Professor Loeb,

I certainly agree that if we were to observe the possibility of sentient life being able to move galaxies, it would raise serious questions as to what is possible and what would be required to attain such capabilities. That is why it would have been fitting for the "What is possible" contest.

Would it really change our understanding of gravity though, since we...

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Professor Loeb,

I certainly agree that if we were to observe the possibility of sentient life being able to move galaxies, it would raise serious questions as to what is possible and what would be required to attain such capabilities. That is why it would have been fitting for the "What is possible" contest.

Would it really change our understanding of gravity though, since we really don't have a satisfactory understanding of how gravity operates? Yes, spacetime provides an effective model, but it should be noted epicycles also provided an effective mathematical model of the action of the heavens, for the very logical reason that we are the center of our own perspective, so modeling it from our point of view would be logical. That didn't make those giant cosmic gearwheels real and I suspect we will eventually come to see correlating measures of duration and distance, using C, does not mean the "fabric of spacetime" is any more real.

My views on this are based on the subject of my own essay, where I point out that by simply treating time as a measure of duration, physics incorporates our intuition of time as a sequence of events into foundational theory. The reality is that what causes this configurational change is action and it is not that the "present" moves/exists on a vector from past to future, but that due to such action, the future becomes past. For example, the earth doesn't travel/exist along a vector from yesterday to tomorrow. Tomorrow becomes yesterday because the earth rotates. Duration only exists within the present, not external to it.

As an effect of action, rate of change, time is similar to temperature, level of activity. The reason clock rates vary is because rates of change vary when levels of activity vary, as velocity or gravity do to atomic activity. The twins simply have different metabolic rates in different environments.

The cat is not both dead and alive, because it is the collapse of probability(future), which yields actuality(present and past), rather than proceeding along that vector from a determined past into a probabilistic future.

Now how would this affect our philosophy, religion and culture? By and large, the intellect is a function of the serial sequence of time, from narrative to cause and effect logic. Our explanations and descriptions of reality, religious, national and now cosmic, are all based on that linear, beginning to end, temporal understanding. When we create such narratives, there is the necessity of trimming away all the loose threads that disrupt from the chosen storyline. Opening up this understanding of time, we would begin to see reality as much more of a tapestry of activity and we simply happen to be one of those threads, weaving our way through it, not so much to any particular destination, but doing our part to hold the larger picture together. How would this affect our understanding of life, if we begin to realize we are where we need to be and not some preferred idealized location, somewhere else? Might we begin to take care of the planet and not just dream of escaping its confines, which make us what we are. As I see it, from a biologically emergent model, life on this planet is trying to grow itself a central nervous system, with human civilization as the potential medium. Obviously we have much growing to do, as we currently act like children, always rushing to see what is ahead and tripping over all we didn't notice.

Regards,

John

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Dear Avi:

Thanks very much for responding! Of course, I agree fairly well with what you wrote. In fact, I probably should have explicitly stated that my point really does not affect the main thrust of your paper, even on the basis of space-time kinematics; i.e., regardless of the physical interpretation of the mathematics, no distant galaxy can be observed after a finite proper time on its...

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Dear Avi:

Thanks very much for responding! Of course, I agree fairly well with what you wrote. In fact, I probably should have explicitly stated that my point really does not affect the main thrust of your paper, even on the basis of space-time kinematics; i.e., regardless of the physical interpretation of the mathematics, no distant galaxy can be observed after a finite proper time on its own clock in a flat LambdaCDM universe, so any two distant neighbours who want to remain in contact for all time will eventually have to do something about it. Furthermore, as you've correctly noted, my point doesn't even matter as much in this regard, as the fact that even at a finite time in the future, the wavelengths of distant photons will be shifted to more than the Hubble radius, so they'll be impossible to observe. I agree with all of this, and therefore see relevance in your examination of this possible unexplored phenomenon in section 3.

My point was that the physical interpretation of the space-time kinematics, as being analogous to the description of a particle that passes through a black hole's event horizon and is forevermore causally disconnected from distant observers while it leaves behind a frozen image of its former self, is incorrect. Before I go on: do you understand my point about the incorrectness of the inferred similarity between this result and the description of a black hole's event horizon? Again, the result you've given is really *basically* due to the facts, that light travels at a finite speed, and that it can only travel a finite comoving coordinate distance through space in infinite time in a flat LambdaCDM universe, despite the fact that the comoving distance to any observer's particle horizon is a monotonically increasing function of cosmic time---i.e., so that no comoving galaxy ever *exits* anyone else's particle horizon. Do you agree with that?

