Terror management theory

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Terror management theory (TMT) is both a social and evolutionary psychology theory originally proposed by Jeff Greenberg, Sheldon Solomon, and Tom Pyszczynski and codified in their book The Worm at the Core: On the Role of Death in Life (2015). It proposes that a basic psychological conflict results from having a self-preservation instinct while realizing that death is inevitable and to some extent unpredictable. This conflict produces terror, which is managed through a combination of escapism and cultural beliefs that act to counter biological reality with more significant and enduring forms of meaning and value.

The most obvious examples of cultural values that assuage death anxiety are those that purport to offer literal immortality (e.g. belief in the afterlife through religion). However, TMT also argues that other cultural values – including those that are seemingly unrelated to death – offer symbolic immortality. For example, values of national identity, posterity, cultural perspectives on sex, and human superiority over animals have been linked to calming death concerns. In many cases these values are thought to offer symbolic immortality, by either a) providing the sense that one is part of something greater that will ultimately outlive the individual (e.g. country, lineage, species), or b) making one's symbolic identity superior to biological nature (i.e. you are a personality, which makes you more than a glob of cells). Because cultural values influence what is meaningful, they are foundational for self-esteem. TMT describes self-esteem as being the personal, subjective measure of how well an individual is living up to their cultural values.

Terror management theory was developed by social psychologists Greenberg, Solomon, and Pyszczynski. However, the idea of TMT originated from anthropologist Ernest Becker's 1973 Pulitzer Prize-winning work of nonfiction The Denial of Death. Becker argues most human action is taken to ignore or avoid the inevitability of death. The terror of absolute annihilation creates such a profound – albeit subconscious – anxiety in people that they spend their lives attempting to make sense of it. On large scales, societies build symbols: Laws, religious meanings, cultures, and belief systems to explain the significance of life, define what makes certain characteristics, skills, and talents extraordinary, reward others whom they find exemplify certain attributes, and punish or kill others who do not adhere to their cultural worldview. Adherence to these created "symbols" aid in relieving stresses associated with the reality of mortality. On an individual level, self-esteem provides a buffer against death-related anxiety.

Background

The idea of death, the fear of it, haunts the human animal like nothing else; it is a mainspring of human activity designed largely to avoid the fatality of death, to overcome it by denying in some way that it is the final destiny for man.

Ernest Becker, 1973

Cultural anthropologist Ernest Becker asserted in his 1973 book The Denial of Death that humans, as intelligent animals, are able to grasp the inevitability of death. They therefore spend their lives building and believing in cultural elements that illustrate how to make themselves stand out as individuals and give their lives significance and meaning. Death creates an anxiety in humans; it strikes at unexpected and random moments, and its nature is essentially unknowable, causing people to spend most of their time and energy to explain, forestall, and avoid it.

Becker expounded upon the previous writings of Sigmund Freud, Søren Kierkegaard, Norman O. Brown, and Otto Rank. According to clinical psychiatrist Morton Levitt, Becker replaces the Freudian preoccupation with sexuality with the fear of death as the primary motivation in human behavior.

People desire to think of themselves as beings of value and worth with a feeling of permanence, a concept in psychology known as self-esteem. This feeling counters the cognitive dissonance created by an individual's realization that they may be no more important than any other living thing. Becker refers to high self-esteem as heroism:

the problem of heroics is the central one of human life, that it goes deeper into human nature than anything else because it is based on organismic narcissism and on the child's need for self-esteem as the condition for his life. Society itself is a codified hero system, which means that society everywhere is a living myth of the significance of human life, a defiant creation of meaning.

The rationale behind decisions regarding one's own health can be explored through a terror management model. A 2008 research article in Psychological Review proposes a three-part model for understanding how awareness of death can ironically subvert health-promoting behaviors by redirecting one's focus towards behaviors that build self-esteem instead:

Proposition 1 suggests that conscious thoughts about death can instigate health-oriented responses aimed at removing death-related thoughts from current focal attention. Proposition 2 suggests that the unconscious resonance of death-related cognition promotes self-oriented defenses directed toward maintaining, not one's health, but a sense of meaning and self-esteem. The last proposition suggests that confrontations with the physical body may undermine symbolic defenses and thus present a previously unrecognized barrier to health promotion activities.

Evolutionary backdrop

Terror management theorists consider TMT to be compatible with the theory of evolution: Valid fears of dangerous things have an adaptive function that helped facilitate the survival of our ancestors' genes. However, generalized existential anxiety resulting from the clash between a desire for life and awareness of the inevitability of death is neither adaptive nor selected for. TMT views existential anxiety as an unfortunate byproduct of these two highly adaptive human proclivities rather than as an adaptation that the evolutionary process selected for its advantages. Just as human bipedalism confers advantages as well as disadvantages, death anxiety is an inevitable part of our intelligence and awareness of dangers.

Anxiety in response to the inevitability of death threatened to undermine adaptive functioning and therefore needed amelioration. TMT posits that humankind used the same intellectual capacities that gave rise to this problem to fashion cultural beliefs and values that provided protection against this potential anxiety. TMT considers these cultural beliefs (even unpleasant and frightening ones, such as ritual human sacrifice) when they manage potential death anxiety in a way that promotes beliefs and behaviors which facilitated the functioning and survival of the collective.

Hunter-gatherers used their emerging cognitive abilities to facilitate solving practical problems, such as basic needs for nutrition, mating, and tool-making. As these abilities evolved, an explicit awareness of death also emerged. But once this awareness materialized, the potential for terror that it created put pressure on emerging conceptions of reality. Any conceptual formation that was to be widely accepted by the group needed to provide a means of managing this terror.

Originally, the emergence of morality evolved to facilitate co-existence within groups. Together with language, morality served pragmatic functions that extended survival. The struggle to deny the finality of death co-opted and changed the function of these cultural inventions. For example, Neanderthals might have begun burying their dead as a means of avoiding unpleasant odors, disease-infested parasites, or dangerous scavengers. But during the Upper Paleolithic era, these pragmatic burial practices appear to have become imbued with layers of ritual performance and supernatural beliefs, suggested by the elaborate decoration of bodies with thousands of beads or other markers. Food and other necessities were also included within the burial chamber, indicating the potential for a belief system that included life after death. In many human cultures today, funerals are viewed primarily as cultural events, viewed through the lens of morality and language, with little thought given to the utilitarian origins of burying the dead.

Evolutionary history also indicates that "the costs of ignoring threats have outweighed the costs of ignoring opportunities for self-development." This reinforces the concept that abstract needs for individual and group self-esteem may continue to be selected for by evolution, even when they sometimes confer risks to physical health and well-being.

Self-esteem

Self-esteem lies at the heart of TMT and is a fundamental aspect of its core paradigms. TMT fundamentally seeks to elucidate the causes and consequences of a need for self-esteem. Theoretically, it draws heavily from Ernest Becker's conceptions of culture and self-esteem. TMT not only attempts to explain the concept of self-esteem, it also tries to explain why we need self-esteem. One explanation is that self-esteem is used as a coping mechanism for anxiety. It helps people control their sense of terror and nullify the realization that humans are just animals trying to manage the world around them. According to TMT, self-esteem is a sense of personal value that is created by beliefs in the validity of one's cultural worldview, and the belief that one is living up to the cultural standards created by that worldview.

Critically, Hewstone et al. (2002) have questioned the causal direction between self-esteem and death anxiety, evaluating whether one's self-esteem comes from their desire to reduce their death anxiety, or if death anxiety arises from a lack of self-esteem. In other words, an individual's suppression of death anxiety may arise from their overall need to increase their self-esteem in a positive manner.

