Ask a physicist anything. (8)

[lesliedellow]

Not every consequence of a theory is known to its developers. I don't doubt that parsimony was one of the reasons people dropped geocentrism, but that doesn't mean it's the only one. Classical gravitation requires the Earth to accelerate around the barycentre of the Earth/Sun system. That Newton was unaware of this is irrelevant.

I imagine Newton would have realised that the Sun and the Earth both orbited a common centre of mass, but that that centre of mass was so close to the Sun's centre as to make no difference.

He doubtless realised that there were other planets involved in determining that centre of mass as well. Whether or not that would have made the calculations too complicated, without using later formulations of Newtonian mechanics, I don't know.

 

[Michael]

Suppose there's an idealized point mass fixed in place (is this no longer practical?) somewhere along your interval.

You want to calculate the work done by gravity as another (test) mass moves from a to b through the point mass.

Work is the integral of F along the path.

Since gravity is an inverse square, F blows up when the test mass passes through the point mass.

You can use symmetry to eliminate the singularity and still get a sensible result. (Or even better, since gravity is a conservative force, the result is path independent, and you can write down the potential energy as a function and just take the difference in potential energy from points a and b.)

OK, now an inverse square is not a simple pole.

But if instead of a point mass, we have an infinite line of mass (is this impractical?), then the force goes as 1/r, and it's a simple pole.

If you add more infinite lines of mass, you can have multiple poles.

FYI, your wealth of understanding over a wide range of areas in physics impresses me quite often. WC is excellent too of course, but you're awesome.

 

[Wiccan_Child]

I imagine Newton would have realised that the Sun and the Earth both orbited a common centre of mass, but that that centre of mass was so close to the Sun's centre as to make no difference.

He doubtless realised that there were other planets involved in determining that centre of mass as well. Whether or not that would have made the calculations too complicated, without using later formulations of Newtonian mechanics, I don't know.

Ah, I read 'Newton' instead of 'Newton's university'. Nevertheless, the Earth is not an inertial frame, even in classical gravitation. To me, this is a more solid foundation for heliocentrism than the fact that heliocentric maths is simpler (unless you're a rocket scientist, of course).

 

[Senator Cheese]

Is the universe determined? I understand that in classical physics, it is determined. I understood the Heisenberg principle to merely state that you cannot know both exact location and speed of a particle, not that it doesn't have one. And I haven't understood basic quantum mechanics at all.

So.. in short, can particles move "arbitrarily", or is their movement always determined by other particles/forces (which are themselves predetermined by other particles/forces, etc).

Thanks!

 

[Justatruthseeker]

Is the universe determined? I understand that in classical physics, it is determined. I understood the Heisenberg principle to merely state that you cannot know both exact location and speed of a particle, not that it doesn't have one. And I haven't understood basic quantum mechanics at all.

So.. in short, can particles move "arbitrarily", or is their movement always determined by other particles/forces (which are themselves predetermined by other particles/forces, etc).

Thanks!

Once a particle is set in motion, it will always retain that course and motion until affected by another particle, which outcome is yes, predetermined. Two particles on set trajectories and velocities, will interact in only one possible way. No matter how many particles you add, there is only one set outcome. Like you mentioned we call some things random, simply because we lack the technology to account for all the variables. In the Heisenberg uncertainty principle, that variable is the energy input into the system from viewing it. It does not mean it can't be known, just that with our current technology we can not calculate it.

 

[Subduction Zone]

Once a particle is set in motion, it will always retain that course and motion until affected by another particle, which outcome is yes, predetermined. Two particles on set trajectories and velocities, will interact in only one possible way. No matter how many particles you add, there is only one set outcome. Like you mentioned we call some things random, simply because we lack the technology to account for all the variables. In the Heisenberg uncertainty principle, that variable is the energy input into the system from viewing it. It does not mean it can't be known, just that with our current technology we can not calculate it.

Not necessarily. Your "reality" may not match real reality. On a quantum scale particles are not "real". They are best represented as "probability packets". That is shown by the double slit experiment performed on particles. Or how would you explain that classically? Even when the particles go through the slits one at a time the particles still "interfere with themselves" and make an interference pattern when they hit an opposing wall.

 

[essentialsaltes]

Is the universe determined? I understand that in classical physics, it is determined. I understood the Heisenberg principle to merely state that you cannot know both exact location and speed of a particle, not that it doesn't have one.

I would maintain that what the Heisenberg Uncertainty Principle entails is that the particle does not have an exact location and momentum simultaneously. It is not merely a limitation on our instruments and measurements -- it is really how things are.

