Distinguishing between acceleration and gravity

[swansont]

No need to think of it as a force? Just becouse Einstein says graity is a spacetime curvature doesn't mean it's not a force. This argument is like saying "a tiger isn't an animal, it's a mammal."

There are only four forces in the universe and gravity it one of them. For extra credit what are the other 3?

There is a view in GR that gravity is not a force. An object in freefall is in an inertial frame. But this is GR; it's really not that simple.

 

 

The equations for Newtonian grvity are correct. The issue is not accuracy. The issue is some experiments, such as the precession of the perihelion of Mercury's orbit, can't be explained by classical physics. Einstein says that precession is relative.

"Can't be explained" is an issue of accuracy. Newtonian gravity does not predict the right amount. It is not as accurate as GR.

[BusaDave9]

Good discussion

"Can't be explained" is an issue of accuracy. Newtonian gravity does not predict the right amount. It is not as accurate as GR.

Newton's equations are accurate. Here is an exaggerated view of precession of the perihelion of Mercury's orbit:

Before Einstein the scientist thought there must be another planet closer to the sun pulling on Mercury. They were so sure that they even named this planet Vulcan (trivia for Star Trek fans) before it was discovered.

For the classical physicists the only other explanation would be that the rest of the universe is rotating around the sun making it look like the perihelion is moving. If that were the case then Newton's equations would be extremely accurate. But of course that would mean distant galaxies would be moving faster than the speed of light. It would also defy the notion that the sun did not have a special place in the universe.

Then Einstein said this precession of the perihelion is relative. He didn't even have to use any math to show that you could consider the orbit to be perfect ellipse but the movement of the perihelion was relative.

[swansont]

Good discussion

Newton's equations are accurate. Here is an exaggerated view of precession of the perihelion of Mercury's orbit:

precession of the perihelion of Mercury.jpeg

Before Einstein the scientist thought there must be another planet closer to the sun pulling on Mercury. They were so sure that they even named this planet Vulcan (trivia for Star Trek fans) before it was discovered.

For the classical physicists the only other explanation would be that the rest of the universe is rotating around the sun making it look like the perihelion is moving. If that were the case then Newton's equations would be extremely accurate. But of course that would mean distant galaxies would be moving faster than the speed of light. It would also defy the notion that the sun did not have a special place in the universe.

 

IOW, Newton does not predict the effect accurately.

[BusaDave9]

 

IOW, Newton does not predict the effect accurately.

Newton did not, could not, predict the movement of the perihelion (even if he had the data collected in the 19 century) but his equations are still extremely accurate IF you consider the ellipse to be moving relative to universe and everything in it. Newton would never have said the orbital ellipse is moving BUT HIS EQUATIONS DO NOT DENY IT.

[swansont]

Newton did not, could not, predict the movement of the perihelion (even if he had the data collected in the 19 century) but his equations are still extremely accurate IF you consider the ellipse to be moving relative to universe and everything in it. Newton would never have said the orbital ellipse is moving BUT HIS EQUATIONS DO NOT DENY IT.

 

If the ellipse is moving, then you don't have an inertial frame of reference, the very concept he introduced in his three laws of motion to tell you when to use equations for forces. IOW you can use his equation when he says that it will not hold. Seems contradictory, wouldn't you say?

[decraig]

[...]If I am standing on a bathroom scale it says I weigh 170 pounds. I am not accelerating.[...]

 

 

But you are accelerating. From the perspective of an observer, who is freely falling, or freely floating, not acted upon by external forces, you are accelerating upward due to the force from the scale upon your feet.

 

One standing upon the surface of the Earth is in an accelerating reference frame. The acceleration is the result off the force of the scale acting upward, not a "gravitational force".

 

In the context of general relativity there are no "gravitational forces". This is a Newtonian concept.

 

[Edited] I see swansont has already commented upon this.

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What's going on with all these posts, comparing gravity and acceleration? Apparently the word 'gravity' is not understood here. Gravity and acceleration are not even in the same phenomenological category.

Edited November 16, 2013 by decraig

[BusaDave9]

 

But you are accelerating. From the perspective of an observer, who is freely falling, or freely floating, not acted upon by external forces, you are accelerating upward due to the force from the scale upon your feet.

Agreed, A force on a bathroom scale due to gravity is the same as a force on a scale due to acceleration. The accelerating motion is relative. A free falling observer could consider himself at rest and consider the persons on the scale to be accelerating. This is Einstein's General Relativity.

 

 

 

 

What's going on with all these posts, comparing gravity and acceleration? Apparently the word 'gravity' is not understood here. Gravity and acceleration are not even in the same phenomenological category.

 

Decraig, it sounds like you don't understand gravity. Einstein called the comparison between gravity and acceleration "The Principle of Equivalence".

If you would like to learn more on the topic check out this link where they say that:

 

Einstein came to realize the principle of equivalence, and it states that an accelerated system is completely physically equivalent to a system inside a gravitational field.

 

Here's where I got the above quote:

http://www.dummies.com/how-to/content/einsteins-general-relativity-theory-gravity-as-acc.html

[decraig]

BusaDave: Agreed, A force on a bathroom scale due to gravity is the same as a force on a scale due to acceleration.

 

Well done. You caught me in an error describing Einstein gravity in terms of forces. We are not in agreement.

 

You won't learn a great deal of physics on a forum. I won't argue much further. You have to do the hard work and let the equations speak rather than the written word. So this is it for me.

 

 

 

(The four forces you spoke of in one post have nothing to do with Newton's F=ma. 'Force' is an anachronism. It's just a name, having nothing to do with Newton's definition of force.

 

The 4 forces are expressed in the standard model using what are called Lagrangians. Force does not appear in a Lagrangian. A Lagrangian formulates a physical theory in terms of kinetic and potential energy rather than force. Einstein's gravity is formulated using a Lagrangian. Try the Wikipedia entry on Hilbert Action to see for yourself.

 

Everybody likes Feynman, right? See what Feynman says about force.)

Edited November 16, 2013 by decraig

[MigL]

"You won't learn a great deal of physics on a forum", but you will be steered in the right direction where you may. The rest of us have tried to steer BusaDave in the right direction and even stating that you "won't argue further" and "this is it" for you ( a little condescending ), you do make a very valid point about modern Lagrangian formulation and the outdated use of the term 'force' for the four elementary forces. Gravity may even turn out to be a fictitious 'force' much like centrifugal or coriolis.

 

If you are willing to make further contributions, they will certainly be appreciated by us all.

If not, thanks for coming out.

[BusaDave9]

As a scientific web site we need to stick to facts. Facts that can be shown and proven. With data to support our positions.

This entire thread is really about the Principal of Equivalence.

If you google the Principal of Equivalence you'll find lots of reading on this topic.

Here's just one link: http://www.dummies.com/how-to/content/einsteins-general-relativity-theory-gravity-as-acc.html

 

Einstein came to realize the principle of equivalence, and it states that an accelerated system is completely physically equivalent to a system inside a gravitational field.

 

 

The 2nd debate that has arisen asks if gravity is a force or a warped spacetime or maybe gravitons or all of the above. Here's what Dr. Sten Odenwald says:

 

Theorists believe that both of these descriptions are valid, in much the same way that we can think of the force of electromagnetism as being either the product of a continuous field, or the exchange of numerous force-carrying particles called photons. For certain 'classical' calculations, the description of electromagnetism as a field is more workable than its 'quantum' description, and vice versa.

 

 

And the above quote came from here: http://einstein.stanford.edu/content/relativity/q1669.html

 

A healthy debate is good. I never meant to come accross as rude.

Edited November 18, 2013 by BusaDave9