## Search found 31 matches

Fri Feb 12, 2016 2:41 am UTC
Forum: Science
Topic: Is the entire universe a black hole?
Replies: 13
Views: 3899

### Re: Is the entire universe a black hole?

All hail Sean Carroll
Tue May 26, 2015 3:01 am UTC
Forum: Science
Topic: PhD study of this forum
Replies: 32
Views: 22976

### Re: PhD study of this forum

Sounds like a cool project! I am a current PhD student in physics. I'm not very active here in any capacity, but I read more than I post. The other online scientific community I occasionally participate in is the physics stackexchange: physics.stackexchange.com, under the same handle. Both the volum...
Fri May 01, 2015 11:39 pm UTC
Forum: Science
Topic: Constants and physical significance
Replies: 12
Views: 3296

### Re: Constants and physical significance

The http://en.wikipedia.org/wiki/Planck_mass, which is roughly the maximum mass an elementary particle could have before it became impossible to describe it without quantum gravity, is about the weight of a grain of sand.
Fri Apr 24, 2015 4:52 am UTC
Forum: Science
Topic: De Broglie Wavelength and humans
Replies: 13
Views: 5089

### Re: De Broglie Wavelength and humans

(Apologies to trumpet for hijacking your thread, but hopefully you're getting something out of this conversation as well) You may be right though. For a hot body moving at speed v slow enough that the de broglie wavelength due to coherent motion is the mean wavelength with the thermal de broglie wav...
Thu Apr 23, 2015 10:20 pm UTC
Forum: Science
Topic: De Broglie Wavelength and humans
Replies: 13
Views: 5089

### Re: De Broglie Wavelength and humans

It's true that you can separate out the centre of mass motion from the internal degrees of freedom and so calculate a de broglie wavelength; that wasn't why I said it wasn't meaningful though. That separation is only going to be possible for pure states where all the internal particles are in a spe...
Wed Apr 22, 2015 4:42 am UTC
Forum: Science
Topic: De Broglie Wavelength and humans
Replies: 13
Views: 5089

### Re: De Broglie Wavelength and humans

The idea that humans have a single meaningful wavelength isn't really true because humans are in an incredibly messy incoherent state. Well, it isn't so obvious that this is true. The property of a DeBroglie wavelength for your center of mass doesn't depend on having a simple internal structure. Se...
Fri Apr 17, 2015 12:36 am UTC
Forum: Science
Topic: De Broglie Wavelength and humans
Replies: 13
Views: 5089

### Re: De Broglie Wavelength and humans

drachefly is completely right, but setting the many other logistical issues aside, here's an exercise for you: -Calculate the speed one would need to travel at to make this happen -At the very least, you have to entirely go through the door for this to happen. That means you'd have to travel, say, 1...
Mon Dec 22, 2014 6:42 pm UTC
Forum: Science
Topic: Fermi's Golden Rule and laser rate equations
Replies: 5
Views: 3255

### Re: Fermi's Golden Rule and laser rate equations

Here is what mean by "it breaks down." For some two level system there is a coefficient describing the population in the initial state, c0 and a coefficient describing the population in the other state, c1. Fermi's golden rule is derived on the assumption that c1<<1. i.e. c1 must be very ...
Tue Sep 23, 2014 5:50 am UTC
Forum: School
Topic: Question for pursuing a PHD in physics
Replies: 8
Views: 11181

### Re: Question for pursuing a PHD in physics

That's what I do! It might be useful to know what country you are in. In the United States, unlike many other places, graduate physics programs are almost always PhD programs, which are taken after undergraduate. There is typically no option for a masters, other than quitting partway through the pro...
Sat Aug 30, 2014 8:27 pm UTC
Forum: Science
Topic: Holomorpic vs Holograpic
Replies: 7
Views: 3237

### Re: Holomorpic vs Holograpic

It's an interesting idea, but not quite as simple as you suggest. Any explanation of the holographic principle has to involve gravity in some essential way, I think. My (decidedly non-expert) intuition for this is that with, say, EM interactions you can have screening of positive and negative charge...
Thu Aug 07, 2014 2:29 am UTC
Forum: Science
Topic: EMDrive and other reactionless propulsion claims
Replies: 71
Views: 16200

