Interaction of Nuclear Radiations with Matter

May 22nd, 2010 No Comments

α, β & γ rays are called nuclear radiations

Interaction of nuclear radiations with matter depends on three characteristics of nuclear radiations.

  1. Mass of particles
  2. Charge
  3. Energy

Interaction of α-Rays

  • α-particle can do ionization in the following two ways
  1. Mechanical collision ( α-particle directly hits electron)
  2. Coulomb’s interaction ( electrostatic interaction )]
  • Mode of ionization by Coulomb’s interaction for α-rays dominates over that by direct collision.
  • The path of ionization followed by α-rays is straight and continuous because of its high ionization power and large mass.
  • During ionization, α-particle continuously looses its energy as a result of which its velocity decreases.
  • In each collision, α-particle loses an average of 35eV energy.
  • 7.7 MeV α-particle produces 2 x 105 ion pairs before stopping.
  • When α-particle has spent all its energy on ionization, it absorbs two electrons from its surroundings gas and becomes a neutral (He atom)
  • Range of α-particle in air is small due to intense ionization.
  • 7.7 Mev α-particle has 7cm range in air at S.T.P, which reduces further in denser medium.
  • Range of 7.7 MeV α-particle in aluminum is only 0.04 mm
  • α-particle produces disintegration in nuclei of some atoms if they have high energy.

Interaction of β-Rays

  • β-particles are fast electrons or positrons coming from nucleus.
  • Range of β-particle is larger than that of α-particle by a factor of 100.
  • Ionization of β-particle is smaller than that of α.
  • Mass of β-particle is equal to that of an electron.
  • Charge of β-particle is equal to 1.6 x 1019 C that may be positive for positron [β+] and negative for electron [β-].
  • β-particle does ionization due to electrostatic repulsion ( in case of β-) and attraction ( in case of β+).
  • Ionization path of β is broken and zigzag due to its smaller mass.
  • Ionization by head-on-collision is very rare.
  • Β-particle looses almost all its energy in a single encounter.
  • Because of lesser ionization encounters, penetration of β is 100 times larger than that of α-particle of same energy.
  • 3 MeV β-particles can pass through 6.5 mm aluminum foil.
  • β-particle can produce fluorescence.

Interaction of γ-Rays

  • γ-rays being photons can’t be stopped by matter ( lead can be used as a shield because of its high electron density)
  • γ-rays have shorter wavelength than X-rays.
  • γ-rays loose their energy by following three ways
  1. Photoelectric effect
  2. Compton effect
  3. pair production
  • The type of interaction depends upon energy range of photon available according to following scheme:
Energy Range Type of Interaction
E < 0.1 MeV Photoelectric effect
E = 0.1 MeV to 1 MeV Compton effect
E > 1.02 MeV Pair production

Interaction of Neutrons

  • Neutrons are more effective radiation than both α and β rays because they bear no charge as regard to penetration.
  • When neutron is captured by a nucleus, it results in the formation of a radioisotope.
  • Neutron causes fission in heavy nuclei.
  • Neutron can knock down electrons out of body cells causing instantaneous death.

interaction-of-radiations

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The Flow of Blood in the Human Body

May 20th, 2010 1 Comment

Human blood consists of plasma, the fluid, and red and white corpuscles are immersed in the plasma. Because blood is a fluid, the laws of physics can be applied to the flow of blood throughout the body. A schematic diagram of the circulatory system, which transport blood and oxygen around the body , is shown in figure.

Diagram

It consists of

  1. Heart, which is the pump that is responsible for supplying the pressure to move the blood.
  2. Lungs, which are the source of oxygen for all the cells of the body.
  3. Arteries, which are connecting blood vessels that pass the blood from the heart to various parts of the body.
  4. Capillaries, which are extremely small blood vessels that bring the oxygenated blood down to the layer of human cells.
  5. Veins, which are blood vessels that return deoxygenated blood to the heart to complete the circulatory system.

The heart is the pump that circulates the blood throughout the body and a diagram of it is shown in figure. Blood, containing carbon dioxide, returns to the heart by the veins and enters the right auricle. It is then pumped from the right ventricle to the pulmonary artery to the lungs where it dumps the waste carbon dioxide and picks up a new supply of oxygen. It then returns to the left auricle of the heart. The left ventricle then pumps this oxygen rich blood to the aorta, the main artery of the body, for distribution to the rest of the body.

