The fine-tuned universe is the hypothesis that, because "life as we know it" could not exist if the constants of nature – such as the electron charge or the gravitational constant – had been even slightly different, the universe must be tuned specifically for life.In practice, this hypothesis is formulated in terms of dimensionless physical constants.
History
In 1913, chemistLawrence Joseph Henderson wrote The Fitness of the Environment,
one of the first books to explore fine tuning in the universe.
Henderson discusses the importance of water and the environment to
living things, pointing out that life as it exists on Earth depends
entirely on Earth's very specific environmental conditions, especially
the prevalence and properties of water.
In 1961, physicist Robert H. Dicke argued that certain forces in physics, such as gravity and electromagnetism, must be perfectly fine-tuned for life to exist in the universe.
Astronomer Fred Hoyle
argued for a fine-tuned universe: "From 1953 onward, Willy Fowler and I
have always been intrigued by the remarkable relation of [...] and your
fixing would have to be just where these levels are actually found to
be. [...] A common sense interpretation of the facts suggests that a
superintellect has monkeyed with physics, as well as with chemistry and
biology, and that there are no blind forces worth speaking about in
nature." In his 1983 book The Intelligent Universe, Hoyle wrote, "The list of anthropic properties, apparent accidents of a
non-biological nature without which carbon-based and hence human life
could not exist, is large and impressive."
Belief in the fine-tuned universe led to the expectation that the Large Hadron Collider would produce evidence of physics beyond the Standard Model, such as supersymmetry, but by 2012 it had not produced evidence for supersymmetry at the energy scales it was able to probe.
Motivation
Physicist Paul Davies
said: "There is now broad agreement among physicists and cosmologists
that the Universe is in several respects 'fine-tuned' for life. But the
conclusion is not so much that the Universe is fine-tuned for life;
rather it is fine-tuned for the building blocks and environments that
life requires". He also said that "'anthropic' reasoning fails to distinguish between minimally biophilic
universes, in which life is permitted, but only marginally possible,
and optimally biophilic universes, in which life flourishes because biogenesis occurs frequently". Among scientists who find the evidence persuasive, a variety of natural explanations have been proposed, such as the existence of multiple universes introducing a survivorship bias under the anthropic principle.
The premise of the fine-tuned universe assertion is that a small
change in several of the physical constants would make the universe
radically different. Stephen Hawking
observed: "The laws of science, as we know them at present, contain
many fundamental numbers, like the size of the electric charge of the
electron and the ratio of the masses of the proton and the electron. ...
The remarkable fact is that the values of these numbers seem to have
been very finely adjusted to make possible the development of life".
For example, if the strong nuclear force were 2% stronger than it is (i.e. if the coupling constant representing its strength were 2% larger) while the other constants were left unchanged, diprotons would be stable; according to Davies, hydrogen would fuse into them instead of deuterium and helium. This would drastically alter the physics of stars,
and presumably preclude the existence of life similar to what we
observe on Earth. The diproton's existence would short-circuit the slow
fusion of hydrogen into deuterium. Hydrogen would fuse so easily that it
is likely that all the universe's hydrogen would be consumed in the
first few minutes after the Big Bang. This "diproton argument" is disputed by other physicists, who calculate
that as long as the increase in strength is less than 50%, stellar
fusion could occur despite the existence of stable diprotons.
The precise formulation of the idea is made difficult by the fact
that it is not yet known how many independent physical constants there
are. The Standard Model of particle physics has 25 freely adjustable parameters and general relativity has one more, the cosmological constant, which is known to be nonzero but profoundly small in value. Because physicists have not developed an empirically successful theory of quantum gravity, there is no known way to combine quantum mechanics, on which the standard model depends, and general relativity.
Without knowledge of this more complete theory suspected to
underlie the standard model, it is impossible to definitively count the
number of truly independent physical constants. In some candidate
theories, the number of independent physical constants may be as small
as one. For example, the cosmological constant may be a fundamental
constant but attempts have also been made to calculate it from other
constants, and according to the author of one such calculation, "the
small value of the cosmological constant is telling us that a remarkably
precise and totally unexpected relation exists among all the parameters
of the Standard Model of particle physics, the bare cosmological constant and unknown physics".
N, the ratio of the electromagnetic force to the gravitational force between a pair of protons, is approximately 1036. According to Rees, if it were significantly smaller, only a small and short-lived universe could exist. If it were large enough, they would repel them so violently that larger atoms would never be generated.
Epsilon (ε), a measure of the nuclear efficiency of fusion from hydrogen to helium, is 0.007: when four nucleons fuse into helium, 0.007 (0.7%) of their mass is converted to energy. The value of ε is in part determined by the strength of the strong nuclear force. If ε
were 0.006, a proton could not bond to a neutron, and only hydrogen
could exist, and complex chemistry would be impossible. According to
Rees, if it were above 0.008, no hydrogen would exist, as all the
hydrogen would have been fused shortly after the Big Bang.
Other physicists disagree, calculating that substantial hydrogen
remains as long as the strong force coupling constant increases by less
than about 50%.
Omega (Ω), commonly known as the density parameter,
is the relative importance of gravity and expansion energy in the
universe. It is the ratio of the mass density of the universe to the
"critical density" and is approximately 1. If gravity were too strong
compared with dark energy and the initial cosmic expansion rate, the
universe would have collapsed before life could have evolved. If gravity
were too weak, no stars would have formed.
Lambda (Λ), commonly known as the cosmological constant, describes the ratio of the density of dark energy
to the critical energy density of the universe, given certain
reasonable assumptions such as that dark energy density is a constant.
In terms of Planck units, and as a natural dimensionless value, Λ is on the order of 10−122. This is so small that it has no significant effect on cosmic structures
that are smaller than a billion light-years across. A slightly larger
value of the cosmological constant would have caused space to expand rapidly enough that stars and other astronomical structures would not be able to form.
Q, the ratio of the gravitational energy required to pull a large galaxy apart to the energy equivalent of its mass, is around 10−5.
