# Apocalyptic doomsday scenarios predicted by physics

05 Apr 2021 - tsp
Last update 05 Apr 2021

This will be a rather unusual article for this page since I won’t go too much into detail - it’s just a short overview article. So we all know physics offers many many doomsday scenarios - we physicists seem to be rather optimistic about the future - so while reading this short summary of possible outcomes (I might have missed some - or really a huge amount - so this article might change in future and since it’s only a fast writeup there might be more information to specific events later on) just remember one important quote of Richard Feynman quotes:

Don’t worry about anything. Go out and have a good time.

And keep in mind that most events described here may only happen somewhere in distant future or are really unpredictable and thus usually unavoidable anyways.

## Just destroying civilization but not life on earth

So on the entry level there are some scenarios that might end our current type of civilization but leave rather large amounts of people alive.

### Kessler syndrome

This is a pretty simple effect. The assumption is that there is more and more debris collected in earth orbit. This increases the probability of collisions and thus - with a given critical density - the probability for an cascade effect at which all shrapnel from the clouds forming after an explosion triggering the next collisions, thus generating even more debris and so on. This sounds like an event happening in a huge distance but it would simply prevent one to use these orbits for a long period of time - so any space bound communication system, navigational systems, observation technology, monitoring systems, etc. would be rendered unusable.

### Solar flare / Coronal mass ejections

A coronal mass ejection is a significant release of plasma (i.e. charged particles) out of the solar corona. Usually they follow a solar flare - which is a localized sudden increase of solar luminescence. Usually solar flares are barely visible and hard to detect when compared with the total solar irradiance. During a solar flare the plasma inside the solar atmosphere - including the corona, chromosphere and photosphere - heat up rapidly to a few tens of millions of Kelvin, charged particles are accelerated nearly instantly near speed of light. The electromagnetic radiation leaked ranges from radio waves up to gamma waves. They can basically be imagined as a discharge of a huge amount of magnetic potential when magnetic fields penetrate the photosphere so stored magnetic energy from the corona regions can be discharged into the solar interior.

The UV radiation as well as X-rays irradiated during a solar flare usually affect the ionosphere of the earth and disrupt long range communication systems. A more major problem that might arise is the high energy flow of charged particles. They can usually be seen as northern lights - aurora borealis - when interacting with the ionosphere. They can cause disruptions in radio links, damage to spaceborn equipment like satellites and they can cause damage to power transmission equipment linked to massive and long lasting power outages.

## The small thing (beginner level): Ending life on earth

A little bit more motivated doomsday approach just affecting life on earth involves scenarios that leave our solar system and galaxy alone - not bugging the universe in any way - but would be rather unpleasant for any living human being.

### Gammabursts

One of the more likely scenarios available. Gamma ray bursts are one of the most energetic electromagnetic energy releases that have been observed in the universe up to today. Bursts usually last from several milliseconds up to a few hours. The initial burst is usually followed by an afterglow at longer wavelengths. These bursts are currently thought to be emitted under specific conditions during supernovae or super-luminous supernovae when super heavy stars collapse into the state of an black hole or neutron star. A specific subclass of GRBs is thought to be emitted in case of ripping up the crust of stars in binary star systems because of acting massive tidal forces before such a system collides.

All observed GRBs that have been detected have been millions of light years away, have been extremely energetic (one can imagine about the same amount of energy as a star is releasing over it’s entire lifetime for an estimated scale) and are extremely rare. As a side note: the first detected GRBs have been detected by satellites that had been brought into orbit to monitor the nuclear test ban treaty. As one has seen the distribution of detected gamma ray bursts has been isotropic which suggests that they’ve been detected from outside of our own galaxy.

So why haven’t these energy eruptions destroyed life in earth up until now? They seem to be highly collimated into two back to back jets with an angle between 2 and 20 degrees. Having an directed beam traveling into the huge empty space radically reduces the probability of being directly hit.

