Hot Dyson spheres vibrate

When Freeman Dyson wrote about the concept of a giant sphere the size of a star, built to enclose the star and use all of the energy from it, he might not have imagined that seventy years later an entire literature would have grown up around such a speculative concept. In fact, the conjecture has thrived. In a recent issue of the International Journal of Astrobiology, Osmanov and Berezhiani (OB) show how a hot Dyson sphere should be optically visible.

Dyson first writes about the spherical concept in the June 1960 issue of Science. He envisions it as visible only in the infrared range. The infrared wavelengths of light are those that are just larger than we can see with our eyes.

Dyson predicts an infrared sphere for a few reasons. A spherical megastructure blocks out all the visible light of the star assuming that the structure is opaque. This distinguishes the concept of a full or complete Dyson sphere from a partial sphere, or swarm, which wouldn’t block out all the light.

Second, Dyson predicted an infrared sphere because of temperature. An object of a habitable temperature for human life on earth radiates light mainly in the infrared. The fact that all objects radiate light is an important finding that requires quantum physics to properly explain. The basic concept itself is simple.

At all temperatures above absolute zero, the atoms in a material are constantly moving about. This includes the electric charges moving inside atoms and charges moving around inside the bulk material. And when charges accelerate, they emit light. The energy of this light scales with the temperature. For Dyson’s habitable temperature sphere, the energy of the light corresponds to the infrared range.

To make a sphere of habitable temperature about a star only requires that you expand the radius to a sufficient degree. The bigger the sphere, the more area it has to distribute the energy of the star, and the cooler it can be.

Dyson estimated that Jupiter alone could supply the raw material needed to create a sphere with the radius of the earth’s orbit around the sun. It is roughly at this radius that the sphere is big enough and thus cool enough to be habitable.

Back to the present. In their paper, 60 years later, Osmanov and Berezhiani recognize that the shell can be made out of a modern material such as graphene. Graphene has exotic properties like a 4200 degree C melting temperature, almost three times higher than steel.

With graphene you can construct a Dyson sphere much closer to the star. This makes it much hotter, which would seem unfavorable for life. But you can create a cool, habitable patch on the sphere to support life. OB show that this is quite doable. The energy costs for your portable A/C are very low, considering that you are sitting on an electric generator the size of the sun.

For a hot Dyson sphere, the charges in the material radiate light of a higher energy, in the optical range. To search for a hot Dyson sphere, you would use optical astronomy.

To date, a number of studies have been performed that search for the signatures of possible Dyson spheres. In 2009, Carrigan took an existing satellite infrared dataset and combed through it for Dyson spheres. The instruments used made the study most favorable for searching a temperature range of 100 to 600 K. OB consider a reference temperature for hot Dyson spheres of 1000 to 4000 K.

What would the hot Dyson spheres look like to our instruments, and studies like Carrigan’s? A simple calculation by OB shows that the hot Dyson spheres vibrate, and thus their light should be variable.

To see this, consider that the outflux from the star exerts a great pressure on the inside of the sphere. When and if the sphere drifts off center, which will inevitably happen, then the pressure increases on the near side to the star. The sphere tends to get pushed back to center, until its momentum makes it overshoot the mark, and these oscillations continue.

A smaller and hotter shell gets hit harder by starlight and oscillates faster than a larger one. This oscillation period is around 1 to 30 years for the hot Dyson sphere temperature range considered by OB. The oscillation should cause variability in its starlight.

Variable luminosity is a common property of stars, and there are many reasons for variability and many different types of stars that exhibit it. Yet unlike other variable stars, OB suggest that the hot Dyson sphere should have some peculiar parameters. At a high rate of oscillation, the surface temperature of the Dyson sphere should be unusually low compared to other variable stars.

The predictions of OB are quite general and based on elementary calculations. More comprehensive models for the stability and dynamics of stellar megastructures would seem possible.

Nevertheless, future Dyson sphere searches might use the OB analysis to motivate taking their search to an expanded parameter range. Hotter spheres, with more visible light, and anomalous variability.

References:

Osmanov Z, Berezhiani VI (2018). On the possibility of the Dyson spheres observable beyond the infrared spectrum. International Journal of Astrobiology 17, 356–360.

Richard A. Carrigan Jr 2009 ApJ 698 2075

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