I often ponder the nature of an alien culture. I contemplate what I can by using my primate mind. What kind of technology could the extraterrestrials have developed? How far ahead of us is their understanding of the Universe. Are the laws we currently think are true actually laws or are they just veiled suggestions.
Could they bypass relativity and travel superluminally? Is what we dream of in our science fiction science fact for them? There is obviously much for us to still learn.
But we have an idea on a few things that we believe is impossible for all intergalactic intelligences.
Detecting a photon, for example, is extremely easy. There many types of devices that are able to detect single photons, such as photomultipliers, used in labs around the world. In fact, you don’t even need any fancy technology; the human eye can, in principle, detect a single photon.
How did Einstein discover the existence of photons, particles of light? Not directly. But, by observing how the photoelectric effect occurs, he deduced that light ought to have quantized properties, that energy from light must be exchanged with atoms and electrons not as a wave but as individual, indivisible “lumps” of energy called photons. The quantized property of a gravity wave is called a “Graviton“.
Now, while the wave equivalent of the electromagnetic force is light, in general relativity, the wave equivalent of gravity is, well, a gravitational wave. These are produced when bodies with mass accelerate in space. Like how moving an object through the surface of water produces waves, the acceleration of bodies with mass produce gravitational waves. When gravitational waves pass, they compress and expand space with a definite frequency, and the magnitude of how space is compressed or expanded can be considered as the wave’s amplitude. Generally, massive bodies like supermassive black holes produce gravitational waves of large amplitude and low frequencies, and the inverse occurs for lower mass neutron stars and stellar mass black holes.
Now, here’s the interesting part: while physicists have already indirectly measured gravitational waves by observing how fast pulsars get closer and closer to each other (decay of the orbit). (Additional point: when accelerating bodies release a gravitational wave, the energy the object has is propagated away by the gravitational wave, ergo, conservation of energy. So, it’s the rotational energy of those two pulsars that are being reduced at a specific rate, and that rate agrees exactly with general relativity, considering that both of them release such waves), we haven’t been able to detect gravitational waves directly, that is, how they propagate. And this is one of the most interesting physics mysteries that still remain. Because the way they propagate can tell us so much about the nature of the universe.
Directly detecting gravitons is many thousands of years ahead of us. But could have E.T. figured it out?
A famous theoretical example considers an ideal G-wave detector with the mass of the planet Jupiter, around kilograms, placed in close orbit around a neutron star, which is a very strong source of gravitons. A back-of-the-envelope calculation reveals that even in this extremely unrealistic scenario, it would take 100 years to detect a single graviton!
Okay, you say, so let’s just make that detector (sometime in the far future when we have the technology to do so) and wait for 100 years. There’s a crucial detail that I forgot to mention, however. The star also emits neutrinos in addition to gravitons; in fact, much more neutrinos than gravitons. And neutrinos are much easier to detect than gravitons. In fact, we can calculate that for every graviton that is detected in this scenario, around neutrinos will be detected. So we will never be able to find the one graviton among the neutrinos.
For a perspective on this…
The electromagnetic radiation of the Sun amounts to a colossal 400 trillion terawatts approximately (that is, a 4 followed by 26 zeroes.) It is easy to detect. Moreover, its quantum nature is easy to detect, as it does not take long to find an atom in which an electron transition to a higher energy level, interacting with a single photon from solar radiation.
The gravitational equivalent of thermal radiation of the Sun has been calculated. As it turns out, the Sun emits a respectable 79 megawatts of heat in the form of thermal gravitational radiation. Of course, it is a tiny, tiny, tiny portion (roughly 0.2 millionths of a trillionth) of the Sun’s electromagnetic output, but it is there.
So can we detect single gravitons from this flux of thermal gravitational radiation? In principle, we can. But it would require a mighty big detector. In fact, if we used the entire Earth as a graviton detector, we might expect to see roughly one electronic transition every billion years or so. So maybe four events to date since the birth of the solar system. All other gravitons will just fly through the Earth without interacting with its constituent elementary particles.
In conclusion, even with insanely advanced futuristic alien technology, it would simply be impossible to detect a graviton.
This is at least the one thing humans and aliens most likely have in common.
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