The universe as we know it might not end unexpectedly and unpredictably. That’s good. But on the other hand, it might. That’s bad. Consider just one way the universe could change unexpectedly. Physicists call it a “vacuum metastability event.”
Unbeknownst to us, all of space (even where there is complete vacuum) could be stuck in a relatively high energy state that has been stable so far, but might not remain so forever. This relatively high energy state is termed a “false vacuum.” At some spot in the universe this state could suddenly transition to a lower energy state (because of chance quantum tunneling, or a high-energy physics experiment involving an accelerator like CERN’s Large Hadron Collider gone awry, or perhaps some other unfortunate meddling by supposedly intelligent beings on another planet far away). Crazy things would then happen fast. The laws governing the universe could change at that spot, radically changing or simply destroying whatever happens to be there. Sound like no big deal if you’re not in the vicinity? It gets worse, and quickly. The lower-energy state of this spot would spread outward at near the speed of light. The changed physical laws, annihilation of matter, or whatever it is that is associated with this lower energy state would also spread outward at the same rate. The Earth would be destroyed more or less instantly if it started on Earth. If it started somewhere else in the universe, the spread would arrive here eventually. When it did, the Earth as we know it would be gone in the blink of an eye.
We don’t know much about what this new state of the universe would be like, but physicists Coleman and de Luccia deduced something about it. They wrote that, heretofore, “one could always draw stoic comfort from the possibility that perhaps in the course of time the new vacuum would sustain, if not life as we know it, at least some structures capable of knowing joy. This possibility has now been eliminated.” Not a pleasant prospect that.
If you feel this is hard to wrap your mind around, and does not exactly make common sense, you are not alone. Even the physicists who investigate the mathematical models from which they draw such conclusions don’t know if their equations really apply or not. As philosophers of science like Karl Popper (who is admired by many scientists) have pointed out, scientific theories cannot be proven absolutely. Of course, evidence may build up in favor or against a theory, and the rational person will go with the evidence. But quality evidence about vacuum metastability events may not be obtainable unless one actually happens, mooting the question. So maybe we have nothing to worry about, and maybe we do. How much do we need to worry?
First, we don’t know if the universe as we know it will unpredictably end or not. If it doesn’t, then we may have other things to worry about but at least we’re ok on that score. If it does end, though, a very important question is “when?” If it happens in billions of years, there is little point in breaking a sweat over it now. If it happens soon, you might want to start eating dessert first. Surprisingly enough, though the detailed physics of a vacuum metastability event is mostly impenetrable to anyone but a specialist, the average reader is quite capable of understanding when it (or some other unpredictable end to the universe) will occur, if it does. You see, its very unpredictability is the key to knowing when it will happen. I said “You see,” but you probably don’t. Yet in the next few minutes, you will. Just read on!
Glance at your watch. There is a 50% chance the second hand will be between the 12 and the 6 (the first half of a minute), and a 50% chance it will be between the 6 and the 12 (the second half). We’ll move from seconds to the life span of the universe in a couple of minutes, but first, another example. You are on the Web shopping for a used book by one of your favorite authors, and notice the following description: “Great condition! This is one of the copies that was numbered and signed by the author.” The price is reasonable so you buy it. Of all the numbered, signed copies, there is a 50% chance that the number of this one is in the lower half of the set of copies the author numbered and signed, and a 50% chance it is in the upper half. Upon receipt, you eagerly open the cover and there it is: the author’s name, signed in firm, distinctive yet flowing script, say, and numbered “94.” Since there is a 50% chance that 94 is in the lower half of the group of signed copies, there is a 50% chance that the full group contains at least 94 times 2, or 188, copies. Similarly, there is a 50% chance that 94 is in the higher half of the signed copies, which therefore must comprise fewer copies. For example, if 150 copies were signed, then 94 would be in the higher half. (You might — or might not — want to think about accounting for the case where the number is in neither the higher nor the lower half, but right in the middle.) Now, let’s get back to the more weighty matter of the life span of the universe.
A randomly chosen point within the life span of the universe has a 50% chance of being in the first half of that life span, and a 50% chance of being in the second half. Suppose that now, the very moment that you read these words, is indeed a point in time, chosen randomly, from within the life span of the universe as we know it. The universe is currently 13.75 billion years old, give or take a hundred million years or so. If 13.75 billion years has a 50% chance of being in the first half of the life span of the universe, then the universe must have a 50% chance of lasting at least 13.75×2 billion years — that is, another 13.75 billion years or more. But don’t breathe a sigh of relief just yet: the universe must also have a 50% chance of lasting less than 27.5 billion years, that is, of ending some time before the next 13.75 billion years is over. So can you breathe easy, or not? To decide this question, we need to extend our analysis a bit further.