Research has demonstrated that self-esteem can play an important role in physical health. In some cases, people may be so concerned with their physical appearance and boosting their self-esteem that they ignore problems or concerns with their own physical health. Arndt et al. (2009) conducted three studies to examine how peer perceptions and social acceptance of smokers contributes to their quitting, as well as if, and why these people continue smoking for outside reasons, even when faced with thoughts of death and anti-smoking prompts. Tanning and exercising were also looked at in the researchers' studies. The studies found that people are influenced by the situations around them. Specifically, Arndt et al. (2009) found in terms of their self-esteem and health, that participants who saw someone exercising were more likely to increase their intentions to exercise. In addition, the researchers found in study two that how participants reacted to an anti-smoking commercial was affected by their motivation for smoking and the situation which they were in. For instance, people who smoked for extrinsic reasons and were previously prompted with death reminders were more likely to be compelled by the anti-smoking message.

Self-esteem as anxiety buffer

An individual's level of self-consciousness can affect their views on life and death. To a point, increasing self-consciousness is adaptive in that it helps prevent awareness of danger. However, research has demonstrated that there may be diminishing returns from this phenomenon. Individuals with higher levels of self-consciousness sometimes have increased death cognition, and a more negative outlook on life, than those with reduced self-consciousness.

Conversely, self-esteem can work in the opposite manner. Research has confirmed that individuals with higher self-esteem, particularly in regard to their behavior, have a more positive attitude towards their life. Specifically, death cognition in the form of anti-smoking warnings weren't effective for smokers and in fact, increased their already positive attitudes towards the behavior. The reasons behind individuals' optimistic attitudes towards smoking after mortality was made salient, indicate that people use positivity as a buffer against anxiety. Continuing to hold certain beliefs even after they are shown to be flawed creates cognitive dissonance regarding current information and past behavior, and the way to alleviate this is to simply reject new information. Therefore, anxiety buffers such as self-esteem allow individuals to cope with their fears more easily. Death cognition may in fact cause negative reinforcement that leads people to further engage in dangerous behaviors (smoking in this instance) because accepting the new information would lead to a loss of self-esteem, increasing vulnerability and awareness of mortality.

Mortality salience

The mortality salience hypothesis (MS) states that if indeed one's cultural worldview, or one's self-esteem, serves a death-denying function, then threatening these constructs should produce defenses aimed at restoring psychological equanimity (i.e., returning the individual to a state of feeling invulnerable). In the MS paradigm, these "threats" are simply experiential reminders of one's own death. This can, and has, taken many different forms in a variety of study paradigms (e.g., asking participants to write about their own death; conducting the experiment near funeral homes or cemeteries; having participants watch graphic depictions of death, etc.). Like the other TMT hypotheses, the literature supporting the MS hypothesis is vast and diverse. For a meta analysis of MS research, see Burke et al. (2010).

Experimentally, the MS hypothesis has been tested in close to 200 empirical articles. After participants in an experiment are asked to write about their own death (vs. a neutral, non-death control topic, such as dental pain), and then following a brief delay (distal, worldview/self-esteem defenses work the best after a delay; see Greenberg et al. (1994) for a discussion), the participants' defenses are measured. In one early TMT study assessing the MS hypothesis, Greenberg et al. (1990) had Christian participants evaluate other Christian and Jewish students that were similar demographically, but differed in their religious affiliation. After being reminded of their death (experimental MS induction), Christian participants evaluated fellow Christians more positively, and Jewish participants more negatively, relative to the control condition. Conversely, bolstering self-esteem in these scenarios leads to less worldview defense and derogation of dissimilar others.

Mortality salience has an influence on individuals and their decisions regarding their health. Cox et al. (2009) discuss mortality salience in terms of suntanning. Specifically, the researchers found that participants who were prompted with the idea that pale was more socially attractive along with mortality reminders, tended to lean towards decisions that resulted in more protective measures from the sun. The participants were placed in two different conditions; one group of participants were given an article relating to the fear of death, while the control group received an article unrelated to death, dealing with the fear of public speaking. Additionally, they gave one group an article pertaining to the message that "bronze is beautiful," one relating to the idea that "pale is pretty," and one neutral article that did not speak of tan or pale skin tones. Finally, after introducing a delay activity, the researchers gave the participants a five-item questionnaire asking them about their future sun-tanning behaviors. The study illustrated that when tan skin was associated with attractiveness, mortality salience positively affected people's intentions to suntan; however, when pale skin was associated with attractiveness people's intentions to tan decreased.

Mortality and self-esteem on health risks

Studies have shown that mortality and self-esteem are important factors of the terror management theory. Jessop et al. (2008) study this relationship within four studies that all examine how people react when they are given information on risks, specifically, in terms of the mortality related to the risks of driving. More specifically, the researchers were exploring how participants acted in terms of self-esteem, and its impact on how mortality-related health-risk information would be received. Overall, Jessop et al. (2008) found that even when mortality is prominent, people who engage in certain behaviors to improve their self-esteem have a greater chance of continuing with these activities. Mortality and self-esteem are both factors that influence people's behaviors and decision-making regarding their health. Furthermore, individuals who are involved in behaviors and possess motivation to enhance their self-worth are less likely to be affected by the importance placed on health risks, in terms of mortality.

Self-esteem is important when mortality is made salient. It can allow people a coping mechanism, one that can cushion individuals' fears; and thus, impacting one's attitudes towards a given behavior. Individuals who have higher levels of self-esteem regarding their behavior(s) are less likely to have their attitudes, and thus their behaviors changed regardless of mortality salience or death messages. People will use their self-esteem to hide behind their fears of dying. In terms of smoking behaviors, people with higher smoking-based self-esteem are less susceptible to anti-smoking messages that relate to death; therefore, mortality salience and death warnings afford them with an even more positive outlook on their behavior, or in this instance their smoking.

In the Hansen et al. (2010) experiment the researchers manipulated mortality salience. In the experiment, Hansen et al. (2010) examined smokers' attitudes towards the behavior of smoking. Actual warning labels were utilized to create mortality salience in this specific experiment. The researchers first gave participants a questionnaire to measure their smoking-based self-esteem. Following the questionnaire, participants were randomly assigned to two different conditions; the first were given anti-smoking warning labels about death and the second control group were exposed to anti-smoking warning labels not dealing with death. Before the participants' attitudes towards smoking were taken the researchers introduced an unrelated question to provide a delay. Further research has demonstrated that delays allow mortality salience to emerge because thoughts of death become non-conscious. Finally, participants were asked questions regarding their intended future smoking behavior. However, one weakness in their conduction was that the final questionnaire addressed opinions and behavioral questions, as opposed to the participants level of persuasion regarding the different anti-smoking warning labels.

Social influences

Many people are more motivated by social pressures, rather than health risks. Specifically for younger people, mortality salience is stronger in eliciting changes of one's behavior when it brings awareness to the immediate loss of social status or position, rather than a loss, such as death that one can not imagine and feels far off. However, there are many different factors to take into consideration, such as how strongly an individual feels toward a decision, his or her level of self-esteem, and the situation around the individual. Particularly with people's smoking behaviors, self-esteem and mortality salience have different effects on individuals' decisions. In terms of the longevity of their smoking decisions, it has been seen that individuals' smoking habits are affected, in the short-term sense, when they are exposed to mortality salience that interrelates with their own self-esteem. Moreover, people who viewed social exclusion prompts were more likely to quit smoking in the long run than those who were simply shown health-effects of smoking. More specifically, it was demonstrated that when individuals had high levels of self-esteem they were more likely to quit smoking following the social pressure messages, rather than the health risk messages. In this specific instance, terror management, and specifically mortality salience is showing how people are more motivated by the social pressures and consequences in their environment, rather than consequences relating to their health. This is mostly seen in young adult smokers with higher smoking-based self-esteems who are not thinking of their future health and the less-immediate effects of smoking on their health.

