
This will be a rather unusual article for this page since I wonât go too much
into detail - itâs 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 itâs only a fast writeup there might be more information to
specific events later on) just remember one important quote of Richard Feynman
quotes:
Donât 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 itâs 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
theyâve been detected from outside of our own galaxy.
So why havenât 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.
What would happen in such an unlikely event?
- High energy gamma rays are simply destructive for any biological or complex
system. Luckily our atmosphere is really good at shielding so itâs just likely
that there would be a really highly energetic and low duration ultraviolet
burst occurring - in theory itâs possible to reach immediately life threatening
levels but thatâs 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. Theyâve 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
theyâre 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 moonâs iron core, identical stable isotope ratios
in lunar and earthâs rocks, etc.). On the other hand itâs 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. Itâs 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 wouldâve been maximized - usually
itâs 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 thatâs also often discussed in politics today. So whatâs the big
deal about climate change?
- Atmospheric effects. Itâs 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 itâs 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 itâs 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 theyâre 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. Itâs 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 doesnât 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. Itâs 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 itâs responsible
of transferring warm water from the Gulf of Mexico towards northern Europe.
First sounding paradoxically this would lead to falling temperatures in Europe - itâs
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. Itâs 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 itâs the same as nuclear winter or nuclear famine.
To âsolveâ that problem some scientists have suggested the idea of a doomsday
device that has also been thematized in the movie âDr. Strangelove or: How I Learned
to Stop Worrying and Love the Bombâ. 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. Itâs assumed to be able to
travel with speeds of up to $800 \frac{km}{h}$ and lead to global destruction - with
itâs 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.
Itâs 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 itâs assumed that there have been hypercanes during the extinction
events in the Permian period or the CretaceousâPaleogene 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 thatâd 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. Itâs only somewhat linked
to physics but since nanotechnology is usually attributed to physicists - letâs
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 thatâs
capable of eating the environment to replicate itself (an ecophag). This ecophag
would eat itâs whole environment, all lifeforms and everything around it that itâs
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
itâs 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 itâs trace inside the galaxy so weâd be able to detect such a
massive object simply by itâs 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).
Solar lifetime end
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 sheâll 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. Itâs 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 wouldnât
be any potential gradients available. Since potential gradients are required to
sustain any form of information processing or chemical reaction the universe wouldâve
just halted at the global minimal reachable energy. Nothing would happen any more.
Big freeze
The big freeze is strongly related to heat death. Itâs 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 itâs assumed that there wonât 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 itâs 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. Itâs 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 theyâre ripped apart by the expanding space. Under this
model also the idea of time makes no sense any more, sometimes itâs 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. Itâs assumed that after the big bang
the universe expands but after a given time it will start to collapse again till
itâs 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 itâs 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.
This is a rather bad one. The idea is that the currently observed vacuum state
thatâs 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 weâd 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 itâs large enough to overcome
a given potential barrier it would start to expand; even if itâs 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.
There is a nice article as a starter
about that phenomenon in the context of the current standard model including
derivations and implications for various aspects and universe models if one wants
to go in depth.
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 itâs 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 weâre currently resting at a false
vacuum state as described above.
This article is tagged: Physics, Doomsday, Apocalypse