If it is too small, no stars can form. If it is too large, no stars can
survive because the universe is too violent, according to Rees.
D, the number of spatial dimensions in spacetime, is 3. Rees claims that life could not exist if there were 2 or 4 spatial dimensions. Rees argues this does not preclude the existence of ten-dimensional strings.
Max Tegmark
argued that if there is more than one time dimension, then physical
systems' behavior could not be predicted reliably from knowledge of the
relevant partial differential equations. In such a universe, intelligent life capable of manipulating technology could not emerge. Moreover, protons and electrons
would be unstable and could decay into particles having greater mass
than themselves. This is not a problem if the particles have a
sufficiently low temperature.
An older example is the Hoyle state, the third-lowest energy state of the carbon-12 nucleus, with an energy of 7.656 MeV above the ground level. According to one calculation, if the state's energy level were lower
than 7.3 or greater than 7.9 MeV, insufficient carbon would exist to
support life. To explain the universe's abundance of carbon, the Hoyle
state must be further tuned to a value between 7.596 and 7.716 MeV. A
similar calculation, focusing on the underlying fundamental constants
that give rise to various energy levels, concludes that the strong force
must be tuned to a precision of at least 0.5%, and the electromagnetic
force to a precision of at least 4%, to prevent either carbon production
or oxygen production from dropping significantly.
Explanations
Some explanations of fine-tuning are naturalistic. First, the fine-tuning might be an illusion: more fundamental physics
may explain the apparent fine-tuning in physical parameters in the
current understanding by constraining the values those parameters are
likely to take. As Lawrence Krauss
put it, "certain quantities have seemed inexplicable and fine-tuned,
and once we understand them, they don't seem to be so fine-tuned. We
have to have some historical perspective". Victor J. Stenger has shown that random selection of physical parameters can still produce universes capable of harboring life. Some argue it is possible that a final fundamental theory of everything will explain the underlying causes of the apparent fine-tuning in every parameter.
Still, as modern cosmology developed, various hypotheses not presuming hidden order have been proposed. One is a multiverse, where fundamental physical constants are postulated to have different values outside of the known universe.
On this hypothesis, separate parts of reality would have wildly
different characteristics. In such scenarios, the appearance of
fine-tuning is explained as a consequence of the weak anthropic principle and selection bias, specifically survivorship bias.
Only those universes with fundamental constants hospitable to life,
such as on Earth, could contain life forms capable of observing the
universe who can contemplate the question of fine-tuning. Zhi-Wei Wang and Samuel L. Braunstein argue that the apparent fine-tuning of fundamental constants could be due to the lack of understanding of these constants.
If the universe is just one of many (possibly infinitely many)
universes, each with different physical phenomena and constants, it is
unsurprising that there is a universe hospitable to intelligent life.
Some versions of the multiverse hypothesis therefore provide a simple
explanation for any fine-tuning, while the analysis of Wang and Braunstein challenges the view that this universe is unique in its ability to support life.
The multiverse idea has led to considerable research into the anthropic principle and has been of particular interest to particle physicists because theories of everything
do apparently generate large numbers of universes in which the physical
constants vary widely. Although there is no evidence for the existence
of a multiverse, some versions of the theory make predictions of which
some researchers studying M-theory and gravity leaks hope to see some evidence soon. According to Laura Mersini-Houghton, the WMAP cold spot could provide testable empirical evidence of a parallel universe. Variants of this approach include Lee Smolin's notion of cosmological natural selection, the ekpyrotic universe, and the bubble universe theory.
It has been suggested that invoking the multiverse to explain fine-tuning is a form of the inverse gambler's fallacy.
Top-down cosmology
Stephen Hawking and Thomas Hertog proposed that the universe's initial conditions consisted of a superposition of many possible initial conditions, only a small fraction of which contributed to the conditions seen today. According to the top-down cosmology
theory, the universe's "fine-tuned" physical constants are inevitable,
because the universe "selects" only those histories that led to the
present conditions. In this way, top-down cosmology provides an
anthropic explanation for why this universe allows matter and life
without invoking the multiverse.
Carbon chauvinism
Some
forms of fine-tuning arguments about the formation of life assume that
only carbon-based life forms are possible, an assumption sometimes
called carbon chauvinism. Conceptually, alternative biochemistry or other forms of life are possible.
Simulation hypothesis
The simulation hypothesis
holds that the universe is fine-tuned simply because the more
technologically advanced simulation operator(s) programmed it that way.
No improbability
Graham Priest, Mark Colyvan, Jay L. Garfield,
and others have argued against the presupposition that "the laws of
physics or the boundary conditions of the universe could have been other
than they are".
Some scientists, theologians, and philosophers, as well as certain religious groups, argue that providence or creation are responsible for fine-tuning. Christian philosopher Alvin Plantinga
argues that random chance, applied to a single and sole universe, only
raises the question as to why this universe could be so "lucky" as to
have precise conditions that support life at least at some place (the
Earth) and time (within millions of years of the present).
One reaction to these apparent enormous coincidences
is to see them as substantiating the theistic claim that the universe
has been created by a personal God and as offering the material for a
properly restrained theistic argument – hence the fine-tuning argument.
It's as if there are a large number of dials that have to be tuned to
within extremely narrow limits for life to be possible in our universe.
It is extremely unlikely that this should happen by chance, but much
more likely that this should happen if there is such a person as God.
— Alvin Plantinga, "The Dawkins Confusion: Naturalism ad absurdum"
William Lane Craig, a philosopher and Christian apologist, cites this fine-tuning of the universe as evidence for the existence of God or some form of intelligence capable of manipulating (or designing) the basic physics that governs the universe. Philosopher and theologian Richard Swinburne reaches the design conclusion using Bayesian probability. Scientist and theologian Alister McGrath observed that the fine-tuning of carbon is even responsible for nature's ability to tune itself to any degree.