Death thought accessibility

Another paradigm that TMT researchers use to get at unconscious concerns about death is what is known as the death thought accessibility (DTA) hypothesis. Essentially, the DTA hypothesis states that if individuals are motivated to avoid cognitions about death, and they avoid these cognitions by espousing a worldview or by buffering their self-esteem, then when threatened, an individual should possess more death-related cognitions (e.g., thoughts about death, and death-related stimuli) than they would when not threatened.he DTA hypothesis has its origins in work by Greenberg et al. (1994) as an extension of their earlier terror management hypotheses (i.e., the anxiety buffer hypothesis and the mortality salience hypothesis). The researchers reasoned that if, as indicated by Wegner's research on thought suppression (1994; 1997), thoughts that are purposely suppressed from conscious awareness are often brought back with ease, then following a delay death-thought cognitions should be more available to consciousness than (a) those who keep the death-thoughts in their consciousness the whole time, and (b) those who suppress the death-thoughts but are not provided a delay. That is precisely what they found. However, other psychologists have failed to replicate these findings.

In these initial studies (i.e., Greenberg et al. (2004); Arndt et al. (1997)), and in numerous subsequent DTA studies, the main measure of DTA is a word fragment task, whereby participants can complete word fragments in distinctly death-related ways (e.g., coff_ _ as coffin, not coffee) or in non death-related ways (e.g., sk_ _l as skill, not skull). If death-thoughts are indeed more available to consciousness, then it stands to reason that the word fragments should be completed in a way that is semantically related to death.

Importance of the Death Thought Accessibility hypothesis

The introduction of this hypothesis has refined TMT, and led to new avenues of research that formerly could not be assessed due to the lack of an empirically validated way of measuring death-related cognitions. Also, the differentiation between proximal (conscious, near, and threat-focused) and distal (unconscious, distant, symbolic) defenses that have been derived from DTA studies have been extremely important in understanding how people deal with their terror.

It is important to note how the DTA paradigm subtly alters, and expands, TMT as a motivational theory. Instead of solely manipulating mortality and witnessing its effects (e.g., nationalism, increased prejudice, risky sexual behavior, etc.), the DTA paradigm allows a measure of the death-related cognitions that result from various affronts to the self. Examples include threats to self-esteem and to one's worldview; the DTA paradigm can therefore assess the role of death-thoughts in self-esteem and worldview defenses. Furthermore, the DTA hypothesis lends support to TMT in that it corroborates its central hypothesis that death is uniquely problematic for human beings, and that it is fundamentally different in its effects than meaning threats (i.e., Heine et al., 2006) and that is death itself, and not uncertainty and lack of control associated with death; Fritsche et al. (2008) explore this idea.

Since its inception, the DTA hypothesis had been rapidly gaining ground in TMT investigations, and as of 2009, has been employed in over 60 published papers, with a total of more than 90 empirical studies.

Death anxiety on health promotion

How people respond to their fears and anxiety of death is investigated in TMT. Moreover, Taubman-Ben-Ari and Noy (2010) examine the idea that a person's level of self-awareness and self-consciousness should be considered in relation to their responses to their anxiety and death cognitions. The more an individual is presented with their death or death cognitions in general, the more fear and anxiety one may have; therefore, to combat said anxiety one may implement anxiety buffers.

Due to a change in people's lifestyles, in the direction of more unhealthy behaviors, the leading causes of death now, being cancer and heart disease, most definitely are related to individuals' unhealthy behaviors (though the statement is over-generalising and certainly cannot be applied to every case). Age and death anxiety both are factors that should be considered in the terror management theory, in relation to health-promoting behaviors. Age undoubtedly plays some kind of role in people's health-promoting behaviors; however, an actual age related effect on death anxiety and health-promoting behaviors has yet to be seen. Although research has demonstrated that for young adults only, when they were prompted with death related scenarios, they yielded more health-promoting behaviors, compared to those participants in their sixties. In addition, death anxiety has been found to have an effect for young adults, on their behaviors of health promotion.

Terror management health model

The terror management health model (TMHM) explores the role that death plays on one's health and behavior. Goldenberg and Arndt (2008) state that the TMHM proposes the idea that death, despite its threatening nature, is in fact instrumental and purposeful in the conditioning of one's behavior towards the direction of a longer life.

According to Goldenberg and Arndt (2008), certain health behaviors such as breast self-exams (BSEs) can consciously activate and facilitate people to think of death, especially their own death. While death can be instrumental for individuals, in some cases, when breast self-exams activate people's death thoughts an obstacle can present itself, in terms of health promotion, because of the experience of fear and threat. Abel and Kruger (2009) have suggested that the stress caused by increased awareness of mortality when celebrating one's birthday might explain the birthday effect, where mortality rates seem to spike around these days.

On the other hand, death and thoughts of death can serve as a way of empowering the self, not as threats. Researchers, Cooper et al. (2011) explored TMHM in terms of empowerment, specifically using BSEs under two conditions; when death thoughts were prompted, and when thoughts of death were non-conscious. According to TMHM, people's health decisions, when death thoughts are not conscious, should be based on their motivations to act appropriately, in terms of the self and identity. Cooper et al. (2011) found that when mortality and death thoughts were primed, women reported more empowerment feelings than those who were not prompted before performing a BSE.

Additionally, TMHM suggests that mortality awareness and self-esteem are important factors in individuals' decision making and behaviors relating to their health. TMHM explores how people will engage in behaviors, whether positive or negative, even with the heightened awareness of mortality, in the attempt to conform to society's expectations and improve their self-esteem. The TMHM is useful in understanding what motivates individuals regarding their health decisions and behaviors.

In terms of smoking behaviors and attitudes, the impact of warnings with death messages depends on:

1. The individuals' level of smoking-based self-esteem
2. The warnings' actual degree of death information

Emotion

People with low self-esteem, but not high self-esteem, have more negative emotions when reminded of death. This is believed to be because these individuals lack the very defenses that TMT argues protect people from mortality concerns (e.g., solid worldviews). In contrast, positive mood states are not impacted by death thoughts for people of low or high self-esteem.

Leadership

It has been suggested that culture provides meaning, organization, and a coherent world view that diminishes the psychological terror caused by the knowledge of eventual death. The terror management theory can help to explain why a leader's popularity can grow substantially during times of crisis. When a follower's mortality is made prominent they will tend to show a strong preference for iconic leaders. An example of this occurred when George W. Bush's approval rating jumped almost 50 percent following the September 11 attacks in the United States. As Forsyth (2009) posits, this tragedy made U.S. citizens aware of their mortality, and Bush provided an antidote to these existential concerns by promising to bring justice to the terrorist group responsible for the attacks.

Researchers Cohen et al. (2004), in their particular study on TMT, tested the preferences for different types of leaders, while reminding people of their mortality. Three different candidates were presented to participants. The three leaders were of three different types: task-oriented (emphasized setting goals, strategic planning, and structure), relationship-oriented (emphasized compassion, trust, and confidence in others), and charismatic. The participants were then placed in one of two conditions: mortality salient or control group. In the former condition the participants were asked to describe the emotions surrounding their own death, as well as the physical act of the death itself, whereas the control group were asked similar questions about an upcoming exam. The results of the study were that the charismatic leader was favored more, and the relationship-oriented leader was favored less, in the mortality-salient condition. Further research has shown that mortality salient individuals also prefer leaders who are members of the same group, as well as men rather than women (Hoyt et al. 2010). This has links to social role theory.

Religion

TMT posits that religion was created as a means for humans to cope with their own mortality. Supporting this, arguments in favor of life after death, and simply being religious, reduce the effects of mortality salience on worldview defense. Thoughts of death have also been found to increase religious beliefs. At an implicit, subconscious level, this is the case even for people who claim to be nonreligious.

Mental health

Some researchers have argued that death anxiety may play a central role in numerous mental health conditions. To test whether death anxiety causes a particular mental illness, TMT researchers use a mortality salience experiment, and examine whether reminding participants of death leads to increased prevalence of behaviors associated with that mental illness. Such studies have shown that reminders of death lead to increases in compulsive handwashing in obsessive-compulsive disorder, avoidance in spider phobias and social anxiety, and anxious behaviors in other disorders, including panic disorder and health anxiety, suggesting the role of death anxiety in these conditions according to TMT researchers.

