It’s a little tongue-in-cheek, but I don’t find it totally out-of-the-question either. This post is a confluence of a few things, only two of which are mentioned in the title.
The best exposition of Fermi’s paradox I’ve come across, is by Prof Brian Cox. In fact, he provides a very good succinct definition in the first 2m of his video.
Notwithstanding the fact that there has been billions of years on billions of worlds for civilisations to arise, we see no evidence of any of them in the Galaxy at all. So the paradox is, Why?
He then uses the remaining 20+ minutes of the video, providing expositions on various reasons why this is the case. Firstly, he gives a history lesson, explaining how life on Earth took close to 4 billion years to evolve to its present state. He doesn’t mention that humans (homo sapiens) only arose in the last 3-400,000 years of that, so the last 0.01% or thereabouts, and it’s only in the last century of that 300k+ years that we developed very rudimentary extra-terrestrial space exploration capabilities (my point, not his). So a blink-of-an-eye in the context of our evolutionary history.
He goes on to give various scenarios why the Fermi paradox exists but the one most pertinent to this post (around 7m) is that the distance between worlds that could have the same properties as our solar system, in being able to create a stable environment over tens of billions of years for life to evolve, are at least thousands of lightyears away (in our galaxy), and therefore beyond the ability to travel here in the timeframe that such distances dictate. He points out that even the time required for receiving radio transmissions are commensurate with the distance to be traversed. Which is why searching for ‘artificial’ signals is the most efficient method to look for extra-terrestrial civilisations.
One of his best arguments involves Von Neumann machines or ‘replicators’, which are self-replicating robots, which could colonise a galaxy within 100s of millions of years (his estimate). He contends that the fact we’ve never seen any, is the best indicator we have that a spacefaring civilisation doesn’t exist in our galaxy.
I won’t elaborate on any of the other scenarios Cox expounds upon, but I recommend watching the full 23m, because it’s very stimulating and informative. He also talks about the ‘Great Filter’ (~10m), which is a scary yet totally plausible scenario, although not relevant to this post. Basically, he says that civilisations like us, who can create the technology for space-travel, also have the technology to destroy ourselves, and may lack the wisdom to do one over the other. In his own words, we’re brought undone by our ‘stupidity’.
His best argument he left to last, which I’ve reported on before, but based on Nick Lane’s excellent book, Life Ascending; The Ten Great Inventions of Evolution. Basically, Cox argues there was 4B years of life before complex life arose, and it’s such a rare possibility that maybe it’s never happened anywhere else. Note: this doesn’t rule out simple life being prolific throughout the galaxy. He cites John Barrow's and Frank Kipler’s book, The Anthropic Principle, which he explains, he grew up with, as supporting that particular view. I read that book and reference it here.
That took longer than I intended, so now I will address the other part of my question, which is time-travel. Another physicist, Jim Al-Khalili, who wrote an excellent book called Paradox (which I’ve read) has a video titled provocatively, Could I Travel Back in Time? where he explores various time-travel scenarios.
Naturally, he starts with the well known grandfather paradox, whereby you travel back in time to kill your grandfather before he met your grandmother, thus breaking the causal chain for your own existence. This idea was exploited in the original Terminator movie. In fact, it’s hard to talk about time-travel without referencing science fiction examples.
He then uses the scenario of having found instructions to build a time machine, actually builds it, which takes him 10 years, then travels back 10 years in time to drop the instructions on his desk. Al-Khalili points out a couple of issues: it defeats free will because if the scientist ‘chooses’ not to travel back in time, it can’t happen at all; and the instructions are never actually created in the first place, so causality is overturned. He also mentions the problem of having 2 versions of the scientist existing at the same time, therefore breaking the first law of thermodynamics: conservation of energy.
He then talks about how travelling into the future is physically possible according to Einstein’s theories of relativity, either by travelling close to the speed of light, as per the famous twin paradox thought experiment, or spending time in a strong gravitational field, as per the movie, Interstellar. A version of the last one, I’ve used in my own science fiction.
Al-Khalili draws a spacetime diagram showing the future and past light cones, which features strongly in my way of viewing this. But without getting ahead of myself, he points out that, according to GR (Einstein’s general theory of relativity), if you could curve spacetime strongly enough you could create a time-loop. In fact, this would be a feature of a rotating universe, as pointed out by Einstein’s friend, Kurt Godel, as well as a possibility for a rapidly spinning black hole. There is good reason to believe we don’t live in a rotating universe, but rotating black holes, one assumes, are quite common. Could one get stuck in a time-loop at the event horizon of a spinning black hole? I don’t know.