The entire biological evolutionary process depends upon
the unusual chemistry of carbon, which allows it to bond to itself, as
well as other elements, creating highly complex molecules that are
stable over prevailing terrestrial temperatures, and are capable of
conveying genetic information (especially DNA). [...] Whereas it might
be argued that nature creates its own fine-tuning, this can only be done
if the primordial constituents of the universe are such that an
evolutionary process can be initiated. The unique chemistry of carbon is
the ultimate foundation of the capacity of nature to tune itself.
Theoretical physicist and Anglican priest John Polkinghorne stated: "Anthropic fine tuning is too remarkable to be dismissed as just a happy accident".
Theologian and philosopher Andrew Loke
argues that there are only five possible categories of hypotheses
concerning fine-tuning and order: (i) chance, (ii) regularity, (iii)
combinations of regularity and chance, (iv) uncaused, and (v) design,
and that only design gives an exclusively logical explanation of order
in the universe. He argues that the Kalam Cosmological Argument strengthens the teleological argument by answering the question "Who designed the Designer?".
Creationist Hugh Ross advances a number of fine-tuning hypotheses. One is the existence of what Ross calls "vital poisons", which are elemental nutrients that are harmful in large quantities but essential for animal life in smaller quantities.
Philosopher and theologian Robin Collins
argues that theism entails the expectation that God would create a
reality structured to allow for scientific discovery to easily happen.
According to Collins, various physical constants such as the fine-structure constant allowing for efficient energy usage, the baryon-to-photon ratio allowing for the cosmic microwave background to be discovered, and the mass of the Higgs boson allowing it to be detected are examples of the laws of physics being fine-tuned for scientific discovery.
Evolutionary biologist Richard Dawkins
dismisses the theistic argument as "deeply unsatisfying" since it
leaves the existence of God unexplained, with a God capable of
calculating the fine-tuning at least as improbable as the fine-tuning
itself. Against this claim, it has been argued that theism is a simple
hypothesis, allowing theists to deny that God is at least as improbable
as the fine-tuning.
Imagine
a puddle waking up one morning and thinking, "This is an interesting
world I find myself in, an interesting hole I find myself in, fits me
rather neatly, doesn't it? In fact, it fits me staggeringly well, must
have been made to have me in it!"
Mutual assured destruction (MAD) is a doctrine of military strategy and national security policy which posits that a full-scale use of nuclear weapons by an attacker on a nuclear-armed defender with second-strike capabilities would result in the complete annihilation of both the attacker and the defender. It is based on the theory of rational deterrence,
which holds that the threat of using strong weapons against the enemy
prevents the enemy's use of those same weapons. The strategy is a form
of Nash equilibrium in which, once armed, neither side has any incentive to initiate a conflict or to disarm.
The result may be a nuclear peace, in which the presence of nuclear weapons
decreases the risk of crisis escalation, since parties will seek to
avoid situations that could lead to the use of nuclear weapons.
Proponents of nuclear peace theory therefore believe that controlled nuclear proliferation may be beneficial for global stability. Critics argue that nuclear proliferation increases the chance of nuclear war through either deliberate or inadvertent use of nuclear weapons, as well as the likelihood of nuclear material falling into the hands of violent non-state actors.
The term "mutual assured destruction", commonly abbreviated "MAD", was coined by Donald Brennan, a strategist working in Herman Kahn's Hudson Institute in 1962. Brennan conceived the acronym cynically, spelling out the English word "mad" to argue that holding weapons capable of destroying society was irrational.
Theory
Under MAD, each side has enough nuclear weaponry to destroy the other
side. Either side, if attacked for any reason by the other, would
retaliate with equal or greater force. The expected result is an
immediate, irreversible escalation of hostilities resulting in both
combatants' mutual, total, and assured destruction. The doctrine
requires that neither side construct shelters on a massive scale. If one side constructed a similar system of shelters, it would violate
the MAD doctrine and destabilize the situation, because it would have
less to fear from a second strike. The same principle is invoked against missile defense.
The doctrine further assumes that neither side will dare to launch a first strike because the other side would launch on warning (also called fail-deadly) or with surviving forces (a second strike),
resulting in unacceptable losses for both parties. The payoff of the
MAD doctrine was and still is expected to be a tense but stable global
peace. However, many have argued that mutually assured destruction is
unable to deter conventional war that could later escalate. Emerging
domains of cyber-espionage, proxy-state conflict, and high-speed missiles threaten to circumvent MAD as a deterrent strategy.
The primary application of this doctrine started during the Cold War (1940s to 1991), in which MAD was seen as helping to prevent any direct full-scale conflicts between the United States and the Soviet Union while they engaged in smaller proxy wars around the world. MAD was also responsible for the arms race, as both nations struggled to keep nuclear parity, or at least retain second-strike capability. Although the Cold War ended in the early 1990s, the MAD doctrine continues to be applied.
Proponents of MAD as part of the US and USSR strategic doctrine believed that nuclear war
could best be prevented if neither side could expect to survive a
full-scale nuclear exchange as a functioning state. Since the
credibility of the threat is critical to such assurance, each side had
to invest substantial capital
in their nuclear arsenals even if they were not intended for use. In
addition, neither side could be expected or allowed to adequately defend
itself against the other's nuclear missiles. This led both to the hardening and diversification of nuclear delivery systems (such as nuclear missile silos, ballistic missile submarines, and nuclear bombers kept at fail-safe points) and to the Anti-Ballistic Missile Treaty.
When the possibility of nuclear warfare between the United States
and Soviet Union started to become a reality, theorists began to think
that mutual assured destruction would be sufficient to deter the other
side from launching a nuclear weapon. Kenneth Waltz,
an American political scientist, believed that nuclear forces were in
fact useful, but even more useful in the fact that they deterred other
nuclear threats from using them, based on mutually assured destruction.
The theory of mutually assured destruction being a safe way to deter
continued even farther with the thought that nuclear weapons intended on
being used for the winning of a war, were impractical, and even
considered too dangerous and risky. Even with the Cold War ending in 1991, deterrence from mutually assured
destruction is still said to be the safest course to avoid nuclear
warfare.
