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A duck walks into a bar. Animal control is promptly called and the duck is released into a near by park.


Feb
7
comment What justifies dimensional analysis?
@KyleKanos: Of course you must give meaning to it, but it's not like that step is avoided in the case of making sense of $3\cdot 2\ kg\ s^{-1}$. You just effectively define "makes sense" as "no person working in a physics department has been able to publish a paper on it that others find valuable". You don't tell the OP something which he doesn't know.
Feb
7
comment What justifies dimensional analysis?
But you don't justify why it doesn't make sense, you just observe that it's of no use for anyone you have heard of. You just have no application for the expression $3kg+2s^{-1}$ and think you can infer from this that it's meaningless, while $3·2\ kg\ s^{-1}$ is "physical". As a side note, I also don't know how to infer if something is physical or unphysical.
Feb
7
comment What justifies dimensional analysis?
Here is a link to Terrence Tao's blog, where he discusses mathematical underpinning of physical units. I personally guess that types might be a better approach. @KyleKanos: I don't see how what you answered clears things up at all. You just say "that's how we do it". The motivation, or why certain things "make sense" and others don't, is lost on me. I, for one, am a PhD student with a physics degree and I don't quite why physical units work either.
Feb
6
comment Does Gravity / curved space cause rotation?
Are you asking if an extended object tilts (and then rotates) when headed in the general direction of another massive object?
Feb
4
comment What is fundamentally physically impossible?
Okay, so you define science to be that what scientists do and know, which can change. And then things "that absolutely cannot happen" are just the things what scientist at the moment where you life don't believe to be possible, right? Otherwise I don't think something we can comprehend could be impossible. For a statements like "mass of a particle can't change" (or whatever your impossible-candidate is) must necessarily be statements made by humans and hence underlie a theory (e.g. the idea of "mass") and every concept eventually gets corrected and replaces, rendering the old statements fuzzy.
Feb
4
comment What is fundamentally physically impossible?
I don't understand the paragraph which is followed by "That is obviously...". And what does it mean for science to change? Are you saying there are things which are physically impossible or not?
Feb
2
accepted In which field theories with fermions do string- and fivebrane structures not come up?
Jan
29
awarded  Popular Question
Jan
28
comment Textbook on the Geometry of Special Relativity
Relativity, Groups, Particles by Sexl and Urbantke will work.
Jan
25
asked What is a Nicolai map?
Jan
23
comment Are Newton's “laws” of motion laws or definitions of force and mass?
@joshphysics: That wasn't supposed to be my theme, but I agree to some extent (while I still think axiomatizations are great fun and important to our understanding, if done carefully). The claim in your answer just caught my eye and something told me to point out the trouble I'm having with it. Just as now I feel I should point out that " If I say unicorns exist, then I'm saying something 'untrue,' because they have not been observed" isn't true, I'm sure people have seen them. "Believe to have seem them", if you will. It's appearently not reproducable anyhow. But okay, let's leave it at that.
Jan
23
comment Are Newton's “laws” of motion laws or definitions of force and mass?
@joshphysics: "...how is any definition in physics a meaningful statement about the world if there isn't an existence statement underlying it?" In the logical sense: If $A$ and if from $A$ follows $B$, it follows $B$. I.e. axiom: $A\land (A\implies B)\implies B$. No need for existence. That's the example: the empty set is postulated to exist and then used to model numbers. I don't quite see where the existence of reference frames is used. We can say "If we see no change in $v$ (to some desired accuracy), then $B$", that's enough. I think your axiomatization just wantso make sense of Newton 1.
Jan
23
comment Are Newton's “laws” of motion laws or definitions of force and mass?
@joshphysics: What is my sentiment that you share? I'm sure trying to axiomize stuff is helpful, specifically if one want to find flaws in the old mindset. It also clarifies "the status of our empirical knowledge" in some way, but I don't quite know how as this topic is difficult. If there is "empirical knowledge" about masses of particles, then we must recognize that this idea presupposes a physical theory (involving stuff like "mass"). "Also, existence statements tell us something about how the world actually is" How? If I say unicorn exist, does this give us info about how the world is?
Jan
22
revised Is the step of analytic continuation unavoidable or can you model around it?
added 21 characters in body
Jan
22
comment Are Newton's “laws” of motion laws or definitions of force and mass?
And the justification "These are frames of reference in which when an object is isolated from all other matter, it's local acceleration is zero. It is an empirical fact that such frames exist, and we'll take this as the first law" is problematic. Firstly, I don't quite know what an empiric fact is, but I have an engineering understanding of it. But then we can never know if some thing are or can be isolated and hence we can not test this concept. If my friend and I jump off a building, he will not accelerate next to me and so I'd not assume there is a force somewhere.
Jan
22
comment Are Newton's “laws” of motion laws or definitions of force and mass?
What, would you say, is the aim of an axiomatization of Newtonian mechanics? And then regsrding the first axiom: How is an unconditional axiom of the form "there exists $X$" of any use? Is there anything constructively build on the inertial frame (analogous to, for example, the empty set, where you can model numbers in terms of set). I don't quite understand physics, so I might just not see the use, but I see potential harm in enforcing an axiomatization in terms of three sentences, just to conform to the frameworks history.
Jan
22
comment Is force a “real thing”, or a tool for explaining changes in measurable phenomena?
Regarding your first line, What exactly is ment by "is simply a definition of force"?
Jan
14
revised In which field theories with fermions do string- and fivebrane structures not come up?
added 16 characters in body
Jan
14
asked In which field theories with fermions do string- and fivebrane structures not come up?
Jan
14
revised Gupta-Bleuler Formalism
langle rangle and some text changes, if you like