Half Life

Half Life is the time required to double in the case of growth and to fall to one half of its initial value in the case of decay.
There is a general tendency, to think about radioactive decay only, when Half life is mentioned. But it is applicable in many fields including social sciences.
Nature is not static, but changing. Change is an inevitable and the Change is only constant factor in the life-cycle of individual, organization or a country.
Our joy, sadness, misery will grow and decay also. Nothing is for ever and things move on.
Our assets, usefule ness of knowledge, product life exhibits growth and decay, governed by its half life.
The half-life of knowledge is decreasing in many fields. One beomes obsolete very fast. Having once undergone training or holding a degree or degrees is not enough. Lifelong learning is essential to competitiveness.

Half-life of a theorey or set of facts, (for knowledge) is a time when half of the knowledge in these fields is going to be overturned.
Languages have half life. Meaning of half the words gets changed over half life period
Social belief systems (religion, Cultural practices and even political systems) have half life. Half the practices get modified or changed to be relevant. Similarly, many becomes irrelevant and issue is how to update contents. We still continue with laws, which are required few centuries ago.
Everything what we Know Has an Expiration Date.
Half of most of the professions in this world would be gone in the next ten years to twenty years time.
The half-life of critical vulnerabilities is imprtant security issue for all industries. For example vulnerability due to cyber threat, remained at 30 days across all industries. Similarly threat due to virus outbreak for a region.

Exponential growth occurs when the growth rate of the value of a mathematical function is proportional to the function's current value. Exponential decay occurs in the same way when the growth rate is negative.
It is also called geometric growth or geometric decay (the function values form a geometric progression).
For example, with a growth rate of r = 5% = 0.05, going from any integer value of time to the next integer causes x at the second time to be 1.05 times (i.e., 5% larger than) what it was at the previous time.
For radioactive decay, we also use an exponential model.However, the rate is now negative to represent decay.
For figures and illustration: http://www.drcruzan.com/MathExponentialFunctions.html

The doubling time of a population exhibiting exponential growth is the time required for a population to double. No matter when you start measuring, the population will always take the same amount of time to double.
Half Life is the time required for the decaying quantity to fall to one half of its initial value. For example, if there were 100 grams of Iodine-131, 8 days ago and now there is only 50 grams, as half life is 8 days.

Traditionally, technologies and training once changed every 20 years. Today the half-life of rapidly advancing technologies may be anywhere between three and five years.
The half-life of one’s Internet/Web knowledge obsoletes every two years.
Half of what we are teaching our students in some fields like computer science is less than 3 years. So whatever they learn becomes obsolete by the time they graduate.
The pace of change in itself will change.
This is applicable to all fields of education. What fields decay the slowest or the fastest matters in developing educational infrastructure and learning/training systems

The number of microorganisms in a culture will increase exponentially until an essential nutrient is exhausted. Typically the first organism splits into two daughter organisms, who then each split to form four, who split to form eight, and so on.
Human population, is exponentially increasinysg at different average rate for different countries.
Genetic complexity of life on Earth doubled every 376 million years (example only)
Exponential decay occurs in a wide variety of situations. Most of these fall into the domain of the natural sciences.
Many decay processes that are often treated as exponential, are really only exponential so long as the sample is large and the law of large numbers holds. For small samples, a more general analysis is necessary, accounting for a Poisson process.
A quantity may decay via two or more different processes simultaneously. In general, these processes are called "decay modes" or "decay routes". The total decay rate of the quantity N is given by the sum of the decay routes.

Economic growth is expressed in percentage terms, implying exponential growth or decay.
Businesses grow or decay exponentially. This should be taken into business strategy, while monitoring any business.
Compound interest at a constant interest rate provides exponential growth of the capital. If interest rate is x%, principla will double in (72/x) years approximately.
Pyramid schemes also show this type of growth resulting in high profits for a few initial investors and losses among great numbers of investors.
A retirement fund will decay exponentially being subject to discrete payout amounts, usually monthly, and an input subject to a continuous interest rate.
Value of your savings will drop with inflation rate. If interest rate does not match inflation rate, it is not worthwhile to save.

When you're one year old, a year is literally forever to you as it is all the time t hat you've ever known. But as you grow older, one year is a smaller and smaller fraction of your total life. It's like watching something shrink in your rear view mirror. It means that parents actually see their children grow up much faster than children perceive themselves to be.
Feeling of joy or sadness have half life. For example, grief is halved every week, when some one close to you dies. This is applicable to memory also.

Rice on a chessboard: According to an old legend, vizier presented an Indian King with a chessboard. The king asked what he would like in return for his gift. vizier asked for one grain of rice on the first square, two grains on the second, four grains on the third etc. The king readily agreed and asked for the rice to be brought. All went well at first, 20th square demanded over a million grains and on the 41st square, more than a million million. There was not enough rice in the whole world for the final squares.
Water lily: French children are told a story in which they imagine having a pond with water lily leaves floating on the surface. The lily population doubles in size every day and if left unchecked will smother the pond in 30 days, killing all the other living things in the water. Day after day the plant seems small and so it is decided to leave it to grow until it half-covers the pond, before cutting it back. They are then asked on what day half-coverage will occur. This is revealed to be the 29th day, and then there will be just one day to save the pond.