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PostPosted: Thu Feb 21, 2013 12:23 am 
Broken Crown Panelist
Broken Crown Panelist

Joined: Wed Oct 17, 2012 1:47 pm
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The Hazards of Black Holes

The longest human spaceflight has been to the moon and back- about half a million miles. The distance from Earth to Titan ranges from about 770 million miles to close to a billion miles. Being able to convey humans from Earth will take a big leap forward, both in keeping humans alive during a long flight and actually having the technology to survive that journey.

Humans have been able to travel in space for more than 50 years at this point, but doing so remains a perilous undertaking. Most of the dangers are understood to a large extent: launching an object filled with highly explosive fuel (as seen with the Challenger), travelling at high speeds through the atmosphere (as in the Columbia disaster), exposure to space debris flying at well over 10,000 miles per hour, and being faced with high levels of radiation from the sun and cosmic rays outside of the safety of the Earth’s environment. While encountering black holes are not expected to present a practical problem that NASA must address in the near future, this blog post explores, somewhat whimsically, what hazards a space traveler might face from a black hole.


Black Holes

Black holes are extremely dense objects which pack a great deal of matter into very small spaces. For example, to turn the Earth into a black hole, you’d have to compress it from a sphere whose radius is about 4000 miles into a black hole and shrink it down to a sphere with a radius of about a tenth of an inch! That means that in order to turn something as massive as Earth into a black hole, it would have be mushed up into something the size of a quinoa seed (and still weigh as much as the entire Earth).

To illustrate- a black hole is something that weighs as much as this:

but the size of one of these:


The way black holes appear in our universe is incredibly strange. Black holes earn their names because light cannot escape from inside. The modern interpretation of black holes comes from Einstein’s theory of relativity, which contains even stranger predictions. Among them are that time seems to slow to a stop near a black hole, light from objects near the black hole turns redder, and light bends wildly near a black hole. This phenomena is called gravitational lensing and an illustration can be seen in the animated image below. In this image, which is a computer simulation and not an actual observation, a black hole moves in between an observer and a distant galaxy (the band of stars that moves across the screen). The light of the galaxy bends as if a strong lens were present – hence, the term gravitational lensing, where the lensing effect is provided by gravity.


Where are the Black Holes?

Black holes are predicted by the theory of General Relativity, which has been around since 1915. However, until the 1930s, black holes were thought to be physically impossible, until it was proven that very large stars would collapse to a point once their fuel ran out and would form a black hole. It took until the 1970s for the first evidence of black holes to emerge, and until the 1990s for the evidence to be convincing.

Observed black holes have generally been one of two types: massive black holes at the center of the galaxy or medium sized black holes (roughly the same mass of our sun) which form a “binary” system with a visible star in which both stars orbit around a common center. Although black holes are invisible, they can be observed through their effects on their neighborhoods. Supermassive black holes are now believed to be at the center of the most galaxies and there is an exciting, growing body of research about the black hole at the center of the Milky Way. Its existence has been confirmed by observing how stars move around it. A simulation of the stars moving around the invisible black hole can be seen here ( Only a black hole could explain the enormous amount of matter required to keep these stars in orbit. A black hole in a binary system can be seen because it sucks gas off of the star and creates a bright disk around it, as seen in the illustration below:


What would happen to a space traveler who had the poor luck to meet up with a large black hole? As a traveler falls into a black hole, so long as the black hole has not “accreted” stuff around it that would mark the boundary of the black hole, the traveller may not notice as he entered the boundary (event horizon). At some point, however, the traveler would begin to feel tidal forces: As he is falling towards the central point of the black hole, termed the “singularity”, gravity would become stronger at his feet than at his head.


Eventually, the differential between the force at his head and his feet would be so strong that the person unfortunate enough to fall in will be stretched to death- the process known as spaghettification. Since there are no known black holes in the neighborhood of our solar system, it is unlikely that any astronauts would suffer this fate, but spaghettification is a reminder that the perils of deep space are unusual and dramatic.

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