So.. in short, can particles move "arbitrarily"

I don't know about arbitrarily, but the standard interpretation of quantum mechanics is not deterministic. Certain quantum events are inherently random and un-pre-determined.

 

[AV1611VET]

WC, congratulations on your 8th thread.

Impressive!

 

[lesliedellow]

At the macroscopic level yes. Although it is theoretically possible for something weighing a few grams to exhibit behaviour not in accord with Newtonian mechanics, in practice you would have to wait longer than the age of the universe to witness such an event.

Whether or not, at the sub atomic level, there are hidden variables, operating in accordance with causal laws, but unobservable in principle by us, that is something debated amongst physicists. The majority view is that there aren't.

 

[Wiccan_Child]

Is the universe determined? I understand that in classical physics, it is determined. I understood the Heisenberg principle to merely state that you cannot know both exact location and speed of a particle, not that it doesn't have one.

No, it states it doesn't have one. It's one of the major misinterpretations of quantum mechanics (alongside quantum mystics being too literal and liberal with the 'observer' in the observer effect) - quantum uncertainty isn't simply limit in technology, it's a limit in what actually exists. It's not that they have a precise position and momentum and we're just limited in how well we can simultaneously know both... it's that those quantities are not discrete.

And I haven't understood basic quantum mechanics at all.

So.. in short, can particles move "arbitrarily", or is their movement always determined by other particles/forces (which are themselves predetermined by other particles/forces, etc).

Thanks!

Quantum mechanics states that particles are described by a wavefunction, and that wavefunction must obey an equation called the Schrödinger equation, and this largely determines how likely the particle is to be at any given point when you measure it. This means that, on average, large groups of particles act classically, but individual particles can be more... erratic.

It's like radioactive decay. There are rules rigidly followed by the aggregate (half of a radioactive substance will decay over a time T[sub]1/2[/sub], implying exponential decay with a time constant λ = ln(2)/t[sub]1/2[/sub], etc), but any individual radioactive particle is utterly unpredictable. We now the probability distribution, but not the final outcome. I know the distribution of wins in a national lottery, but not who will win.

 

[Wiccan_Child]

WC, congratulations on your 8th thread.

Impressive!

Thank you! I actually know nothing about physics, I'm just very good at bluffing

 

[Zippy the Wonderslug]

Could someone give me a few algorithms or formulas that might make for some interesting art design on a drawing website?

It has this feature that draws a line between two points so I'm hoping for something creative using that tool.

The dimensions of the drawing will be 939x597.

I'll be using Google Spreadsheets and a hex editor to make a script for a mouse recording program to do this.

Cheers.

 

[Wiccan_Child]

I was trying to work out how long I've been on CF, and it occurs to me that today marks the end of the ninth full year. Nine! Years!

So just a quick message to say THANK YOU to all the friends (and foes!) here at CF, to AV1611VET for showing me how far down the rabbit hole of Christian thought goes, to Michael for showing me the Electric Universe paradigm, to dad for forcing me to back up even the most basic suppositions (PO fishbowl science!), to Maxwell for reviving this thread and getting it to its eighth incarnation (8,733 posts!), to people to numerous to mention, to everyone who's changed my mind, and to those who cared enough to poke holes in my most cherished and deeply held beliefs (I was, after all, once Wiccan).

Have some cake!

 

[pgp_protector]

...snip...
Have some cake!

Isn't that cake like 5 years old

 

[Michael]

I was trying to work out how long I've been on CF, and it occurs to me that today marks the end of the ninth full year. Nine! Years!

Hey, Congrats! That's actually pretty cool IMO. You've contributed a great deal to this board and this specific forum and I appreciate it. We may disagree about a few things, but I enjoy most of your posts, and the rest I just try to ignore.

Have some cake!

Yum.

 

[selfinflikted]

I actually know nothing about physics, I'm just very good at bluffing

A likely story, I'm sure.

 

[Wiccan_Child]

Hey, Congrats! That's actually pretty cool IMO. You've contributed a great deal to this board and this specific forum and I appreciate it. We may disagree about a few things, but I enjoy most of your posts, and the rest I just try to ignore.

Agreed!

 

[lesliedellow]

I was trying to work out how long I've been on CF, and it occurs to me that today marks the end of the ninth full year. Nine! Years!

You must have done something pretty bad to be sentenced to nine years on here.

 

[TerranceL]

Have some cake!

What are the equations on the cake?

Oh and happy... erm.... account birthday?

 

[Elendur]

What are the equations on the cake?

Oh and happy... erm.... account birthday?

My tip:
Feynman diagram - Wikipedia, the free encyclopedia

Here's also something that's related, which I found interesting:
New Discovery Simplifies Quantum Physics

And I second that motion