Effectiveness of the theory according to empirical studies
A study published in the Journal of Conflict Resolution in 2009 quantitatively evaluated the nuclear peace hypothesis and found support for the existence of the stability-instability paradox. The study determined that nuclear weapons promote strategic stability and prevent large-scale wars but simultaneously allow for more low intensity conflicts.
If a nuclear monopoly exists between two states, and one state has
nuclear weapons and its opponent does not, there is a greater chance of
war. In contrast, if there is mutual nuclear weapon ownership with both
states possessing nuclear weapons, the odds of war drop precipitously.
The concept of MAD had been discussed in the literature for nearly a
century before the invention of nuclear weapons. One of the earliest
references comes from the English author Wilkie Collins, writing at the time of the Franco-Prussian War
in 1870: "I begin to believe in only one civilizing influence—the
discovery one of these days of a destructive agent so terrible that War
shall mean annihilation and men's fears will force them to keep the
peace." The concept was also described in 1863 by Jules Verne in his novel Paris in the Twentieth Century,
though it was not published until 1994. The book is set in 1960 and
describes "the engines of war", which have become so efficient that war
is inconceivable and all countries are at a perpetual stalemate.
MAD has been invoked by more than one weapons inventor. For example, Richard Jordan Gatling patented his namesake Gatling gun in 1862 with the partial intention of illustrating the futility of war. Likewise, after his 1867 invention of dynamite, Alfred Nobel
stated that "the day when two army corps can annihilate each other in
one second, all civilized nations, it is to be hoped, will recoil from
war and discharge their troops." In 1937, Nikola Tesla published The Art of Projecting Concentrated Non-dispersive Energy through the Natural Media, a treatise concerning charged particle beam weapons. Tesla described his device as a "superweapon that would put an end to all war."
The March 1940 Frisch–Peierls memorandum,
the earliest technical exposition of a practical nuclear weapon,
anticipated deterrence as the principal means of combating an enemy with
nuclear weapons.
Early Cold War
Aftermath of the atomic bomb explosion over Hiroshima (August 6, 1945), to date one of the only two times a nuclear strike has been performed as an act of war
In August 1945, the United States became the first nuclear power after the nuclear attacks on Hiroshima and Nagasaki. Four years later, on August 29, 1949, the Soviet Union detonated its own nuclear device.
At the time, both sides lacked the means to effectively use nuclear
devices against each other. However, with the development of aircraft
like the American Convair B-36 and the Soviet Tupolev Tu-95,
both sides were gaining a greater ability to deliver nuclear weapons
into the interior of the opposing country. The official policy of the
United States became one of "Instant Retaliation", as coined by
Secretary of State John Foster Dulles,
which called for massive atomic attack against the Soviet Union if they
were to invade Europe, regardless of whether it was a conventional or a
nuclear attack.
By the time of the 1962 Cuban Missile Crisis,
both the United States and the Soviet Union had developed the
capability of launching a nuclear-tipped missile from a submerged
submarine, which completed the "third leg" of the nuclear triad
weapons strategy necessary to fully implement the MAD doctrine. Having a
three-branched nuclear capability eliminated the possibility that an
enemy could destroy all of a nation's nuclear forces in a first-strike attack; this, in turn, ensured the credible threat of a devastating retaliatory strike against the aggressor, increasing a nation's nuclear deterrence.
Campbell Craig and Sergey Radchenko argue that Nikita Khrushchev
(Soviet leader 1953 to 1964) decided that policies that facilitated
nuclear war were too dangerous to the Soviet Union. His approach did not
greatly change his foreign policy or military doctrine but is apparent
in his determination to choose options that minimized the risk of war.
B-52D Stratofortress being refueled by a KC-135 Stratotanker, 1965
Beginning in 1955, the United States Strategic Air Command
(SAC) kept one-third of its bombers on alert, with crews ready to take
off within fifteen minutes and fly to designated targets inside the Soviet Union
and destroy them with nuclear bombs in the event of a Soviet
first-strike attack on the United States. In 1961, President John F.
Kennedy increased funding for this program and raised the commitment to 50 percent of SAC aircraft.
During periods of increased tension in the early 1960s, SAC kept
part of its B-52 fleet airborne at all times, to allow an extremely fast
retaliatory strike against the Soviet Union in the event of a surprise
attack on the United States. This program continued until 1969. Between
1954 and 1992, bomber wings had approximately one-third to one-half of
their assigned aircraft on quick reaction ground alert and were able to
take off within a few minutes. SAC also maintained the National Emergency Airborne Command Post
(NEACP, pronounced "kneecap"), also known as "Looking Glass", which
consisted of several EC-135s, one of which was airborne at all times
from 1961 through 1990. During the Cuban Missile Crisis the bombers were dispersed to several different airfields, and sixty-five B-52s were airborne at all times.
During the height of the tensions between the US and the USSR in
the 1960s, two popular films were made dealing with what could go
terribly wrong with the policy of keeping nuclear-bomb-carrying
airplanes at the ready: Dr. Strangelove (1964) and Fail Safe (1964).
The strategy of MAD was fully declared in the early 1960s, primarily by United States Secretary of DefenseRobert McNamara.
In McNamara's formulation, there was the very real danger that a nation
with nuclear weapons could attempt to eliminate another nation's
retaliatory forces with a surprise, devastating first strike and
theoretically "win" a nuclear war relatively unharmed. The true
second-strike capability could be achieved only when a nation had a guaranteed ability to fully retaliate after a first-strike attack.
The United States had achieved an early form of second-strike
capability by fielding continual patrols of strategic nuclear bombers,
with a large number of planes always in the air, on their way to or from
fail-safe points close to the borders of the Soviet Union. This meant
the United States could still retaliate, even after a devastating
first-strike attack. The tactic was expensive and problematic because of
the high cost of keeping enough planes in the air at all times and the
possibility they would be shot down by Soviet anti-aircraft missiles before reaching their targets. In addition, as the idea of a missile gap existing between the US and the Soviet Union developed, there was increasing priority being given to ICBMs over bombers.