You've noted the importance of being "careful of over-interpreting spacetime diagrams without checking if the signal is at all detectable". I knew of your result when I made my criticism, but I do think the entire physical description wants a logically consistent interpretation: I think that if the interpretation of a particular result that one makes is logically inconsistent with other related results, it is important to try to understand what the correct interpretation of the theory should be, which would regain logical consistency. Therefore, together with the (relevant) results like the one you've mentioned about the redshift of incident-but-unobservable photons, and the fact that distant galaxies are only observable up to finite values of their own proper times, I think it is important to note that the latter is really *basically* due to the fact that the light that any galaxy emitted even right after the big bang will only travel a finite comoving spatial coordinate distance in infinite cosmic time. Thus, it's not accurate to just think of the distant galaxy as continuing its existence in some unobservable region of spacetime after that final image of itself has been transmitted, and describe the situation as being similar to a black hole. Rather, the correct description is that the two galaxies exist together, separated by a finite comoving coordinate distance and a finite physical distance at t_{em,max}; then, the light emitted by the distant galaxy finally traverses that finite comoving coordinate distance by crossing an infinite physical distance through expanding space in infinite cosmic time.

Now, you may disagree with me that it matters at all which basic physical interpretation of the mathematics is actually consistent with the overall theory. In that case, you wouldn't be alone: not every physicist sees relevance in making logically consistent interpretations of our physical theories, so long as we have an accurate description of the observed phenomena. What truly goes on behind the scenes is an irrelevant matter to many physicists, who would agree with you that what really counts is whether the signal is at all detectable. The reason I take issue with this, is because of the fact that one incorrect interpretation does often breed another, until the description that's given does become very mis-construed, and, although the theory has been required to remain consistent with observed *phenomena*, there is little in the basic understanding of what's going on that resembles what is *actually* going on, even according to a logically consistant interpretation of the mathematical theory that was there from the outset!

It's relevant, therefore, to note that the black hole analogy to your result is wrong, and that it has already led to the production of mis-information about what the physical theory tells us is going on; e.g., in 2007, Krauss and Scherrer wrote, "Both analytic [7] and numerical [10] calculations indicate that the Local Group remains gravitationally bound in the face of the accelerated Hubble expansion. All more distant structures will be driven outside of the de Sitter event horizon in a timescale on the order of 100 billion years ([4], see also Refs. [8,9]). While objects will not be observed to cross the event horizon, light from them will be exponentially redshifted, so that within a time frame comparable to the longest lived main sequence stars all objects outside of our local cluster will truly become invisible [4]."

While these objects do truly become invisible (due to redshifts, as Krauss and Starkman showed), the expansion of space does not drive them outsite an event horizon, as I've mentioned: ever more comoving galaxies will *enter* our particle horizon for all time, although at infinite time we will finally "see" (by adding quotation marks, I mean that I understand that the light will be redshifted beyond detectability) the light from some very distant galaxy only as it was emitted at the Big Bang. The three sentences I've quoted above are, respectively, correct, incorrect, and correct, according to the physical theory.

A year after this, Krauss and Scherrer wrote an article for Scientific American, where they stated, "The

cosmological constant produces a fixed “event horizon,” an imaginary surface beyond which no matter or radiation can reach us. The universe comes to resemble an inside-out black hole, with matter and radiation trapped outside the horizon rather than inside it... all the expanding matter in the universe will be driven outside the event horizon. This process has been studied by Abraham Loeb and Kentaro Nagamine, both then at Harvard University, who found that our so-called Local Group of galaxies (the Milky Way, Andromeda and a host of orbiting dwarf galaxies) will collapse into a single enormous supercluster of stars. All the other galaxies will disappear into the oblivion beyond the event horizon. This process takes about 100 billion years, which may seem long but is fairly short compared to the wilderness of eternity."

The galaxies are not simply trapped behind an imaginary surface where they can no longer be seen: the finite limit on the time when the last image of a distant galaxy was transmitted is really just due to the combined effect of asymptotic exponential expansion, in a universe where light travels at a constant finite speed; i.e., because of that constant finite speed, we always observe distant galaxies as they were in the distant past, rather than as they presently exist; and because of the asymptotic exponential expansion, the light that's received from any one at infinite cosmic time will have come from a finite comoving coordinate distance, and will have been emitted at a finite cosmic time.

If you understand this, and answered "yes" to my above questions, do you not now see that I'm not simply "over-interpreting spacetime diagrams", but that this incorrect inference has led to the production of mis-information that is potentially detrimental to the progress of physics? The present essay contest is about questioning which of our basic physical assumptions are wrong. Physical assumptions are based on empirical evidence and interpretation of the mathematical theories that best describe them. Do you agree that there is an incorrect interpretation that is being perpetuated here, and that the physics hasn't been correctly understood? Such mis-understandings, which often emerge when one is not aware of all the relevant facts, but only considers a few bits of knowledge, often lead to incorrect physical assumptions about what's actually going on in our Universe.

As I've argued in my essay (which I made a link to in my first comment), such was the case with Einstein's interpretation of the meaning of relativity of simultaneity, which should be interpreted differently according to cosmology. Then, with a clear understanding of that different interpretation of the relativity of simultaneity, one sees that the Robertson-Walker line-element was derived partly on the basis of a redundant assumption that was made as the result of a view that was sympathetic to Einstein's interpretation of the relativity of simultaneity, so that it may be possible to derive a different background metric that's consistent with the empirical data, but where the many well-known problems do not manifest themselves as they have.

I invite you to read my essay. I'm quite interested to know your thoughts on it, and would be very honoured if you were to offer some feedback.

Best regards,

Daryl

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