Search found 415 matches

by ThinkerEmeritus
Fri Apr 05, 2013 1:31 pm UTC
Forum: Individual XKCD Comic Threads
Topic: 1190: "Time"
Replies: 107235
Views: 50132217

Re: 1190: "Time"

Check out "Building Castles in the Sand" at http://www.youtube.com/watch?v=XV4fQyW8gcU. This is a 2006 song. A copy of the lyrics are at http://artists.letssingit.com/the-philosopher-kings-lyrics-castles-in-the-sand-r5zx6r6#axzz2PalGYwkN. The picture in the earlier reference and the lyric...
by ThinkerEmeritus
Fri Apr 05, 2013 12:46 pm UTC
Forum: Individual XKCD Comic Threads
Topic: 1190: "Time"
Replies: 107235
Views: 50132217

Re: 1190: "Time"

Check out "Building Castles in the Sand" at http://www.youtube.com/watch?v=XV4fQyW8gcU.
by ThinkerEmeritus
Thu Mar 28, 2013 11:35 pm UTC
Forum: Individual XKCD Comic Threads
Topic: 1190: "Time"
Replies: 107235
Views: 50132217

Re: 1190: "Time"

Tass wrote:
ThinkerEmeritus wrote:
mscha wrote:He's taking the flag. :( No other changes, sea level constant.


The stretch at the right of the picture isn't water; Cueball walked along the top of it after the "swim."


No he didn't. Look at his height. He was walking in it.


You are right. My bad.
by ThinkerEmeritus
Thu Mar 28, 2013 11:23 pm UTC
Forum: Individual XKCD Comic Threads
Topic: 1190: "Time"
Replies: 107235
Views: 50132217

Re: 1190: "Time"

mscha wrote:He's taking the flag. :( No other changes, sea level constant.


The stretch at the right of the picture isn't water; Cueball walked along the top of it after the "swim."
by ThinkerEmeritus
Sat Sep 24, 2011 2:52 am UTC
Forum: Individual XKCD Comic Threads
Topic: 0955: "Neutrinos"
Replies: 159
Views: 43979

Re: 0955: "Neutrinos"

I am a first year undergrad physicist, and so my knowledge of particle physics is fairly rudimentary. Are neutrinos more or less weakly interacting than photons? That is, is it possible that neutrinos interact with an imperfect vacuum less than photons do, explaining the minor speed difference? Neu...
by ThinkerEmeritus
Fri Sep 23, 2011 11:59 pm UTC
Forum: Individual XKCD Comic Threads
Topic: 0955: "Neutrinos"
Replies: 159
Views: 43979

Re: 0955: "Neutrinos"

What we want or are comfortable with has nothing to do with what is. That is not an argument for or against any findings, simply a statement that considerations about "ease" are meaningless when it comes to arriving at scientific truth. I read that mostly as Thinker trying to give his tho...
by ThinkerEmeritus
Fri Sep 23, 2011 11:13 pm UTC
Forum: Individual XKCD Comic Threads
Topic: 0955: "Neutrinos"
Replies: 159
Views: 43979

Re: 0955: "Neutrinos"

There are problems with tachyons. They allow you to communicate, in principle at least, instantaneously with anyone else moving below c. This fact combined with special relativity can cause major problems with causality. You may determine that some kind of event always "causes" a differen...
by ThinkerEmeritus
Fri Sep 23, 2011 9:43 pm UTC
Forum: Individual XKCD Comic Threads
Topic: 0955: "Neutrinos"
Replies: 159
Views: 43979

Re: 0955: "Neutrinos"

Strictly speaking, the CERN experiment does not by itself violate special relativity. Relativity permits particles that always travel faster than light. What is forbidden is for an object to be capable of going both below and above the speed of light (c). Particles that always move (relative to us) ...
by ThinkerEmeritus
Mon Dec 14, 2009 7:40 pm UTC
Forum: Science
Topic: Scientific Method
Replies: 97
Views: 14046

Re: Scientific Method

I hate learning about the six-step process of the scientific method every damn year in every damn science class. There has been a lot of complaining lately in Science and other journals that high schools in the U.S. tend to teach science as a batch of facts to be memorized, without any attempt to e...
by ThinkerEmeritus
Wed Dec 09, 2009 6:46 pm UTC
Forum: Science
Topic: Homework thread. [Physics-->Statics]
Replies: 15
Views: 1297

Re: Homework thread. [Physics-->Statics]

The diagram is a cross-section of the system. The solid parts of the pipes are the two circles, and they extend toward and away from the viewer. Hence the 1.8 m doesn't show in the diagram -- it tells you how far apart (perpendicular to the paper) that there are similar supports, each with their own...
by ThinkerEmeritus
Fri Dec 04, 2009 6:03 pm UTC
Forum: Science
Topic: Why do we torture ourselves with Minkowski metric?
Replies: 36
Views: 5862

Re: Why do we torture ourselves with Minkowski metric?

