In 30 seconds
Roger Penrose describes how an ever-expanding, ever-emptier universe loses its sense of scale, and why that makes its far future look like a Big Bang. He argues that signals from a previous eon, especially from black-hole collisions, could reach us, and he speculates this opens a new place to look for alien civilizations.
- 1Penrose frames the universe's far future as relentlessly dull: black holes swallow galaxies, then Hawking evaporation removes even them, leaving only photons in an ever colder, emptier cosmos.
- 2For massless particles like photons, infinity behaves like just another place, a boundary that can be squashed into a finite picture, much as Escher's circle limit squashes an infinite hyperbolic world.
- 3Because mass and frequency are equivalent, no mass means no clocks, and no clocks means no rulers, so the far future retains shape (conformal structure) but loses any sense of scale.
- 4The Big Bang loses scale for the opposite reason: particles moving almost at light speed behave as if massless, so the hottest, densest state and the coldest, emptiest state can be rescaled into one another.
- 5Penrose says signals from a previous eon, most notably gravitational waves from colliding black holes, could pass through into our own eon and be observable today.
- 6He suggests that if the universe cycles, the search for aliens might include civilizations so advanced they could send signals across eons, a speculation he explored in a paper with a colleague.
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The far future of an expanding universe is a long slide into boredom
Penrose starts with the universe's fate under the cosmological constant. Supernova observations early in this century showed that the universe is expanding exponentially, which vindicated the term Einstein introduced for the wrong reason and later called his greatest blunder. In Penrose's telling, it was not a blunder at all.
As the expansion continues, black holes become the most interesting objects around. They swallow clusters of galaxies, including our own galaxy after it collides with Andromeda. Eventually Hawking's black-hole evaporation takes over, and each black hole disappears "with a pop" at the end.
What remains is a colder and colder universe with nothing in it but photons. Penrose admits that calling this boring is emotional rather than scientific. His real question is who, if anyone, would be bored by it, and he points out that the photons would not be. Relativity means they reach infinity without experiencing the passage of time.
For massless particles, infinity is just another place
Penrose draws on his early work on gravitational radiation, where infinity could be squashed down and treated as an ordinary boundary. Photons and gravitons reach it as long as no mass is involved. The usual view is that nothing exists on the other side, because infinity is only a mathematical boundary. Penrose wants to take the trick seriously.
To make this concrete he uses M.C. Escher's circle-limit pictures of angels and devils. The figures crowd toward the edge of a circle, and the edge is their infinity. To them it is an endless hyperbolic world, but from outside we see a finite boundary. Penrose says he found this squashing trick useful for describing gravitational radiation, and now claims the trick becomes real.
He grants that the idea is hard to get one's mind around and says many colleagues find it difficult to take him seriously. When Lex Fridman asks whether infinity ever shows up physically, Penrose replies that mathematicians get used to different kinds of infinity and play with them without trouble. The physical question, he says, is what lies beyond that boundary.
Without mass there are no clocks, so the far future and the Big Bang look alike
Penrose explains why photons may be allowed to cross that boundary by pointing to two famous equations. Einstein's E equals mc squared makes energy and mass equivalent, and Planck's E equals h nu makes energy and frequency equivalent. Put together, mass and frequency are equivalent, so any massive particle is effectively a precise clock. Atomic and nuclear clocks are scaled-down versions of that principle.
The reverse follows too. If nothing has mass there are no clocks, therefore no rulers and no measure of scale in space and time. What survives is the conformal structure, in which shapes are preserved while sizes can be scaled up or down. Penrose compares it to the devils near the circle's boundary, which keep the same shape at a smaller size.
He then runs the argument backward to the Big Bang. As the early universe gets hotter and denser, particles move almost at light speed and effectively lose their mass, which again removes the sense of scale. His "crazy idea" is that the remote future and the Big Bang, though they feel completely different, can be matched by conformal rescaling. The far future of one eon would then become the Big Bang of the next.
Signals from a previous eon might reach us, and could even be alien
If the Big Bang was the remote future of an earlier eon, Penrose says, signals from that earlier time may be coming through to us. He identifies two main kinds, both connected to black holes. The first comes from collisions between black holes, which release large amounts of energy as gravitational waves while the black holes spiral into each other. Those waves, he says, pass in a certain form into the next eon.
Fridman picks up on the implication that information could travel across eons, and links it to the Fermi paradox. Penrose says he and a colleague wrote a paper on exactly this. If the universe cycles with each Big Bang as a punctuation mark, then intelligent systems might be able to communicate across the boundary.
Asked why we have not heard from aliens, Penrose says the fundamental reason may be that we do not know how to look, and he stresses that this is speculation. SETI was a reasonable effort aimed at civilizations that got there first. His new suggestion is to look instead for civilizations so successful, perhaps having handled problems like their own global warming, that they could send signals across eons for their own reasons. As for what such a signal would look like, he says he has no idea.
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