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ensabah6
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Would GR apply to a universe without any matter or energy? Would such a universe have time?
Bob_for_short said:What is a universe without matter and energy? No theory would apply.
ensabah6 said:-- in this geddenaken -- GR alone applies. Can GR make sense without any matter or energy? What would a universe described by GR but no matter or energy
look like?
But that is what the OP asks. Physics makes predictions about observations. If there is nothing to observe, physics makes no predictions. Does this mean that physics still applies? It is at least not wrong. It predicts nothing and there is nothing.lalbatros said:Further assuming that no test-mass would even exist is of course more like brain-washing that thinking physics.
Not approximately empty, but approximately flat. This solution serves to calculate (estimate) the matter and radiation behavior. The flat ST is widely used in SR. As soon as there is no matter/radiation, this solution or whatever else is just not practiced.lalbatros said:Of course the GR theory applies to this situation.
This solution of Einstein's equations is flat space-time.
This is the most commonly used solution of Einstein's equations !
"Day-to-day physics" simply neglects the influence of mass and energy on the structure of space-time. This is the same as assuming the universe is approximately empty.
"GR" refers to General Relativity, a theory proposed by Albert Einstein to explain the force of gravity. In a universe without matter or energy, there would be no objects or particles to exert a gravitational force, making it an ideal scenario for studying the fundamental principles of GR.
In a universe without matter or energy, gravity is still present as a curvature of spacetime. This means that even in the absence of physical objects, the fabric of the universe is still curved and can be described by the equations of GR.
Studying GR in a universe without matter or energy can help us better understand the fundamental principles of gravity and how it affects the fabric of spacetime. It can also provide insights into the behavior of the universe in extreme conditions, such as the early stages of the Big Bang.
While it is currently not known if a universe without matter or energy exists, it is a theoretical possibility. Some scientists propose that in the very early stages of the universe, there was a period of rapid expansion called inflation, during which matter and energy were not yet present.
In our current understanding, gravity is the force that attracts objects with mass towards each other. However, in GR, gravity is described as a curvature of spacetime caused by the presence of matter and energy. In a universe without these factors, the equations of GR would still hold, but the effects of gravity would be purely geometric rather than being caused by the presence of matter and energy.