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Timeline of the universe

An alien planet could conceivably have formed as early as 1 billion years after the big bang, meaning that there are likely aliens with a head start on us by billions of years.

An alien civilization could easily build a fission or fusion rocket that travels at 1/10 the speed of light, which would take 1 million years to cross the galaxy. The aliens have plenty of time to get here.

                Millions of years ago

Big bang             13700
First planets formed 13000
Earth formed          4500
Photosynthesis        3000
Oxygen atmosphere      600
Multicellular life     600
Vertebrates            480
Tetrapod vertebrates   400       Mammals, birds, and reptiles are all tetrapods
Mammals                170
Dinosaur extinction     66
Cats                    25
Cheetahs                 6       Fastest land animal
Tigers                   1.8
Humans                   1
Lions                     .9
Agriculture               .01
Civilization              .005
Calculus                  .0004
Smartphones               .00001


We presently possess the technology to build a fission and fusion rocket, each of which can reach a speed of .1 times the speed of light, and such a rocket can cross the Milky Way galaxy in a time that is a small fraction of the age of the universe. If aliens had built such a rocket they could easily have already colonized the galaxy.

Speed of light                       =  C
Speed of a fission or fusion rocket  =  V  =  .1 C
Diameter of the Milky Way            =  X           =     .1 million light years
Time to cross the galaxy             =  T  =  X/V   =      1 million years
Age of the universe                                 =  13800 million light years


The divinity hypothesis becomes persuasive if there is a physical mechanism allowing it to happen, a mechanism that obeys the known laws of physics. Such a mechanism exists. An advanced alien civilization is equivalent to a diety.


The human brain contains 10⋅11 neurons and 10⋅14 synapses. It would be an easy thing for an advanced civilization to scan the neural state of a brain and reproduce it on a computer. Such a scenario is equivalent to an afterlife.

Prime directive

Kirk's interpretation of the Prime Directive is often overly-liberal. You could say that he considers it to be the "Prime Suggestion". If aliens exist it's reasonable to suppose that they may have something like a Prime Directive because as yet, no evidence for them has been found.

Doomsday machine

One could imagine the aliens creating a "doomsday machine" with orders to replicate itself and travel to every star in the galaxy, and upon arriving, exterminate all life present. If this had happened then we wouldn't be here right now. The non-existence of a doomsday machine is one of the few solid assertions that we can make about the aliens.

The "Matrix" hypothesis

The matrix hypthesis is that we live in a computer simulation. It's not unreasonable because humans like to create their own computer simulations.


This Astrobiology textbook provides a discussion of what we can expect for alien physiology and biochemistry. Alien life is necessarily carbon based and intelligent life likely requires an oxygen atmosphere.

One of the goals of physics is to discover new laws of physics, and one of the techniques used is to look for extensions of the existing laws of physics, extensions that are mathematically consistent with the existing laws. These kinds of hypotheses are useful for telling experimenters where to look. In the following sections we give a few examples.


The discovery of the battery paved the way for the discovery of electromagnetism.

1800       Volta constructs the first "battery" by connecting multiple electrochemical
           cells in parallel, increasing the output power and voltage.
           In 1836 Daniell refined the battery, making it suitable for industrial use
1820       Oersted finds that an electric current produces a magnetic field
1826       Ampere finds that electric currents attract each other
1831       Faraday finds that a changing magnetic field produces an electric field
1861       Maxwell finds that a changing electric field produces a magnetic field
1862       Maxwell unifies the previous discoveries about currents and magnetic
           fields with "Maxwell's equations"
1864       Maxwell finds that light is an electromagnetic wave
           This theory predicts a paradox, that the speed of light is invariant
At Maxwell's time, it was known that a changing magnetic field produces an electric field. Maxwell hypothesized that a changing electric field produces a magnetic field, a hypothesis that was consistent with the earlier laws but had not yet been tested. Maxwell tested it and found it to be true.
Special relativity

In Newton's time, Newtonian mechanics correctly predicted the outcome of experiments. It fails at high speeds, speeds approaching the speed of light, but there was no way for anyone to know this because there were no known high-speed objects at the time. If data for high-speed objects had been available they would have noticed the discrepancy and they would have discovered special relativity.

Suppose someone had postulated special relativity in Newton's time. The laws of special relativity reduce to Newtonian mechanics at low speeds and so it's consistent with Newtonian mechanics. Since it is consistent, such a hypothesis is worthy of consideration.

Subsequent to Newton's time, the discovery of electromagnetism gave credence to the special relativistic hypothesis. Electromagnetism is inconsistent in Newtonian mechanics and is consistent in special relativity. Subsequent experiments on high-speed objects confirmed special relativity.

General relativity

Einstein sought an extension of special relativity that is both consistent with special relativity and also includes gravity, and with this motivation he postulated the equations of general relativity. Subsequent experiments on gravity proved to be consistent with general relativity.

String theory

The current laws of physics consist of the "Standard Model", which describes particle interactions, and "general relativity", which covers gravity, but there are regimes where they are known to be inconsistent. String theory is an attempt to construct new laws of physics that are consistent with both the Standard Model and general relativity.

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