boat doesn't sink because it's mostly air and air is less dense than water, therefor it floats on top of the water, right? But what if you had like a million tons of air compressed into one cubic inch? Wouldn't that sink in water since it's so heavy? Or why doesn't a lead brick just sink through the surface of a wood table. I mean, surely the lead is more dense than wood?
[QUOTE=cqbcat;44735699]Does surface area play a role in whether something sinks or not? I was thinking a boat doesn't sink because it's mostly air and air is less dense than water, therefor it floats on top of the water, right? But what if you had like a million tons of air compressed into one cubic inch? Wouldn't that sink in water since it's so heavy?[/QUOTE] Buoyancy depends on density. That compressed air would sink not because it's heavy, but because it's more dense than the water. [QUOTE=cqbcat;44735699]Or why doesn't a lead brick just sink through the surface of a wood table. I mean, surely the lead is more dense than wood?[/QUOTE] Those are solids though. [editline]6th May 2014[/editline] Whoops, though I neglected to say that surface area does play a part as well. An object is pushed upward by bouyant force equal to the weight of the water it displaces. That's why the bottom of a boat is so large. Even though the boat may be more dense than water, it displaces a lot of water, and as long as the water isn't coming up into the boat, the buoyant force large. [editline]6th May 2014[/editline] Essentially the boat uses the air in it to lower its effective density, but that's why if the boat is allowed to fill with water, it's fucked. It's not [I]really[/I] surface area that's doing it from a physics standpoint, but that's practically that's how we keep boats afloat.
The inability of people to use a common convention for spherical coordinates really bugs me. It causes so much unnecessary confusion.
I'm inconsistent myself. In polar coordinates I use theta, yet in spherical coordinates theta is the angle with the z axis, not the one in the x-y plane. I think it's because my syllabi all do it their own way so e.g. a course with lots of polar coordinates used theta while another one with lots of spherical coordinates used it differently and I got used to both. I also hate rho as the polar/spherical radius.
[QUOTE=Number-41;44739346]I'm inconsistent myself. In polar coordinates I use theta, yet in spherical coordinates theta is the angle with the z axis, not the one in the x-y plane. I think it's because my syllabi all do it their own way so e.g. a course with lots of polar coordinates used theta while another one with lots of spherical coordinates used it differently. I also hate rho as the polar/spherical radius.[/QUOTE] Physics uses theta as the polar angle, but math use phi. It's just confusing being concurrently enrolled in two classes teaching similar things with such whacky conventions.
[img]http://puu.sh/8D19l.jpg[/img] That sure is a scary headline.
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