Tired light is a class of hypothetical
redshiftIn physics and astronomy, redshift occurs when electromagnetic radiation—usually visible light—emitted or reflected by an object is shifted towards the red end of the electromagnetic spectrum due to the Doppler effect or other gravitationally-induced effects...
mechanisms that was proposed as an alternative explanation for the
redshift-distance relationshipHubble's law describes the observation in physical cosmology that the velocity at which various galaxies are receding from the Earth is proportional to their distance from us. The law was first formulated by Edwin Hubble in 1929 after nearly a decade of observations...
as alternatives to the
Big BangThe Big Bang is the cosmological model of the initial conditions and subsequent development of the Universe that is supported by the most comprehensive and accurate explanations from current scientific evidence and observation...
and the
Steady StateIn cosmology, the Steady State theory is a model developed in 1948 by Fred Hoyle, Thomas Gold, Hermann Bondi and others as an alternative to the Big Bang theory...
cosmologiesPhysical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of our universe and is concerned with fundamental questions about its formation and evolution. Cosmology involves itself with studying the motions of the celestial bodies and the first cause....
, both of which proposed that
Hubble's lawHubble's law describes the observation in physical cosmology that the velocity at which various galaxies are receding from the Earth is proportional to their distance from us. The law was first formulated by Edwin Hubble in 1929 after nearly a decade of observations...
was associated with a
metric expansion of spaceThe metric expansion of space is the averaged increase of metric distance between distant objects in the universe with time. It is an intrinsic expansion—that is, it is defined by the relative separation of parts of the universe and not by motion "outward" into preexisting space...
. The concept was first proposed in 1929 by
Fritz ZwickyFritz Zwicky was born in Bulgaria, America-based Swiss astronomer. He was an original thinker, with many important contributions in theoretical and observational astronomy.-Biography:...
, who suggested that photons lose energy over time through collisions with other particles. Alternative models of gravity that do not produce an expanding universe sometimes appeal to tired light to explain Hubble's Law. Today, tired light is remembered mainly for historical interest, and almost no scientist accepts tired light as a viable explanation for Hubble's Law.
History and reception
Tired
lightLight is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye ....
was an idea that came about due to the observation made by
Edwin HubbleEdwin Powell Hubble was an American astronomer. He profoundly changed our understanding of the universe by demonstrating the existence of other galaxies besides the Milky Way. He also discovered that the degree of redshift observed in light coming from a galaxy increased in proportion to the...
that
distant galaxiesA galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The name is from the Greek root galaxias [γαλαξίας], meaning "milky," a reference...
had
redshiftIn physics and astronomy, redshift occurs when electromagnetic radiation—usually visible light—emitted or reflected by an object is shifted towards the red end of the electromagnetic spectrum due to the Doppler effect or other gravitationally-induced effects...
s proportional to their
distanceDistance measures are used in physical cosmology to give a natural notion of the distance between two objects or events in the universe. They are often used to tie some observable quantity to another quantity that is not directly...
. Redshift is a shift in the spectrum of
electromagnetic radiationElectromagnetic radiation is a ubiquitous phenomenon that takes the form of self-propagating waves in a vacuum or in matter. It consists of electric and magnetic field components which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation...
toward lower energies and frequencies, usually attributed to
recessional velocityRecessional Velocity is a term used to describe the rate at which an object is moving away, typically from Earth.-Application to Cosmology:This term is generally only used in reference to distant Galaxies. The most common reason for the use of this term is Hubble's Law, which states that the...
. Before Hubble's observations, all observed redshifts were attributable directly to a mechanism similar to the
Doppler EffectThe Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer...
. In these situations, objects moving away from us exhibit a shift toward the red end of the optical spectrum while objects moving toward us exhibit a shift toward the blue end (blueshift). Hubble and other scientists observing the redshifts associated with distant galaxies attributed their redshifts to motion away from us. Bizarrely, the relation held in all directions and so could not be attributed to normal movement with respect to a background which would show redshifts and blueshifts. Everything was moving
away.