For a person at rest, the heart pumps approximately 5.00 liters of blood per minute (8.33 x 10-5 m3/s) at a rate of about 70 beats per minute. For a person engaged in very strenuous exercise the heart can pump up to 25.0 liters of blood per minute (41.7 x 10-5 m3/s) at a rate of about 180 beats per minute. We can determine the speed of the blood as it enters the aorta by an eq.

ΔV/Δt  = AAVA

Where ΔV/Δt is the rate at which the blood is flowing from the heart into the aorta, AA is the cross-sectional area of aorta, and VA is the speed of the blood in the aorta. The diameter of the aorta is about 2.00 cm.

So, Area of cross-section of aorta is

Diameter   = 2.00 cm

Radius = r = 1.00 cm  = 0.01 m

A  =      πr2

A  =   3.14 x ( 0.01 m )2 = 3.14 x 10-4 m2

Hence speed of blood in aorta is

VA =       (ΔV/Δt )/AA

= (8.33 x 10-5 m3/s ) / (3.14 x 10-4 m2 )

= 0.265 m/s

= 26.5 cm/s

We can determine the speed of the blood in the capillaries by the continuity eq.

AAVA         = AbVb

Where AA is the cross-sectional area of the aorta, which was just determined as

3.14 x 10-4 m2 ; VA is the speed of the blood in the aorta, which was just found to be 26.5 cm/s ; and Ab is the cross-sectional area of a capillary tube, which is quite small.

However, because there are literally billions of these capillaries the effective cross-sectional area of all these capillaries combined approximately 2500 x 10-4 m2. The speed of blood in the capillary becomes

Vb =   (AA /Ab) VA

= (3.14 x 10-4 m2 / 2500 x 10-4 m2 ) ( 26.5 cm/s)

Thus, the blood moves relatively slowly at the level of the capillaries.

Finally, we should note that the body controls the flow of blood through the arteries by muscles that surround the arteries. When the muscles contract, the diameter of the artery is reduce. From the equation of continuity, Av = constant. By decreasing the diameter of the artery, the cross-sectional area of the artery decrease and hence the speed of blood must increase through the artery. Alternatively, when the muscles are relaxed, the diameter of the artery increases to its former size, the cross-sectional area increases, and the speed of the blood decreases. With advancing age the arterial muscles lose some of this ability to contract, a situation called hardening of the arteries, and the control of blood flow is somewhat diminished.

A good indication of how well the heart is functioning is obtained by measuring the pressure that heart exerts when pumping blood and when at rest. The device used to measure blood pressure is called sphygmomanometer.

The device consists of an air bag, called a cuff that is wrapped around the upper arm of the patient at the level of the heart. A hand pump is used to inflate the cuff, and the pressure exerted by the cuff on the arm is measured by the mercury manometer. The pressure exerted by the cuff is increased until the pressure is great enough to collapse the brachial artery in the arm, cutting off the blood supply to the rest of the arm. A stethoscope is placed over the brachial artery and the pressure in the cuff is slowly decreased. When the pressure in the cuff becomes low enough, the pressure exerted by heart is large enough to force the artery open and some blood squirts through. This blood flowing through the narrow restriction becomes turbulent and makes a noise as it enters the open portion of the artery. The physician hears this noise through the stethoscope, and simultaneously observes the pressure indicated on the manometer, expressed in terms of mm of Hg. At this point the pressure exerted by the heart, called the systolic pressure, is equal to the pressure exerted by the cuff. A normal systolic pressure is around 120 mm of Hg.

As the pressure in the cuff is decreased the turbulent flow noise is still heard in the stethoscope until the lowest pressure exerted by the heart, the diastolic pressure, is equal to the pressure exerted by the cuff. At this point the artery is completely open and the blood is no longer in turbulent flow and the characteristic noise disappears. The pressure is read from the mercury manometer at this point. This pressure is the pressure that the heart exerts when it is at rest. The combined systolic and diastolic pressures are usually indicated in form 120/80. If the systolic pressure becomes too high, above about 150 mm of Hg, the patient has high blood pressure. If the systolic pressure becomes too large fro a long period of time, damage can be done to the different organs of the body. If the systolic pressure becomes extremely large, arteries in the brain can rupture and the person will have a stroke. If the diastolic pressure exceeds 90 mm of Hg, the person also said to have high blood pressure. This type of high blood pressure causes eventual damage to the heart itself because it is operating under high pressures even while it is supposed to be resting.

