Does Light Possess Energy?

Intellectual Discussion about Light & Energy:

The CMB radiation is the oldest (or first) light ever emitted when our universe was only 380'000 years old and had a temperature of 3'000 Kelvin.

Q) Does light have energy?

A) Yes, light does have energy. It's made up of particles called photons, and each photon carries energy.

Q) How much energy does a photon carry?

A) The amount of energy a photon carries depends on its frequency. The higher the frequency, the higher the energy. This relationship is described by the equation
 
  E = hf  
 
where E is energy, h is Planck's constant, and f is the frequency of the light photon.

Q) What unit is E measured in?

A) The energy (E) of a photon is typically measured in joules (J).

Q) Why not use electron volts as a unit here?

A) Energy can indeed be measured in electron volts (eV), especially in the context of particles like photons and electrons. It's a common unit used in particle physics because it's convenient for describing energies on the atomic and subatomic scale.

Q) So why isn't that unit of measurement used for the photon?

A) Electron volts (eV) are often used in particle physics because they provide a convenient scale for describing energies at the atomic and subatomic level. However, joules (J) are more commonly used in broader scientific contexts, including when discussing light and its interactions with matter. Both units are valid; it's just a matter of convention and context.

Q) So if I have some quantity of energy in eV, how do I convert it to joules?

A) To convert energy from electron Volts (eV) to joules (J), you can use the conversion factor:
 
  1 eV = 1.602 x 10^-19 J  
 
So, if you have a value in electron volts (eg 511'000 eV/c² for the mass of an electron), you can multiply it by 1.602 x 10^-19 J to get the equivalent value in joules (thus 8.2 x 10^-14 J for an electron).

Q) But an electron-volt unit is also used to measure mass !?!

A) Yes, electron volts (eV) can also be used to measure mass, particularly in particle physics where energies and masses are often interrelated. This usage arises from the principle of mass-energy equivalence, famously described by Einstein's equation E=mc².
In this context, the mass of particles (such as electrons, protons, or neutrinos) can be expressed in terms of their energy using the relationship:
 
  E = mc²  
 
where E is the energy, m is the mass, and c is the speed of light in a vacuum. When expressing mass in units of energy (eV), it's typically done using the same conversion factor as before:
 
  1 eV = 1.602 x 10^-19 joules  
 
So, for example, if you know the energy of a particle in eV, you can rearrange the equation to solve for its mass.

Q) So what's the lightest particle in the universe?

A) The lightest known particle in the universe is the neutrino. While neutrinos have tiny masses, they are not precisely zero, but they are incredibly small compared to other particles like electrons.

Q) What's the lifetime of a Neutrino?

A) Neutrinos are incredibly stable particles with lifetimes that are practically infinite. They are known to interact very weakly with matter, which makes them extremely long-lived.

Q) Does gravity have an impact on matter like neutrinos?

A) Yes, gravity does have an impact on matter, including neutrinos. Gravity affects all forms of matter with mass, including particles like neutrinos, although neutrinos have very little mass compared to other particles. However, because neutrinos have such small mass and interact very weakly with matter, their response to gravity is usually negligible in most situations.

Q) But if photons don't have any mass, how come they are affected by gravity?

A) Photons are indeed affected by gravity, even though they themselves are massless. This phenomenon is a consequence of Einstein's theory of general relativity, which describes gravity as the curvature of spacetime caused by mass and energy.


According to general relativity, mass and energy curve the fabric of spacetime, and objects with mass or energy (like photons) follow paths determined by this curvature. So, even though photons have no mass, they still travel along curved paths in the presence of gravitational fields, which is why they are affected by gravity. This effect is commonly observed in phenomena like gravitational lensing, where the path of light is bent by the gravity of massive objects like galaxies.

Q) So what's the difference between general relativity and special relativity?

A) General relativity and special relativity are both theories developed by Albert Einstein to describe the behavior of objects in space and time, but they address different aspects of physics.


⇒ Special relativity, formulated in 1905, deals with the relationship between space and time for objects moving at constant speeds in straight lines (inertial frames). It introduces concepts like time dilation, length contraction, and the equivalence of mass and energy (E=mc²). Special relativity applies to situations where gravity is negligible.