• High energy gamma rays are simply destructive for any biological or complex system. Luckily our atmosphere is really good at shielding so it’s just likely that there would be a really highly energetic and low duration ultraviolet burst occurring - in theory it’s possible to reach immediately life threatening levels but that’s really unlikely.
• The long term effects would be really bad though. The delivered energy would trigger a reaction producing massive amounts of nitrogen dioxide. This gas would at first deplete the ozone layer up to 25-75% - way enough to raise UV levels on the earths surface to dangerous levels. The photochemical smog which would darken the sky might - even though not hindering photosynthesis due to the absorbed spectrum - lead to a cosmic winter (the same idea as impact winter) due to falling temperatures. The acid rain caused by the nitrogen dioxide would wash out huge amounts of nitric acid.
• DNA damage is assumed to be raised by about a factor of 16 after a gamma burst has hit the earth.

### Anoxic events

Anoxic events occurred multiple times during earth history until now. They’ve always been accompanied by mass extinction events. During an anoxic event oceans have been completely depleted of dissolved oxygen which lead them to be big pool of sulphidic and anoxic ponts. These events seem to be strongly linked to climate warming, slowing of oceanic circulation as well as an increase of greenhouse gasses - and they’re also the times at which fossil fuels have been produced in larger quantities by dying organisms sinking to the bases of the oceans and fouling there.

Since anoxic events are usually linked to atmospheric changes as well as climate warming this would usually also lead to major problems for humans ecosystem as well as food supply and eventually also lead to human extinction.

### Impact events

The idea is simple - a rather huge astronomical object impacts earth. For most smaller objects this has none to minimal effect - small objects burn up in earths atmosphere, larger objects might reach the ground but usually do not do any damage.

On the other hand major impact events are already thought to have had an major influence on the development of todays earth - for example there are theories that the formation of the moon has been triggered by a collision between the proto-Earth and a about mars sized planetesimal (sometimes referred to as Theia - i.e. the mother of Selene, the goddess of the moon) about 4.5 billion years ago. There is some evidence to support this thesis (like similar orientation of the moon orbit and earths spin, the anomalously high angular momentum, geological indications on the moon, the moon’s iron core, identical stable isotope ratios in lunar and earth’s rocks, etc.). On the other hand it’s assumed that an impact of such huge energy should have produced a global magma ocean on earth (there is some indication that this might have been the case) or the question why only a single moon formed out of this impact event which would be rather unusual.

There are theories that other impact events are for example responsible for the delivery of initial water to earth by impact from icy planetesimal - and of course there is the cretaceous-paleogene extinction event - a mass extinction event around 66 million years ago where about 3/4 of all animal species on earth vanished.

### Impact winter

This is one of the possible outcomes of an impact event. The idea is that an impact event of a medium sizes asteroid or comet - usually assumed to be about 5 km in diameter or larger - might eject enormous amounts of ash and dust into the atmosphere. This would cause an radical temperature drop on earth as well as block any or most sunlight reaching the surface. It’s assumed to wipe out most of the worlds existing species (like it has been proposed for the Cretaceous-Paleogene extinction).

Besides the dust such an impact - an asteroid with about 10km in diameter would have an explosive force of more than $10^8 Mt$ - might also trigger massive fire storms with global reach. These fires might release huge amounts of water, ash, carbon dioxide, etc. and contribute to the change in climate on their own.

### Kinetic bombardment

This is just a human made variation of an impact event. The idea was to use kinetic projectiles that are dropped from space at high speed at a very steep angle (thus hard to defend against) onto earth. Due to maximized velocity of the projectiles the impact energy would’ve been maximized - usually it’s proposed to use tungsten rods due to high temperature capability during reentry so as much mass survives at highest possible speed. Smaller projectiles such as 6 x 0.3 meter tungsten rods would be able to deliver around 50 GJ directed onto their target. On the other hand it would require really much mass (refer to impact events) to trigger some kind of apocalyptic event.

### Climate Change

This is one that’s also often discussed in politics today. So what’s the big deal about climate change?