Frictional effect between different layers of a fluid is known as viscosity. A fluid in which frictional effects are significant is called a viscous fluid and the fluid flow is referred to as laminar flow, flow in layers. Flow rate is inversely proportional to the coefficient of viscosity of the fluid. Thus, a very viscous fluid flows very slowly compared to a fluid of low viscosity. That is, everything else being equal, molasses flows at a slower rate than water. Human blood is a viscous fluid, the greater the number of red corpuscles in the blood the greater the viscosity. The viscosity of human blood varies from about (1.50 x 10-3 N-s/m2) for plasma, to about 4.00 x 10-3 N-s/m2) for whole blood.

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Transfer of Thermal energy by Convection on the surface of Earth

May 14th, 2010 1 Comment

Convection is the main mechanism of thermal energy transfer in the atmosphere. On a global basis, the non-uniform temperature distribution on the surface of the earth causes convection cycles that result in the prevailing winds. If the earth were not rotating, a huge convection cell would be established. The equator is the hottest portion of the earth because it gets the maximum radiation from the sun. Hot air at the equator expands and rises into the atmosphere. Cooler air at the surface flows toward the equator to replace the rising air. Colder air at the poles travels toward the equator. Air aloft over the poles descends to replace the air at the surface that just moved toward the equator. The initial rising air at the equator flows towards the pole, completing the convection cycle. The net result of the cycle is to bring hot air at the surface of the equator, aloft, then north to the poles, returning cold air at the polar surface back to the equator.

The simplest picture of convection on the surface of the earth is not quite correct, because the effect produced by the rotating earth, called the Coriolis effect, has been neglected. The Coriolis effect is caused by the rotation of the earth and can best be described by an example.

If a projectile, aimed at New York, were fired from the North Pole, its path through space would be in a fixed vertical plane that has the North Pole as the starting point of the trajectory and New York as the ending point at the moment that the projectile is fired. However, be the time that the projectile arrived at the end point of its trajectory, New York would no longer be there, because while the projectile was in motion, the earth was rotating, and New York will have rotated away from the initial position it was in when the projectile was fired. A person fixed to the rotating earth would see the projectile veer away to the right of its initial path, and would assume that a force was acting on the projectile toward the right of its trajectory. This fictitious force is called the Coriolis force and this seemingly strange behavior occurs because the rotating earth is not an inertial coordinate system.

The Coriolis effect can be applied to the global circulation of air in the atmosphere, causing winds in the northern hemisphere to be deflected to the right of their original path. The global convection cycle described above still occurs, but instead of one huge convection cell, there are three smaller ones. The winds from the North Pole flowing south at the surface of the earth are deflected to the right of their path and become the polar easterlies. As the air aloft at the equator flows north it is deflected to the right of its path and eventually flows in a easterly direction at approximately 30o north latitude. The piling up of air at this latitude causes the air aloft to sink to the surface where it emerges from a semi permanent high-pressure area called subtropical high. The air at the surface that flows north from this high-pressure area is deflected to the right of its path producing the mid-latitude westerlies. The air at the surface that flows south from this high-pressure area is also deflected to the right of its path and produces the northeast trade winds. Thus, it is the non uniform temperature distribution on the surface of the earth that is responsible fro the global winds.

Transfer of thermal energy by convection is also very important in the process called the sea breeze. Water has a higher specific heat than land and fro the same radiation from the sun, the temperature of the water does not rise as high as the temperature of the land. Therefore, the land mass becomes hotter than the neighboring water. The hot air over the land rises and a cool breeze blows off the ocean to replace the rising hot air. Air aloft descends to replace this cooler air and complete the cycle. The net result of the process is to replace hot air over the land surface by cool air from the sea. This is one of the reasons why so many people flock to the ocean beaches during the hot summer months. The process reverses at night when the land cools faster than the water. The air then flows from the land to the sea and this is called land breeze.

This same process of thermal energy transfer takes place on a smaller scale in any room in your home or office. Let us assume there is a radiator situated at one wall of the room. The air in contact with the heater is warmed, and then rises. Cooler air moves in to replace the rising air and a convection cycle is started. The net result of the cycle is to transfer thermal energy from the heater to the rest of the room. All these cases are examples of what is called natural convection.