Since we’ve already done a 50/50 split of the life span of the universe, let’s try another one, 75/25. The question then becomes, how long are the shortest 25% of the set of all possible life spans? This is important because, while there is a 75% chance the universe lasts longer than that (whew!) the 25% chance of it ending sooner is still pretty high — considering it’s the existence of the entire universe we’re talking about here. Focusing on the 25% chance that we are in the last 25% of the life span of the universe, the first 75% must therefore be within the past 13.75 billion years. See the figure.
From the figure, we can conclude that there is a 25% chance of the universe as we know it ending within the next 4.58 billion years, if it does end unpredictably (which again, it might or might not do). The worst case is that it ends within the next few minutes, but that sounds rather unlikely considering the billions of years worth of other possible times. But how unlikely is an uncomfortably soon end? Let’s continue the analysis…
We discussed a 50/50 split and a 75/25 split already. How about a 99/1 split? In that case there is a 1% chance we are in the last 1% of the life of the universe. Then, the same type of reasoning behind the 75/25 analysis applies, except this time the best case is that the universe lasts 13.89 billion years, of which the current 13.75 billion is 99% and the remaining 1% is another 140 million years. Still a long time although it’s getting short enough to give some pause — dinosaurs roamed the planet 140 million years ago, for example. Nevertheless, 140 million years is not exactly tomorrow.
What about a split that creates a time frame that one might actually worry about? Say, one which would induce you to at least think seriously about eating dessert first. Here is the straight dope: if the universe as we know it is destined to end at a random and unpredictable time, there is one chance in 100 million it will be within the next 137 years and 6 months.
You heard it here first.
Recommendations. Please continue to enjoy dessert in its traditional role of a post-main course treat. While 137.5 years is soon enough to potentially be of concern, the probability level of 1 in 100 million is not. Your chances of being hit by lightning, for example, are many thousands of times greater, roughly 1 in 6,000, making it a much greater concern. Take precautions during storms. Seek shelter, but not under a tree as lightning may target the tree. Long, thin metal objects such as umbrellas and golf clubs also may tend to attract lightning, particularly when raised above the head. If you feel your hair standing on end, you have become electrically charged and lightning may strike. Crouch down close to the ground immediately, because lightning seeks targets that are higher than the ground.
Interestingly, the same type of analysis described above can be applied to human events as well. For example, suppose the year 2020 rolls around with the universe still more or less intact. The United States, born in 1776, will be 244 years old. Assuming 2020 is a random year in the lifespan of the US, there is a 5% chance that the country will end by just under 13 years later, and a 1% chance it will end by a week or two shy of 2 years and 6 months. Think that’s crazy? As I write this, a large fireball that appeared last night in the skies over portions of the midwest US, accompanied by a loud sonic boom, is in the news. Probably caused by a meteorite, it broke up into smaller fireballs before disappearing. Although it caused no damage, had it been big enough, it would have leveled many square miles (as happened in Tunguska in 1908), ended the aforementioned life span, or even devastated the Earth (as happened to the dinosaurs).
If you’re Canadian, your country was born in 1867. There is a 5% chance the end would be within 8 years and a couple of weeks, and a 1% chance it would be within just over a year and a half. The apparent greater stability of the US (as measured by the historical trend of having existed for longer) gives it a greater than even chance of outliving Canada. What is that chance? In the case where the life spans of the two countries are independent, a mathematical technique called “convolution” can actually compute the chance of the US outlasting Canada: about 58%. On the other hand, Canada’s corresponding probability of outliving the US is 42%, not that much less.
Now you know.
Hopefully, 2020 is not a random year in these national life spans. For example, the life spans might be tied to the life span of western civilization, which started much longer ago and would thus lead to longer estimates of remaining life. That is certainly possible — but there’s no guarantee. Because we are in the “Recommendations” section, I suggest careful study of this issue, perhaps funded by the US and Canada governments.
Maybe eating dessert first is not such a bad idea after all.
Thanks to George Kahrimanis for useful discussion.
“As philosophers of science like Karl Popper (who is admired by many scientists) have pointed out, scientific theories cannot be proven absolutely.” A readable introduction to Popper appears in, for example, chapter 4 of P. Godfrey-Smith, Theory and Reality, University of Chicago Press, 2003.
“one could always draw stoic comfort from the possibility that perhaps in the course of time”: S. Coleman and F. de Luccia, Gravitational effects on and of vacuum decay, Phys. Rev. D21:3305, 1980; preprint available as report SLAC-PUB-2463, SLAC National Accelerator Laboratory, http://www.slac.stanford.edu/pubs/slacpubs/2000/slac-pub-2463.html.
“Your chances of being hit by lightning are roughly 1 in 6,000…”: Lightning safety, National Weather Service, http://www.lightningsafety.noaa.gov/medical.htm.
“Take precautions during storms.” See, e.g., Personal lightning safety tips, National Lightning Safety Institute, http://www.lightningsafety.com/nlsi_pls/lst.html.