Immediately upon publishing this results,
Willem de SitterWillem de Sitter was a Dutch mathematician, physicist and astronomer.Born in Sneek, De Sitter studied mathematics at the University of Groningen and then joined the Groningen astronomical laboratory. He worked at the Cape Observatory in South Africa . Then, in 1908, de Sitter was appointed to the...
who was working with Einstein's Theory of General Relativity recognized that these observations fit with a particular solution to the Einstein equations now known as the FRW metric. This solution explains that distant objects are observed with greater wavelengths than were emitted by the objects because they are moving with an expansion of the universe. In this formulation, there was still an analogous effect to the
Doppler EffectThe Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer...
, though relative velocities are not really well-defined for such situations.
Not everyone accepted this interpretation immediately. General relativity was notoriously difficult to understand and many of the astronomers were skeptical that this was the only possible explanation. Some, like
Edward MilneEdward Milne may refer to:*Edward Arthur Milne , British mathematician and astrophysicist*Eddie Milne , British Labour Party Member of Parliament for Blyth, afterwards re-elected as an independent...
proposed that there was a giant explosion that could explain redshifts (see Milne universe). Others thought that there might be
systematic effectsBiological systematics is the study of the diversification of life on the planet Earth, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees...
that made the observation suspect. Along this line,
Fritz ZwickyFritz Zwicky was born in Bulgaria, America-based Swiss astronomer. He was an original thinker, with many important contributions in theoretical and observational astronomy.-Biography:...
proposed a "tired light" mechanism in 1929. Zwicky suggested that
photonIn physics, a photon is an elementary particle, the quantum of the electromagnetic field and the basic "unit" of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s might slowly lose
energyIn physics, energy is a scalar physical quantity that describes the amount of work that can be performed by a force, an attribute of objects and systems that is subject to a conservation law...
as they travel vast
distanceDistance is a numerical description of how far apart objects are. In physics or everyday discussion, distance may refer to a physical length, a period of time, or an estimation based on other criteria . In mathematics, a distance function or metric is a generalization of the concept of physical...
s through a
static universeThe idea of a static universe or "Einstein's universe" is one which demands that space is not expanding nor contracting but rather is dynamically stable...
by interaction with matter or other photons, or by some novel physical mechanism. Since a decrease in
energyIn physics, energy is a scalar physical quantity that describes the amount of work that can be performed by a force, an attribute of objects and systems that is subject to a conservation law...
corresponds to an increase in light's
wavelengthIn physics, the wavelength of a sinusoidal wave is the spatial period of the wave – the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
, this effect would produce a
redshiftIn physics and astronomy, redshift occurs when electromagnetic radiation—usually visible light—emitted or reflected by an object is shifted towards the red end of the electromagnetic spectrum due to the Doppler effect or other gravitationally-induced effects...
in
spectral lineA spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...
s that increase
proportionallyIn mathematics, two quantities are said to be proportional if they vary in such a way that one of the quantities is a constant multiple of the other, or equivalently if they have a constant ratio.Proportion also refers to the equality of two ratios....
with the distance of the source. The term "tired light" was coined by Richard Tolman in the early 1930s as a way to refer to this idea
http://books.google.com/books?vid=ISBN0792340442&id=2-0p1eOCYeIC&pg=PA29&lpg=PA29&dq=%22tired+light%22+coined+tolman&sig=wi74K_Qan8pC-k15hfxl8ipWbog.
Tired light mechanisms were among the proposed alternatives to the
Big BangThe Big Bang is the cosmological model of the initial conditions and subsequent development of the Universe that is supported by the most comprehensive and accurate explanations from current scientific evidence and observation...
and the
Steady StateIn cosmology, the Steady State theory is a model developed in 1948 by Fred Hoyle, Thomas Gold, Hermann Bondi and others as an alternative to the Big Bang theory...
cosmologiesPhysical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of our universe and is concerned with fundamental questions about its formation and evolution. Cosmology involves itself with studying the motions of the celestial bodies and the first cause....
, both of which relied on the general relativistic expansion of the universe of the FRW metric. Through the middle of the twentieth century, most cosmologists supported one of these two
paradigmThe word paradigm has been used in linguistics and science to describe distinct concepts....
s, but there were a few scientists who worked with the tired light alternative. As the discipline of
observational cosmologyObservational cosmology is the study of the structure, the evolution and the origin of the universe through observation, using instruments such as telescopes and cosmic ray detectors.-Early observations:...
developed in the late twentieth century and the associated data became more numerous and accurate, the Big Bang emerged as the cosmological theory most supported by the observation evidence, and it remains the accepted
consensus modelScientific consensus is the collective judgement, position, and opinion of the community of scientists in a particular field of study. Consensus implies general agreement, though not necessarily unanimity. Scientific consensus is not by itself a scientific argument, and it is not part of the...
in the current
parametrizationΛCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter. It is frequently referred to as the concordance model of big bang cosmology, since it attempts to explain cosmic microwave background observations, as well as large scale structure observations and supernovae observations of...
of the state and evolution of the universe. Additionally, a number of studies have shown that the "tired light" hypothesis is not a viable explanation for cosmological redshifts.