The deployment of fleets of ballistic missile submarines
established a guaranteed second-strike capability because of their
stealth and by the number fielded by each Cold War adversary—it was
highly unlikely that all of them could be targeted and preemptively
destroyed (in contrast to, for example, a missile silo with a fixed
location that could be targeted during a first strike). Given their
long-range, high survivability
and ability to carry many medium- and long-range nuclear missiles,
submarines were credible and effective means for full-scale retaliation
even after a massive first strike.
This deterrence strategy and the program have continued into the 21st century, with nuclear submarines carrying Trident II ballistic missiles as one leg of the US strategic nuclear deterrent
and as the sole deterrent of the United Kingdom. The other elements of
the US deterrent are intercontinental ballistic missiles (ICBMs) on
alert in the continental United States, and nuclear-capable bombers.
Ballistic missile submarines are also operated by the navies of China,
France, India, and Russia.
The US Department of Defense anticipates a continued need for a sea-based strategic nuclear force. The first of the current Ohio-class SSBNs are expected to be retired by 2029, meaning that a replacement platform must already be seaworthy by that
time. A replacement may cost over $4 billion per unit compared to the
USS Ohio's $2 billion. The USN's follow-on class of SSBN will be the Columbia class, which began construction in 2021 and enter service in 2031.
In the 1960s both the Soviet Union (A-35 anti-ballistic missile system) and the United States (LIM-49 Nike Zeus) developed anti-ballistic missile systems. Had such systems been able to effectively defend against a retaliatory second strike,
MAD would have been undermined. However, multiple scientific studies
showed technological and logistical problems in these systems, including
the inability to distinguish between real and decoy weapons.
The multiple independently targetable re-entry vehicle
(MIRV) was another weapons system designed specifically to aid with the
MAD nuclear deterrence doctrine. With a MIRV payload, one ICBM could hold many separate warheads. MIRVs were first created by the United States in order to counterbalance the Soviet A-35 anti-ballistic missile systems
around Moscow. Since each defensive missile could be counted on to
destroy only one offensive missile, making each offensive missile have,
for example, three warheads (as with early MIRV systems) meant that
three times as many defensive missiles were needed for each offensive
missile. This made defending against missile attacks more costly and
difficult. One of the largest US MIRVed missiles, the LGM-118A Peacekeeper, could hold up to 10 warheads, each with a yield of around 300 kilotons of TNT (1.3 PJ)—all together, an explosive payload equivalent to 230 Hiroshima-type
bombs. The multiple warheads made defense untenable with the available
technology, leaving the threat of retaliatory attack as the only viable
defensive option. MIRVed land-based ICBMs tend to put a premium on
striking first. The START II agreement was proposed to ban this type of weapon, but never entered into force.
In the event of a Soviet conventional attack on Western Europe, NATO planned to use tactical nuclear weapons.
The Soviet Union countered this threat by issuing a statement that any
use of nuclear weapons (tactical or otherwise) against Soviet forces
would be grounds for a full-scale Soviet retaliatory strike (massive retaliation). Thus it was generally assumed that any combat in Europe would end with apocalyptic conclusions.
Land-based MIRVed ICBMs threaten MAD
MIRVed land-based ICBMs are generally considered suitable for a first strike (inherently counterforce) or a counterforce second strike, due to:
Their high accuracy (low circular error probable), compared to submarine-launched ballistic missiles which used to be less accurate, and more prone to defects;
Their fast response time, compared to bombers which are considered too slow;
Their ability to carry multiple MIRV warheads at once, useful for
destroying a whole missile field or several cities with one missile.
MIRVed land-based ICBMs
are considered destabilizing because they tend to put a premium on
striking first. When a missile is MIRVed, it is able to carry many warheads (up to eight in existing US missiles, limited by New START, though Trident II is capable of carrying up to 12)
and deliver them to separate targets. If it is assumed that each side
has 100 missiles, with five warheads each, and further that each side
has a 95 percent chance of neutralizing the opponent's missiles in their
silos by firing two warheads at each silo, then the attacking side can
reduce the enemy ICBM force from 100 missiles to about five by firing 40
missiles with 200 warheads, and keeping the rest of 60 missiles in
reserve. As such, this type of weapon was intended to be banned under
the START II
agreement; however, the START II agreement was never brought into
force, and neither Russia nor the United States ratified the agreement.
Late Cold War
The original US MAD doctrine was modified on July 25, 1980, with US President Jimmy Carter's adoption of countervailing strategy with Presidential Directive 59. According to its architect, Secretary of Defense Harold Brown,
"countervailing strategy" stressed that the planned response to a
Soviet attack was no longer to bomb Soviet population centers and cities
primarily, but first to kill the Soviet leadership, then attack
military targets, in the hope of a Soviet surrender before total
destruction of the Soviet Union (and the United States). This modified
version of MAD was seen as a winnable nuclear war, while still
maintaining the possibility of assured destruction for at least one
party. This policy was further developed by the Reagan administration with the announcement of the Strategic Defense Initiative
(SDI, nicknamed "Star Wars"), the goal of which was to develop
space-based technology to destroy Soviet missiles before they reached
the United States.
SDI was criticized by both the Soviets and many of America's allies (including Prime Minister of the United Kingdom Margaret Thatcher)
because, were it ever operational and effective, it would have
undermined the "assured destruction" required for MAD. If the United
States had a guarantee against Soviet nuclear attacks, its critics
argued, it would have first-strike capability, which would have been a
politically and militarily destabilizing position. Critics further
argued that it could trigger a new arms race, this time to develop
countermeasures for SDI. Despite its promise of nuclear safety, SDI was
described by many of its critics (including Soviet nuclear physicist and
later peace activist Andrei Sakharov)
as being even more dangerous than MAD because of these political
implications. Supporters also argued that SDI could trigger a new arms
race, forcing the USSR to spend an increasing proportion of GDP on
defense—something which has been claimed to have been an indirect cause
of the eventual collapse of the Soviet Union. Gorbachev
himself in 1983 announced that “the continuation of the S.D.I. program
will sweep the world into a new stage of the arms race and would
destabilize the strategic situation.”