⇒ General relativity, developed later by Einstein in 1915, extends special relativity to include the effects of gravity. It describes gravity as the curvature of spacetime caused by mass and energy. General relativity predicts the bending of light by gravity, the warping of spacetime around massive objects, and phenomena like black holes and gravitational waves.


In summary, special relativity deals with the behavior of objects in inertial frames at constant speeds, while general relativity includes the effects of gravity and describes the curvature of spacetime caused by mass and energy.

Q) So where does the Hubble constants come in to these equations?

A) The Hubble constant, denoted by 𝐻₀, is a fundamental parameter in cosmology that describes the rate at which the universe is expanding at the present time. It relates the velocity of distant galaxies to their distance from us.


While it's not directly part of the equations of special relativity or general relativity, the Hubble constant is crucial for understanding the large-scale structure and evolution of the universe. It plays a significant role in cosmological models and helps us understand the expansion history of the cosmos.

Q) And does this constant appear in any of the equations from Einstein?

A) The Hubble constant itself does not appear explicitly in Einstein's equations of special relativity or general relativity. However, it is indirectly related to these equations through their implications for the behavior of the universe as a whole.


In general relativity, the equations govern the dynamics of spacetime and the distribution of matter and energy within it. The expansion of the universe, described by the Hubble constant, is a consequence of these equations operating on a cosmological scale. So, while the Hubble constant is not directly part of Einstein's equations, it arises from the solutions to those equations when applied to the universe as a whole.

Q) So if it's a consequence of his equations, what did Einstein think of this constant?

A) Einstein initially introduced a cosmological constant (denoted by the Greek letter lambda, Λ) into his equations of general relativity to allow for a static universe. However, when it was later discovered that the universe is expanding, Einstein reportedly referred to the introduction of the cosmological constant as his "greatest blunder."


The cosmological constant can be thought of as a sort of energy density associated with empty space, and it has the effect of producing a repulsive force that counteracts gravity on cosmological scales. In modern cosmology, the cosmological constant is often interpreted as the energy density of the vacuum and is closely related to dark energy, the mysterious force driving the accelerated expansion of the universe.


So, while Einstein initially introduced the cosmological constant to maintain a static universe, he later abandoned it when the expanding universe was observed. However, the concept of a cosmological constant has seen a resurgence in modern cosmology due to its role in explaining the accelerated expansion of the universe.

Q) This cosmological constant, is it related to the expansion of the universe lambda? If so, how?

A) Yes, the cosmological constant (Λ) is indeed related to the expansion of the universe. In the context of general relativity, the cosmological constant represents a constant energy density that fills empty space and exerts a repulsive gravitational force. This force counteracts the attractive force of gravity and leads to an accelerated expansion of the universe.


The cosmological constant is often included as a term in Einstein's field equations of general relativity, alongside the terms representing matter, energy, and curvature of spacetime. When the cosmological constant is nonzero, it contributes to the overall energy density of the universe and affects the dynamics of its expansion.


In modern cosmology, the cosmological constant is often interpreted as a form of dark energy, a mysterious force that permeates the universe and drives its accelerated expansion. Observations of distant supernovae, cosmic microwave background radiation, and large-scale structure all support the presence of dark energy and the accelerated expansion of the universe, which can be effectively modeled using the cosmological constant.

Q) So this dark energy, does it have mass?

A) Dark energy is a mysterious form of energy that is thought to permeate all of space and drive the accelerated expansion of the universe. Unlike matter and radiation, which contribute to the gravitational attraction that slows down the expansion of the universe, dark energy behaves as a repulsive force, causing the expansion to accelerate.


As far as current understanding goes, dark energy does not have mass in the traditional sense. It is characterized by its energy density and pressure, which determine its gravitational effects on the universe. Dark energy is thought to be uniformly distributed throughout space and does not clump together like matter, so it doesn't behave like particles with mass.

Q) So if it exerts pressure, how does it do so when it doesn't have mass?

A) The pressure exerted by dark energy arises from its energy density and its equation of state, which relates its pressure to its energy density. In the context of general relativity, pressure is a fundamental component of the stress-energy tensor, which describes the distribution of energy and momentum in spacetime.


While dark energy itself may not have mass in the traditional sense, it still contributes to the curvature of spacetime and affects the dynamics of the universe through its gravitational effects. The pressure associated with dark energy influences the expansion rate of the universe and plays a crucial role in cosmological models that aim to explain its behavior.