Al-Khalili cites Russian astrophysicist, Igor Novikov, who proposed his own rule, regarding CTCs (Closed-Timelike-Curves): ‘Only “logically self-consistent” solutions are allowed’.
Al-Khalili then introduces us to Dr Fabio Costa, who has published a paper where he talks about taking time-slices, where cause-and-effects create boundary conditions, which neither he nor Al-Khalili explain in detail. But Costa concludes by saying, “Nature is smart enough to prevent such paradoxes” (referring to the grandfather paradox, specifically). My takeaway, which admittedly is rather simplistic, even superficial, is that cause-and-effect is such a fundamental principle in physics, that any scenario that sidesteps it, needs to be treated with extreme caution. I should point out that there are scientists and philosophers (like Donald Hoffman) who think that causality, as a principle, is past its use-by date.
As Al-Khalili sums up, Fabio Costa argues that “time paradoxes disappear, not because they’re logically inconsistent, but because they’re physically not allowed.”
Al-Khalili also refers to the Many Worlds Interpretation (MWI) of quantum mechanics, as providing another loophole, for want of a better expression, to allow time-travel without creating paradoxes. In fact, this is the scenario invoked in the Back to the Future movie franchise. When Marty McFly goes back to the past and intervenes, he subsequently finds himself in a different universe when he returns to the present. Al-Khalili explains how this resolves the grandfather paradox very neatly (similar to the Marty McFly scenario), but like me, he has issues with MWI as a reality.
Lastly, Al-Khalili talks to Dr Lorenzo Gavassino at the Department of Mathematics, Vanderbilt University, Nashville, Tennessee, who took a serious look at the Groundhog Day scenario, but proposes that the individual was in a closed box that underwent a time loop. Specifically, he looked at how this is impacted by the 2nd Law of Thermodynamics or entropy, because when the individual went back in time their entropy would decrease and their memories would be erased. Again, I might be simplifying it for the sake of explication, but it’s what I expect a time loop would create. The way Gavassino describes it, including his literal handwaving, the time loop has 2 nodes, where at one node the entropy is at its lowest and at the other node, entropy is highest. And effectively, the individual oscillates between these two extremes.
Right at the end, Al-Khalili makes the point that I’ve heard before, which is that if someone did invent a time machine, it could never go back to earlier than the time it was invented, which is why we’ve never seen one.
Again, I spent much longer on this than I intended but I’m now going to reference yet another video that talks about the edge of time for the entire universe. In essence, I’m going to discuss a video by Curt Jaimungal who discusses an aspect of Einstein’s GR that I’d never heard about before. Curt often discusses very esoteric topics, but I’m confident he knows what he’s talking about, because I’ve seen him hold his own with such luminaries as Stephen Wolfram, Gregory Chaitin, Sir Roger Penrose and Sabine Hossenfelder. In fact, I watched a 2hr video conversation he had with her after he sent me a link, and they were very chatty and friendly.
I had to watch this more than once to get my head around it, and I’ll rely heavily on quotes to get my points across. As a side-issue, he addresses a conundrum I’ve raised many times before, which is that the Universe has an edge in time, but not space (as best we can tell), yet relativity theory tells us that there is no universal now. I’ll get to that, but first I’ll address what the video is primarily about.
What the video is about, contrary to what everyone seems to believe, including Einstein himself, based on what I know, is that his GR equations don’t provide for a deterministic universe at the very limit or edge of the Universe in time. In fact, Curt contends that it is even less deterministic than quantum mechanics with an infinity of possibilities. At one point, he quips, “God does not even play dice, He just shrugs.”
But what I want to focus on, is what Curt calls a ‘Cauchy surface,’ which, if I understand him correctly, is the hypersurface that we are all familiar with in the spacetime diagram that Al-Khalili referred to in his video, which also shows the past and future light cones for any observer at time, t.
Significantly, Curt distinguishes between local determinism and global determinism, because he argues that GR allows for one and not the other, and this is also relevant to determining what ‘now’ means. Significantly, global determinism "determines the future of the entire universe", whereas local determinism refers to "what happens in the immediate future of that region [of space]".
Curt defines a ‘Cauchy surface’ as “a spacelike slice, that every causal curve hits exactly once.” He emphasises this with beats of his hand for every single word. An obvious consequence of this is that there can be no time loops.