Redshift is directly observable and used by cosmologists as a direct measure of
timeTime is a component of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify the motions of objects...
, often referring to age in terms of redshift rather than years. The
Big BangThe Big Bang is the cosmological model of the initial conditions and subsequent development of the Universe that is supported by the most comprehensive and accurate explanations from current scientific evidence and observation...
is the end of this scale of time corresponding to a redshift of infinity. The
relation between universal expansion and redshiftHubble's law describes the observation in physical cosmology that the velocity at which various galaxies are receding from the Earth is proportional to their distance from us. The law was first formulated by Edwin Hubble in 1929 after nearly a decade of observations...
is observationally confirmed through a great variety of tests, though alternative hypotheses such as tired light remain historical interest. So-called alternative theories of gravity that do not have an expanding universe need an alternative explanation to explain the correspondence between redshift and distance that is
sui generis to
expanding metricsThe metric expansion of space is the averaged increase of metric distance between distant objects in the universe with time. It is an intrinsic expansion—that is, it is defined by the relative separation of parts of the universe and not by motion "outward" into preexisting space...
of general relativity. Such cosmologies are sometimes referred to as "tired-light cosmologies", though not all authors are necessarily aware of the historical antecedents.
Zwicky's models
ZwickyFritz Zwicky was born in Bulgaria, America-based Swiss astronomer. He was an original thinker, with many important contributions in theoretical and observational astronomy.-Biography:...
investigated a number of redshift explanations, ruling out some himself (Zwicky, 1929):
- "... light coming from distant nebulae would undergo a shift to the red by Compton effect on those free electrons [in interstellar spaces] [...] But then the light scattered in all directions would make the interstellar space intolerably opaque which disposes of the above explanation. [...] it is evident that any explanation based on a scattering process like the Compton effect or the Raman effect, etc., will be in a hopeless position regarding the good definition of the images"
- "One might expect a shift of spectral lines due to the difference of the static gravitational potential at different distances from the center of a galaxy. This effect, of course, has no relation to the distance of the observed galaxy from our own system and, therefore, cannot provide any explanation of the phenomenon discussed in this paper."
- The Gravitational "Drag" of Light:
- "... [a] gravitational analogue of the Compton effect [...] It is easy to see that the above redshift should broaden these absorption lines asymmetrically toward the red. If these lines can be photographed with a high enough dispersion, the displacement of the center of gravity of the line will give the redshift independent of the velocity of the system from which the light is emitted."
Zwicky also notes, in the same paper, that according to a tired light model a distance-redshift relationship would necessarily be present in the light from sources within our own galaxy (even if the redshift would be so small that it would be hard to measure), that do not appear under a recessional-velocity based theory. He writes, referring to sources of light within our galaxy: "It is especially desirable to determine the redshift independent of the proper velocities of the objects observed".
Hubble and Tolman's "energy loss" treatment
Following after
Fritz ZwickyFritz Zwicky was born in Bulgaria, America-based Swiss astronomer. He was an original thinker, with many important contributions in theoretical and observational astronomy.-Biography:...
in 1935,
Edwin HubbleEdwin Powell Hubble was an American astronomer. He profoundly changed our understanding of the universe by demonstrating the existence of other galaxies besides the Milky Way. He also discovered that the degree of redshift observed in light coming from a galaxy increased in proportion to the...
and Richard Tolman compare recessional redshift with a non-recessional one, writing that they:
- "... both incline to the opinion, however, that if the red-shift is not due to recessional motion, its explanation will probably involve some quite new physical priciples [.. and] use of a static Einstein model of the universe, combined with the assumption that the photons emitted by a nebula lose energy on their journey to the observer by some unknown effect, which is linear with distance, and which leads to a decrease in frequency, without appreciable transverse deflection"
Finlay-Freundlich Red Shift Hypothesis
In the early 1950s,
Erwin Finlay-FreundlichErwin Finlay-Freundlich [Scottish name:"Finlay"] was a German astronomer, a pupil of Felix Klein. He was born in Biebrich, Germany...
proposed a redshift as "the result of loss of energy by observed photons traversing a radiation field.". R.A. Alpher noted "No generally accepted physical mechanism has been proposed for this loss" , though P.F. Brown ".. proposed that the energy lost reappears as neutrino pairs resulting from the exchange of a graviton between two photons".