Criticisms

Criticisms of terror management theory have been based on several lines of arguments:

• Suppression of fear and anxiety is implausible from an evolutionary point of view.
• The observed psychological responses to terrifying cues are better explained by coalitional psychology and theories of collective defense.
• The responses can be explained as fear of uncertainty and the unknown.
• The responses can be explained as search for meaning of life and mortality.
• The experimental results are difficult to replicate.

These arguments are discussed in the following sections.

Evolutionary argument

Anxiety and fear are psychological responses that have evolved because they help us avoid danger. A mechanism to suppress anxiety and fear, as postulated by TMT, is unlikely to have evolved because it would reduce the chances of survival. It is argued that TMT relies on misguided assumptions about evolved human nature originating from psychoanalytic theory. Proponents of TMT argue that it is not responses to immediate danger that are suppressed, but the thought that we are all mortal. However, critics argue that the observed responses are not only evoked by cues of essential mortality, but more generally by cues of danger or insecurity.

Coalitional psychology and collective defense as alternative explanations

TMT posits that people respond to cues about mortality by strengthening shared worldviews. Critics believe that such a worldview defense is better explained by coalitional psychology. People confronted with danger tend to build shared worldviews and a pro-normative orientation in order to garner social support and to build coalitions and alliances. Proponents of TMT argue that the coalitional psychology theory relies on limited evidence, is unable to explain the diversity of responses to mortality cues, and is unable to explain the symbolic and supernatural worldview defenses found in all cultures. The coalitional theory is supported, however, by a large statistical study finding that conservatism, traditionalism, and other responses represented by TMT theory are connected with collective danger, while individual danger has very different and often opposite effects. The observed connection with collective danger supports the coalitional theory, while it contradicts the TMT which explicitly deals with individual danger only. Similarly, another study has found that the response of system justification postulated by TMT theorists is increased by salience of terrorism, not by salience of individual mortality. Earlier experimental findings can be explained by the fact that individual danger and collective danger are seriously confounded. The findings that the observed responses are connected with collective danger rather than individual danger was predicted by regality theory. This finding is in agreement with authoritarianism theory, realistic group conflict theory, and Ronald Inglehart's theory of modernization, but not in agreement with terror management theory.

Prevalence of death

Since findings on mortality salience and worldview defense were first published, other researchers have claimed that the effects may have been obtained due to reasons other than death itself, such as anxiety, fear, or other aversive stimuli such as pain. The experimental manipulations in TMT research are likely to elicit a mixture of different types of negative emotions, including fear, anxiety, sadness, and anger.

Other studies have found effects similar to those that mortality salience results in – for example, thinking about difficult personal choices to be made, being made to respond to open-ended questions regarding uncertainty, thinking about being robbed, thinking about being socially isolated, and being told that one's life lacks meaning. While these cases exist, thoughts of death have since been compared to various aversive experimental controls, such as (but not limited to) thinking about: failure, writing a critical exam, public speaking with a considerable audience, being excluded, paralysis, dental pain, intense physical pain, etc.

With regards to the studies that found similar effects, TMT theorists have argued that in the previously mentioned studies where death was not the subject thought about, the subjects would quite easily be related to death in an individual's mind due to "linguistic or experiential connection with mortality" (p. 332). For example, being robbed invokes thoughts of violence and being unsafe in one's own home – many people have died trying to protect their property and family. A second possible explanation for these results involves the death-thought accessibility hypothesis: these threats somehow sabotage crucial anxiety-buffering aspects of an individual's worldview or self-esteem, which increases their death thought accessibility. For example, one study found increased death thought accessibility in response to thoughts of antagonistic relations with attachment figures. However, this makes it difficult or impossible to isolate the effect of mortality salience.

While many TMT theorists claim that affective responses to mortality salience are suppressed and pushed out of consciousness, later studies contradict this and show that affective responses are indeed observable.

Meaning maintenance model

The meaning maintenance model (MMM) was initially introduced as a comprehensive motivational theory that claimed to subsume TMT, with alternative explanations for TMT findings. Essentially, it posits that people automatically give meaning to things, and when those meanings are somehow disrupted, it causes anxiety. In response, people concentrate on "meaning maintenance to reestablish their sense of symbolic unity" and that such "meaning maintenance often involves the compensatory reaffirmation of alternative meaning structures". These meanings, among other things, should "provide a basis for prediction and control of our...environments, help [one] to cope with tragedy and trauma...and the symbolic cheating of death via adherence to the enduring values that these cultures provide".

While TMT regards the search for meaning as a defense mechanism, meaning management theory regards the quest for meaning as a primary motive because we are meaning-seeking and meaning-making creatures living in a world of meanings. When people are exposed to mortality salience, both TMT and meaning management theory would predict an increase in pro-culture and pro-esteem activities, but for very different reasons. The latter theory is replacing death denial by death acceptance.

TMT theorists argue that meaning management theory cannot describe why different sets of meaning are preferred by different people, and that different types of meaning have different psychological functions. TMT theorists argue that unless something is an important element of a person's anxiety-buffering worldview or self-esteem, it will not require broad meaning maintenance. TMT theorists believe that meaning management theory cannot accurately claim to be an alternative to TMT because it does not seem to be able to explain the current breadth of TMT evidence.

Offensive defensiveness

Some theorists have argued that it is not the idea of death and nonexistence that is unsettling to people, but the fact that uncertainty is involved. For example, these researchers posited that people defend themselves by altering their fear responses from uncertainty to an enthusiasm approach. Other researchers argue for distinguishing fear of death from fear of dying and, therein, posit that ultimately the fear of death has more to do with some other fear (e.g., fear of pain) or reflects uncertainty avoidance or fear of the unknown.

TMT theorists agree that uncertainty can be disconcerting in some cases and it may even result in defense responses, but note that they believe the inescapability of death and the possibility of its finality regarding one's existence is most unsettling. They also note that people actually seek out some types of uncertainty, and that being uncertain is not always very unpleasant. In contrast, there is substantial evidence that, all things being equal, uncertainty and the unknown represent fundamental fears and are only experienced as pleasant when there is sufficient contextual certainty. For example, a surprise involves uncertainty, but is only perceived as pleasant if there is sufficient certainty that the surprise will be pleasant.

Though TMT theorists acknowledge that many responses to mortality salience involve greater approaches (zealousness) towards important worldviews, they also note examples of mortality salience which resulted in the opposite, which offensive defensiveness cannot account for: when negative features of a group to which participants belong were made salient, people actively distanced themselves from that group under mortality salience.

Replication failure

In addition to the criticisms from alternative theoretical perspectives, a large-scale attempt by Many Labs 4 to replicate published findings failed to replicate the mortality salience effect on worldview defense under any condition. The test is a multi-lab replication of Study 1 of Greenberg et al. (1994). Psychologists in 21 labs across the U.S. re-executed the original experiment among a total of 2,200 participants. In response to the Many Labs 4 paper, Tom Pyszczynski (one of the founding psychologists of TMT), criticized the study for insufficient sample sizes, failure to follow the advice of researchers, and deviation from a preregistered protocol.

Popularity

Psychologist Yoel Inbar summarized the popularity of the theory:

I can not explain to people who were not around during this time - which I would say was roughly 2004 to 2008 - how much everything at the time was about terror management theory. You would go to SPSP and it seemed like half of the posters were about terror management theory. It was just everywhere. There is just an explosion of terror management theory stuff. And then it sort of receded. And now you barely see it. Which is also kind of weird. We were obsessed with this for a period of 3-5 years, then we moved on to other things.

Modal realism

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Modal_realism

Modal realism is the view propounded by philosopher David Lewis that all possible worlds are real in the same way as is the actual world: they are "of a kind with this world of ours." It is based on four tenets: possible worlds exist, possible worlds are not different in kind from the actual world, possible worlds are irreducible entities, and the term actual in actual world is indexical, i.e. any subject can declare their world to be the actual one, much as they label the place they are "here" and the time they are "now".