The Euclidean metric with imaginary time used to be used a lot, and it works fine in Special Relativity. It even gives a heuristic reason for the occurence of hyperbolic functions of time/four-vector length in the transformations involving time. It's use has declined a lot. I agree with JWalker that...
by ThinkerEmeritus
Wed Dec 02, 2009 9:58 pm UTC
Forum: Science
Topic: Solution calculations - Help?
Replies: 4
Views: 1161

Re: Solution calculations - Help?

OK, to be more specific about my question, in the reaction equation, what does the "2" in 2NaHCO 3 mean exactly? Japp's questions are very good and should be helpful. Don't expect a template for solving your assigned problems from the textbook or from us [I know that comment may be unfair,...
by ThinkerEmeritus
Wed Dec 02, 2009 6:01 pm UTC
Forum: Science
Topic: Solution calculations - Help?
Replies: 4
Views: 1161

Re: Solution calculations - Help?

What does the reaction equation tell you about the relative amount of sulphuric acid and sodium hydrogencarbonate in the reaction? How to get that will be in your textbook, if necessary. Remember that the textbook tells you what happens, and it is your job to figure out how to use that information i...
by ThinkerEmeritus
Tue Nov 24, 2009 1:54 am UTC
Forum: Science
Topic: Centrifugal Force?
Replies: 8
Views: 3171

Re: Centrifugal Force?

If you are forbidden to use centrifugal force, which is a good thing to begin with, you must work the problem in a stationary frame. Use the ground under the the merry-go-round. In this stationary, inertial frame, the weight on the string is travelling in a circle around the center of the merry-go-r...
by ThinkerEmeritus
Mon Nov 23, 2009 6:58 pm UTC
Forum: Science
Topic: Is electric charge a scalar or vector?
Replies: 39
Views: 17567

Re: Is electric charge a scalar or vector?

"Is electric charge a scalar or vector?" "Yes" is the best answer. [Edit - ninja'd] OK, if I am not trying to be mean and funny at the same time, the answer is "It depends on what you are interested in." The question is subtle and interesting, and I have had to think qu...
by ThinkerEmeritus
Fri Nov 20, 2009 8:24 pm UTC
Forum: Science
Topic: Distance for matter-antimatter annihilation
Replies: 10
Views: 2046

Re: Distance for matter-antimatter annihilation

Let's see if I can say this reasonably correctly without resorting to a "[math]" tag. At the level of particles, no particle is ever at a single place. There is only a smeared out probability distribution of its being anywhere inside some region, low towards the fuzzy edges of the region a...
by ThinkerEmeritus
Sat Nov 14, 2009 1:18 am UTC
Forum: Science
Topic: Gravity, and such.
Replies: 28
Views: 3181

Re: Gravity, and such.

I think the original problem has been overlooked. Not really overlooked, since the Gauss's Law stuff addresses it, but no one has explained the ideas. So let's look at F g = GMm/r^2. This equation is valid for point masses, and you can't get inside point masses and sit at r=0. For an extended mass, ...
by ThinkerEmeritus
Fri Nov 13, 2009 10:04 pm UTC
Forum: Science
Topic: Special Relativity...learning from an older textbook
Replies: 5
Views: 922

Re: Special Relativity...learning from an older textbook

No, their time variable is still t. However, for measuring time they often use "time light takes to travel one meter", or mtt for short. And then length is defined by a standard meter. Never seen that before and not sure if it's standard. Yeah, it is standard but not often put that way. I...
by ThinkerEmeritus
Fri Nov 13, 2009 5:45 pm UTC
Forum: Science
Topic: Special Relativity...learning from an older textbook
Replies: 5
Views: 922

Re: Special Relativity...learning from an older textbook

Also check to see whether their time variable is ct instead of t.
by ThinkerEmeritus
Tue Nov 10, 2009 11:41 pm UTC
Forum: Science
Topic: Screwing up ampere's law
Replies: 1
Views: 493