General features of tired light models
The simplest form of a tired light theory assumes an exponential decrease in photon energy with distance traveled:
where is the energy of the photon at distance
x from the source of light, is the energy of the photon at the source of light, and
R is a large constant characterizing the "resistance of the space". To correspond to
Hubble's lawHubble's law describes the observation in physical cosmology that the velocity at which various galaxies are receding from the Earth is proportional to their distance from us. The law was first formulated by Edwin Hubble in 1929 after nearly a decade of observations...
, the constant
R must be several giga
parsecThe parsec is a unit of length, equal to just under 31 million million kilometres , or about 3.26 light-years. The parsec measurement unit is used in astronomy and is defined as the length of the adjacent side of an imaginary right triangle in space...
s.
Criticisms
Any "tired light" mechanism must solve some basic problems, in that the observed redshift must:
- admit the same measurement in any wavelength-band
- not exhibit blurring
- follow the detailed Hubble-relation observed with Supernova
A supernova is a stellar explosion. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months. During this short interval, a supernova can radiate as much energy as the Sun could emit over...
data (see accelerating universeThe accelerating universe is the observation that the universe appears to be expanding at an increasing rate. In 1998 observations of Type Ia supernovae suggested that the expansion of the universe is accelerating since around redshift of z~0.5....
)
- explain associated time dilation
Time dilation is a phenomenon described by the theory of relativity. It can be illustrated by supposing that two observers are in motion relative to each other, and/or differently situated with regard to nearby gravitational masses. They each carry a clock of identically similar construction and...
of cosmologically distant events.
As part of a broader
alternative cosmologyA non-standard cosmology is any physical cosmological model of the universe that has been, or still is, proposed as an alternative to the big bang model of physical cosmology...
, other observations that need explanation include:
- the detail observations of the cosmic microwave background radiation
In cosmology, cosmic microwave background radiation is a form of electromagnetic radiation filling the universe. With a traditional optical telescope, the space between stars and galaxies is pitch black...
- the abundance of light elements
In physical cosmology, Big Bang nucleosynthesis refers to the production of nuclei other than those of H-1 during the early phases of the universe...
- large-scale structure statistics
In physical cosmology, the large-scale structure of the universe refers to the characterization of observable distributions of matter and light on the largest scales...
To date, no established mechanism to produce such a drop in energy has been proposed that reproduces all the observations associated with the redshift-distance relation.
ScatteringScattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
by known mechanisms from gas or dust does not reproduce the observations. For example, scattering by any mechanism would blur an object more than observed. In general, cosmologists consider classical tired light models to have too many problems to be worth serious consideration. Tired light alone does not provide a full cosmological explanation and so cannot reproduce all the successes of the standard
big bangThe Big Bang is the cosmological model of the initial conditions and subsequent development of the Universe that is supported by the most comprehensive and accurate explanations from current scientific evidence and observation...
cosmology. No tired light theory is known that by itself correctly accounts for the observed time dilation of distant
supernovaA supernova is a stellar explosion. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months. During this short interval, a supernova can radiate as much energy as the Sun could emit over...
e light curves , the black body spectrum or anisotropy of the cosmic microwave background, and the observed change in the morphology, number count, and surface brightness of high redshift
galaxiesA galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The name is from the Greek root galaxias [γαλαξίας], meaning "milky," a reference...
and
quasarA quasi-stellar radio source is a powerfully energetic and distant galaxy with an active galactic nucleus. Quasars were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than extended...
s. Furthermore, the fact that the age of the oldest stars is roughly equal to the inverse of the Hubble constant emerges naturally from a Big Bang cosmology, but is an unexplained coincidence with most tired light models.
In 2008, Blondin
et al. showed that even processes that take macroscopic amounts of time such as various elapsed times are slowed down by the same redshift factor as the light frequency.