To help the transfer of thermal energy by convection, fans can be used to blow the hot air into the room. Such a hot air heating system is called a forced convention system. A metal plate is heated to a high temperature in the furnace. A fan blows air over the hot metal plate, then through some ducts, to a low-level vent in the room to be heated.  The hot air emerges from the vent and rises into the room. A cold air return duct is located near the floor on the other side of the room, returning cool air to the furnace to start the convection cycle over again. The final result of the process is the transfer of thermal energy from the hot furnace to the cool room.

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Humidity and the Cooling of the Human Body

May 13th, 2010 No Comments

Have you ever wondered why you feel so uncomfortable on those dog days of August when the weatherman says that it is very hot and humid? what has humidity got to do with your being comfortable? What is humidity in the first place?

To understand the concept of humidity, we must first understand the concept of evaporation. consider the two bowls. Both are filled with water. bowl 1 is open to the environment, whereas a glass plate is placed over bowl 2. If we leave the two bowls overnight, on returning the next day we would find bowl 1 empty while bowl 2 would still be filled with water. What happened to the water in bowl 1? The water in bowl 1 has evaporated into the air and is gone.

Evaporation is a process by which water goes from the liquid state to the gaseous state at any temperature.

Boiling, as you recall, is the process by which water goes from the liquid state to the gaseous state at the boiling point of 100 Celsius. that is, it is possible for liquid water to go to the gaseous state at any temperature.

As latent heat of vaporization for boiling water is Lv = 540 kcal / Kg  and water at 0oC is Lv = 600 kcal/ kg. The latent heat at any in-between temperature can be found by interpolation. Thus, in order to evaporate 1 kg of water into the air at 0oC, One would have to supply 600 kcal of thermal energy to the water.

The molecules in the water in bowl1 are moving about in a random order. But their attractive molecular forces still keep them together. These molecules can now absorb heat from the surroundings. This absorbed energy shows up as an increase in the kinetic energy of the molecules, and hence an increase in the velocity of the molecule. When the liquid molecule has absorbed enough energy it moves right out of the liquid water into the air above as a molecule of water vapor. (Water molecule is same either it is solid, liquid or gas)

Since the most energetic of the water molecules escape from the liquid, the molecules left behind have lower energy, hence the temperature of the remaining liquid decreases. Hence, evaporation is a cooling process. The water molecule that evaporated took the thermal energy with it, and the water left behind is just that much cooler.

The remaining water in bow 1 now absorbs energy from the environment, thereby increasing the temperature of the water in the bowl. This increased thermal energy is used by more liquid water molecules to escape into the air as more water vapor. The process continues until all the water in bowl 1 is evaporated.

Now when we look at bowl 2, the water is still there. Why didn’t all that water evaporate into the air? To explain this we do the following experiment.

We place water in a container and place a plate over the water. Then we allow dry air, air that does not contain water vapor, to fill the top portion of the closed container. Using a thermometer, we measure the temperature of the air as t = 20oC, and using a pressure gauge we measure the pressure of the air po, in the container. Now we remove the plate separating the dry air from the water by sliding it out of the closed container. As time goes by, we observe that the pressure recorded by the pressure gauge increases. This occurs because some of the liquid water molecules evaporate into the air as water vapor. Water vapor is a gas like any other gas and it exerts a pressure. It is this water vapor pressure that is being recorded as the increased pressure on the gauge. The gauge is reading the air pressure of the dry air plus the actual water vapor pressure of the gas, po + pwp. Subtracting po from po + pwp, gives the actual water vapor pressure, pwp. As time goes on, the water vapor pressure increases as more and more water molecules evaporate into the air. However, after a while, the pressure indicated by the gauge becomes a constant. At this point the air contains the maximum amount of water vapor that it can hold at that temperature. As new molecules evaporate into the air, some of the water vapor molecules condense back into the liquid. An equilibrium condition is established, whereby just as many water vapor molecules are condensed as liquid water molecules are evaporating. At this point, the air is said to be saturated. That is, the air contains the maximum amount of water vapor that it can hold at that temperature. The vapor pressure read by the gauge is now called the saturation vapor pressure pwp.

