Everyone knows the famous example of gravity where two weights are placed on a stretched out sheet of fabric, and the two weights come together exemplifying gravity, but does anyone know who originally came up with this idea? I think I read that it was Einstein, but with all the false info out there who knows. Anybody?
Metaphors, like the fabric of space example are useful for help us to visualize. However, one needs to be careful not to take the metaphor too literally, since this will lead the curious mind to ask other questions, that can make or break the application of the metaphor.
For example, this metaphor at some level assumes the fabric, that is space, is somehow anchored at its perimeter and suspended in the middle like a trampoline. Why is the fabric not lying flat on a table? If space was a fabric laying flat on a table, this gravity metaphor would not work. Or, if it was not anchored at the perimeter, but only suspended, the two masses would both fall, without attraction. Attachment or not makes or breaks it.
The next question that comes to mind about the gravity and the fabric of space, is how is space fastened to the perimeter; edge of the universe, and what is that edge made of? The metaphor works better only if you do not think too deep and allow the unspoken premises of the fabric to remain unspoken. I am a conceptual modeler which causes me to seek any extra hidden premises that were consciously or unconsciously ignored.
As far as the end of the universe, where the material universe ends and where the fabric of inertial space is attached, one possible boundary condition is the speed of light reference. Mass cannot go the speed of light, and therefore, if the speed of light reference was at the perimeter, it would, by default, need to exclude mass and matter. This is simple and compact.
How would we attach the fabric of space to the speed of light reference? Is there special stitching that can bridge the gap? The answer appears to be yes, and is expressed by photons. Photons travel at the speed of light, but they also have inertial properties associated with finite distance and time; finite wavelength and frequency, These can be impacted by gravity. Photons are a possible bridge since it shares quality of the fabric and edge. Photons would be a good conceptually consistent material for the stitching. All things equal, energy and photons will reach the perimeter before the mass. This is why we measure energy and not mass where we look out at the distant universe; energy is much faster and mass is very slow.
In the fabric metaphor, the stretching of the fabric, due to the weight of mass, so the two masses can slide together, would create a force vector in the fabric, that would pull the fabric of space away from the perimeter. If two people are on a trampoline, the perimeter springs will stretch. This may be observed as the red shift of energy. The stitches that hold it together, stretch. When the two mass attract each other, the local space-time between the two mass gets more compact, therefore the doubled mass moves closer to the speed of light reference via the change of the reference, at the same time the boundary sees stress; red shift. It appears both are heading toward the speed of light reference.
Can this force away from the perimeter, due to gravity, cause the fabric of space or the stitches, to tear, where they are fastened at the boundary? Or do these photon stitches continue to stretch so the boundary fasteners only gets thinner, but never break? The observed red shift or stretching of the photon stitches can theoretically reach infinite wavelength. After that we would get tearing. All this extrapolation implies heading back to the speed of light reference; ground state.
With a trampoline, if it isused it a lot, the perimeter springs rarely break, but they do stretch out and can cause slack in the fabric. A slack trampoline fabric makes it harder to stay apart. Gravity will quicken as the perimeter stretches.
The fabric also tends to get weaker and can puncture. I have a friend who shattered his ankle, when he went too high and his leg poked through the fabric and his foot hit the ground. This metaphor appear to describe the black hole. The black hole does not cause the perimeter to detach. However, it can break through the fabric of space, and isolate itself from the fabric, suspended near the boundary conditions. The space-tine contractor of a black hole approaches that of the speed of light boundary; among the stitches. The boundary is not just out there deep in space, but also close by, just beyond the local fabric.
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