Tuesday, June 11, 2019

A Future Telescope

This post describes an idea for a telescope that can see where heavenly objects will be in the future. This may sound crazy, like something out of a science-fiction story, but I believe it is based on solid theory. Unless, or course, I have misinterpreted something. Read on if you enjoy considering surprising extrapolations of theory.


Collective Electrodynamics

Carver Mead's book Collective Electrodynamics, first published in 2002, puts forth a theory of electrodynamics based on four-vectors. As with many other low-level aspects of physics, this theory is time-symmetric, making no claims about how to distinguish between the past and the future.

I found Carver's theory and his exposition of it to be elegant and convincing. Even if you don't agree with my interpretation and conclusions in this post, I recommend you read this book if you are generally interested in physics.

Carver's description of the process of photon emission and absorption includes a few comments noting that a photon will not be emitted without a destination that will absorb the photon at some point in the future, because the emitter and absorber are a coupled pair forming a single resonator.
  • In section 4.8: "Any energy leaving one resonator is transferred to some other resonator, somewhere in the universe."
  • In section 4.12: "The spectral density of distant resonators acting as absorbers is, of necessity, identical to that of the resonators producing the local random field, because they are the same resonators."
  • In the Epilogue: "It is by now a common experimental fact that an atom, if sufficiently isolated from the rest of the universe, can stay in an excited state for an arbitrarily long period. ... The mechanism for initiating an atomic transition is not present in the isolated atom; it is the direct result of coupling with the rest of the universe."
Part 5 describes how two atoms couple electromagnetically as resonators.

Interpreting the Theory

As a thought experiment, if we were out in space in some part of the universe in which there were no matter in one direction, we would not be able to shine a flashlight in that direction because there would be nothing to absorb the photons, therefore they would not be emitted. If we were able to measure all of the other energy going into or out of the flashlight, we would be able to notice that energy leaves the flashlight when we point it towards other things, but not when we point it towards truly empty space.

Coming back to our current location in the universe, there is a finite amount of matter between us and the Hubble sphere. Consider a line segment from our location to a point on the Hubble sphere. If there are no atoms on the intersection of said line segment and our future light cone, then it should not be possible to emit a photon in that direction. More restricted, if there are no atoms in that intersection that are capable of absorbing a photon of the frequency our source atom is attempting to emit, then we will not be able to emit said photon in that direction.

The Big Idea

Assume, then, that we have a highly directional monochromatic light source that we can point accurately, and that we can accurately know how much light we are emitting based on energy input measurements. What would happen if we were to provide that light with a suitable input power signal, then scan the sky? If there are any differences in the density of atoms in different directions that are capable of absorbing photons of the frequency we are sending, would we be able to produce a map of the sky showing those differences? Would there be any anisotropism, as there is for the background radiation?

Given how much matter there is in the universe, I suspect it would be hard to find one of those line segments out to the Hubble sphere without a single atom capable of absorbing one of our photons, but perhaps if we are trying to send out a great many photons, there will be enough of a statistical variation to measure.

The thing that I find fascinating about this is that, if it did in fact work, we would be "seeing the future", because whatever map we produced would be a function of where the absorbing atoms are going to be when the light we emit reaches them. For planets in our solar system that would be minutes or hours in the future, but for distant nebulae that could be millions or billions of years from now.

The Details

The devil is in the details. Even if, in principle, the theory supports this conclusion, would it be possible to build such a device?

In addition to the statements of theory, I make two assumptions above:
  1. We can accurately point our light source, such that we can perform a raster scan on a portion of the sky.
  2. We can determine how much light energy is leaving our light source by measuring the input energy to that source.
The first assumption seems straightforward: the optics involved in sending out a beam of light to a small portion of the sky should be the same as receiving light from a small portion of sky, which we do on a regular basis to form images of space. But I am not an astronomer, so I may be missing something. For example, I know that some modern telescopes use a guide laser shining up through the atmosphere to allow for dynamic adjustments to the mirrors to compensate for atmospheric distortion. Would this also work when sending out a signal beam alongside the reference beam? I don't know why not, but, as mentioned, this is not my area of expertise.

I think the second assumption may require more effort to solve. The typical advice for powering a laser is to use a current source in order to get a stable output. For my experiment, however, I specifically don't want a stable source. Instead, I want a source that can output more or less light based on how much the space into which it is shining can accept.

Since I can't directly measure the light output, I also need a light source where I can accurately judge how much light is being output by measuring the input power. This means I need to know the power transfer characteristics of the light source. How much of the input power is transformed into light, and how much into heat or other forms of energy? Is that relationship constant over time, or might it vary such that at one point in time I get x% of the input turning into heat, and moments later I get 2x% turning into heat? Alas, I am not a solid-state physicist (assuming my light source is a solid-state laser), so I don't know the answers to these questions.

An Invitation

So, what do you think? Is there a fatal flaw to my understanding of the theory? A fundamental reason why it would not be possible to build such a "future telescope"? A technical limitation making it not currently possible?

I have talked to a few people about this idea, and the ones who I know have a good understanding of Carver's theory have said that, in principle, they don't see anything wrong with my reasoning.

AsI mentioned above, I'm not an astronomer or solid-state physicist, so I don't have the background to take this concept to the practical stage. But perhaps someone else does.

This seems like it would be a very exciting thing if it worked, but I think it would require a significant investment of time and access to some expensive equipment to take the next step. Would anyone like to give it a try? If you do, I'd love to hear about it.