The amount of water vapor in the air is called humidity.A measure of the amount of water vapor in the air is given by the relative humidity, RH, and is defined as:

“The ratio of the amount of water vapor actually present in the air to the amount of water vapor that the air can hold at a given temperature and pressure, times 100%”

The amount of water vapor in the air is directly proportional to the water vapor pressure. Therefore, we can determine the relative humidity, RH, of the air as

RH =  [(Actual vapor pressure)  /  (Saturation vapor pressure) ]x 100%

When the air is saturated, the actual vapor pressure recorded by the gauge is equal to the saturation vapor pressure and hence, the relative humidity is 100%.

If the air in the container is heated, we notice that the pressure indicated by the pressure gauge increases. Part of the increased pressure is caused by the increase of the pressure of the air. This increase can be calculated by the ideal gas equation and subtracted from the gauge reading, so that we can determine any increase in pressure that would come from an increasing the air temperature to 25oC, the water vapor pressure also increases. After a while, however, the water vapor pressure again becomes a constant. The air is again saturated. We see from this experiment that the ‘maximum amount of water vapor that the air can hold is a function of temperature’. At low temperatures the air can hold only a little water vapor, while at high temperatures the air can hold much more water vapor.

We can now see why the water in bowl 2 did not disappear. Water evaporated from the liquid into the air above, increasing the relative humidity of the air. However, once the air became saturated, the relative humidity was equal to 100%. And no more water vapor could evaporate into it. This is why you can still see the water in bowl 2, there is no place for it to go.

Because of the temperature dependence of water vapor in the air, when the temperature of the air is increased, the capacity of the air to hold water increases. Therefore, if no additional water is added to the air, the relative humidity will decrease because the capacity of the air to hold water vapor has increased. Conversely, when the air temperature is decreased, its capacity to hold water vapor decreases, and therefore the relative humidity of the air increases. This temperature dependence causes a decrease in the relative humidity during the day light hours, and an increase in the relative humidity during the night time hours, with the maximum relative humidity occurring in the early morning hours just before sunrise.

The amount of evaporation depends on the following factors:

  1. The vapor pressure. Whenever the actual vapor pressure is less than the maximum vapor pressure allowable at that temperature, the saturation vapor pressure, then evaporation will readily occurs. Greater evaporation occurs whenever the air is dry, that is, at low relative humidities. Less evaporation occurs when the air is moist, that is, at high relative humidities.
  2. Wind movement and turbulence. Air movement and turbulence replaces air near the water surface with less moist air and increases the rate of evaporation.

Cooling of human body

Now by keeping in mind the concept relative humidity we can understand how the body cools itself. Through the process of perspiration, the body secrets microscopic droplets of water onto the surface of the skin of the body. As these tiny droplets of water evaporate into the air, they cool the body. As long as the relative humidity of the air is low, evaporation occurs readily, and the body cools itself. However whenever the relative humidity becomes high, it is more difficult for the microscopic droplets of water to evaporate into the air. The body can not cool itself, and the person feels very uncomfortable.

We are all aware of the discomfort caused by the hot and humid days of June to August. The high relative humidity prevents the normal evaporation and cooling of the body. As some evaporation occurs from the body, the air next to the skin becomes saturated, and no further cooling can occur. If a fan is used, we feel more comfortable because the fan blows the saturated air next to our skin away and replaces it with air that is slightly less saturated. Hence, the evaporation process can continue while the fan is in operation and the body cools itself. Another way to cool the human body in the summer is to use an air conditioner. The air conditioner not only cools the air to a lower temperature, but it also removes a great deal of water vapor from the air, thereby decreasing the relative humidity of the air and permitting the normal evaporation of moisture from the skin.

In the hot summertime, people enjoy swimming as a cooling experience. Not only the immersion of the body in the cool water is so satisfying, but when the person comes out of the water, evaporation of the sea or pool water from the person adds to the cooling. It is also customary to wear loose clothing the summertime. The reason for this is to facilitate the flow of air over the body and hence assist in the evaporation process. Tight fitting clothing prevents this evaporation process and the person feels hotter.

What many people do not realize is that you can also feel quite uncomfortable even the wintertime, because of the humidity of the air. If the relative humidity is very low in your home then evaporation occurs very rapidly, cooling the body perhaps more than desirable. As an example, the air temperature might be 70 oF but if the relative humidity is low, say 30%, then evaporation readily occurs from the skin of the body, and the person feels cold even though the air temperature is 70 oF. In this case the person can feel more comfortable if he or she uses a humidifier. A humidifier is a device that adds water vapor the air. By increasing the water vapor in the air and hence increasing the relative humidity, the rate of evaporation from the body decreases. The person no longer feels cold at 70 oF, but feels quite comfortable. If too much water vapor is added to the air, increasing the relative humidity to near a 100%, then evaporation from the body is hampered, the body is not able to cool itself, and the person feels too hot even though the temperature is only 70 oF. Thus too high or too low a relative humidity makes the human body uncomfortable.