Extended modal realism is a form of modal realism that involves ontological commitments not just to possible worlds but also to impossible worlds. Objects are conceived as being spread out in the modal dimension, i.e. as having not just spatial and temporal parts but also modal parts. This contrasts with Lewis' modal realism according to which each object only inhabits one possible world.

Common arguments for modal realism refer to their theoretical usefulness for modal reasoning and to commonly accepted expressions in natural language that seem to imply ontological commitments to possible worlds. A common objection to modal realism is that it leads to an inflated ontology, which some think to run counter to Occam's razor. Critics of modal realism have also pointed out that it is counterintuitive to allow possible objects the same ontological status as actual objects. This line of thought has been further developed in the argument from morality by showing how an equal treatment of actual and non-actual persons would lead to highly implausible consequences for morality, culminating in the moral principle that every choice is equally permissible.

The term possible world

Main article: Possible world

The term goes back to Leibniz's theory of possible worlds, used to analyse necessity, possibility, and similar modal notions. In short: the actual world is regarded as merely one among an infinite set of logically possible worlds, some "nearer" to the actual world and some more remote. A proposition is necessary if it is true in all possible worlds, and possible if it is true in at least one.

Main tenets

At the heart of David Lewis's modal realism are six central doctrines about possible worlds:

1. Possible worlds exist – they are just as real as our world;
2. Possible worlds are the same sort of things as our world – they differ in content, not in kind;
3. Possible worlds cannot be reduced to something more basic – they are irreducible entities in their own right.
4. Actuality is indexical. When we distinguish our world from other possible worlds by claiming that it alone is actual, we mean only that it is our world.
5. Possible worlds are unified by the spatiotemporal interrelations of their parts; every world is spatiotemporally isolated from every other world.
6. Possible worlds are causally isolated from each other.

Details and alternatives

In philosophy possible worlds are usually regarded as real but abstract possibilities (i.e. platonism), or sometimes as a mere metaphor, abbreviation, or as mathematical devices, or a mere combination of propositions.

Lewis himself not only claimed to take modal realism seriously (although he did regret his choice of the expression modal realism), he also insisted that his claims should be taken literally:

By what right do we call possible worlds and their inhabitants disreputable entities, unfit for philosophical services unless they can beg redemption from philosophy of language? I know of no accusation against possibles that cannot be made with equal justice against sets. Yet few philosophical consciences scruple at set theory. Sets and possibles alike make for a crowded ontology. Sets and possibles alike raise questions we have no way to answer. [...] I propose to be equally undisturbed by these equally mysterious mysteries.

How many [possible worlds] are there? In what respects do they vary, and what is common to them all? Do they obey a nontrivial law of identity of indiscernibles? Here I am at a disadvantage compared to someone who pretends as a figure of speech to believe in possible worlds, but really does not. If worlds were creatures of my imagination, I could imagine them to be any way I liked, and I could tell you all you wished to hear simply by carrying on my imaginative creation. But as I believe that there really are other worlds, I am entitled to confess that there is much about them that I do not know, and that I do not know how to find out.

Extended modal realism

Main article: Extended modal realism

Extended modal realism, as developed by Takashi Yagisawa, differs from other versions of modal realism, such as David Lewis' views, in several important aspects. Possible worlds are conceived as points or indices of the modal dimension rather than as isolated space-time structures. Regular objects are extended not only in the spatial and the temporal dimensions but also in the modal dimension: some of their parts are modal parts, i.e. belong to non-actual worlds. The concept of modal parts is best explained in analogy to spatial and temporal parts. My hand is a spatial part of myself just as my childhood is a temporal part of myself, according to four-dimensionalism. These intuitions can be extended to the modal dimension by considering possible versions of myself which took different choices in life than I actually did. According to extended modal realism, these other selves are inhabitants of different possible worlds and are also parts of myself: modal parts.

Another difference to the Lewisian form of modal realism is that among non-actual worlds within the modal dimension are not just possible worlds but also impossible worlds. Yagisawa holds that while the notion of a world is simple, being a modal index, the notion of a possible world is composite: it is a world that is possible. Possibility can be understood in various ways: there is logical possibility, metaphysical possibility, physical possibility, etc. A world is possible if it doesn't violate the laws of the corresponding type of possibility. For example, a world is logically possible if it obeys the laws of logic or physically possible if it obeys the laws of nature. Worlds that don't obey these laws are impossible worlds. But impossible worlds and their inhabitants are just as real as possible or actual entities.

Arguments for modal realism

Reasons given by Lewis

Lewis backs modal realism for a variety of reasons. First, there doesn't seem to be a reason not to. Many abstract mathematical entities are held to exist simply because they are useful. For example, sets are useful, abstract mathematical constructs that were only conceived in the 19th century. Sets are now considered to be objects in their own right, and while this is a philosophically unintuitive idea, its usefulness in understanding the workings of mathematics makes belief in it worthwhile. The same should go for possible worlds. Since these constructs have helped us make sense of key philosophical concepts in epistemology, metaphysics, philosophy of mind, etc., their existence should be accepted on pragmatic grounds.

Lewis believes that the concept of alethic modality can be reduced to talk of real possible worlds. For example, to say "x is possible" is to say that there exists a possible world where x is true. To say "x is necessary" is to say that in all possible worlds x is true. The appeal to possible worlds provides a sort of economy with the least number of undefined primitives/axioms in our ontology.

Taking this latter point one step further, Lewis argues that modality cannot be made sense of without such a reduction. He maintains that we cannot determine that x is possible without a conception of what a real world where x holds would look like. In deciding whether it is possible for basketballs to be inside of atoms we do not simply make a linguistic determination of whether the proposition is grammatically coherent, we actually think about whether a real world would be able to sustain such a state of affairs. Thus we require a brand of modal realism if we are to use modality at all.

Argument from ways

Possible worlds are often regarded with suspicion, which is why their proponents have struggled to find arguments in their favor. An often-cited argument is called the argument from ways. It defines possible worlds as "ways how things could have been" and relies for its premises and inferences on assumptions from natural language, for example:

(1) Hillary Clinton could have won the 2016 US election.

(2) So there are other ways how things could have been.

(3) Possible worlds are ways how things could have been.

(4) So there are other possible worlds.

The central step of this argument happens at (2) where the plausible (1) is interpreted in a way that involves quantification over "ways". Many philosophers, following Willard Van Orman Quine, hold that quantification entails ontological commitments, in this case, a commitment to the existence of possible worlds. Quine himself restricted his method to scientific theories, but others have applied it also to natural language, for example, Amie L. Thomasson in her easy approach to ontology. The strength of the argument from ways depends on these assumptions and may be challenged by casting doubt on the quantifier-method of ontology or on the reliability of natural language as a guide to ontology.

Criticisms

A number of philosophers, including Lewis himself, have produced criticisms of (what some call) "extreme realism" about possible worlds.

Lewis's own critique

Lewis's own extended presentation of the theory (On the Plurality of Worlds, 1986) raises and then counters several lines of argument against it. That work introduces not only the theory, but its reception among philosophers. The many objections that continue to be published are typically variations on one or other of the lines that Lewis has already canvassed.

