Ex 8 Answers

This answer page refers to the post 8: Welcome to the solar system. If you haven’t done so already, read the post and try the questions before studying the answers below.

What is the claim?
In practical terms, we are alone in the universe.

What are the main premises?
1. The nearest inhabited planet is likely to be at least 200 light years away.
2. 200 light years is a distance too great to travel or to make contact with other thinking beings.
(Hence, even if we are not really alone, we are practically alone)

What kind of argument is it?
It is a mixture of argument from authority and inductive reasoning (see below).

Is the argument convincing?
Let’s examine how the argument works.
The author uses the idea that the universe is huge to draw out two contrasting consequences. On the one hand, the fact that the universe is huge means there is a high statistical probability of advanced life existing on other planets simply because there are just so many planets. On the other hand, the fact that the universe is huge means we can probably never have any contact with life on other planets simply because the vast size of the universe means the planets are so widely separated.

The author quotes an inductive argument from an expert (argument from authority). He also quotes scientific estimations and scientific facts (i.e., authority) about the likely distances between two inhabitable planets, and how long conventional travel would take between them.

So is it a good argument or not? You could argue that it’s a bad argument on the grounds that neither of the premises are uncontroversial:

i). No one supposes that we would travel from one planet to another at the rate of a conventional jet aeroplane. Even current spaceships, using ‘gravity slings’ and conventional rocket power, travel at speeds far greater than 800km/h, so the example is misleading.

ii) Drake’s equation looks very ‘shaky’: there are so many variables in it that, arguably, it’s conclusion is worthless.

However, in defence it might be replied that

i) the example about jet aeroplanes was just an illustration. The point is that even if we developed some way of travelling at the speed of light, the journey would still take 200 hundred years, i.e., longer than a human lifespan; and

ii) The point of Drake’s equation is that even if you take the most pessimistic values for each of the variables, it still turns out to be millions of planets that likely have advanced life.

However, even if we accept the premises, even a 200 light year gap still does not rule out some kind of radio contact (albeit one that would take generations to send and receive) through which important information about species, biology, civilisations and thought could be exchanged. Such contact would radically alter our cosmic view of ourselves. When the author says ‘practically alone’, it seems he has in mind the possibility of physical contact, but almost any kind of empirical evidence that there exist other intelligent life forms would have a profound effect on life and culture on Earth.

However, the main problem with this passage is that it is impossible to weigh the crucial piece of information: i.e., how good is the evidence that the nearest inhabited planet is likely to be 200 light years away? Everything hangs on this because it is this distance that makes it impractical for us to either contact or travel to another inhabited planet (notice this piece of information is in both the premises above).

This crucial bit of information we have to take on authority, but even the authority only claims it is an estimate.

Therefore, as it stands, the argument is not wholly convincing. In order to accept it, it would need some supporting argument or evidence to suggest that the figure of 200 light years was about right. Anything less than a human lifespan (i.e., 70 light years), and the idea that we are ‘practically alone’ wouldn’t work.

If you have any comments on the text or this analysis, please post them below.

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