Is Spacetime the Cause of Inertia?

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In summary, Brian Greene argues in The Fabric of the Cosmos that spacetime itself, rather than the "fixed stars," is the cause of inertial effects. However, the notion of an expanding universe and an infinitesimally small matter density brings up potential problems with this idea. Despite this, the mathematical equations of general relativity do not support the idea that inertia will disappear in low density portions of the universe.
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PhizzicsPhan
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Brian Greene describes in The Fabric of the Cosmos how spacetime itself is now thought to be the cause of the inertial effects behind Newton's bucket experiment (rising water, etc.). That is, rather than the "fixed stars" being the cause, as was the commonly held notion before Einstein, spacetime itself, a 4D construct, is now thought to be the cause.

I see at least one problem with this notion, however: if we live in an increasingly expanding universe, as we apparently do, the very large majority of the duration of our universe will consist of an infinitesimally small matter density. This is the case because, as galaxies continue to hurtle away from each other, we reach over the course of billions and trillions of years a state in which all matter is eventually spread out fairly uniformly, and then the final heat death...

It's not a pretty picture, by any means, and it also seems to lead to a problem with the notion of spacetime itself as causing inertial effects. This is the case because if matter density over the entire course of the existence of our universe is on average infinitesimally small, the gravitational effects exerted by our 4D universe (inertia in this case) will also be infinitesimally small.

Any thoughts?
 
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PhizzicsPhan said:
if matter density over the entire course of the existence of our universe is on average infinitesimally small, the gravitational effects exerted by our 4D universe (inertia in this case) will also be infinitesimally small
While this sounds nice, the math doesn’t bear it out, at least not for GR. Even a vacuum solution will lead to all of the usual observables in Newton’s bucket, so there is no minimum density required and the idea that the usual cosmological solution will have a low density portion does not suggest that inertia will disappear.
 

1. What is spacetime?

Spacetime is a concept in physics that combines the three dimensions of space and the dimension of time into a single four-dimensional continuum. It is the framework through which events are viewed in the universe.

2. How does Newton's bucket experiment relate to spacetime?

Newton's bucket experiment was a thought experiment used by Isaac Newton to demonstrate his theory of absolute space and time. It showed that in a rotating reference frame, the water in a bucket would be curved due to the force of rotation, which contradicted Newton's theory of absolute space. This led to the development of the concept of spacetime.

3. What is the significance of spacetime in Einstein's theory of relativity?

In Einstein's theory of relativity, spacetime is a fundamental concept that describes the geometry of the universe. It explains how gravity is not a force between masses, but rather a curvature of spacetime caused by the presence of mass and energy.

4. Can spacetime be visualized?

Spacetime cannot be fully visualized in the traditional sense because it involves four dimensions, whereas humans can only perceive three dimensions. However, mathematical models and diagrams can be used to represent certain aspects of spacetime.

5. How does the concept of spacetime impact our understanding of the universe?

The concept of spacetime has revolutionized our understanding of the universe. It has allowed us to explain phenomena such as gravity, black holes, and the expansion of the universe. It also plays a crucial role in modern physics, including Einstein's theory of relativity and the development of quantum mechanics.

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