The Equivalence Principle (EP)

[Alan Grayson]

As precisely as possible, can anyone describe the function and value of the EP in the construction of GR? Alternatively, how are the field equations implied by the EP? TY, AG

[Alan Grayson]

Maybe the EP just showed Einstein that graity is not a force, so it must be modeled differently, possibly by geometry? AG

[Alan Grayson]

On Friday, March 14, 2025 at 3:59:42 AM UTC-6 Alan Grayson wrote:

Maybe the EP just showed Einstein that graity is not a force, so it must be modeled differently, possibly by geometry? AG

No. The EP just shows that gravity is equivalent to acceleration ignoring tidal forces, but the fact that no forces are exerted on a body in free fall is what showed Einstein that gravity couldn't be modeled as a force. I have to check; this might be called the Weak Equivalent Principle (WEP). AG 

[Alan Grayson]

On Friday, March 14, 2025 at 6:46:05 AM UTC-6 Alan Grayson wrote:

On Friday, March 14, 2025 at 3:59:42 AM UTC-6 Alan Grayson wrote:

Maybe the EP just showed Einstein that graity is not a force, so it must be modeled differently, possibly by geometry? AG

No. The EP just shows that gravity is equivalent to acceleration ignoring tidal forces, but the fact that no forces are exerted on a body in free fall is what showed Einstein that gravity couldn't be modeled as a force. I have to check; this might be called the Weak Equivalent Principle (WEP). AG 

The WEP states that gravitaitonal and inertial mass are equivalent, which is implied by the fact that all bodies, regardless of their mass, fall at the same acceleration in a gravitiational field. Now I am puzzled by the fact that an orbiting body is in constant free fall, or constant acceleration, yet its velocity remains essentially unchanged.. Can someone explain this? AG 

[Cosmin Visan]

Of course gravity is not a force. And neither the chair in front of you is not a chair. They are all ideas in consciousness.

[Alan Grayson]

On Friday, March 14, 2025 at 8:31:55 AM UTC-6 Alan Grayson wrote:

On Friday, March 14, 2025 at 6:46:05 AM UTC-6 Alan Grayson wrote:

On Friday, March 14, 2025 at 3:59:42 AM UTC-6 Alan Grayson wrote:

Maybe the EP just showed Einstein that graity is not a force, so it must be modeled differently, possibly by geometry? AG

No. The EP just shows that gravity is equivalent to acceleration ignoring tidal forces, but the fact that no forces are exerted on a body in free fall is what showed Einstein that gravity couldn't be modeled as a force. I have to check; this might be called the Weak Equivalent Principle (WEP). AG 

The WEP states that gravitaitonal and inertial mass are equivalent, which is implied by the fact that all bodies, regardless of their mass, fall at the same acceleration in a gravitiational field. Now I am puzzled by the fact that an orbiting body is in constant free fall, or constant acceleration, yet its velocity remains essentially unchanged. Can someone explain this? AG 

This is the explanation: an orbiting object is in constant free fall and is constantly accelerating, but the acceleration vector is perpendicular to its velocity vector along its trajectory, pointing in the direction of the center of mass. Hence, the magnitude of its velocity vector is NOT constantly increasing. For example, for perfect circular motion of an orbiting object, its velocity is constantly accelerating, but the magnitude of its velocity remains constant. AG   

[Alan Grayson]

On Friday, March 14, 2025 at 9:23:39 AM UTC-6 Cosmin Visan wrote:

Of course gravity is not a force. And neither the chair in front of you is not a chair. They are all ideas in consciousness.

If you were really conscious, you'd realize that your comment is worthless. You could just as well say gravity IS a force, just an idea in consciousness. The issue is how does one distinguish between a true or false idea about a gravity, and your "of course" just shows

your ignorance. AG

[Brent Meeker]

Except he didn't show any such thing and he generally did regard it as a force, although he saw merit in Minkowski's geometric interpretation.

Brent

--
You received this message because you are subscribed to the Google Groups "Everything List" group.
To unsubscribe from this group and stop receiving emails from it, send an email to everything-li...@googlegroups.com.
To view this discussion visit https://groups.google.com/d/msgid/everything-list/b240a3a0-9a4b-4e11-9de0-7aab5a740e65n%40googlegroups.com.