Proponents of ballistic missile defense
(BMD) argue that MAD is exceptionally dangerous in that it essentially
offers a single course of action in the event of a nuclear attack: full
retaliatory response. The fact that nuclear proliferation has led to an increase in the number of nations in the "nuclear club", including nations of questionable stability (e.g. North Korea), and that a nuclear nation might be hijacked by a despot
or other person or persons who might use nuclear weapons without a sane
regard for the consequences, presents a strong case for proponents of
BMD who seek a policy which both protect against attack, but also does
not require an escalation into what might become global nuclear war.
Russia continues to have a strong public distaste for Western BMD
initiatives, presumably because proprietary operative BMD systems could
exceed their technical and financial resources and therefore degrade
their larger military standing and sense of security in a post-MAD
environment. Russian refusal to accept invitations to participate in NATO BMD may be indicative of the lack of an
alternative to MAD in current Russian war-fighting strategy due to the
dilapidation of conventional forces after the breakup of the Soviet Union.
Proud Prophet
Proud Prophet
was a series of war games played out by various American military
officials. The simulation revealed MAD made the use of nuclear weapons
virtually impossible without total nuclear annihilation, regardless of
how nuclear weapons were implemented in war plans. These results
essentially ruled out the possibility of a limited nuclear strike, as
every time this was attempted, it resulted in a complete expenditure of
nuclear weapons by both the United States and USSR. Proud Prophet marked
a shift in American strategy; following Proud Prophet, American
rhetoric of strategies that involved the use of nuclear weapons
dissipated and American war plans were changed to emphasize the use of
conventional forces.
TTAPS Study
In 1983, a group of researchers including Carl Sagan
released the TTAPS study (named for the respective initials of the
authors), which predicted that the large scale use of nuclear weapons
would cause a “nuclear winter”.
The study predicted that the debris burned in nuclear bombings would be
lifted into the atmosphere and diminish sunlight worldwide, thus
reducing world temperatures by “-15° to -25°C”. These findings led to theory that MAD would still occur with many fewer
weapons than were possessed by either the United States or USSR at the
height of the Cold War. As such, nuclear winter was used as an argument
for significant reduction of nuclear weapons since MAD would occur
anyway.
After the fall of the Soviet Union, the Russian Federation
emerged as a sovereign entity encompassing most of the territory of the
former USSR. Relations between the United States and Russia were, at
least for a time, less tense than they had been with the Soviet Union.
While MAD has become less applicable for the US and Russia, it has been argued as a factor behind Israel's acquisition of nuclear weapons.
Similarly, diplomats have warned that Japan may be pressured to
nuclearize by the presence of North Korean nuclear weapons. The ability
to launch a nuclear attack against an enemy city is a relevant deterrent
strategy for these powers.
The administration of US President George W. Bush withdrew from the Anti-Ballistic Missile Treaty
in June 2002, claiming that the limited national missile defense system
which they proposed to build was designed only to prevent nuclear blackmail
by a state with limited nuclear capability and was not planned to alter
the nuclear posture between Russia and the United States.
While relations have improved and an intentional nuclear exchange
is more unlikely, the decay in Russian nuclear capability in the post–Cold War era
may have had an effect on the continued viability of the MAD doctrine. A
2006 article by Keir Lieber and Daryl Press stated that the United
States could carry out a nuclear first strike on Russia and would "have a
good chance of destroying every Russian bomber base, submarine, and
ICBM." This was attributed to reductions in Russian nuclear stockpiles
and the increasing inefficiency and age of that which remains. Lieber
and Press argued that the MAD era is coming to an end and that the
United States is on the cusp of global nuclear primacy.
However, in a follow-up article in the same publication, others criticized the analysis, including Peter Flory,
the US Assistant Secretary of Defense for International Security
Policy, who began by writing "The essay by Keir Lieber and Daryl Press
contains so many errors, on a topic of such gravity, that a Department
of Defense response is required to correct the record." Regarding reductions in Russian stockpiles, another response stated
that "a similarly one-sided examination of [reductions in] U.S. forces
would have painted a similarly dire portrait".
A situation in which the United States might actually be expected
to carry out a "successful" attack is perceived as a disadvantage for
both countries. The strategic balance between the United States and
Russia is becoming less stable, and the objective, the technical
possibility of a first strike by the United States is increasing. At a
time of crisis, this instability could lead to an accidental nuclear
war. For example, if Russia feared a US nuclear attack, Moscow might
make rash moves (such as putting its forces on alert) that would provoke
a US preemptive strike.
An outline of current US nuclear strategy toward both Russia and other nations was published as the document "Essentials of Post–Cold War Deterrence" in 1995.
In November 2020, the US successfully destroyed a dummy ICBM outside the atmosphere with another missile. Bloomberg Opinion writes that this defense ability "ends the era of nuclear stability".
India and Pakistan
MAD does not entirely apply to all nuclear-armed rivals. India and Pakistan
are an example of this; because of the superiority of conventional
Indian armed forces to their Pakistani counterparts, Pakistan may be
forced to use their nuclear weapons on invading Indian forces out of
desperation regardless of an Indian retaliatory strike. As such, any
large-scale attack on Pakistan by India could precipitate the use of
nuclear weapons by Pakistan, thus rendering MAD inapplicable. However,
MAD is applicable in that it may deter Pakistan from making a “suicidal”
nuclear attack rather than a defensive nuclear strike.
North Korea
Since the emergence of North Korea as a nuclear state,
military action has not been an option in handling the instability
surrounding North Korea because of their option of nuclear retaliation
in response to any conventional attack on them, thus rendering
non-nuclear neighboring states such as South Korea and Japan incapable
of resolving the destabilizing effect of North Korea via military force. MAD may not apply to the situation in North Korea because the theory
relies on rational consideration of the use and consequences of nuclear
weapons, which may not be the case for potential North Korean
deployment.
