“For the wise man looks into space and he knows there is no limited dimensions.” –Lao Tzu
You know the deal: it’s the end of the week, so it’s time for another Ask Ethan! You’ve continued to send in your questions-and-suggestions, much to my delight, and I’m pleased to tell you that this week’s question comes from Peter Tibbles. Peter asks about obtaining information from beyond where Einstein’s theory of relativity allows us to see:
Because of the expansion of the universe there is an event horizon beyond which we can know nothing. There’s been one instance of intelligent life evolved (us); let’s assume another in a galaxy far, far away. They have the same size event horizon (that includes us). On the “other side” of them (away from us) they would be able to see parts of the universe that’s beyond our event horizon.
Now, they see something there and say “That’s interesting, let’s share it with everyone”. So, they send out signals about this. We pick them up and decode them (we are theoretically intelligent). Thus we would be able to know something of features beyond our event horizon. […] Is this theoretically possible, or have I made a blunder somewhere?
This is a really interesting possibility Peter has brought up! Let’s walk through what he’s asking.
On the one hand, we can — if we’re clever — look back all the way to the beginning of our Universe, at least as it can first be described by the Big Bang, some 13.8 billion years ago. We have direct images of the Universe from when it was just 380,000 years old (of the Cosmic Microwave Background), below…
and we can see stars and galaxies from nearly that long ago as well, up to the limits of modern telescope technology. The current record-holder‘s light is just reaching us now after a journey of 13.4 billion years, placing it an unbelievably large 33 billion light years away, a record that will only be broken once the James Webb Space Telescope comes online in a few years.
The thing is, we’re limited by the part of the Universe that’s observable to us. Since the Big Bang, light has been on a journey for up to 13.8 billion years, which means we can see the light that’s traveled for up to that long in any direction we look.
But that doesn’t mean there isn’t more Universe beyond that; in fact, we’re sure there is, and the only question we have is how much!
Now, here’s where Peter’s question comes in. If there’s a limit to what we can see, then that limit must be the same for the observers whose light we’re seeing!
In other words, if we look at two galaxies in opposite directions, both of which have had their light traveling for, say, 13.7 billion years, we can see both of them, but they can’t see each other.
And if there’s an intelligent person in either one of them, couldn’t they send us information about the things they’ve seen that are inaccessible to us, therefore “cheating” the limits of space and time, and giving us information about the Universe that we wouldn’t have otherwise?
It’s a fun question to ponder! After all, wouldn’t we be doing that exact thing if, say, we sent an image of the Hubble eXtreme Deep Field to a galaxy in the complete opposite direction of us?
Or, for that matter, a map of the large-scale structure of the Universe as we see it? And, for that matter, couldn’t we keep passing that information on and on and on, learning about the Universe far beyond what any one observer, on their own, could see?
It’s a remarkably tempting thought, but — unfortunately — it’s a flawed line of thinking. Can you spot where it goes wrong?
I’ll give you a hint: think about the light that we can see from something extremely far away.
Yes, it’s a fact that distant galaxy is emitting light right now, 13.8 billion years into our Universe. But we’ll never receive it.
You see, not only is the Universe expanding, but that expansion is accelerating thanks to dark energy, and light emitted from galaxies beyond a certain distance will no longer reach us.
If it weren’t for dark energy, we would eventually be able see that light (and receive that information) they’re sending today… but only at-or-after the light from those galaxies they observed would have reached us anyway.So yes, we can “get information” from objects that are beyond our apparent horizon today, but only once they move inside our horizon in the first place. And because of dark energy, everything that’s ever going to be inside our apparent horizon is already here!