Author Topic: Re: BP: War in Heaven discussion  (Read 1221 times)

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Offline Luis Dias

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Re: BP: War in Heaven discussion
Well presumably if you've got artificial gravity you can help deal with those Gs. Gravitational force due to acceleration and mass are physically identical.

That's why they are called "Gs"....

 

Offline General Battuta

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Re: Re: BP: War in Heaven discussion
Well presumably if you've got artificial gravity you can help deal with those Gs. Gravitational force due to acceleration and mass are physically identical.

That's why they are called "Gs"....

They're called G-forces to indicate that the force experienced is one multiple of Earth's standard gravity; that alone doesn't imply the above equivalence. The equivalence between gravity and acceleration is set out in general relativity.

 

Offline Luis Dias

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Re: Re: BP: War in Heaven discussion
Well presumably if you've got artificial gravity you can help deal with those Gs. Gravitational force due to acceleration and mass are physically identical.



That's why they are called "Gs"....

They're called G-forces to indicate that the force experienced is one multiple of Earth's standard gravity; that alone doesn't imply the above equivalence. The equivalence between gravity and acceleration is set out in general relativity.

If they weren't equivalent, they wouldn't be measured in the same units. I think I'm stating quite the obvious thing.

 

Offline General Battuta

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Re: Re: BP: War in Heaven discussion
Well presumably if you've got artificial gravity you can help deal with those Gs. Gravitational force due to acceleration and mass are physically identical.

That's why they are called "Gs"....

They're called G-forces to indicate that the force experienced is one multiple of Earth's standard gravity; that alone doesn't imply the above equivalence. The equivalence between gravity and acceleration is set out in general relativity.

If they weren't equivalent, they wouldn't be measured in the same units. I think I'm stating quite the obvious thing.

You're not. You've got something backwards, which is the statement that G-forces, a unit of acceleration, are called G-forces because gravity and acceleration are physically indistinguishable.

You're confusing two separate but related branches of thought. Acceleration and gravity could be quite separated on the physical level and it would still be meaningful to refer to G-forces as multiples of the forces of gravity. One G says 'you are experiencing acceleration which produces a force on your body equivalent to the force (and acceleration) generated by Earth's gravity.' You could instead refer to them as M-forces, multiples of the force produced by the magnetic attraction of a given amount of magnetic substance on a chunk of iron. It would be equally meaningful in the aerospace context, just less convenient; one could be pulling .6 Ms. The only physical knowledge you need to generate this analogy are Newton's laws of motion.

However, the equivalence principle is something different. It states that the pseudoforce experienced by an observer in a non-IRF is equivalent to the force generated on an observer by the gravity of a local object, because both are examples of objects attempting to follow spacetime geodesics. You cannot draw this conclusion using only Newtonian mechanics, because an observer stationary on the surface of a non-rotating massive object in Newtonian mechanics is in an IRF.

Therefore the Einsteinian statement of the equivalence of the non-IRF pseudoforce and the force of gravity is quite meaningfully different from the statement that G-forces are called G-forces because they measure acceleration (and thus force) in multiples of the force exerted by Earth's gravity.

It's rarely wise to state something is 'quite the obvious thing' when it was only pinned down as a result of the geometric nature of the general theory of relativity.  ;)

 

Offline Luis Dias

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Re: Re: BP: War in Heaven discussion
Hmm no. I appreciate the lesson of general relativity, despite having been myself a physics olympiads finalist back in high school days and having read a gazillion of books about it, but I'm afraid that such lenghty and thoughtful comment is sligthly irrelevant.

You see, if we are to be rigorous, which you are forcing me to be now ;), a G-Force is an acceleration that is measured against any einsteinian spacetime geodesic. A "gravitational" G-Force is exactly equal to an "acceleration" of a spaceship because it's exactly the same phenomena, i.e., you are being pulled (pushed?) over your own natural geodesic with a constant force. In the case of a gravity well, like the Earth, the 1G you are experiencing right now is due to the existence of a natural body that is stopping your own body from following its own natural geodesic line towards the center of the Earth. This "natural body" is exerting a constant force to maintain you in the "same" position every second. Likewise, in your spaceship, its hull is exerting a force against your own body, stopping you from going in the geodesic line your body wished to go to instead. :)

The phenomena is "equivalent" because it is "exactly the same". It matters not that it was Einstein or anyone else who figured this out, it's this "sameness" that makes it possible for us to quantify these different experiences in the exact same unit. You'll notice that in physics, when same units are applied, it's because it's the exact same phenomena that is occurring, even though in the middle of the derivations you get somewhat surprised at some so-called "equivalences". ;)

 

Offline Snail

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Re: Re: BP: War in Heaven discussion
I did the Maths Olympiad at high school and consistently got gold award. And yet today I think I am the most Mathematically retarded person I know.



Anyway


what are the spinny things on UEF ships for

 

Offline General Battuta

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Re: Re: BP: War in Heaven discussion
You see, if we are to be rigorous, which you are forcing me to be now ;), a G-Force is an acceleration that is measured against any einsteinian spacetime geodesic. A "gravitational" G-Force is exactly equal to an "acceleration" of a spaceship because it's exactly the same phenomena, i.e., you are being pulled (pushed?) over your own natural geodesic with a constant force. In the case of a gravity well, like the Earth, the 1G you are experiencing right now is due to the existence of a natural body that is stopping your own body from following its own natural geodesic line towards the center of the Earth. This "natural body" is exerting a constant force to maintain you in the "same" position every second. Likewise, in your spaceship, its hull is exerting a force against your own body, stopping you from going in the geodesic line your body wished to go to instead. :)

Congratulations, by invoking Einsteinian theory you just proved my argument.