Here are some of the major categories of objection:

• Catastrophic counterintuitiveness. The theory does not accord with our deepest intuitions about reality. This is sometimes called "the incredulous stare", since it lacks argumentative content, and is merely an expression of the affront that the theory represents to "common sense" philosophical and pre-philosophical orthodoxy. Lewis is concerned to support the deliverances of common sense in general: "Common sense is a settled body of theory — unsystematic folk theory — which at any rate we do believe; and I presume that we are reasonable to believe it. (Most of it.)" (1986, p. 134). But most of it is not all of it (otherwise there would be no place for philosophy at all), and Lewis finds that reasonable argument and the weight of such considerations as theoretical efficiency compel us to accept modal realism. The alternatives, he argues at length, can themselves be shown to yield conclusions offensive to our modal intuitions.
• Inflated ontology. Some object that modal realism postulates vastly too many entities, compared with other theories. It is therefore, they argue, vulnerable to Occam's razor, according to which we should prefer, all things being equal, those theories that postulate the smallest number of entities. Lewis's reply is that all things are not equal, and in particular competing accounts of possible worlds themselves postulate more classes of entities, since there must be not only one real "concrete" world (the actual world), but many worlds of a different class altogether ("abstract" in some way or other).
• Too many worlds. This is perhaps a variant of the previous category, but it relies on appeals to mathematical propriety rather than Occamist principles. Some argue that Lewis's principles of "worldmaking" (means by which we might establish the existence of further worlds by recombination of parts of worlds we already think exist) are too permissive. So permissive are they, in fact, that the total number of worlds must exceed what is mathematically coherent. Lewis allows that there are difficulties and subtleties to address on this front (1986, pp. 89–90). Daniel Nolan ("Recombination unbound", Philosophical Studies, 1996, vol. 84, pp. 239–262) mounts a sustained argument against certain forms of the objection; but variations on it continue to appear.
• Island universes. On the version of his theory that Lewis strongly favours, each world is distinct from every other world by being spatially and temporally isolated from it. Some have objected that a world in which spatio-temporally isolated universes ("island universes") coexist is therefore not possible, by Lewis's theory (see for example Bigelow, John, and Pargetter, Robert, "Beyond the blank stare", Theoria, 1987, Vol. 53, pp. 97–114). Lewis's awareness of this difficulty discomforted him; but he could have replied that other means of distinguishing worlds may be available, or alternatively that sometimes there will inevitably be further surprising and counterintuitive consequences — beyond what we had thought we would be committed to at the start of our investigation. But this fact in itself is hardly surprising. Plantinga also wonders why we would think that possibility is grounded in some other multi-verse counterpart to me if we were to discover other universes. If not, then why think the same would apply to possible worlds as a whole?

Finally, some of these objections can be combined. For example, one can think that modal realism is unnecessary because multiverse theory can do all the modal work (e.g. many "worlds" interpretation of quantum mechanics).

A pervasive theme in Lewis's replies to the critics of modal realism is the use of tu quoque argument: your account would fail in just the same way that you claim mine would. A major heuristic virtue of Lewis's theory is that it is sufficiently definite for objections to gain some foothold; but these objections, once clearly articulated, can then be turned equally against other theories of the ontology and epistemology of possible worlds.

Stalnaker's response

Robert Stalnaker, while he finds some merit in Lewis's account of possible worlds, finds the position to be ultimately untenable. He himself advances a more "moderate" realism about possible worlds, which he terms actualism (since it holds that all that exists is in fact actual, and that there are no "merely possible" entities). In particular, Stalnaker does not accept Lewis's attempt to argue on the basis of a supposed analogy with the epistemological objection to mathematical Platonism that believing in possible worlds as Lewis imagines them is no less reasonable than believing in mathematical entities such as sets or functions.

Kripke's response

Saul Kripke described modal realism as "totally misguided", "wrong", and "objectionable". Kripke argued that possible worlds were not like distant countries out there to be discovered; rather, we stipulate what is true according to them. Kripke also criticized modal realism for its reliance on counterpart theory, which he regarded as untenable. Specifically, Kripke states that Lewis' modal realism implies that when we refer to possibilities regarding persons like you or me, we're not referring to you or me. Instead, we're referring to counterparts who are similar to us but not identical. This seems problematic because it seems like when, for example, we say that, 'Humphrey could have become President', we are talking about Humphrey (and we're not talking about a person that is like Humphrey). Lewis responds by saying this objection (i.e. The Humphrey Objection) wouldn't apply to modal realists who believe that the identity of persons can "overlap" in multiple worlds, even though Lewis thinks that view is problematic. Secondly, Lewis doesn't seem to share the intuition that there is any problem, as evidenced by the fact that he calls it an "alleged" intuition.

Argument from morality

The argument from morality, as initially formulated by Robert Merrihew Adams, criticizes modal realism on the grounds that modal realism has very implausible consequences for morality and should therefore be rejected. This can be seen by considering the principle of plenitude: the thesis that there is a possible world for every way things could be. The consequence of this principle is that the nature of the pluriverse, i.e. of reality in the widest sense, is fixed. This means that whatever choices human agents make, they have no impact on reality as a whole. For example, assume that during a stroll at a lake you spot a drowning child not far from the shore. You have a choice to save the child or not to. If you choose to save the child then a counterpart of you at another possible world chooses to let it drown. If you choose to let it drown then the counterpart of you at this other possible world chooses to save it. Either way, the result for these two possible worlds is the same: one child drowns and the other is saved. The only impact of your choice is to relocate a death from the actual world to another possible world. But since, according to modal realism, there is no important difference between the actual world and other possible worlds, this shouldn't matter. The consequence would be that there is no moral obligation to save the child, which is drastically at odds with common-sense morality. Worse still, this argument can be generalized to any decision, so whatever you choose in any decision would be morally permissible.

David Lewis defends moral realism against this argument by pointing out that morality, as commonly conceived, is only interested in the actual world, specifically, that the actual agent doesn't do evil. So the argument from morality would only be problematic for an odd version of utilitarianism aiming at maximizing the "sum total of good throughout the plurality of worlds". But, as Mark Heller points out, this reply doesn't explain why we are justified in morally privileging the actual world, as modal realism seems to be precisely against such a form of unequal treatment. This is not just a problem for utilitarians but for any moral theory that is sensitive to how other people are affected by one's actions in the widest sense, causally or otherwise: "the modal realist has to consider more people in moral decision making than we ordinarily do consider". Bob Fischer, speaking on Lewis' behalf, concedes that, from a modally unrestricted point of view of morality, there is no obligation to save the child from drowning. Common-sense morality, on the other hand, assumes a modally restricted point of view. This disagreement with common-sense is a cost of modal realism to be considered in an overall cost-benefit calculation, but it is no knockdown argument.

Tests of special relativity

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(Redirected from Status of special relativity)

Special relativity is a physical theory that plays a fundamental role in the description of all physical phenomena, as long as gravitation is not significant. Many experiments played (and still play) an important role in its development and justification. The strength of the theory lies in its unique ability to correctly predict to high precision the outcome of an extremely diverse range of experiments. Repeats of many of those experiments are still being conducted with steadily increased precision, with modern experiments focusing on effects such as at the Planck scale and in the neutrino sector. Their results are consistent with the predictions of special relativity. Collections of various tests were given by Jakob Laub, Zhang, Mattingly, Clifford Will, and Roberts/Schleif.

Special relativity is restricted to flat spacetime, i.e., to all phenomena without significant influence of gravitation. The latter lies in the domain of general relativity and the corresponding tests of general relativity must be considered.

Experiments paving the way to relativity

The predominant theory of light in the 19th century was that of the luminiferous aether, a stationary medium in which light propagates in a manner analogous to the way sound propagates through air. By analogy, it follows that the speed of light is constant in all directions in the aether and is independent of the velocity of the source. Thus an observer moving relative to the aether must measure some sort of "aether wind" even as an observer moving relative to air measures an apparent wind.

First-order experiments

Main article: Luminiferous aether § First order experiments

Fizeau experiment, 1851

Beginning with the work of François Arago (1810), a series of optical experiments had been conducted, which should have given a positive result for magnitudes of first order in $v/c$ (i.e., of ${\displaystyle (v/c{)}^1}$) and which thus should have demonstrated the relative motion of the aether. Yet the results were negative. An explanation was provided by Augustin Fresnel (1818) with the introduction of an auxiliary hypothesis, the so-called "dragging coefficient", that is, matter is dragging the aether to a small extent. This coefficient was directly demonstrated by the Fizeau experiment (1851). It was later shown that all first-order optical experiments must give a negative result due to this coefficient. In addition, some electrostatic first-order experiments were conducted, again having a negative results. In general, Hendrik Lorentz (1892, 1895) introduced several new auxiliary variables for moving observers, demonstrating why all first-order optical and electrostatic experiments have produced null results. For example, Lorentz proposed a location variable by which electrostatic fields contract in the line of motion and another variable ("local time") by which the time coordinates for moving observers depend on their current location.

