381–390 of 451 posts
Physicist here, gravity is a force, just a different one. Also, like everything else in physics: it depends how you observe it. For instance, electromagnetism comes from the curvature of a U(1) bundle over space time, the (local) U(1) symmetry yields electromagnetic interactions. For gravity the symmetry is the (local) Pointcaré (SO(1,3) + translations) symmetry and curvature of spacetime itself. Also gravity on Eart…
No , is not a force
Physicist here, gravity is a force, just a different one. Also, like everything else in physics: it depends how you observe it. For instance, electromagnetism comes from the curvature of a U(1) bundle over space time, the (local) U(1) symmetry yields electromagnetic interactions. For gravity the symmetry is the (local) Pointcaré (SO(1,3) + translations) symmetry and curvature of spacetime itself. Also gravity on Eart…
Correct. And just making the trajectory to appear straight by modifying the reference grid doesn't change anything. For example, once can see even exponential curve appear in a logarithmic grid. Force is a relation between the reference frame and the object. It is possible to consider either one as straight or flat.
This affects our understanding of forces: Accelerations live in the double tangent bundle, which gets split into a horizontal and vertical part by the connection. The horizontal part is the contribution by inertia, yielding pseudo-forces (including gravity according to GR). The vertical part is due to non-inertial forces.
Earlier quoted context omitted.
It's impossible for an object with mass to go the speed of light. But you can observe what happens as you approach it: Let's say I put you in a spaceship and accelerate you to 50% the speed of light toward the sun. From an inertial viewer's perspective you are travelling toward the sun at half the speed of light and it takes you ~16 minutes to crash into the sun. But from your perspective it only took ~14 minutes to…
> you can observe what happens as you approach it Yes, but you cannot extrapolate from this to say that the time lapse for a photon would be zero. A photon is not the limit of objects with mass going closer and closer to the speed of light, because "closer and closer to the speed of light" is frame-dependent, but a photon's speed being c is not. I can find an inertial frame in which each of your objects is at rest, a…
I don't believe that, and have never heard it before. There are many ways in which light actually behaves just like particles with mass traveling at speed c. It has to or conservation of momentum is violated.
This video really cleared up a lot of misunderstandings for me: https://www.youtube.com/watch?v=wrwgIjBUYVc I had never visualized it this way in my head before, and it all makes a lot more sense now. Highly recommend!
Great video. Reminded me of the arguments still going on about ‘whether this is true or not’ in the sense that the mathematics is painting this more accurate picture than what Newton’s math painted, but the math can’t explain most of the universe’s lack of observable mass/energy, so there might be some higher level of mathematics that describes a different but ‘more true’ state of events.
The inconsistency points towards an actual type of matter as opposed to systematic error.
Physicist here, gravity is a force, just a different one. Also, like everything else in physics: it depends how you observe it. For instance, electromagnetism comes from the curvature of a U(1) bundle over space time, the (local) U(1) symmetry yields electromagnetic interactions. For gravity the symmetry is the (local) Pointcaré (SO(1,3) + translations) symmetry and curvature of spacetime itself. Also gravity on Eart…
Physicist here, gravity is a force, just a different one. Also, like everything else in physics: it depends how you observe it. For instance, electromagnetism comes from the curvature of a U(1) bundle over space time, the (local) U(1) symmetry yields electromagnetic interactions. For gravity the symmetry is the (local) Pointcaré (SO(1,3) + translations) symmetry and curvature of spacetime itself. Also gravity on Eart…
But the principle of equivalence is not included in the Poincaré group.
Light is without mass, but not without energy. That energy causes what you could describe as "propulsion". Even the slightest bit of propulsion in the emptiness of space will cause matter to move around.
It's like light being a big ladle. And using that ladle to spin the water + tiny floating objects in a bath. They will all affect one another until the heat death of the bath occurs and all life grinds to a halt.
Physicist here, gravity is a force, just a different one. Also, like everything else in physics: it depends how you observe it. For instance, electromagnetism comes from the curvature of a U(1) bundle over space time, the (local) U(1) symmetry yields electromagnetic interactions. For gravity the symmetry is the (local) Pointcaré (SO(1,3) + translations) symmetry and curvature of spacetime itself. Also gravity on Eart…
Newton's first law tells us that in the absence of external forces, a body will keep going in the same direction, uniformly, ie its wordline will be straight. However, that concept is not available in arbitrary manifolds, you need additional structure: The covariant connection, which allows you to parallel transport velocity vectors, enabling you to define the concept of straight lines (autoparallels, which will be g…
but if you introduce a derivative which is covariant under diffeomorphisms and u(1) transformations
D = d + \omega + A
you see that gravitation (connection \omega) enters on a somewhat same footing as the electromagnetic potential (A). there are nuances ofc, but the view (force/curvature) depends from where you're looking.
also in real life particles don't real exist... they're excitations in a field
Physicist here, gravity is a force, just a different one. Also, like everything else in physics: it depends how you observe it. For instance, electromagnetism comes from the curvature of a U(1) bundle over space time, the (local) U(1) symmetry yields electromagnetic interactions. For gravity the symmetry is the (local) Pointcaré (SO(1,3) + translations) symmetry and curvature of spacetime itself. Also gravity on Eart…
Explain like I'm five, how-and-why a charged particle emits EM radiation when passing through a medium at speeds greater than light.
Earlier quoted context omitted.
Newton's first law tells us that in the absence of external forces, a body will keep going in the same direction, uniformly, ie its wordline will be straight. However, that concept is not available in arbitrary manifolds, you need additional structure: The covariant connection, which allows you to parallel transport velocity vectors, enabling you to define the concept of straight lines (autoparallels, which will be g…
correct! but if you introduce a derivative which is covariant under diffeomorphisms and u(1) transformations D = d + \omega + A you see that gravitation (connection \omega) enters on a somewhat same footing as the electromagnetic potential (A). there are nuances ofc, but the view (force/curvature) depends from where you're looking. also in real life particles don't real exist... they're excitations in a field
"No, Mr Bond, I expect you to die."
Obligatory https://xkcd.com/123/