• Atmospheric effects. It’s thought that global warming reduces the oceans capability to absorb oxygen. This would lead to high carbon dioxide concentrations in the water and thus to production of large amounts of hydrogen sulfide gas by anaerobic microbes. At some critical threshold huge amounts of this toxic gas would be released into atmosphere leading to mass extinction of animals and plants. There are also models out there that suggest that this would also damage the ozone layer exposing the remaining lifeforms to dangerously high ultraviolet radiation levels - there are indications this might have happened during the Permian extinction event. At the end of Paleocene and Triassic extinction events happened around 1000ppm carbon dioxide concentration.
• Ocean acidification would also be a major consequence of large carbon dioxide concentrations. This would for example suppress reproduction of all sealife with calcium carbonate shells - and thus have a huge impact on all oceanic life (and food production - again).
• Rising temperatures increase the risk for epidemics and pandemics - for example by spread of typical distributors such as mosquitoes and ticks. On the other hand epidemics are more likely after events such as floodings and might also be supported by malnutrition - thus also supported by agricultural impact.
• Famines. This is most likely the first effect of climate change to see. Already up of today it’s already been seen that climate change cut the global agricultural productivity by more than 20% since the 1960s.
• Uninhabitable areas due to excessive heat. Since the human body usually uses evaporational cooling due to sweating it’s not possible to cool down under high humidity conditions properly. Reports which predict heat increase of around 3 to 5 degree Celsius up until 2100 would predict that around 70% of worlds population would be living in areas in which they’re exposed to lethal conditions.
• This would also trigger mass displacements. These would trigger social instabilities and conflicts as huge amounts of people want to move to new areas. It’s already assumed that climate change and natural disasters trigger around 60% of all internal displacements.
• Sea level rise. A rise of around 6 degree Celsius would lead to a rise of the sea level of around 20 meters. This doesn’t sound that much but would trigger huge scale mass migrations due to high number of inhabitants near coastal lines and large amount of land loss.
• Shutdown of oceanic currents. It’s already assumed that climate change has reduced the gulf streams velocity by about 15 to 20%. Halting of the gulf stream would have a major impact on earths climate since it’s responsible of transferring warm water from the Gulf of Mexico towards northern Europe. First sounding paradoxically this would lead to falling temperatures in Europe - it’s assumed for example that temperature in the United Kingdom would drop by around 3.4 degree Celsius, rainfall would be reduced by around 123mm during the growing season - and thus arable land would be reduced from 32% to 7% leading to huge problems with food supply.
• All of this would of course lead to society collapsing and also wars arising again for sure.

Note there is currently a discussion about the amount of human influence - during this one tends to forget that even if human influence is low it might trigger an unstoppable feedback circle as soon as permafrost starts to melt or oceans are warming up. One of the major questions is of course if this point has already been reached or if human influence and ability to change is large enough to stop that development.

### Nuclear winter

Being a little bit more under our control nuclear winter is a human made way of eradicating humans from earth. The idea is that the dropping of too many nuclear weapons raises huge quantities of dust into earths upper atmospheres. Large scale wildfires also caused by nuclear weapon use would produce huge amounts of dense smoke, burning cities will even contribute more to that smoke - all effects blocking sunlight already in upper atmospheric layers. It’s assumed to take weeks to months till the particles would sink down to ground level again - leading to a temperature decrease of 10-20 Kelvin worldwide which would lead to crop failure and famines. Nearly all regions that are relevant for food production would be affected for many years (some models assume more than a decade).

In addition to cooling the destruction of the ozone layer would allow more UV light to pass - with the usual results of higher rate of cancer, more reproductive failure, etc.

### Nuclear famine

See nuclear winter.

### Nuclear holocaust

This is another term used to describe the destruction of civilization and depending on the scale also of mankind at a whole by the massive usage of nuclear weapons. In the most simple case it’s the same as nuclear winter or nuclear famine.

To “solve” that problem some scientists have suggested the idea of a doomsday device that has also been thematized in the movie “Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb”. The idea was to surround a number of hydrogen bombs with cobalt casings - the cobalt would be activated into a highly radiative state with around 5 years of half life time. This highly radioactive material was proposed to be distributed all around the world - with a high enough radiation density to clear the whole earth from any human life. Some more modern variants also added certain kinds of aerosols that would prolong the effects of nuclear winter to make survival even less likely.

### Super volcano eruption

A super volcano eruption is just another way of transferring huge amounts of dust into the atmosphere.

### Hypercane

A hypercane is a theoretical tropical hurricane. It’s assumed to be able to travel with speeds of up to $800 \frac{km}{h}$ and lead to global destruction - with it’s height reaching up to $30 km$ into the atmosphere it might lead to major damage to the ozone layer and thus again high ultraviolet radiation levels.

It’s assumed that an hypercane can only be triggered with water temperatures in the ocean reaching up to 50 degree Celsius (hurricanes start at temperatures of around 26 degrees) and thus might only happen in combination with other apocalyptic events like impact events or super volcano eruptions.

