Quantum mechanics as-we-know-it predicts that the vacuum energy density, the so-called `vacuum zero-point’ or `dark energy’, should have been much larger than the value inferred from the expansion rate of the Universe. If the value had been indeed much higher, as expected, the Universe would have expanded too rapidly for galaxies like the Milky-Way to form and give birth to stars like the Sun, and for us to have fun under the Sun.
The measured mass density of the vacuum based on the accelerated cosmic expansion, 6.5x10^{-30} grams per cubic centimeter, is lower than air density by 26 orders of magnitude. The vacuum energy is so tiny that even if we were to harvest it in full across a huge cube which is 20 kilometers on a side — as long as Manhattan Island and as thick as twice the maximum altitude of commercial airplanes, and convert all the dark energy in this volume into electricity, it would have been able to power merely a single 100-Watt light bulb for less than a minute. Despite claims to the contrary, the vacuum energy density is so dilute that it cannot power alien craft across our sky. There is a limit to how much we can contemplate technologies that go beyond the standard model of physics. Energy conservation must be satisfied.
Some speculators argue that the Casimir effect provides evidence for the potential of advanced technologists to tap the energy of the vacuum. The effect results from placing two conducting plates parallel to each other and excluding electromagnetic vacuum fluctuations with wavelengths larger than their separation. As a result of the boundary conditions imposed by the plates, the vacuum energy density is lowered between them, giving the illusion that one can manipulate the vacuum to artificially create a negative mass. However, the energy associated with the conducting plates far exceeds the energy density of the vacuum between them, implying that the total energy of the system is positive. No physics can give rise to a negative mass object, as far as we know.
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