Riding on a laser beam ...

...  to chase the Starshot interstellar flight dream?

In the nearly 60 years of spaceflight we have accomplished wonderful feats of exploration that have shown the incredible spirit of the human drive to explore and understand our universe. Yet in those 60 years we have barely left our solar system with the Voyager 1 spacecraft launched in 1977 finally leaving the solar system after 37 years of flight at a speed of 17 km/s or less than 0.006% the speed of light. As remarkable as this is we will never reach even the nearest stars with our current propulsion technology in even 10 millennium. We have to radically rethink our strategy or give up our dreams of reaching the stars, or wait for technology that does not currently exist. While we all dream of human spaceflight to the stars in a way romanticized in books and movies, it is not within our power to do so, nor it is clear that this is the path we should choose. We posit a technological path forward, that while not simple, it is within our technological reach. We propose a roadmap to a program that will lead to sending relativistic probes to the nearest stars and will open up a vast array of possibilities of flight both within our solar system and far beyond. Spacecraft from gram level complete spacecraft on a wafer ("wafersats") that reach more than 1/4 c and reach the nearest star in 20 years to spacecraft with masses more than 10^5 kg (100 tons) that can reach speeds of greater than 1000 km/s. These systems can be propelled to speeds currently unimaginable with existing propulsion technologies. To do so requires a fundamental change in our thinking of both propulsion and in many cases what a spacecraft is. In addition to larger spacecraft, some capable of transporting humans, we consider functional spacecraft on a wafer, including integrated optical communications, imaging systems, photon thrusters, power and sensors combined with directed energy propulsion.

...Directed energy systems are ubiquitous, used throughout science and industry to melt or vaporize solid objects, such as for laser welding & cutting, as well as in defense. Recent advances in photonics now allow for a 2D array of phase locked laser amplifiers fed by a common low power seed laser that have already achieved near 50% wall plug conversion efficiency. It is known as a MOPA (Master Oscillator Power Amplifier) design. 

Schematic design of phased array laser driver. Wavefront sensing from both local and extended systems combined with the system metrology are critical to forming the final beam.

The technology is proceeding on a "Moore's Law" like pace with power per mass at 5 kg/kW with the size of a 1 kW amplifier not much larger than a textbook. There is already a roadmap to reduce this to 1 kg/kW in the next 5 years and discussions for advancing key aspects of the technology to higher TRL are beginning. These devices are revolutionizing directed energy applications and have the potential to revolutionize many related applications. Due to the phased array technology the system can simultaneous send out multiple beams and thus is inherently capable of simultaneous multitasking as well as multi modal.  

The laser system can be built and tested at any level from desktop to extremely large platforms. This is radically different than the older "use a huge laser" approach to photon propulsion. This is the equivalent to modern parallel processing vs an older single processor supercomputer. 

Parameters for full class 4 [a 10 km array] system with 1 g wafer SC and 1 m sail. Craft achieves 0.2 c in about 10 min (assuming an extended illumination) and takes about 20 years to get to Alpha Centauri. Communications rate assumes class 4 drive array is also used for reception with a 1 watt short burst from a 100 mm wafer SC. Here we use the 1 meter drive reflector as the transmit and receive optical system on the spacecraft. We also assume a photon/bit ratio near unity. In this case we get a data rate at Andromeda of about 65 kb/s. In the previous figure for the same wafer scale spacecraft the only optical system on the spacecraft was the 100 mm wafer. The data rate received at the Earth from Alpha Centauri is about 0.65 kbs during the burst assuming we can use the DE-STAR 4 driver as the receiver and only the wafer itself for the transmission optic. The plot above shows a much more conservative photon/bit ratio of 40 while unity has been achieved but never over the extremely long distances discussed here.

A Roadmap to Interstellar Flight

Philip Lubin

(Submitted on 5 Apr 2016 (v1), last revised 7 Apr 2016 (this version, v2))

//added on April 13, 2016: I guess some of the advances in photonics are spin-off applications of the late Strategic Defense Initiative but it is fascinating that they could be the backbone of the new Yuri Milner scientific/speculative breakthrough initiative publicized yesterday.

Anyway the future of large phased-array lasers in Earth orbit could definitely be in Directed Energy System for Targeting of Asteroids and exploRation at a time when nuclear ballistic missiles are likely not the most pressing threat anymore...

After astrophysics and condensed matter theory, holography meets biology

No comment // almost
Holographic Biology 

There are successful applications of the holographic AdS/CFT correspondence to high energy and condensed matter physics. We apply the holographic approach to photosynthesis that is an important example of nontrivial quantum phenomena relevant for life which is being studied in the emerging field of quantum biology. Light harvesting complexes of photosynthetic organisms are many-body quantum systems, in which quantum coherence has recently been experimentally shown to survive for relatively long time scales even at the physiological temperature despite the decohering effects of their environments. We use the holographic approach to evaluate the time dependence of entanglement entropy and quantum mutual information in the Fenna-Matthews-Olson (FMO) protein-pigment complex in green sulfur bacteria during the transfer of an excitation from a chlorosome antenna to a reaction center. It is demonstrated that the time evolution of the mutual information simulating the Lindblad master equation in some cases can be obtained by means of a dual gravity describing black hole formation in the AdS-Vaidya spacetime. The wake up and scrambling times for various partitions of the FMO complex are discussed.