Q) How much of it is there in the universe?

A) The exact amount of dark energy in the universe is not precisely known, but it's believed to constitute a significant portion of the total energy density of the cosmos. Current estimates suggest that dark energy makes up approximately 68% of the total energy density of the universe, with dark matter accounting for about 27%, and ordinary matter (the stuff we can see and interact with) making up the remaining 5%. These percentages are based on observations of the cosmic microwave background radiation, the large-scale distribution of galaxies, and the accelerated expansion of the universe.

Q) When this measurement was made, it was supposed to reflect what time of the cosmological history.

A) When these measurements of the composition of the universe were made, they reflected the state of the universe at the present time, or very close to it. The observations used to determine the proportions of dark energy, dark matter, and ordinary matter are based on data collected from various cosmological surveys and experiments conducted over the past few decades. These observations provide a snapshot of the universe as it exists today, allowing scientists to infer the relative abundances of different components throughout cosmic history.

Q) Do we have a picture of the Cosmos?

A) Yes, scientists have constructed detailed maps and images of the cosmos using data from telescopes and space probes. These images provide a glimpse into the vastness and complexity of the universe on different scales. For example:


⇒ Cosmic Microwave Background (CMB): This is a snapshot of the universe when it was only about 380,000 years old. It's the afterglow of the Big Bang, and it provides valuable information about the early universe's temperature fluctuations, which later seeded the formation of galaxies and other cosmic structures.


⇒ Galaxy Surveys: Telescopes have surveyed millions of galaxies across the observable universe, creating 3D maps that show the distribution of galaxies on large scales. These maps reveal cosmic structures like filaments, clusters, and voids, giving insight into the large-scale structure of the universe.


⇒ Hubble Space Telescope Images: The Hubble Space Telescope has captured stunning images of galaxies, nebulae, and other celestial objects across the visible universe. These images provide detailed views of individual galaxies, star clusters, and supernova remnants, helping scientists study the processes shaping the evolution of galaxies.


⇒ Radio Telescopes: Radio telescopes have mapped the universe's radio emissions, revealing phenomena like active galactic nuclei, pulsars, and the cosmic microwave background.


These and other observations help astronomers understand the universe's history, structure, and evolution, from its earliest moments to the present day.

Q) If the CMB is the earliest picture we have of our universe. What was the cosmos like before then?

A) Before the cosmic microwave background (CMB) radiation was emitted, the universe was in a hot, dense state known as the "hot big bang" phase. During this phase, the universe was filled with a hot, opaque plasma consisting of protons, neutrons, electrons, and photons.


Prior to about 380,000 years after the Big Bang, the universe was so hot and dense that protons and electrons couldn't combine to form neutral atoms. Instead, the universe was filled with a sea of charged particles that interacted strongly with photons, preventing them from traveling freely through space.


As the universe expanded and cooled, it eventually reached a critical temperature and density where protons and electrons could combine to form neutral hydrogen atoms. This event, known as recombination, marked the transition from the hot, opaque plasma to a transparent gas. It's at this point, about 380,000 years after the Big Bang, that the cosmic microwave background radiation was emitted and began freely traveling through space.


So, before the CMB was emitted, the universe was too hot and dense for light to travel freely, making it impossible to directly observe or image. Our understanding of this early phase of the universe comes from theoretical models, observations of the CMB, and experiments conducted in particle accelerators that recreate the conditions of the early universe.

Q) So during this opaque, young universe what was the size of the universe suring that time?

A) During the opaque phase of the early universe, when it was filled with a hot, dense plasma, the entire cosmos was much smaller and more compact compared to its current size. The universe underwent a period of rapid expansion known as cosmic inflation shortly after the Big Bang, which caused it to expand exponentially in size.


The precise size of the universe during the opaque phase is difficult to determine precisely, as it depends on various cosmological parameters and models of inflation. However, it's believed that the universe underwent significant expansion during this period, growing from a tiny, incredibly dense state to a size much larger than what we observe today.


While we can't directly measure the size of the universe during the opaque phase, our understanding of cosmology suggests that it was much smaller and denser than it is now, with all matter and energy concentrated in a much smaller volume.

Source: ChatGPT