Second-order experiments

Main article: Luminiferous aether § Second order experiments

Michelson-Morley interferometer

The stationary aether theory, however, would give positive results when the experiments are precise enough to measure magnitudes of second order in $v/c$ (i.e., of $(v/c{)}^2$). Albert A. Michelson conducted the first experiment of this kind in 1881, followed by the more sophisticated Michelson–Morley experiment in 1887. Two rays of light, traveling for some time in different directions were brought to interfere, so that different orientations relative to the aether wind should lead to a displacement of the interference fringes. But the result was negative again. The way out of this dilemma was the proposal by George Francis FitzGerald (1889) and Lorentz (1892) that matter is contracted in the line of motion with respect to the aether (length contraction). That is, the older hypothesis of a contraction of electrostatic fields was extended to intermolecular forces. However, since there was no theoretical reason for that, the contraction hypothesis was considered ad hoc.

Besides the optical Michelson–Morley experiment, its electrodynamic equivalent was also conducted, the Trouton–Noble experiment. By that it should be demonstrated that a moving condenser must be subjected to a torque. In addition, the Experiments of Rayleigh and Brace intended to measure some consequences of length contraction in the laboratory frame, for example the assumption that it would lead to birefringence. Though all of those experiments led to negative results. (The Trouton–Rankine experiment conducted in 1908 also gave a negative result when measuring the influence of length contraction on an electromagnetic coil.)

To explain all experiments conducted before 1904, Lorentz was forced to again expand his theory by introducing the complete Lorentz transformation. Henri Poincaré declared in 1905 that the impossibility of demonstrating absolute motion (principle of relativity) is apparently a law of nature.

Refutations of complete aether drag

Main article: Aether drag hypothesis

Lodge's ether machine. The steel disks were one yard in diameter. White light was split by a beam splitter and ran three times around the apparatus before reuniting to form fringes.

The idea that the aether might be completely dragged within or in the vicinity of Earth, by which the negative aether drift experiments could be explained, was refuted by a variety of experiments.

• Oliver Lodge (1893) found that rapidly whirling steel disks above and below a sensitive common path interferometric arrangement failed to produce a measurable fringe shift.
• Gustaf Hammar (1935) failed to find any evidence for aether dragging using a common-path interferometer, one arm of which was enclosed by a thick-walled pipe plugged with lead, while the other arm was free.
• The Sagnac effect showed that aether wind caused by earth drag cannot be demonstrated.
• The existence of the aberration of light was inconsistent with aether drag hypothesis.
• The assumption that aether drag is proportional to mass and thus only occurs with respect to Earth as a whole was refuted by the Michelson–Gale–Pearson experiment, which demonstrated the Sagnac effect through Earth's motion.

Lodge expressed the paradoxical situation in which physicists found themselves as follows: "...at no practicable speed does ... matter [have] any appreciable viscous grip upon the ether. Atoms must be able to throw it into vibration, if they are oscillating or revolving at sufficient speed; otherwise they would not emit light or any kind of radiation; but in no case do they appear to drag it along, or to meet with resistance in any uniform motion through it."

Special relativity

Overview

Eventually, Albert Einstein (1905) drew the conclusion that established theories and facts known at that time only form a logical coherent system when the concepts of space and time are subjected to a fundamental revision. For instance:

• Maxwell-Lorentz's electrodynamics (independence of the speed of light from the speed of the source),
• the negative aether drift experiments (no preferred reference frame),
• Moving magnet and conductor problem (only relative motion is relevant),
• the Fizeau experiment and the aberration of light (both implying modified velocity addition and no complete aether drag).

The result is special relativity theory, which is based on the constancy of the speed of light in all inertial frames of reference and the principle of relativity. Here, the Lorentz transformation is no longer a mere collection of auxiliary hypotheses but reflects a fundamental Lorentz symmetry and forms the basis of successful theories such as Quantum electrodynamics. Special relativity offers a large number of testable predictions, such as:

Principle of relativity	Constancy of the speed of light	Time dilation
Any uniformly moving observer in an inertial frame cannot determine his "absolute" state of motion by a co-moving experimental arrangement.	In all inertial frames the measured speed of light is equal in all directions (isotropy), independent of the speed of the source, and cannot be reached by massive bodies.	The rate of a clock C (= any periodic process) traveling between two synchronized clocks A and B at rest in an inertial frame is retarded with respect to the two clocks.
Also other relativistic effects such as length contraction, Doppler effect, aberration and the experimental predictions of relativistic theories such as the Standard Model can be measured.

Fundamental experiments

The Kennedy–Thorndike experiment

The effects of special relativity can phenomenologically be derived from the following three fundamental experiments:

• Michelson–Morley experiment, by which the dependence of the speed of light on the direction of the measuring device can be tested. It establishes the relation between longitudinal and transverse lengths of moving bodies.
• Kennedy–Thorndike experiment, by which the dependence of the speed of light on the velocity of the measuring device can be tested. It establishes the relation between longitudinal lengths and the duration of time of moving bodies.
• Ives–Stilwell experiment, by which time dilation can be directly tested.

From these three experiments and by using the Poincaré-Einstein synchronization, the complete Lorentz transformation follows, with $\gamma =1/\sqrt{1-v^2/c^2}$ being the Lorentz factor:

$${\displaystyle x^{\prime }=\gamma (x-vt),y^{\prime }=y,z^{\prime }=z,t^{\prime }=\gamma \left(t-\frac{vx}{c^2}\right)}$$

Besides the derivation of the Lorentz transformation, the combination of these experiments is also important because they can be interpreted in different ways when viewed individually. For example, isotropy experiments such as Michelson-Morley can be seen as a simple consequence of the relativity principle, according to which any inertially moving observer can consider himself as at rest. Therefore, by itself, the MM experiment is compatible to Galilean-invariant theories like emission theory or the complete aether drag hypothesis, which also contain some sort of relativity principle. However, when other experiments that exclude the Galilean-invariant theories are considered (i.e. the Ives–Stilwell experiment, various refutations of emission theories and refutations of complete aether dragging), Lorentz-invariant theories and thus special relativity are the only theories that remain viable.

Constancy of the speed of light

Interferometers, resonators

Michelson-Morley experiment with cryogenic optical resonators of a form such as was used by Müller et al. (2003), see Recent optical resonator experiments

See also: Recent Michelson-Morley experiments and Recent Kennedy–Thorndike experiments

Modern variants of Michelson-Morley and Kennedy–Thorndike experiments have been conducted in order to test the isotropy of the speed of light. Contrary to Michelson-Morley, the Kennedy-Thorndike experiments employ different arm lengths, and the evaluations last several months. In that way, the influence of different velocities during Earth's orbit around the Sun can be observed. Laser, maser and optical resonators are used, reducing the possibility of any anisotropy of the speed of light to the 10⁻¹⁷ level. In addition to terrestrial tests, Lunar Laser Ranging Experiments have also been conducted as a variation of the Kennedy-Thorndike-experiment.

Another type of isotropy experiments are the Mössbauer rotor experiments in the 1960s, by which the anisotropy of the Doppler effect on a rotating disc can be observed by using the Mössbauer effect (those experiments can also be utilized to measure time dilation, see below).

No dependence on source velocity or energy

The de Sitter double star experiment, later repeated by Brecher under consideration of the extinction theorem.