### Re: EMDrive and other reactionless propulsion claims

It can't be pushing off the quantum vacuum. If you want to get anything out of the quantum vacuum, you need to provide all of the energy and momentum yourself. How exactly did conservation of momentum work when we knocked virtual photons into real photons with a superconducting quantum interference...
Sun Jun 08, 2014 9:53 pm UTC
Forum: Science
Topic: Why do only some fields generate particles?
Replies: 19
Views: 3721

### Re: Why do only some fields generate particles?

I have heard a particle is supposed to be a self interaction of a field so why do only some fields self interact? There are also some fields that seem to self interact but don't produce particles. For example I have heard the wind could be described as a vector field but if you place 2 fans facing ...
Thu Mar 20, 2014 10:53 pm UTC
Forum: Science
Topic: Planck lengths and length dilation
Replies: 11
Views: 4099

### Re: Planck lengths and length dilation

Say you have an object in space of a certain length. That length could be given in natural units as a number of Planck lengths, right? Depending on your state of motion and thus reference frame, that length will be different, right? Does that mean that the number of Planck lengths along the object ...
Thu Mar 06, 2014 8:15 pm UTC
Forum: Science
Topic: Rate of Entropy
Replies: 18
Views: 4460

### Re: Rate of Entropy

My pedantic correction: it would be better to phrase this as "rate of entropy production." My short answer: No. My long, rambling answer: Well, first of all for this to be meaningful you'd have to specify what constraints you mean by "as fast as possible." For example, if you jus...
Mon Aug 26, 2013 11:55 pm UTC
Topic: 1256: "Questions"
Replies: 215
Views: 42809

### Re: 1256: "Questions"

omgryebread wrote:Why are there mustaches on cars?
???

'Cause Lyft.
Thu Jun 20, 2013 3:25 pm UTC
Forum: Science
Topic: Conservation of information
Replies: 13
Views: 5096

### Re: Conservation of information

We have to be careful here. Information (in the information theory sense) increases with entropy. (i.e. entropy and information are measured in the same units, so higher entropy = higher information. Of course, we can always ask "which information is entropy measuring?" and the answer is ...
Wed Jun 19, 2013 4:25 am UTC
Forum: Science
Topic: Conservation of information
Replies: 13
Views: 5096

### Re: Conservation of information

I've heard the statement that "information cannot be [created or] destroyed" on a number of occasions and in a few different contexts, but I'm curious: in precisely what sense is this true? I've read a fair bit (no pun intended) about information theory (though it has been a while), and t...
Thu Jan 24, 2013 12:08 am UTC
Forum: Science
Topic: Quick question about photonic interference
Replies: 5
Views: 1940

### Re: Quick question about photonic interference

So that's a definite yes, though it's hard to recreate artificially? That's what I got from the paper. So far as I can tell, if a photon is in phase with another it will interfere with it. Which means a theoretically superpositioned photon would still interfere with another photon in phase with it ...
Tue Dec 18, 2012 10:06 pm UTC
Forum: Science
Topic: Photon frequency, Faraday cages, and relativity
Replies: 4
Views: 2223

### Re: Photon frequency, Faraday cages, and relativity

Qaanol, I think your initial thought experiment is basically valid. Photon number is not Lorentz-invariant even in a vacuum, due to the way fields transform, so you've just constructed a situation when it's extra-super-not-Lorentz-invariant. If it worked as you describe, then a pinhole would provide...
Thu Oct 25, 2012 3:11 am UTC
Forum: Science
Topic: Speed of gravity
Replies: 16
Views: 9292

### Re: Speed of gravity

I surrender on the term of radiation! Yeesh :cry: Also, I suppose I'd been expecting something explicitly mentioning the speed of gravity. But then, they're so close I wonder how much any variation in speed of C versus a hypothetical infinite speed would even change the resulting observations. Goin...
Wed Jun 20, 2012 12:51 pm UTC
Forum: Language/Linguistics
Topic: Why do other writing systems use Roman punctuation?
Replies: 13
Views: 5149

### Re: Why do other writing systems use Roman punctuation?

Is "ma" used in all questions? No. There are quite a few ways to make questions in Chinese. Every question (that I can think of, at least) is unambiguously identifiable as a question without a ?, but in many it is only from word order instead of the use of special 'question' words (that i...
Sat May 12, 2012 9:59 am UTC
Forum: Science
Topic: Quantum Entanglement
Replies: 39
Views: 6253