[Alan Grayson]

On Friday, March 14, 2025 at 11:03:37 PM UTC-6 Brent Meeker wrote:

Except he didn't show any such thing and he generally did regard it as a force, although he saw merit in Minkowski's geometric interpretation.

Brent

Poor choice of a word on my part; he didn't "show" gravity wasn't a force; rather he concluded it from a thought experient of man in free fall. You can conclude this from his direct quote, which I recently read. AG 

[Alan Grayson]

On Friday, March 14, 2025 at 11:41:45 PM UTC-6 Alan Grayson wrote:

On Friday, March 14, 2025 at 11:03:37 PM UTC-6 Brent Meeker wrote:

Except he didn't show any such thing and he generally did regard it as a force, although he saw merit in Minkowski's geometric interpretation.

Brent

Poor choice of a word on my part; he didn't "show" gravity wasn't a force; rather he concluded it from a thought experient of man in free fall. You can conclude this from his direct quote, which I recently read. AG 

In any event, I don't think there is any direct relation between the EP and the GR field equations. What apparently happened is that Einstein's conclusion that gravity isn't a force, forced him to look elsewhere for a new theory of gravity. That elsewhere turned out to be the geometry of spacetime. AG

[Alan Grayson]

On Friday, March 14, 2025 at 11:41:45 PM UTC-6 Alan Grayson wrote:

On Friday, March 14, 2025 at 11:03:37 PM UTC-6 Brent Meeker wrote:

Except he didn't show any such thing and he generally did regard it as a force, although he saw merit in Minkowski's geometric interpretation.

Brent

Poor choice of a word on my part; he didn't "show" gravity wasn't a force; rather he concluded it from a thought experient of man in free fall. You can conclude this from his direct quote, which I recently read. AG 

Here is the quote I referred to above:

https://en.wikipedia.org/wiki/Equivalence_principle

"Soon after completing work on his theory of gravity (known as general relativity)^[6]: 111 and then also in later years, Einstein recalled the importance of the equivalence principle to his work:

The breakthrough came suddenly one day. I was sitting on a chair in my patent office in Bern. Suddenly a thought struck me: If a man falls freely, he would not feel his weight. I was taken aback. This simple thought experiment made a deep impression on me. This led me to the theory of gravity.

— Einstein, 1922^{[7]  "}

CMIIAW, but Wiki incorrectly says this is the EP. Doesn't Einstein's statement above just show how he concluded gravity isn't a force, not that gravity is equivalent to acceleration, and not that all bodies fall at the same rate in gravitational fields (the WEP)? AG 

[Brent Meeker]

That was his inspiration, but it doesn't mean he couldn't later also look at it as a force.  Why worry about what it's called or what Einstein thought about it.  Einstein is certainly not the last word on general relativity.  He thought black holes couldn't form, much less radiate.

Brent

--

You received this message because you are subscribed to the Google Groups "Everything List" group.
To unsubscribe from this group and stop receiving emails from it, send an email to everything-li...@googlegroups.com.

To view this discussion visit https://groups.google.com/d/msgid/everything-list/e3972b13-1e0d-40fb-b647-3d4befcc0b66n%40googlegroups.com.

[Alan Grayson]

On Sunday, March 16, 2025 at 6:28:31 PM UTC-6 Brent Meeker wrote:

That was his inspiration, but it doesn't mean he couldn't later also look at it as a force.  Why worry about what it's called or what Einstein thought about it.  Einstein is certainly not the last word on general relativity.  He thought black holes couldn't form, much less radiate.

Brent

Really? So after the happiest thought of his life, that gravity isn't a force, you think he still thought of gravity as a force and developed GR, a theory of gravity as geometry? Incidentally, I care about this issue because I want to understand how he developed GR. Do you agree with me that the Wiki article is wrong in characterizing his happy thought -- which doesn't imply gravity and acceleration are equivalent, and doesn't imply the WEP -- as his insight about the EP? ISTM, that his realization that gravity isn't a force, is not related to the EP. AG