Holographic Photosynthesis

Irina Aref'eva, Igor Volovich

(Submitted on 30 Mar 2016)

Holographic Mystique

Thus far, in spite of many interesting developments, the overall progress towards a systematic study and classification of various ’strange’ metallic states of matter has been rather limited. To that end, it was argued that a recent proliferation of the ideas of holographic correspondence originating from string theory might offer a possible way out of the stalemate. However, after almost a decade of intensive studies into the proposed extensions of the holographic conjecture to a variety of condensed matter problems, the validity of this intriguing approach remains largely unknown. This discussion aims at ascertaining its true status and elucidating the conditions under which some of its predictions may indeed be right (albeit, possibly, for a wrong reason).

... some limited form of a bulk-boundary relationship might, in fact, be quite robust and hold regardless of whether or not the systems in question possess any particular symmetries, unlike in the original AdS/CFT construction. Naively, this form of correspondence can even be related to the Einstein’s equivalence principle (i.e., ’curvature equals force’), according to which free motion in a curved space should be indistinguishable from the effect of a physical interaction (only, this time around, in the tangential direction). 

...

Together with the systematic comparison between the predictions of the condensed matter theory holography and other, more traditional, approaches and/or experimental data it would be a necessary step towards vetting the intriguing, yet speculative, holographic ideas before the latter can be rightfully called a novel technique for tackling the ’strange metals’ and other strongly correlated materials. In any event, though, it would seem rather unlikely that a hands-on expertise in string theory will become a mandatory prerequisite for understanding the cuprates.

Demystifying the Holographic Mystique

D. V. Khveshchenko

(Submitted on 31 Mar 2016)

Holographic Universe

... the AdS/CFT correspondence and holographic dualities have aroused immense enthusiasm in the string theory community. This constitutes, after all, normal scientific research. The phenomenon is still puzzling[45]. At a minimum, the holographic duality is an interesting tool for calculating fundamental physics. The dictionary the duality offers—between a world in flat space-time and a curved world with gravity—works in both directions. Some calculations are simpler with supergravity than in dual gauge theory. 

Gauge/gravity duality has enhanced the stature of Albert Einstein’s own theory...But, despite its recognized elegance, general relativity has been used by only a small portion of the scientific community. This is not surprising. After all, general relativity seemed confined to cases of strongly curved space-time: compact stars, the big bang, gravitational waves. Its effects were utterly negligible at the scales at work in condensed matter physics and nuclear physics. Why should gravity play a role in the quantum world? Yet, over the last twenty years general relativity has finally penetrated the world of modern physics. Specialists in condensed matter, nuclear physics, fluid turbulence, and quantum information are actively interested in general relativity. 

Why this dramatic turnaround? 

As science progresses, the virtues of cross-fertilization between different areas of knowledge have become widely appreciated. But this is the result rather than the cause. The key factor has been the AdS/CFT correspondence. Thanks to general relativity and its string theory extensions, one can now describe phenomena that have nothing to do with gravity in strong fields. 

On the other hand, the AdS/CFT correspondence has not been mathematically demonstrated. The holographic principle remains a conjecture. 

Its degree of experimental verification is zero. 

String theorists believe in it because their theory supports a specific version of holography, and, under certain important restrictions, black hole thermodynamics suggests it as well. But to conclude that it is a correct representation of nature is an enormous leap. 

We still do not know if string theory is correct. Let us suppose it is. Different formulations of the holographic principle have been tested only in situations that do not correspond to our world. The ensuing equations describe possible worlds that are similar, but not identical to our own. Extant solutions have allowed us to test the principles of string theory in limiting cases and to show the consistency of the theory, but never in situations corresponding exactly to the world in which we live. 

Suppose, on the other hand, that string theory proves false. What would become of the AdS/CFT correspondence? In loop quantum gravity, space-time emerges as the coarse-graining of fundamental structures made discrete, the atoms of space. The formation of a black hole and its ultimate evaporation are described by a unitary process that respects the laws of quantum mechanics [46] Similarly, the black holes described in the approach to non-commutative geometry do not evaporate completely and therefore escape the information paradox [47] The holographic conjecture has nonetheless improved the epistemological status of black holes. 

Their status was not always so elevated. The first exact solution of general relativity describing the space-time of a black hole was discovered by Karl Schwarzschild in 1916. Until the 1950s, general relativity theorists were embarrassed by black holes because of their singularities. Later they seemed esoteric objects, hardly believable. Then, in the decades between 1960 and 1990, they became both relevant to astrophysics, and fascinating in their own right. As we have seen, black holes have proven key for understanding quantum gravity, and the deep dualities between distant fields of theoretical physics. Perhaps someday they will become ubiquitous, because they have become useful in the description of everyday systems.  

The Holographic Universe Jean-Pierre Luminet 

Volume Two, Issue One, published February 9, 2016

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