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Some physics questions

#42
bburk said:
Instinctively, that's what I would think too. But seriously, the Earth would orbit an empty bit of space for those 8 and some odd minutes.



No problem, IMO TheBigO gave a better explanation. ;)
Click to expand...

really? care to elaborate? :)

and also see edit:smug:

Edit: Brad kinda beat you to it. thanks but do tell more.

we never got to Einstein last year:(
that really screwed me over on the ap test
 
#43
ThatGermanDude said:
really? care to elaborate? :)

and also see edit:smug:

Edit: Brad kinda beat you to it. thanks but do tell more.

we never got to Einstein last year:(
that really screwed me over on the ap test
Click to expand...

lol, yeah, you don't see sophisticated treatments of electrodynamics in AP physics!

maybe you've heard of maxwell's equations? well, they're the equations governing electromagnetism. if you fiddle with them, you get the result that electric and magnetic fields travel at the speed of light.

with a little tinkering of history, the story is that einstein realized that this implied that the influence an electromagnetic body exerted on another had a finite speed of propagation, while the prevailing view of mechanics was that it should take no time at all.

einstein held maxwell's equations to be true and then modified mechanics accordingly, leading to special relativity.*

however, it still remained that one massive body could influence another massive body instantaneously through newton's law of gravity. so einstein reworked gravity for it to travel with a finite velocity, and you get general relativity.



i'm struggling through a superficial reading of landau and lifshitz classical theory of fields. it is not gentle AT ALL. :crying:



*i've committed the ultimate sin in talking about relativity by not mentioning "intertial frames of reference," and i don't care to.
 
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#46
ThatGermanDude said:
really? care to elaborate? :)

and also see edit:smug:

Edit: Brad kinda beat you to it. thanks but do tell more.

we never got to Einstein last year:(
that really screwed me over on the ap test
Click to expand...

Hehe, sorry about that.. I initially brought it up as a kind of joke between myself and theBigO because I knew what a can a worms it would open up. It's pretty difficult to explain (especially in this format).

I knew I'd reel someone in with that one :).

Basically, the example demonstrates the Einsteinian view of the speed of gravity. Here's a fairly simple treatment of the idea: http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

In essence, Einstein set the galactic speed limit firmly at the speed of light. This idea is integral to relativity. The idea that anything can possibly travel faster than that wreaks all kinds of havoc with our understanding of relativity. So, when he noticed the old Newtonian idea that gravity can propagate instantaneously, he took exception and claimed that in actuality it can not propagate faster than the speed of light. Unfortunately, gravitational force is so weak that (as far as I know) there have actually been no definitive experiments proving Einstein right (Though there have been plenty proving other applications of general relativity).

Anyway, the problem stems from the fact that we don't really know what gravity *is*. We know what light is. We know what electro-magnitism is. But we are thus far only able to detect the fact that gravity actually happens, not what causes it.


I'm really not the person to give a dissertation on this subject. There obviously a TON of stuff I'm just glossing over. However, I personally find the whole subject fascinating.

I suggest you grab a book on the subject if you are still interested. Hawking's A Brief History of Time is a good start for 'laymen'. If you can grok that, you can proceed as far down the rabbit hole as you care to... :D
 
#48
Jazz Ad said:
Einstein's book are very simply written and interesting.
A good read, even if some of their content is outdated with today's knowledge.
Click to expand...

I really should read some of his books. Could take me a while I get bored reading my Physics text book and it not super heavy duty.

This semester I'm doing a Physics subject as an elective in my degree, its Electromagnetism, optics, Quantum Physics and Relativity (from memory). So far I've found it interesting yet fairly confusing (I end up understanding the content but I have to study into it a bit). What I find confusing is that fact that this course is centred on understanding the concepts and how to derive equations from the concepts behind them. Conversely all my previous physics has been centred more on being familiar with a concept and being able to use it in order to solve problems.


A good example of this Electric Flux. Previously I was just told electric flux is Electric field through and Area. However in this course we look much deeper into Flux and how to derive gauss's law by integrating the component of the electric field perpendicular to the surface for the area of the surface.
 
#50
bburk said:
Ahh yes, the Newton-cannonball thing. I forgot about that. Nice work.



Don't forget to mention why it would take more that 8 minutes from the time that the Sun 'disappears' for the earth to stop orbiting the place where the Sun used to be... :ninja:
Click to expand...


This is an example of trying to answer a question that is paradoxical. The "8 minutes until we notice" answer assumes that relativity prevents the Sun's disappearance from affecting us until the 8 light minute distance elapses. But that also assumes that it's physically possible for the sun to "instantaneously" disappear. It can't, due to the same principal of relativity, hence it's like asking "What happens if an irresistable force acts on an immovable object?"
 
