The Cosmic Now
Aguirre (Anthony)
Source: Aeon, 20 June, 2019
Paper - Abstract

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Author’s Concluding Paragraphs

  1. This commingling of the present and the most distant past makes it clear that there is a complex relationship, and there can be an enormous difference, between what is happening right now and what we are observing right now. This does not feel terribly troubling, but it is a good reason to be very cautious. In particular, it suggests that we pay heed only to things that happen at the same time and the same place (the same ‘event’). For example, perceiving a leaf ‘now’ means that the light is reaching our eyes at (very close to) the same time and place as our internal perception of ‘now’ is taking place; the actual impact of the leaf on the ground is a different event. Likewise, if you disconnected your analogue TV a few years ago and had your last peek at the cosmic dawn then, this event was quite different from the initial launch of that radiation from the early cosmic fireball.
  2. But so what? you might ask. Despite these facts about what we know of the universe right now, our intuition tells us that there is something like a big cosmic clock ticking away in the background, that depends neither on our perception nor on our knowledge of when events happened. As Isaac Newton put it: ‘Absolute, true, and mathematical time, of itself, and from its own nature flows equably without regard to anything external …’ That is, although we see the stars in a delayed sense, there is still some particular way that the stars are right now; we would just have to wait a while to find out. Underlying this intuition is the idea that we could have some sort of signal that travels as fast as we like and that, if we used that signal, we could see the stars as they truly shine at this very moment.
  3. Even if no such signal exists, we can still imagine the set of events that happen right now, at least in retrospect, as it were. Suppose it is 5:00pm, and a NASA technician sends a signal to an exploratory vehicle on Mars, instructing it to take a photo and send it back. When, at 5:20pm, the technician receives a photo showing a waving tentacle about to grab the camera, she can infer that first contact occurred at 5:10pm. At 5:10, though she did not know it, amazing things were happening on Mars ‘right now’. Similarly, we could think of all sorts of events that are happening right now, by giving the corresponding definition, invented by Albert Einstein: if we sent a light signal to an event a time (t) ago, and if that signal reaches the event, turns around, and returns to us at exactly the same time (t) in the future, then we say that the event is happening now. Events for which this is not true would be in the future or the past.
  4. Taking this to its logical conclusion, by imagining signals all throughout the universe, we would think it possible to construct – in principle at least – the cosmic now. That is, we can imagine all sorts of events, all across the cosmos, even those that we cannot actually observe for a long time, if ever. And we feel quite strongly that each such event either is happening right now, or is not.
  5. This construction is perfectly sensible, and accords with your intuition – but it is an illusion we maintain. Events such as the falling leaf or shining stars that we can see here and now are those from which light is reaching us now, and lie along the diagonal line labelled ‘events we can see’. Events such as the baby being born in India, the Martian waving its tentacles and a planet falling into a black hole are happening ‘now’, but not here; we cannot yet see them (we’ll have to wait a time (t), for example, to possibly see that the baby in India has been born). These ‘happening now’ events are defined to be those events for which we can imagine a light signal sent by us a time (t) ago that reaches the event, turns around, and returns to us exactly a time (t) from now. This is how we extend our sense of the cosmic ‘now’. Yet the universal now, the set of all events everywhere that are happening at the same moment as you read this – so clear in your mind – is just a dream.


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