He then asks the obvious question: can you extrapolate local determinism to global determinism? His answer is very informative in that he contends that in flat spacetime this is ‘trivial’ because you can, but not in curved spacetime, which is what GR describes. I think this is also relevant in attempting to understand what a universal now means.
In fact, Curt says, specifically, “There are solutions to the field equations of Einstein [in GR] where you literally can’t define a moment of time across the entire universe.” He then emphasises the point, by saying: “In general relativity, there may not be a way to slice spacetime in all of space at time t.” Note, this is a consequence of spacetime curvature and not SR (special theory of relativity) as most arguments claim.
And there’s a term for this, which is ‘global hyperbolicity’, and that’s where he introduces his definition of a Cauchy surface which I referenced earlier. So the spacelike slice where every ‘causal curve’ only crosses once, applies to the entire universe.
Then, in an apparent contradiction to what he told us before, he says “the Cauchy surface is like a snapshot of the universe at one time.” He then emphasises again that “every world line [for a particle] and every light ray; they all cross the surface exactly once.” He then says, “This is what even allows you to say, the state of the Universe at time t.” And I think this is the closest you will get to the idea of a universal now. But there is a caveat, which is that “not all solutions to Einstein’s equations are globally hyperbolic.”
He then provides examples which mostly include black holes, including rotating black holes, which I mentioned in reference to Al-Khalili’s video. He also mentions Godel’s rotating universe, which I think we can confidently say, we don’t inhabit.
Curt has an understanding of this topic that makes me realise how little I really know. What I do know is that clocks measure time at different rates depending on gravity, so if you happened to live near a black hole, you would measure the age of the Universe as younger than someone on Earth, so you would axiomatically get a different t.
What Curt doesn’t describe is the possibility that there is a wavefunction for the entire universe, which some scientists believe (like Sean Carroll, for example), and, if it describes the future, as Freeman Dyson believed, then maybe entanglement provides an edge in time for the entire universe. But I’m speculating way beyond the limits of my knowledge.
This post has already become too long with detours down various rabbit holes, but there is one more video I want to reference, which has specific relevance to the title.
I grew up in a small rural town in Australia, and I’m old enough to remember when TV arrived, though we didn’t own a set until I was 16. The saving grace for me were books, including both the primary school and high school libraries, which I devoured. So it was around 16 years of age that I read a book called The Truth About Flying Saucers by Aime Michel, first published in 1956 (I looked it up), and I admit to being profoundly influenced by it. (I’m not sure you’d find a book on flying saucers in a modern high school in Australia.) It so happened that I had a science teacher who was also interested in UFOs – he was a very good science teacher, by the way, and I’m not the only one who would say that.
I’ve since become more sceptical about the whole UFO scene, as I’ve developed what I believe is a healthy scepticism towards conspiracy theories of any ilk. So it’s in that context that I reference an episode of Australian Story on ABC TV, where a group of people from a small town in Australia recollect a particular ‘incident’ from their childhood, involving phenomena that couldn’t be explained at the time, or even now.
When I watched this and listened to their testimony, I thought that one explanation, which is even more out-there than the possibility of visitors from outer space, is the possibility that some ultra-advanced technological species has learned how to travel through time from the future. Perhaps only a sci-fi writer would even consider that.
But there is another possibility, which I thought of when I watched Brian Cox’s video. If you look at the spacetime diagram depicting past and future light cones, it’s obvious that most of the events in the so-called ‘observable’ universe are ‘unobservable.’ It’s possible, therefore, that there are civilisations billions of light years away, who have developed technologies that exceed ours, which we can’t possibly know about using radio waves, Von Neumann machines or whatever. Unless, that is, they’ve developed the technology to travel through time as well as space.
The thing is that if someone (or some object) was able to visit us from outside our past or future light cones, it wouldn’t create a causal loop, unlike all the examples that Al-Khalili provided in his video (except MWI).
My conjecture, if I can call it that, which is admittedly more sci-fi than science reality, is: What if a technologically advanced enough extra-terrestrial species developed the ability to ride the hyperplane on the Cauchy surface, while avoiding black holes, and was able to visit our world line? As Brian Cox said at the end of his video, a scientist has to be able to admit they’re wrong, but it would require a visitor in a flying saucer to convince him.
Footnote: The last video I referenced, Australian Story, is very good reporting, and I recommend you watch it so you can make your own judgement.