### Re: Quantum Entanglement

Or, to sum all such interpretations of quantum physics up "We don't know. We probably don't have enough actual data to really know, so we've come up with as many theories as we can; and then strung them up to their logical breaking points. We can't even test the theories we have now, some of t...
Fri Apr 13, 2012 3:42 pm UTC
Forum: Science
Topic: How long until 'Peak Science'?
Replies: 75
Views: 38403

### Re: How long until 'Peak Science'?

The Earth was first circumnavigated in 1522. Bell first demonstrated his telephone in 1876. The Wright brothers' first flight was in 1903. The first atom bomb was tested in 1945. So you've picked four events that took place over the course of > 400 years, and you're lamenting that you haven't seen ...
Thu Mar 15, 2012 2:59 pm UTC
Forum: Science
Topic: [SOLVED] Refraction of light
Replies: 14
Views: 4787

### Re: Refraction of light

It looks like everyone else is happy, but none of these explanations really do it for me. Qaanol's explains the change in wavefront, but not the change in momentum direction, since in any configuration other than an infinite evenly spaced line his cows have a different group velocity and phase veloc...
Mon Feb 27, 2012 4:08 pm UTC
Forum: Science
Topic: Uniform charge density in all of space
Replies: 5
Views: 2219

### Re: Uniform charge density in all of space

Sat Dec 24, 2011 10:50 pm UTC
Forum: Science
Topic: Pauli Exclusion Principle and Brian Cox
Replies: 16
Views: 8866

### Re: Pauli Exclusion Principle and Brian Cox

Why accept something that's probably wrong, jut because someone's smart, or the subject is complicated? You shouldn't, of course. The claim seems pretty nobberish, anyhow. The exclusion principle keeps electrons from being in the same state, but many different states can have the same energy. For e...
Wed Dec 21, 2011 2:02 pm UTC
Forum: Science
Topic: Pauli Exclusion Principle and Brian Cox
Replies: 16
Views: 8866

### Re: Pauli Exclusion Principle and Brian Cox

He's absolutely, incontrovertibly right. Here is a good example of the concept, using a double-well potential: http://www.hep.man.ac.uk/u/forshaw/BoseFermi/Double%20Well.html The key point is that we think of an electron in an atom as being 'isolated', but that's not how quantum mechanics actually w...
Sun Dec 11, 2011 8:25 am UTC
Forum: Science
Topic: Hawking radiation and conservation of charge
Replies: 20
Views: 3449

### Re: Hawking radiation and conservation of charge

Now, a superposition of eigenstates with the same eigenvalue is always another eigenstate. I added an important part here. In terms of energy, momentum and so on, you can look at every linear combination you want and it is an eigenstate. For spin in an (arbitrary) direction, this is not true. You h...
Sat Dec 10, 2011 3:19 pm UTC
Forum: Science
Topic: Hawking radiation and conservation of charge
Replies: 20
Views: 3449

### Re: Hawking radiation and conservation of charge

Then I'm probably misunderstanding what the spin of the photon, and its helicity represents. On wikipedia, they say it correspond to either right or left handed circular position. My understanding is that these are the eigenstates, and that linear polarization is a superposition on these two eigens...
Wed Oct 26, 2011 11:25 am UTC
Forum: Science
Topic: Why does light move at the speed it does?
Replies: 83
Views: 13966

### Re: Q about the speed of light

I've only had light relativity (pun intended?), but maybe I can give a slightly better answer than "because." First, a fact about space-time: everything travels through it at the same overall speed, which of course is c. An object in its rest frame can be said to be moving through space at...
Sun Oct 09, 2011 2:42 pm UTC
Forum: Science
Topic: [Rather long] Reciprocal lattice -> band structure?
Replies: 6
Views: 3955

### Re: [Rather long] Reciprocal lattice -> band structure?

Okay, well I don't think I can summarize a whole semester's class in one post (or want to!), but maybe I can help with the big picture. It sounds like you don't really have any physics background, right? That is part of the problem. For example, wavenumber is a ubiquitous idea in physics. It is just...