Sometimes it’s assumed that there have been hypercanes during the extinction events in the Permian period or the Cretaceous–Paleogene extinction event.

### Overpopulation

One of the more likely events to happen - of course not as early as estimated by many people. The carriage capabilities of earth that are estimated usually with a rather high variance. The most optimist upper bound has been estimated by Fremlin who estimated around 60 quadrillion people would be capable of living on the earth as long as the whole surface would be used for industrial style farming, traveling would be limited to a few hundred meter over the lifetime of every person as well as proper plumbing for supply and wastewater handling. He arrived at this values by thermodynamic considerations so they can be considered to be an absolute upper bound.

On the other hand other overpopulation effects might end the life as we know it way before reaching a theoretical limit that’d only be achievable by global cooperation - caused by political instabilities, famines, epidemics, etc. that may threaten our stable communities and eradicate all higher society functionality by conflicts and wars.

### Gray goo

Not really realistic to happen - gray goo is the name for a human triggered doomsday scenario. It’s only somewhat linked to physics but since nanotechnology is usually attributed to physicists - let’s put it on the list.

The idea - that has also been picked up in many science fiction TV shows - is that someone had the brilliant idea of building a molecular nanobot that’s capable of eating the environment to replicate itself (an ecophag). This ecophag would eat it’s whole environment, all lifeforms and everything around it that it’s capable of consuming till there is nothing left to consume any more - just waiting for something to arrive from the outside.

But as it turns out to be really challenging to build something self replicatable it’s highly unlikely to work even if one desires to build such a nanomachine.

### Falling into a black hole

Falling into a black hole is not as bad as it sounds - at least not from an observer point of view. The interesting thing about a black hole is that for an observer information is just accumulated at the event horizon and objects are slowly fading whereas from their own point of view they get pulled into the black hole and ripped apart into their subatomic parts - and of course compressed to infinitely small volume.

Basically it would be entirely possible that the earth (or better said our whole solar system) could collide or interact with a black hole that crosses our way throughout the solar system. In this case this would really mean the end of our solar system since all matter would be sucked up - after the planets and also the sun would have been teared apart.

This event is pretty unlikely to occur during our lifetime since a massive black hole would leave it’s trace inside the galaxy so we’d be able to detect such a massive object simply by it’s effect on other matter inside the galaxy. Even if such an object would be able to travel near the speed of light it would take billions of years to reach us even when originating from the same solar system. Our own sun on the other hand will never ever turn into a black hole because of her low mass - at the end of her lifetime she will expand into a red dwarf with an radius larger than earths radius around the sun so this event will destroy earth anyways rather early (approximated to be around 120 million years from now on).

The lifetime of our sun is basically set. Since our sun - like every star - is basically driven by a huge fusion reaction that fuses nitrogen to heavier elements there will be a point where her hydrogen supply is not sufficient to keep up with her demand any more. This will start in around 5 billion years from now - during the first phase the sun will run out of hydrogen and start to collapse. In this phase she’ll be fusing heavier elements in her core while continuing to fuse hydrogen on her outside shell. During this phase the temperature of the sun will increase and the outer layers of her outer hydrogen atmosphere will start to expand to about 256 times her current size - which is obviously bad for us since this is larger than earths orbit around the sun - there will be a new habitable zone for this short (a few billion years again) phase somewhere in the Kuiper belt though. After the red giant phase the sun will collapse into a white dwarf as soon as the helium fused inside the inner part of the sun is gone (totally fused). In this state she will slowly cool down and fade out - another step towards the heat death of the universe.

## Being more consequent (expert level): The end of the universe

The last category of doomsday scenarios involves some impossible to escape scenarios. One of them will most likely happen in the far future.

### Heat death of the universe (thermal equilibrium)

Heat death is a rather boring end for our universe. It’s compatible with the assumption of an expanding, contracting or statically sized universe. The basic idea is that the universe reaches a state of maximum entropy - in this state everything (especially energy levels) would be uniformly distributed, there wouldn’t be any potential gradients available. Since potential gradients are required to sustain any form of information processing or chemical reaction the universe would’ve just halted at the global minimal reachable energy. Nothing would happen any more.

