Light sails, aka Solar sails is method to propel a spacecraft using light. The light from the Sun exerts a radiation pressure on these sails that accelerates the spacecraft. However, the magnitude of force is quite low. A sail of dimensions 800m x 800m will experience a force of about 5 newtons at Earth’s distance from the Sun.
What exactly happens is that only a fraction of the photons that hit a light sail is reflected. Some of it are absorbed. And this causes a transfer in momentum from the photon to the light sail. For the momentum to be of any considerable value, there has to be a lot of photons and the light sails have to be as light as it can be.
Omuamua is the first interstellar object detected as a passing object through our solar system. Interstellar, meaning that it originated outside our solar system. These objects are hard to detect before-hand that we can be ready to send a spacecraft behind it. In addition to that, the speed of these objects are quite high to keep up with conventional propulsion methods. Light-sails could be a good candidate for probes that can be used to patrol the boundaries of our system for such objects in the future.
Temperature inversion is a meteorological phenomenon where a deviation occurs from normal change of an atmospheric property with altitude. It usually refers to the inversion of the thermal lapse rate. The lapse rate is the rate at which temperature falls with altitude. The temperature drops as you gain altitude due to the lowering of pressure, following the ideal gas law. An inversion would mean that the temperature would be higher at a higher altitude than it is in a lower altitude. This traps air pollution like smog closer to the ground. Inversion can occur when a warm less-dense air mass moves over a more dense cold air mass. This in turn can cut off any convection in the region stopping the air form circulating. As a consequence of inversion, it is seen that radio waves can be refracted making it possible to receive radio signals from long distances.
Radioactive molecules lose energy by radiation from its nuclei. This is known as radioactive decay and is a stochastic process at an atomic level. It is impossible to predict when an atom will decay but for a significant number of identical atoms it is possible to predict the average amount of decay that would occur. The overall decay rate is known as the half life. In other words, half life is the time taken for something to halve its quantity. An element can have molecules with different number of neutrons. These are known as isotopes and different isotopes have its own half life. Carbon-14 has a half life of 5730 years. It is commonly used to date organic material.
This concept so far is pertaining to physics. But can be applied to ideas as well. Ideas and knowledge have a half life as well. What we learn today might not be true in some years. When a field is in its growing stage, the half life is quiet small. Indicating new theories are being found that disproves existing ones. In the past it used to take longer to disprove something and change the commonly prevailing notion. Now with a low cost to spreading and sharing ideas, the half life has reduced even more. Secrets also have a half life. It is becoming harder and harder to keep secrets with the exposure to a global medium like the internet around.