Re: Screwing up ampere's law

Check the direction of your current, and remind yourself of what direction the current must go to count in Amperes Law.
EDIT: If that doesn't work, draw what you think B should be and check your integration of B around the curve, especially the direction of the curve.
by ThinkerEmeritus
Tue Nov 10, 2009 8:54 pm UTC
Forum: Science
Topic: Mechanics help
Replies: 6
Views: 1022

Re: Mechanics help

The force "-F app " should not be included in the free-body diagram, since it isn't a force exerted on the body in addition to the others shown. It might be helpful to your thinking to show it in a separate diagram, but it isn't a physical force. Its relevance comes from sum(Forces) = 0 as...
by ThinkerEmeritus
Mon Nov 09, 2009 7:28 pm UTC
Forum: Science
Topic: Simple Harmonic Motion
Replies: 9
Views: 1880

Re: Simple Harmonic Motion

Not so dumb, actually. The problem did after all say "assume no damping," and in that case just getting in the car would set off simple harmonic motion.The real problem is that the first sentence, if true, requires damping, and then the second sentence poses a problem without damping. More...
by ThinkerEmeritus
Mon Nov 09, 2009 2:38 pm UTC
Forum: Science
Topic: Kepler's 1st Law - Why Earth revolves in Ellipse,not circle?
Replies: 39
Views: 7425

Re: Kepler's 1st Law - Why Earth revolves in Ellipse,not circle?

I agree with Mr. Jack on this one. The OP wanted to understand why planets move in ellipses; proof was not the issue. A good qualitative discussion is the best response. If you are interested in the ideas and wouldn't be able or willing to follow the math, make sure that the person you listen to has...
by ThinkerEmeritus
Fri Oct 30, 2009 2:52 pm UTC
Forum: Science
Topic: I cant do kinematics?
Replies: 15
Views: 1986

Re: I cant do kinematics?

Solve it symbolically first, before you insert values. Then use latex math to present your solution. It is 400 times more difficult to analyze what you're doing wrong when you insert values straight into the calculation. I'm surprised that so few took this excellent advice. Among other things, if y...
by ThinkerEmeritus
Thu Oct 22, 2009 3:10 am UTC
Forum: Science
Topic: Conceptual physics help! (filled bucket going in circles)
Replies: 7
Views: 1288

Re: Conceptual physics help!

You are right, you can't spin a water bucket exactly parallel to the ground. Another way of saying what rflrob already said is that what is really meant by a problem stated like this is that the bucket is spun around fast enough that the rope is very nearly parallel to the ground, and you are intere...
by ThinkerEmeritus
Mon Oct 19, 2009 12:13 pm UTC
Forum: Science
Topic: So You Want To: Evaluate A University's Physics Program
Replies: 28
Views: 3666

Re: So You Want To: Evaluate A University's Physics Program

(btw TE: is there any such rankings for non-US universities? If so, where could I find them?).. I'm sure there must be, but I have never had any need for them and don't know where to find them. I'd start looking with the web sites of national and international Physics Societies, or ask a physics fa...
by ThinkerEmeritus
Sun Oct 18, 2009 10:39 pm UTC
Forum: Science
Topic: So You Want To: Evaluate A University's Physics Program
Replies: 28
Views: 3666

Re: So You Want To: Evaluate A University's Physics Program

First, if you haven't looked at the USNews and World Report rankings of graduate programs http://grad-schools.usnews.rankingsandreviews.com/best-graduate-schools/top-physics-schools/rankings , by all means take a look. They will give you a good idea of the research strength of the departments. There...
by ThinkerEmeritus
Thu Oct 15, 2009 9:29 pm UTC
Forum: Science
Topic: You might be a physics major if...
Replies: 540
Views: 62553

Re: You might be a physics major if...

Narius wrote:YMBAPM if you've ever had the opportunity to say "dammit I'm a physicist not a doctor!" and relished it.

YMBAPM if a physician has asked you what you are a doctor of, and then said "Darn, that stuff gave me fits in college."
by ThinkerEmeritus
Thu Oct 15, 2009 6:07 pm UTC
Forum: Science
Topic: Gravity and small particles question
Replies: 12
Views: 1156

Re: Gravity and small particles question

In an effort to track down what I orginially heard, I came across: Space is not curved. http://metaresearch.org/cosmology/gravity/spacetime.asp In this form, we can see the space-time interval ds as a purely time-like interval dt that was merely made to look length-like through multiplying it by c....
by ThinkerEmeritus
Wed Oct 14, 2009 1:08 am UTC
Forum: Science
Topic: Delta_0 and the neutron
Replies: 14
Views: 2055