China
Since
2020, China has undertaken an ambitious expansion and modernization of
its nuclear arsenal. As of March 2025, it is estimated to possess
approximately 600 nuclear warheads. It has developed new variants of intercontinental ballistic missiles
and is capable of delivering nuclear warheads via land-based ballistic
missiles, sea-based ballistic missiles, and bombers. A 2023 Pentagon report estimated that China could possess 1,000 operational warheads by 2030. China also has the world’s second-largest economy and a highly capable military force. China’s intense development of its nuclear program complicates mutual
assured destruction with other countries, including the United States.
As its nuclear program expands, the prospect of a credible MAD
relationship with the US is likely to increase.China seeks to develop second-strike capabilities to counter other
nations, following years of adhering to a declared no-first-use policy.
Several analysts have cited China’s nuclear developments as a means of
leveraging power to bolster China’s demands due to an increased threat. However, others have claimed that China is simply seeking to boost its
deterrence to fortify its own security in a rapidly developing world. China’s nuclear arsenal is currently smaller than the arsenals of Russia
and the United States. Historically, the United States has possessed a
strong nuclear advantage over China. Despite the various different
analyses by defense experts and academics of China’s nuclear buildup,
its exact intentions remain largely up to speculation. There does not exist an official consensus on whether or not the United
States and China have full mutual assured destruction. However, China’s
ambitious nuclear policy signals that the country is potentially seeking
to establish a MAD relationship with the United States. China’s nuclear buildup also plays a role in regional nuclear dynamics.
In a conflict involving Taiwan, for example, the presence of nuclear
forces could lead to a rapid escalation in the situation. When the
stakes of a conflict become existential, urgency on all sides increases
rapidly and intensely. This similarly complicates security guarantees
and other forms of alliances with countries across the world,
potentially involving allies and/or strategic partners across the world.
Official policy
Whether
MAD was the officially accepted doctrine of the United States military
during the Cold War is largely a matter of interpretation. The United States Air Force,
for example, has retrospectively contended that it never advocated MAD
as a sole strategy, and that this form of deterrence was seen as one of
numerous options in US nuclear policy. Former officers have emphasized that they never felt as limited by the
logic of MAD (and were prepared to use nuclear weapons in smaller-scale
situations than "assured destruction" allowed), and did not deliberately
target civilian cities (though they acknowledge that the result of a
"purely military" attack would certainly devastate the cities as well).
However, according to a declassified 1959 Strategic Air Command
study, US nuclear weapons plans specifically targeted the populations
of Beijing, Moscow, Leningrad, East Berlin, and Warsaw for systematic
destruction. MAD was implied in several US policies and used in the political
rhetoric of leaders in both the United States and the USSR during many
periods of the Cold War:
To continue to deter in an era of
strategic nuclear equivalence, it is necessary to have nuclear (as well
as conventional) forces such that in considering aggression against our
interests any adversary would recognize that no plausible outcome would
represent a victory or any plausible definition of victory. To this end
and so as to preserve the possibility of bargaining effectively to
terminate the war on acceptable terms that are as favorable as
practical, if deterrence fails initially, we must be capable of fighting
successfully so that the adversary would not achieve his war aims and
would suffer costs that are unacceptable, or in any event greater than
his gains, from having initiated an attack.
The doctrine of MAD was officially at odds with that of the USSR, which had, contrary to MAD, insisted survival was possible.The Soviets believed they could win not only a strategic nuclear war, which they planned to absorb with their extensive civil defense planning, but also the conventional war that they predicted would follow after their strategic nuclear arsenal had been depleted. Official Soviet policy, though, may have had internal critics towards
the end of the Cold War, including some in the USSR's own leadership:
Nuclear use would be catastrophic.
— 1981, the Soviet General Staff
Other evidence of this comes from the Soviet minister of defense, Dmitriy Ustinov,
who wrote that "A clear appreciation by the Soviet leadership of what a
war under contemporary conditions would mean for mankind determines the
active position of the USSR." The Soviet doctrine, although being seen as primarily offensive by
Western analysts, fully rejected the possibility of a "limited" nuclear
war by 1975.
Criticism
Nuclear weapon test Apache (yield 1.85 Mt or 7.7 PJ)
Deterrence
theory has been criticized by numerous scholars for various reasons. A
prominent strain of criticism argues that rational deterrence theory is
contradicted by frequent deterrence failures, which may be attributed to
misperceptions. Critics have also argued that leaders do not behave in ways that are
consistent with the predictions of nuclear deterrence theory. For example, it has been argued that it is inconsistent with the logic
of rational deterrence theory that states continue to build nuclear
arsenals once they have reached the second-strike threshold. For a more inconsistent example, Mao Zedong urged the socialist camp not to fear nuclear war
with the United States since, even if "half of mankind died, the other
half would remain while imperialism would be razed to the ground and the
whole world would become socialist."
Additionally, many scholars have advanced philosophical objections against the principles of deterrence theory on purely ethical grounds. Included in this group is Robert L. Holmes who uses a reductio ad absurdum
argument to observe that mankind's reliance upon a system of preventing
war which is based exclusively upon the threat of waging war is
inherently irrational and must be considered immoral according to
fundamental deontological
principles. In addition, he questions whether it can be conclusively
demonstrated that such a system has in fact served to prevent warfare in
the past and may actually serve to increase the probability of waging
war in the future due to its reliance upon the continuous development of
new generations of technologically advanced nuclear weapons.
A first strike must not be capable of preventing a retaliatory
second strike or else mutual destruction is not assured. In this case, a
state would have nothing to lose with a first strike or might try to
preempt the development of an opponent's second-strike capability with a
first strike. To avoid this, countries may design their nuclear forces
to make decapitation strike almost impossible, by dispersing launchers
over wide areas and using a combination of sea-based, air-based, underground, and mobile land-based launchers.