Emission theories, according to which the speed of light depends on the velocity of the source, can conceivably explain the negative outcome of aether drift experiments. It wasn't until the mid-1960s that the constancy of the speed of light was definitively shown by experiment, since in 1965, J. G. Fox showed that the effects of the extinction theorem rendered the results of all experiments previous to that time inconclusive, and therefore compatible with both special relativity and emission theory. More recent experiments have definitely ruled out the emission model: the earliest were those of Filippas and Fox (1964), using moving sources of gamma rays, and Alväger et al. (1964), which demonstrated that photons didn't acquire the speed of the high speed decaying mesons which were their source. In addition, the de Sitter double star experiment (1913) was repeated by Brecher (1977) under consideration of the extinction theorem, ruling out a source dependence as well.

Observations of Gamma-ray bursts also demonstrated that the speed of light is independent of the frequency and energy of the light rays.

One-way speed of light

Main article: One-way speed of light

A series of one-way measurements were undertaken, all of them confirming the isotropy of the speed of light. However, only the two-way speed of light (from A to B back to A) can unambiguously be measured, since the one-way speed depends on the definition of simultaneity and therefore on the method of synchronization. The Einstein synchronization convention makes the one-way speed equal to the two-way speed. However, there are many models having isotropic two-way speed of light, in which the one-way speed is anisotropic by choosing different synchronization schemes. They are experimentally equivalent to special relativity because all of these models include effects like time dilation of moving clocks, that compensate any measurable anisotropy. However, of all models having isotropic two-way speed, only special relativity is acceptable for the overwhelming majority of physicists since all other synchronizations are much more complicated, and those other models (such as Lorentz ether theory) are based on extreme and implausible assumptions concerning some dynamical effects, which are aimed at hiding the "preferred frame" from observation.

Isotropy of mass, energy, and space

⁷Li-NMR spectrum of LiCl (1M) in D₂O. The sharp, unsplit NMR line of this isotope of lithium is evidence for the isotropy of mass and space.

See also: Hughes–Drever experiment

Clock-comparison experiments (periodic processes and frequencies can be considered as clocks) such as the Hughes–Drever experiments provide stringent tests of Lorentz invariance. They are not restricted to the photon sector as Michelson-Morley but directly determine any anisotropy of mass, energy, or space by measuring the ground state of nuclei. Upper limit of such anisotropies of 10⁻³³ GeV have been provided. Thus these experiments are among the most precise verifications of Lorentz invariance ever conducted.

Time dilation and length contraction

Ives–Stilwell experiment (1938).)

Main articles: Ives–Stilwell experiment, Experimental testing of time dilation, and Confirmations of length contraction

The transverse Doppler effect and consequently time dilation was directly observed for the first time in the Ives–Stilwell experiment (1938). In modern Ives-Stilwell experiments in heavy ion storage rings using saturated spectroscopy, the maximum measured deviation of time dilation from the relativistic prediction has been limited to ≤ 10⁻⁸. Other confirmations of time dilation include Mössbauer rotor experiments in which gamma rays were sent from the middle of a rotating disc to a receiver at the edge of the disc, so that the transverse Doppler effect can be evaluated by means of the Mössbauer effect. By measuring the lifetime of muons in the atmosphere and in particle accelerators, the time dilation of moving particles was also verified. On the other hand, the Hafele–Keating experiment confirmed the resolution of the twin paradox, i.e. that a clock moving from A to B back to A is retarded with respect to the initial clock. However, in this experiment the effects of general relativity also play an essential role.

Direct confirmation of length contraction is hard to achieve in practice since the dimensions of the observed particles are vanishingly small. However, there are indirect confirmations; for example, the behavior of colliding heavy ions can only be explained if their increased density due to Lorentz contraction is considered. Contraction also leads to an increase of the intensity of the Coulomb field perpendicular to the direction of motion, whose effects already have been observed. Consequently, both time dilation and length contraction must be considered when conducting experiments in particle accelerators.

Relativistic momentum and energy

Bucherer's experimental setup for measuring the specific charge e/m of β⁻ electrons as a function of their speed v/c. (Cross-section through the axis of a circular capacitor with a beta-source at its center, at an angle α with respect to the magnetic field H)

Main articles: Tests of relativistic energy and momentum and Kaufmann–Bucherer–Neumann experiments

Starting with 1901, a series of measurements was conducted aimed at demonstrating the velocity dependence of the mass of electrons. The results actually showed such a dependency but the precision necessary to distinguish between competing theories was disputed for a long time. Eventually, it was possible to definitely rule out all competing models except special relativity.

Today, special relativity's predictions are routinely confirmed in particle accelerators such as the Relativistic Heavy Ion Collider. For example, the increase of relativistic momentum and energy is not only precisely measured but also necessary to understand the behavior of cyclotrons and synchrotrons etc., by which particles are accelerated near to the speed of light.

Sagnac and Fizeau

Original Sagnac interferometer

Main articles: Sagnac interferometer and Fizeau experiment

Special relativity also predicts that two light rays traveling in opposite directions around a spinning closed path (e.g. a loop) require different flight times to come back to the moving emitter/receiver (this is a consequence of the independence of the speed of light from the velocity of the source, see above). This effect was actually observed and is called the Sagnac effect. Currently, the consideration of this effect is necessary for many experimental setups and for the correct functioning of GPS.

If such experiments are conducted in moving media (e.g. water, or glass optical fiber), it is also necessary to consider Fresnel's dragging coefficient as demonstrated by the Fizeau experiment. Although this effect was initially understood as giving evidence of a nearly stationary aether or a partial aether drag it can easily be explained with special relativity by using the velocity composition law.

Test theories

Main article: Test theories of special relativity

Several test theories have been developed to assess a possible positive outcome in Lorentz violation experiments by adding certain parameters to the standard equations. These include the Robertson-Mansouri-Sexl framework (RMS) and the Standard-Model Extension (SME). RMS has three testable parameters with respect to length contraction and time dilation. From that, any anisotropy of the speed of light can be assessed. On the other hand, SME includes many Lorentz violation parameters, not only for special relativity, but for the Standard model and General relativity as well; thus it has a much larger number of testable parameters.

Other modern tests

Main article: Modern searches for Lorentz violation

Due to the developments concerning various models of Quantum gravity in recent years, deviations of Lorentz invariance (possibly following from those models) are again the target of experimentalists. Because "local Lorentz invariance" (LLI) also holds in freely falling frames, experiments concerning the weak Equivalence principle belong to this class of tests as well. The outcomes are analyzed by test theories (as mentioned above) like RMS or, more importantly, by SME.

• Besides the mentioned variations of Michelson–Morley and Kennedy–Thorndike experiments, Hughes–Drever experiments are continuing to be conducted for isotropy tests in the proton and neutron sector. To detect possible deviations in the electron sector, spin-polarized torsion balances are used.
• Time dilation is confirmed in heavy ion storage rings, such as the TSR at the MPIK, by observation of the Doppler effect of lithium, and those experiments are valid in the electron, proton, and photon sector.
• Other experiments use Penning traps to observe deviations of cyclotron motion and Larmor precession in electrostatic and magnetic fields.
• Possible deviations from CPT symmetry (whose violation represents a violation of Lorentz invariance as well) can be determined in experiments with neutral mesons, Penning traps and muons, see Antimatter Tests of Lorentz Violation.
• Astronomical tests are conducted in connection with the flight time of photons, where Lorentz violating factors could cause anomalous dispersion and birefringence leading to a dependency of photons on energy, frequency or polarization.
• With respect to threshold energy of distant astronomical objects, but also of terrestrial sources, Lorentz violations could lead to alterations in the standard values for the processes following from that energy, such as Vacuum Cherenkov radiation, or modifications of synchrotron radiation.
• Neutrino oscillations (see Lorentz-violating neutrino oscillations) and the speed of neutrinos (see measurements of neutrino speed) are being investigated for possible Lorentz violations.
• Other candidates for astronomical observations are the Greisen–Zatsepin–Kuzmin limit and Airy disks. The latter is investigated to find possible deviations of Lorentz invariance that could drive the photons out of phase.
• Observations in the Higgs sector are under way.