In my last post, I postulated that gravity arises from any curved spacetime that has an activation layer that constantly moves forward in time.
In MTW Gravitation, this activation layer would be called a hypersurface that is isotropic in time (a fancy way of saying that the surface is formed such that the time value is constant at all points in space), one of an infinite set of curved 3D surfaces through 4D spacetime. What I add to the concept is that existence as we think of it consists of objects and observers in only one (current time) hypersurface called the activation layer. I think the concept of gravity emerging from a moving hypersurface is a great idea, but as always in science, the devil is in the details. Let’s dive in a bit to see if the activation layer hypersurface has anything new worth investigating.
First, I want to make it clear that my activation layer thinking has to lie strictly within the math of general relativity. I am only proposing a constraint to what it means to exist in spacetime, but I am not proposing any changes to how general relativity computes world lines for objects or observers. The activation layer concept attempts to explain why we only can observe one point in time at a time–something that has to be added to 4D spacetime models of our existence. One interesting consequence of this addition is how gravitational potentials would then arise solely from the timewise motion of the hypersurface activation layer through its enclosing 4D spacetime.
To further clarify, I am proposing that time travel like what was shown in the Interstellar movie won’t work as expected–that any given time point, the only hypersurface that has anything in it is the “current time” hypersurface–the activation layer. Time travel is possible to other hypersurfaces via some mechanism like wormholes, but there will not be activated objects, particles or fields there. There is only one activated hypersurface–the one we are currently in.
This is where my activation layer idea starts, and as I postulated in the previous post, the timewise motion of the hypersurface through successive hypersurfaces in 4D spacetime will trace out worldlines that act as if there was a gravitational force (the Einstein equivalence principle of gravitational forces being indistinguishable from inertial acceleration forces). With this activation layer methodology, I am saying that the equivalence principle is not really an equivalence but rather an identity–they are the same. There is no actual gravitational force, just like there’s no real centripetal force being applied to an orbiting object.
However, for the application layer concept to work, there must be a difference between the current time hypersurface and all other hypersurfaces in 4D spacetime. This is what I am studying and researching. I’m especially interested in how this thinking might be used to determine the gravitational constant from the rate of time passing and spacetime curvature–in other words, the stress-energy tensor G = 8 Pi T.
One thing for sure, and what got me started on this whole thing, the 4D tesseract portrayed in Instellar has to be just completely wrong. Wrong even when accounting for Hollwood dramatization. Wrong even just from an observation point of view, never mind the problem of applying forces from one hypersurface (the activation layer) to another. And where did the photons come from that allows Cooper to see the tesseract? And how is Cooper’s 2D retinal sensors going to select which hypersurface it sees? And how is Cooper’s finger going to selectively apply forces to a watch that apparently exists in all hypersurfaces? The premise that gravitational forces cut across timelines to affect a ticking watch makes no sense–world lines are a reflection of how a hypersurface travels through curved spacetime. I see gravity as a pseudo-force like centripetal force, it’s an illusion.
Update: It has been several years since I read his book (“The Science of Interstellar”), so, because of my current issues with the movie science, I did a re-read of how Dr. Thorne worked out the details. I had forgotten how conscientously he and the movie people tried to get the science right. I feel a renewed push to come to a deeper understanding of where we differ in our thinking, especially in how gravitational waves can travel back in time.
The interesting thing about all this is if the equivalence principle is actually an identity principle, and gravity is just a pseudo force, maybe that will shed new insight on how to unify the math of gravity (general relativity) with quantum theory.
I actually wrote Dr. Thorne about these issues, and to give him credit, he did write back! All he said was… “Read my book.” 🙂
Agemoz
Agemoz's Physics Blog 