In 1950, a learned lunchtime conversation set the stage for decades of astronomical exploration. Physicist Enrico Fermi submitted to his colleagues around the table a couple contentions, summarized as 1) The galaxy is very old and very large, with hundreds of billions of stars and likely even more habitable planets. 2) That means there should be more than enough time for advanced civilizations to develop and flourish across the galaxy.
So where the heck are they?
This simple, yet powerful argument became known as the Fermi Paradox, and it still boggles many sage minds today. Aliens should be common, yet there is no convincing evidence that they exist.
Here are twelve possible reasons why this is so.
1. There aren’t any aliens to find. As unlikely as it seems in a galaxy with hundreds of billions of stars and as many as 40 billion Earth-size planets in habitable zones, we could be alone.
2. There is no intelligent life besides us. (This assumes, of course, that humans count as intelligent.) Life may exist, but it could simply take the form of miniscule microbes or other cosmically “quiet” animals.
The Schwarzschild solution describes spacetime in the vicinity of a non-rotating massive spherically-symmetric object. Of the solutions to the Einstein field equations, it is considered by some to be one of the simplest and most useful. As a result of this, some textbooks omit the rigorous derivation of this metric, provided below.
Assumptions and notation
Working in a coordinate chart with coordinates labelled 1 to 4 respectively, we begin with the metric in its most general form (10 independent components, each of which is a smooth function of 4 variables). The solution is assumed to be spherically symmetric, static and vacuum. For the purposes of this article, these assumptions may be stated as follows
- A spherically symmetric spacetime is one that is invariant under rotations and taking the mirror image.
- A static spacetime is one in which all metric components are independent of the time coordinate (so that) and the geometry of the spacetime is unchanged under a time-reversal .
- A vacuum solution is one that satisfies the equation . From the Einstein field equations (with zero cosmological constant), this implies that since contracting yields .
- Metric signature used here is (+,+,+,−).
Diagonalising the metric
The first simplification to be made is to diagonalise the metric. Under the coordinate transformation, , all metric components should remain the same. The metric components ) change under this transformation as:
But, as we expect (metric components remain the same), this means that:
Similarly, the coordinate transformations and respectively give:
Putting all these together gives:
and hence the metric must be of the form:
where the four metric components are independent of the time coordinate (by the static assumption).
Asteroid impact avoidance comprises a number of methods by which near-Earth objects (NEO) could be diverted, preventing destructive impact events. A sufficiently large impact by an asteroid or other NEOs would cause, depending on its impact location, massive tsunamis, multiple firestorms and an impact winter caused by the sunlight-blocking effect of placing large quantities of pulverized rock dust, and other debris, into the stratosphere.
A collision between the Earth and an approximately 10-kilometre-wide object 66 million years ago is thought to have produced the Chicxulub Crater and the Cretaceous–Paleogene extinction event, widely held responsible for the extinction of the dinosaurs.
While the chances of a major collision are not great in the near term, there is a high probability that one will happen eventually unless defensive actions are taken. Recent astronomical events—such as the Shoemaker-Levy 9 impacts on Jupiter and the 2013 Chelyabinsk meteor along with the growing number of objects on the Sentry Risk Table—have drawn renewed attention to such threats. NASA warns that the earth is unprepared.
Most deflection efforts for a large object require from a year to decades of warning, allowing time to prepare and carry out a collision avoidance project, as no known planetary defense hardware has yet been developed. It has been estimated that a velocity change of just 3.5/t × 10−2 m·s−1 (where t is the number of years until potential impact) is needed to successfully deflect a body on a direct collision trajectory. In addition, under certain circumstances, much smaller velocity changes are needed. For example, it was estimated there was a high chance of 99942 Apophis swinging by Earth in 2029 with a 10−4 probability of passing through a ‘keyhole’ and returning on an impact trajectory in 2035 or 2036. It was then determined that a deflection from this potential return trajectory, several years before the swing-by, could be achieved with a velocity change on the order of 10−6 ms−1.
An impact by a 10 kilometres (6.2 mi) asteroid on the Earth has historically caused an extinction-level event due to catastrophic damage to the biosphere. There is also the threat from comets coming into the inner Solar System. The impact speed of a long-period comet would likely be several times greater than that of a near-Earth asteroid, making its impact much more destructive; in addition, the warning time is unlikely to be more than a few months. Impacts from objects as small as 50 metres (160 ft) in diameter, which are far more common, are historically extremely destructive regionally.