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The phenomena is "equivalent" because it is "exactly the same". It matters not that it was Einstein or anyone else who figured this out, it's this "sameness" that makes it possible for us to quantify these different experiences in the exact same unit. You'll notice that in physics, when same units are applied, it's because it's the exact same phenomena that is occurring, even though in the middle of the derivations you get somewhat surprised at some so-called "equivalences". ;)

It mattered because you made a claim about the term G-force which was patently incorrect. The term G-force is not derived from Einsteinian equivalence at all. It's a Newtonian term that was (iirc) in meaningful use before Einstein. It's applied in aerospace engineering, where Einsteinian relativity is generally not even invoked. You don't need Einstein to make sense of it.

You're now arguing a separate point, so I assume you've conceded the original. That was a really long and roundabout concession, involving repeating things I already spoke in my last post; you could just say 'oh you're right' next time.

Actually I just want to make fun of this

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You'll notice that in physics, when same units are applied, it's because it's the exact same phenomena that is occurring

lol

 

Offline Luis Dias

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Re: Re: BP: War in Heaven discussion
Oh sure, I meant finalist in the national competition.

 

Offline General Battuta

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Re: Re: BP: War in Heaven discussion
Actually I'm going to make fun of this some more.

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You see, if we are to be rigorous, which you are forcing me to be now ;), a G-Force is an acceleration that is measured against any einsteinian spacetime geodesic. A "gravitational" G-Force is exactly equal to an "acceleration" of a spaceship because it's exactly the same phenomena, i.e., you are being pulled (pushed?) over your own natural geodesic with a constant force. In the case of a gravity well, like the Earth, the 1G you are experiencing right now is due to the existence of a natural body that is stopping your own body from following its own natural geodesic line towards the center of the Earth. This "natural body" is exerting a constant force to maintain you in the "same" position every second. Likewise, in your spaceship, its hull is exerting a force against your own body, stopping you from going in the geodesic line your body wished to go to instead. :)

Why did you feel the need to restate my post, with several smileys in it, as if it was something I didn't know? Where did you get the idea this would somehow illustrate something I didn't know, when I'd just described it to you?

Oh sure, I meant finalist in the national competition.

You're on a forum with actual physicists, this is like telling Lance Armstrong you won your library fundraiser bike race. Don't make this into a dickmeasuring thing.

 

Offline Luis Dias

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Re: Re: BP: War in Heaven discussion
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You're now arguing a separate point, so I assume you've conceded the original. That was a really long and roundabout concession, involving repeating things I already spoke in my last post; you could just say 'oh you're right' next time.

Why would I concede a falsehood? :D

You could remember that this little humurous deviation started when I poked you with a "That's why they are called "Gs"....", to which you cried foul with no reason at all. Look, all this G talk could be explained in newtonian principles just the same, the fact that I invoked einstein is just because it is more precise.

If you want, you can mostly substitute "Einsteinian" for "Newtonian" in my reply, it all works just the same. Curiously enough, you invoke the counter example of "magnetism", and it is somewhat funny for it is precisely electromagnetism that is creating all this "G-Force" between your own body and the Earth's/Spaceship's. ;)

 

Offline Luis Dias

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Re: Re: BP: War in Heaven discussion
Actually I'm going to make fun of this some more.


That's the spirit, lad!

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You're on a forum with actual physicists, this is like telling Lance Armstrong you won your library fundraiser bike race. Don't make this into a dickmeasuring thing.

It's usually something I have to do, since I'm an architect and when I try to explain physics to someone, they don't take me seriously. I also wouldn't take any of my colleagues' opinion on anything regarding physics seriously, so... it's not a "dickmeasuring" thing, it's more of a lack of self-confidence issue.

  

Offline General Battuta

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Re: Re: BP: War in Heaven discussion
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You're now arguing a separate point, so I assume you've conceded the original. That was a really long and roundabout concession, involving repeating things I already spoke in my last post; you could just say 'oh you're right' next time.

Why would I concede a falsehood? :D

I have no idea why you'd do that, fortunately you conceded a truth instead.

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You could remember that this little humurous deviation started when I poked you with a "That's why they are called "Gs"....", to which you cried foul with no reason at all. Look, all this G talk could be explained in newtonian principles just the same, the fact that I invoked einstein is just because it is more precise.

I cried foul because you were wrong. Gs aren't called Gs because someone looked up the principle of equivalence. They're defined as

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A force acting on a body as a result of acceleration or gravity, informally described in units of acceleration equal to one g. For example, a 12 pound object undergoing a g-force of 2g experiences 24 pounds of force.

That definition can hold if the principle of equivalence is violated. It doesn't require the principle of equivalence. Acceleration and gravity can be distinct mechanisms in some notional fake reality and G-force is still a meaningful term. The statement you were responding to was a statement of the principal of equivalence. Your statement indicated you didn't grasp the theory; hopefully now you do.

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If you want, you can mostly substitute "Einsteinian" for "Newtonian" in my reply, it all works just the same. Curiously enough, you invoke the counter example of "magnetism", and it is somewhat funny for it is precisely electromagnetism that is creating all this "G-Force" between your own body and the Earth's/Spaceship's. ;)

Wow, look, you feel the need to describe the reason matter is solid. Did you know that you are reading this page because light falls upon your eyes? Did you know that touch is one of the five senses?

(This is also tangential; many resisting forces can create the sensation of G-force, including but not limited to to electromagnetism.)

Class dismissed.
« Last Edit: March 11, 2011, 02:51:02 pm by General Battuta »