#53
hbarcat said:
This is an example of trying to answer a question that is paradoxical. The "8 minutes until we notice" answer assumes that relativity prevents the Sun's disappearance from affecting us until the 8 light minute distance elapses. But that also assumes that it's physically possible for the sun to "instantaneously" disappear. It can't, due to the same principal of relativity, hence it's like asking "What happens if an irresistable force acts on an immovable object?"
Click to expand...

well, of course the problem is pathological, but it at least demonstrates a principle.

feel free to solve the more realistic case where the mass of the sun slowly disappears. (of course, to be absolutely accurate, you can't neglect the gravitational effects of the escaping gas particles!)

even then, the retardation effect that the toy problem shows will still be present, and that's why it's worth considering to begin with.
 
#54
This is all such fascinating stuff. I'm definitely going to have to check out that Hawking book. This stuff confuses the crap out of me, but once I actually think about it for a minute, it makes sense and it's simply amazing. And no matter how much I begin to understand, it only spurs more questions. It's so cool...:hyper:


Here's another question I have, does the sun orbit anything? Or does it just stay in the exact same place all the time?
 
#55
Brad Barker said:
well, of course the problem is pathological, but it at least demonstrates a principle.

feel free to solve the more realistic case where the mass of the sun slowly disappears. (of course, to be absolutely accurate, you can't neglect the gravitational effects of the escaping gas particles!)

even then, the retardation effect that the toy problem shows will still be present, and that's why it's worth considering to begin with.
Click to expand...


Of course! When you solve the problem for the case where the mass of the sun slowly disappears you need to come up with a physical explanation for the process before you can solve the problem of what happens to an object in motion around the sun. Suppose the sun loses mass by slowly ejecting it due to increased radiation pressure. As the mass leaves the vicinity of the sun and expands outward past the Earth's orbit it will exert less of a gravitational pull inward and result in a larger orbit. But this process is not going to pose any relativistic problems for "instantaneous" effects on Earth's orbit. Any hypothetical problem of lessening mass that does pose such a problem will also necessarily be paradoxical.
 
#57
hbarcat said:
Of course! When you solve the problem for the case where the mass of the sun slowly disappears you need to come up with a physical explanation for the process before you can solve the problem of what happens to an object in motion around the sun. Suppose the sun loses mass by slowly ejecting it due to increased radiation pressure. As the mass leaves the vicinity of the sun and expands outward past the Earth's orbit it will exert less of a gravitational pull inward and result in a larger orbit. But this process is not going to pose any relativistic problems for "instantaneous" effects on Earth's orbit. Any hypothetical problem of lessening mass that does pose such a problem will also necessarily be paradoxical.
Click to expand...

I feel like you are just being overly argumentative for kicks... ;)

Of course we realize that realistically the Sun cannot just disappear instantaneously. It's a thought experiment... For this purpose we can ignore certain aspects of the problem in order to demonstrate a principle.

BTW, I didn't make this thought experiment up, Einstein did... He seemed like a pretty smart guy and didn't seem to have any qualms about approaching an obviously complex and incredibly unintuitive theory in this manner. In fact, I'd go so far as to say that the majority of Einsteins breakthroughs were initially approached in this way.

The man didn't start with the whole theory of relativity and then set about to experimentally prove it in it's entirety. He thought up little thought experiments (initially ones having to do with light) and worked out the implications, gradually building the full theory.


Do you approach other hypothetical queries in the same manner?

"Would you kill Hitler in 1935 if you could"
"It's the year 2007, besides I wasn't even born then. Thats not possible."

"Imagine what your life would be like if you were born [rich/a different race/a man/woman]"
"But I am already me, there is no way to make that happen, what a silly thing to say. Do you need mental health treatment, you are talking crazy."

Hypothetical situations and thought experiments serve a purpose for scientists. They let them explore the implications of theories without having to worry about all the extraneous details. This is especially useful for sciences that require a more 'creative mind' such as quantum or astro-physics because attempting to hold the whole thing in your head is a ticket to insanity.
 
#58
The Lynchinator said:
Here's another question I have, does the sun orbit anything? Or does it just stay in the exact same place all the time?
Click to expand...

The sun orbits the center of the galaxy and moves around it about about 200 km/s or so. I think it takes about a quarter of a billion years to complete one orbit. The galaxy is also moving around within our Local Group of galaxies which contains around 30 or so galaxies.

Of course, on a technical level the sun orbits the center of mass of the solar system.
 
#59
i get the feeling that hbarcat insists on including the entirety of the universe in physics problems about pushing blocks up inclined planes. :p

i love how in physics, every problem is a new universe, so to speak. for my intermediate EM1 class, i had to calculate the energy required to assemble a spherical object with charge Q and radius R by bringing in little bits of infinitesimal charge dq from an infinite distance.

this sort of requirement for abstraction is a strength of physics, not a weakness. (in fact, the development of physics was precipitated by galileo's abstractions about frictionless motion, etc.)
 

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