- #71
robinpike
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andrien said:
Thanks for the picture.
If, in that picture, gravity is a consequence of following the curved spacelines, then what is the force is pulling the spacelines down in the picture?
andrien said:
robinpike said:If, in that picture, gravity is a consequence of following the curved spacelines, then what is the force is pulling the spacelines down in the picture?
It is the opposite: the properties of space have an influence on the motion of everything that results in a contact force if you counter it. Newton called such a motion changing influence a force, but not everyone uses the exact same definition.robinpike said:Thanks for the picture.
If, in that picture, gravity is a consequence of following the curved spacelines, then what is the force is pulling the spacelines down in the picture?
Nugatory said:There isn't one. That picture can be used to give you a mental model of what curved space does, but doesn't help with why the space curves.
A spacetime line is a graphical sketch of the mathematics. The underlying concept is that space has properties that are influenced by nearby matter - in other words, GR is a field theory.robinpike said:So curved spacetime lines are a means to perform the calculation of how things behave near objects with mass, but the spacetime line concept in itself is not the explanation for gravity?
georgir said:posts are getting deleted by database errors?
HmmTheCat said:If photons make up light (electromagnetic radiation) and radiation can be measured as energy, and energy can be measured as a mass, then why couldn't light have, at least, an extremely small mass?
Say you have an x-watt lightbulb
watts = Joules/second (power/sec)
Power is the conversion of energy (also radiant energy).
Energy is defined
- mechanically by ΔE=work
-- Work=Force X Distance
--- Force=mass X acceleration
- as E=mass X (speed of light)^2
HmmTheCat said:If photons make up light (electromagnetic radiation) and radiation can be measured as energy, and energy can be measured as a mass, then why couldn't light have, at least, an extremely small mass?
HmmTheCat said:If photons make up light (electromagnetic radiation) and radiation can be measured as energy, and energy can be measured as a mass, then why couldn't light have, at least, an extremely small mass?
Say you have an x-watt lightbulb
watts = Joules/second (power/sec)
Power is the conversion of energy (also radiant energy).
Energy is defined
- mechanically by ΔE=work
-- Work=Force X Distance
--- Force=mass X acceleration
- as E=mass X (speed of light)^2
harve said:Do you mean that light can be accelerated by gravity? Curvating motion also means acceleration.
With regards to the black hole gravity, that would probably mean that the curvature in spacetime in that locale is so great that even the speed of light cannot escape it.
TomTelford said:Now if I remember what I read correctly then the photon should be increasing in energy as it falls into the gravity well, if not velocity, effictively blue-shifting it off the scale. Let me know if this is incorrect.
In fact it does, as expressed in "non-local" coordinates; the change of direction was first predicted as due to the gradient in speed (Huygens construction).Nugatory said:There's no substitute for actually doing the math, but the above is a pretty decent summary of how gravity affects light. You might want to be careful with that word "accelerated" - are you thinking that gravity can change the speed at which light moves? It doesn't, it just changes the direction of travel; this effect has actually been observed.
That is only true in "local" coordinates, which do not conserve energy. As a matter of fact, the observed blueshift is ascribed to gravitational time dilation of the clocks at lower gravitational potential. There have been several discussions with detailed clarifications on that topic in this forum.TomTelford said:Now if I remember what I read correctly then the photon should be increasing in energy as it falls into the gravity well, if not velocity, effictively blue-shifting it off the scale. Let me know if this is incorrect.
Tom.
georgir said:tl;dr
But the idea that something needs mass to be affected by gravity is obviously false - all things that have mass are affected absolutely identically by gravity, they receive exactly the same acceleration, regardless of their mass. So even if they had zero mass, it would be normal to assume they will still be affected in the same manner and get the same acceleration.
harve said:Do you mean that light can be accelerated overcoming its constant speed,or simply follows the space curvature? But curvating motion also means acceleration.