Re: Delta_0 and the neutron

So can you have neutron<--->Delta_0 with input of some energy to flip the spin of one of the quarks? Yes, changing one or more spins and doing some subtle things with the way you handle which quark is the u-quark and which ones are d-quarks, so that the overall wave-function remains antisymmetric u...
by ThinkerEmeritus
Tue Oct 13, 2009 11:33 pm UTC
Forum: Science
Topic: [Help] Physics problem (changing radius and gravity)
Replies: 5
Views: 883

Re: [Help] Physics problem

It might be easier to see how it works out if you call the initial values m 1 , V 1 , and r 1 and then call the final values m 2 . V 2 , and r 2 . Then constant density means P = m 1 / V 1 = m 2 / V 2 , One of your equations that have "?" in them will involve V 1 /V 2 , and the other m 1 /...
by ThinkerEmeritus
Tue Oct 13, 2009 11:17 pm UTC
Forum: Science
Topic: Delta_0 and the neutron
Replies: 14
Views: 2055

Re: Delta_0 and the neutron

Yep. Spin is the angular momentum of a particle, measured in units of [imath]\hbar[/imath]. Since the [imath]\Delta[/imath] and the nucleon have different angular momenta, they are different particles.
by ThinkerEmeritus
Thu Oct 08, 2009 10:35 pm UTC
Forum: Science
Topic: visible galaxy
Replies: 16
Views: 2036

Re: visible galaxy

Actually 190 minutes of arc[ ' is minutes, not degrees], so a little over 3 degrees.
by ThinkerEmeritus
Wed Oct 07, 2009 9:54 pm UTC
Forum: Science
Topic: A particle in a changing potential
Replies: 9
Views: 2020

Re: A particle in a changing potential

Well, changing the size of the box doesn't have to involve something moving, for expanding the box at least. You can just have the walls disappear (or turn into an expanding cloud of neutrinos or something), revealing more walls behind them. This doesn't work quite so well for reducing the size sin...
by ThinkerEmeritus
Mon Oct 05, 2009 8:07 pm UTC
Forum: Science
Topic: A particle in a changing potential
Replies: 9
Views: 2020

Re: A particle in a changing potential

The adiabatic theorem might be of interest here. If the box width a(t) changes sufficiently slowly (adiabatically) then the system remains in the appropriate energy eigenstate, so the wavefunction at time t looks something like \psi \sim \sin(n \pi x/a(t)) as you might expect (with ...
by ThinkerEmeritus
Wed Sep 30, 2009 1:18 pm UTC
Forum: Science
Topic: quantum mechanics doubts
Replies: 27
Views: 3449

Re: quantum mechanics doubts

I don't understand that sorry :oops:, I guess I'll just wait for the derivation, if there will ever be one during the course. Thank you anyway! I didn't give you anywhere near enough to understand the momentum operator, so don't be sorry. If anyone needs to be sorry, it's me. The point was to give ...
by ThinkerEmeritus
Tue Sep 29, 2009 11:28 pm UTC
Forum: Science
Topic: quantum mechanics doubts
Replies: 27
Views: 3449

Re: quantum mechanics doubts

Don't worry about your English. Your writing is at least as good as my native-English effort. 1. I can tell you where \hat p\psi=p\psi comes from in words, which may help you be comfortable until a mathematical derivation shows up. Once you have expectation values set up and have seen some calculat...
by ThinkerEmeritus
Mon Sep 28, 2009 11:55 am UTC
Forum: Science
Topic: Water discovered on Moon - they say.
Replies: 13
Views: 2037

Re: Water discovered on Moon - they say.

At least some of the data are shown on the Astronomy Picture of the Day for September 28.
by ThinkerEmeritus
Thu Sep 24, 2009 12:54 am UTC
Forum: Science
Topic: Displacement to path?
Replies: 9
Views: 994

Re: Displacement to path?

It really is simple in principle. However, you seem to have made an algebra error, probably when solving for t 2 in terms of x. It would actually be a bit simpler if, when you started the problem, you had given algebraic names to your constants instead of putting in the numbers. For example, take x ...
by ThinkerEmeritus
Wed Sep 23, 2009 9:36 pm UTC
Forum: Science
Topic: Energy! Since fourth grade, I still don't get it.
Replies: 12
Views: 4866

Re: Energy! Since fourth grade, I still don't get it.

JWalker wrote:Energy is a consequence of a time translation symmetry fundamental to all systems at the core of physics.


[Much beautiful stuff follows.]

Bravo! I don't think I have ever seen that fact explained so well without equations.

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