Another method of ensuring second strike capability is through the use of dead man's switch or "fail-deadly:" in the absence of ongoing action from a functional command structure—such as would occur after suffering a successful decapitation strike—an automatic system defaults to launching a nuclear strike upon some target. A particular example is the Soviet (now Russian) Dead Hand system, which has been described as a semi-automatic "version of Dr. Strangelove's Doomsday Machine"
which, once activated, can launch a second strike without human
intervention. The purpose of the Dead Hand system is to ensure a second
strike even if Russia were to suffer a decapitation attack, thus
maintaining MAD.
Perfect detection
No
false positives (errors) in the equipment and/or procedures that must
identify a launch by the other side. The implication of this is that an
accident could lead to a full nuclear exchange. During the Cold War
there were several instances of false positives, as in the case of Stanislav Petrov.
Perfect attribution. If there is a launch from the Sino-Russian
border, it could be difficult to distinguish which nation is
responsible—both Russia and China have the capability—and, hence,
against which nation retaliation should occur. A launch from a
nuclear-armed submarine could also be difficult to attribute.
Perfect rationality
No
rogue commanders will have the ability to corrupt the launch decision
process. Such an incident very nearly occurred during the Cuban Missile Crisis when an argument broke out aboard a nuclear-armed submarine cut off from radio communication. The second-in-command, Vasili Arkhipov, refused to launch despite an order from Captain Savitsky to do so.
All leaders with launch capability seem to care about the survival of their citizens. Winston Churchill is quoted as saying that any strategy will not "cover the case of lunatics or dictators in the mood of Hitler when he found himself in his final dugout."
Inability to defend
No fallout shelter networks of sufficient capacity to protect large segments of the population and/or industry.
No development of anti-missile technology or deployment of remedial protective gear.
Inherent instability
Another
reason is that deterrence has an inherent instability. As Kenneth
Boulding said: "If deterrence were really stable... it would cease to
deter." If decision-makers were perfectly rational, they would never
order the largescale use of nuclear weapons, and the credibility of the
nuclear threat would be low.
However, that apparent perfect rationality criticism is countered and so is consistent with current deterrence policy. In Essentials of Post-Cold War Deterrence,
the authors detail an explicit advocation of ambiguity regarding "what
is permitted" for other nations and its endorsement of "irrationality"
or, more precisely, the perception thereof as an important tool in
deterrence and foreign policy. The document claims that the capacity of
the United States, in exercising deterrence, would be hurt by portraying
US leaders as fully rational and cool-headed:
The fact that some elements may
appear to be potentially 'out of control' can be beneficial to creating
and reinforcing fears and doubts in the minds of an adversary's decision
makers. This essential sense of fear is the working force of
deterrence. That the U.S. may become irrational and vindictive if its
vital interests are attacked should be part of the national persona we
project to all adversaries.
Terrorism
The threat of foreign and domestic nuclear terrorism
has been a criticism of MAD as a defensive strategy. Deterrent
strategies are ineffective against those who attack without regard for
their life. Furthermore, the doctrine of MAD has been critiqued in regard to
terrorism and asymmetrical warfare. Critics contend that a retaliatory
strike would not be possible in this case because of the
decentralization of terrorist organizations, which may be operating in
several countries and dispersed among civilian populations. A misguided
retaliatory strike made by the targeted nation could even advance
terrorist goals in that a contentious retaliatory strike could drive
support for the terrorist cause that instigated the nuclear exchange.
However Robert Gallucci, the president of the John D. and Catherine T. MacArthur Foundation,
argues that although traditional deterrence is not an effective
approach toward terrorist groups bent on causing a nuclear catastrophe,
"the United States should instead consider a policy of expanded
deterrence, which focuses not solely on the would-be nuclear terrorists
but on those states that may deliberately transfer or inadvertently lead
nuclear weapons and materials to them. By threatening retaliation
against those states, the United States may be able to deter that which
it cannot physically prevent."
Graham Allison
makes a similar case and argues that the key to expanded deterrence is
coming up with ways of tracing nuclear material to the country that
forged the fissile material: "After a nuclear bomb detonates, nuclear forensic
cops would collect debris samples and send them to a laboratory for
radiological analysis. By identifying unique attributes of the fissile
material, including its impurities and contaminants, one could trace the
path back to its origin." The process is analogous to identifying a criminal by fingerprints:
"The goal would be twofold: first, to deter leaders of nuclear states
from selling weapons to terrorists by holding them accountable for any
use of their own weapons; second, to give leaders every incentive to
tightly secure their nuclear weapons and materials."
Space weapons
Strategic
analysts have criticized the doctrine of MAD for its inability to
respond to the proliferation of space weaponry. First, military space
systems have unequal dependence across countries. This means that
less-dependent countries may find it beneficial to attack a
more-dependent country's space weapons, which complicates deterrence.
This is especially true for countries like North Korea which have
extensive ballistic missiles that could strike space-based systems. Even across countries with similar dependence, anti-satellite weapons (ASATs) have the ability to remove the command and control of nuclear weapons. This encourages crisis-instability and pre-emptive nuclear-disabling
strikes. Third, there is a risk of asymmetrical challengers. Countries
that fall behind in space weapon advancement may turn to using chemical
or biological weapons. This may heighten the risk of escalation,
bypassing any deterrent effects of nuclear weapons.
Entanglements
Cold-war
bipolarity no longer is applicable to the global power balance. The
complex modern alliance system makes allies and enemies tied to one
another. Thus, action by one country to deter another could threaten the
safety of a third country. "Security trilemmas" could increase tension
during mundane acts of cooperation, complicating MAD.
Emerging hypersonic weapons
Hypersonic
ballistic or cruise missiles threaten the retaliatory backbone of
mutual assured destruction. The high precision and speed of these
weapons may allow for the development of "decapitory" strikes that
remove the ability of another nation to have a nuclear response. In
addition, the secretive nature of these weapons' development can make
deterrence more asymmetrical.
Failure to retaliate
If it was known that a country's leader would not resort to nuclear retaliation, adversaries may be emboldened. Edward Teller,
a member of the Manhattan Project, echoed these concerns as early as
1985 when he said that "The MAD policy as a deterrent is totally
ineffective if it becomes known that in case of attack, we would not
retaliate against the aggressor."
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