### Big freeze

The big freeze is strongly related to heat death. It’s assumed that a continuously expanding universe will - on average - asymptotically approaching the absolute zero temperature point. In absence of dark energy it might occur under hyperbolic or flat geometries - or with positive cosmological constant also in any closed universe. Stars are usually expected to form for about $10^12$ to $10^14$ years after the big bang, after this time spawn it’s assumed that there won’t be any more gas supply for star formation. As existing stars burn out and black holes would decay by radiative processes the Poincare recurrence theorem would predict the system to fall into it’s initial minimal state of minimum entropy via a spontaneous process.

### Big rip (Space expanding indefinitely)

For the big rip to occur a special type of dark energy - often called phantom energy ($w < -1$) - is required to be present. It’s assumed that the universe will expand indefinite. In this case dark energy could expand infinitely so that it would at one point be large enough to overcome all forces that are currently gluing together the universe. Since the expanding space is continuously accelerating the distances over which forces - which are only mediated at the speed of light - is shrinking and shrinking so the sphere of interaction gets smaller and smaller (one can think of space expanding faster than the speed of light at some point). When the size of this influence sphere becomes smaller than any structure no fundamental interactions are able to occur any more. Since the structures cannot be kept together any more they’re ripped apart by the expanding space. Under this model also the idea of time makes no sense any more, sometimes it’s said that the time has sopped.

In this model - after a finite amount of time - the universe will be a final singularity in which all distances will diverge to infinity and the observable universe reaches zero size.

As measurements suggest that $w \approx -0.991$ changes for the big rip to happen are really low.

### Big crunch (Universe collapsing)

This is a symmetric approach to the big bang. It’s assumed that after the big bang the universe expands but after a given time it will start to collapse again till it’s being squished into a single point again. The assumption for this to happen is that the density of the universe will be large enough to counter any diverging forces such that the universe will begin to contract again.

The final state would be a dimensionless point sized singularity containing all energy of the universe.

### Big bounce (Universe collapsing)

The big bounce is an extension of the big crunch. The idea is that immediately following the big crunch there might be another big bang happening, forming another universe restarting the cycle again.

Usually it’s assumed to be impossible due to violation of the second law of thermodynamics and since there are many hints that point towards the universe being open.

### Metastable false vacuum

This is a rather bad one. The idea is that the currently observed vacuum state that’s assumed to be located at a energetic minimum has in fact formed at a metastable local energy minimum instead of the global minimum. Such a metastable state could be stable for a really long time duration - but if it decays into the lower energy vacuum state some minor problems might arise:

• The fundamental forces may cease to exist or work differently, the families of elementary particles that make up matter may change, particle families might cease to exist, new ones might form, interaction parameters might change.
• Some false vacuum models predict the total destruction of all Baryonic matter or the total gravitational collapse of the universe.
• Other models may indicated possible proton decay - so the previously stable fundamental nucleon would be unstable.

Unfortunately the only time we’d experience or measure such an event would be when it occurs. So this is another candidate that might happen any time instantaneously.

The event that would cause such an decay is usually called bubble nucleation. A small bubble of true vacuum state would form - if it’s large enough to overcome a given potential barrier it would start to expand; even if it’s not able to overcome the potential barrier on the other hand there would be a tunneling probability that would allow expansion to occur. There has been a theory that black holes and high energetic particle collisions might work as nucleation seeds - the latter being highly unlikely since then it would have already happened with a high probability.

Note that this fate is sometimes also called big slurp since one can imagine the bubble expanding from the bubble nucleation event can be imagined of slurping up the remaining universe.

### Proton decay

Proton decay is a unlikely form of particle decay. Up to our knowledge protons seem to be stable - but we can only specify a lower bound of the proton half-life time of about $1.67 * 10^{34}$ years in case of decay through a positron channel for example. Since this decay has never been observed it’s assumed to be not possible by experimental physicists.

A related process - positron emission - is not the same as proton decay since in this case the proton decays only after an interaction process with other particles. On the other hand some beyond standard model theories that try to do the grand unification step of all forces do break the Baryon symmetry and allow decay of the proton through new bosons, the Higgs mechanism or in combination with magnetic monopoles - this is assumed to happen on a timescale of more than $10^{31}$ years which is around 20 magnitudes larger than the current estimated age of the universe.

There are some modern theories that may require and predict proton decay without an apocalypse - but on the other hand it might also simply indicate that the vacuum state is not stable - i.e. that we’re currently resting at a false vacuum state as described above.