Medical point of view

We should also note that the evaporation is also used to cool the human body for medical purposes. If a person is running a high fever, then an alcohol rub down helps cool the body down to normal temperature. The principle of evaporation as a cooling device is the same, only alcohol is very volatile and evaporates very rapidly. This is because the saturation vapor pressure of alcohol at 20 oC is much higher than the saturation vapor pressure of water. At 20 oC, water has a saturation vapor pressure of 17.4 mm of Hg, whereas ethyl alcohol has a saturation vapor pressure of 44 mm of Hg. The larger the saturation vapor pressure of a liquid, the greater is the amount of its vapor that the air can hold and hence the greater is the rate of evaporation. Because the alcohol evaporates much more rapidly than water, much greater cooling occurs than when water evaporates. Ethyl ether and ethyl chloride have saturation vapor pressures of 442 mm and 988 mm of Hg, respectively. Ethyl chloride with its very high saturation vapor pressure, evaporate so rapidly that it freezes the skin, and is often used as a local anesthetic for minor surgery.

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Bing Bang and the Creation of the Universe

May 8th, 2010 No Comments

Have you ever wondered how the world was created? In every civilization throughout time and throughout the world, there has always been an account of the creation of the world. Such discussions have always belonged to religion and philosophy. It might seen strange that astronomers, astrophysicists, physicists have now become involved in the discussion of the creation of the universe. Of course, if we think about it, it is not strange at all. Since physics is a study of the entire physical world; it is only natural that physics should try to say something about the world’s birth.

The story starts in 1923 when the American astronomer, Edwin Hubble, using the Doppler effect ( Apparent change in the frequency of waves by the motion of source or observer) for light, observed that all the galactic clusters, outside our own, in the sky were receding away from the earth. When we studied the Doppler effect we see that e.g when a train recedes from us frequency of its sound decreases ( we hear less sound) . A decreases in the frequency means that there is an increase in the wavelength. Similarly, a Doppler effect for light can be derived. In case of light waves the effect is same. That is, a receding source that emits light at a frequency f, is observed by the stationary observer to have a frequency f ‘, where f < f ‘. Thus, since the frequency decrease, the wavelength increase. Because long waves are associated with the red end of the visible spectrum (ROYGBIV), all the observed wavelengths are shifted toward the red end of the spectrum. The effect is called the cosmological red shift. Hubble found that the light from the distant galaxies were all red shifted indicating that the distant galaxies were receding from us.

It can, therefore, be concluded that if all the galaxies are receding from us, the universe itself must be expanding. Hubble was able to determine the rate at which the universe is expanding. If the universe is expanding now, then in some time in the past it must have been closer together. If we look far enough back in time, we should be able to find when the expansion began.

(Imagine taking a movie picture of an explosion showing all the fragments flying out from the position of the explosion. If the movie is run backward, all the fragments would be seen moving backward toward the source of the explosion.)

The best estimate f or the creation of the universe , is that the universe began as a great bundle of energy that exploded outward about 15 billion years ago. this great explosion has been called the Big Bang. It was not an explosion of matter into an already existing space and time, rather it was the very creation of space and time, or spacetime, and matter themselves.

As the universe expanded from this explosion, all objects became farther and farther apart. A good analogy to the expansion of spacetime is the expansion of a toy ballon. A rectangular coordinate system can be drawn on an unstretched balloon, locating three arbitrary points, A, b and C. The bloon is then blown up. As the balloon expands the distance between points A and B, A and C, and B and C increases. So no matter where you were on the surface of the balloon you would find all other points moving away from you. This is similar to the distant galaxies moving away from the earth.

If everything in the universe is spread out and expanding, the early stages of the universe must have been very compressed. To get all these masses of stars of the present universe back into a small compressed state, that compressed state must have been a state of tremendous energy and exceedingly high density and temperature. Matter and energy would be transforming back forth through Einstein’s mass-energy formula, E=mc2. Work done by particle physicists at very high energies allows us to speculate what the universe must have looked like at these very high energies at the beginning of the universe.

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