The discussion centers on the distinction between gravitational mass and inertial mass, particularly in the context of Einstein's general theory of relativity (GR). Participants emphasize that gravity is a pseudo-force, not a traditional force as understood in Newtonian physics, and that free-falling objects are not considered accelerated in GR. The equivalence principle is highlighted as a key concept, indicating that no experiment can distinguish between inertial and gravitational effects in a non-inertial frame. The conversation also critiques the clarity of Wikipedia's explanations regarding these concepts.
PREREQUISITESPhysics students, educators, and anyone interested in the foundational concepts of general relativity and the nature of gravitational interactions.
Albert Einstein developed his general theory of relativity starting from the assumption that this correspondence between inertial and (passive) gravitational mass is not accidental: that no experiment will ever detect a difference between them (the weak version of the equivalence principle). However, in the resulting theory gravitation is not a force and thus not subject to Newton's third law, so "the equality of inertial and active gravitational mass [...] remains as puzzling as everhttp://en.wikipedia.org/wiki/Gravitational_mass#Gravitational_mass
Inertial and Gravitational Mass
What do you all think about this? Isn't that a bit of an exaggeration??
Stating that "gravitation is not a force" without mentioning that free falling objects are not considered accelerated in GR anymore, is telling just half of the story. This omission also causes the puzzling:However, in the resulting theory gravitation is not a force and thus not subject to Newton's third law, so "the equality of inertial and active gravitational mass [...] remains as puzzling as ever
http://en.wikipedia.org/wiki/Gravitational_mass#Gravitational_mass
Inertial and Gravitational Mass
If you feel a force on your back (while laying down) then you are being accelerated upwards.daytripper said:Was there a recent redefinition of gravity or something that I'm unaware of? What's all this about gravity not being a force? If it's not, then why am I being accelerated downward? If I'm not, then why do I feel a force on my back (I'm laying down)?
Gravity is a pseudo-force in general relativity. Nothing new here, just something you haven't been taught. Imagine you are floating around weightlessly in a spaceship with no windows. Is there any experiment you can conduct in the confines of this spaceship that will let you determine whether you are in in some region of (nearly) flat space between galaxies or in orbit around some massive object such as a planet or a star?daytripper said:Was there a recent redefinition of gravity or something that I'm unaware of? What's all this about gravity not being a force?
Because you aren't in an inertial frame. Gravity is a pseudo-force. All pseudo-forces (e.g., centrifugal force, coriolis effect, inertial force (frame acceleration)) result from attempting to explain physical laws that strictly apply in inertial frames only in a non-inertial frame. All pseudo-forces have one thing in common: The force acting on some object is proportional to the mass of the object. Clue #1 that gravitation is a pseudo-force: The gravitational force acting on some object is proportional to the mass of the object. Clue #2 is the equivalence principle.If it's not, then why am I being accelerated downward?
That is a real force; it's called the normal force.If I'm not, then why do I feel a force on my back (I'm laying down)?
Gravity is a pseudo-force in general relativity.
Naty1 said:DH posted:
THAT must be what Wikipedia was trying to say...I still think it's an obtuse statement...
It's no wonder there are lengthy threads here where we are trying to understand each others context and choice of words...not to mention different interpretations of some mathematics...
"All pseudo-forces (e.g., centrifugal force, coriolis effect, inertial force (frame acceleration)) result from attempting to explain physical laws that strictly apply in inertial frames only in a non-inertial frame..."
Nicely laid out!..haven't seen that description before
Thanks DH.
Okay, here the "active graviational mass" in fact means the energy stress tensor, causing space time curvature. That was not clear in the wiki text.atyy said:The "puzzling as ever" is a phrase from Rindler, Relativity: Special, General, and Cosmological, OUP 2001, p22: http://books.google.com.sg/books?id=fUj_LW51GfQC&printsec=frontcover#PPA22,M1.