Monday, August 22, 2011

Wittgenstein on relativity

William has given us an interesting link here.

Meeting a friend in the corridor, Ludwig Wittgenstein (1889-1951) said: "Tell me, why do people always say that it was natural for men to assume that the sun went around the earth rather than the earth was rotating?"

His friend said: "Well, obviously, because it just looks as if the sun is going around the earth."

To which the philosopher replied: "Well, what would it look like if it had looked as if the earth were rotating?"
How clever of Wittgenstein. He is not asking what it would have looked like if the earth were rotating, but what it would have looked like if it had looked as if the earth were rotating. Implying that to look as though the observer is moving with respect to their environment is the same as looking as though the environment is moving with respect to the observer.

But is that true? As a postgraduate I house-shared with a research assistant who was working on the neurophysiology of perception. Some of his work showed that the world actually looks different dependent on whether the observer is moving with respect to a stationary environment, or the other way round. There are a number of kinaesthetic sensors in the body which respond to bodily motion or rotation, and these interact with the visual sense in various ways. You can fool these sensors in all sorts of ways – for example you can create the illusion of acceleration by tilting their seat backwards, which is how flight simulators like this work. The answer to Wittgenstein’s question could well be “well, it would look exactly like that” – namely, looking as if the earth were moving, rather than the sun.


Bill Wallace said...

Wittgenstein's first question was inadequately answered. The answer should have been something like: "Well, since we had no terrestrial method before Foucault's pendulum (1851) of detecting earth's rotation it was natural to assume there was no rotation and since the sun does appear to though the heavens daily, why not."

The lack of rotation is certainly is sensible naive understanding of the situation. If a person never observed the night sky, it might even seem logical. The moon would be just another object moving around the earth.

However, Ptolemy (90-168) had a large amount of data gathered from the night skies and still thought the Earth did not rotate, but the heavenly bodies moved about Earth.

Later, Copernicus (1473-1543) proposed the heliocentric model: earth rotates about its axis and revolves around the sun. Kepler (1571-1630) improved upon Copernicus' model, replacing circles with ellipses. Kepler had worked with Tycho Brahe (1546-1601), who had accumulated a vast amount of observational data. This data pointed Kepler to the heliocentric system we understand now while it pointed Brahe to a variation of the Ptolemaic geocentric system. In a book relating this is a description of the two men watching the sun set. Kepler explains the sunset as the sun appearing to sink because the Earth is rotating (their location) away from the sun and Brahe explains it as the sun moving down below the horizon.

So, the answer to Wittgenstein's second question is: it will look exactly the same.

I have to wonder if the conversation ended there. If it did, was the question a trick question?

Edward Ockham said...

>>natural to assume there was no rotation

Why would it be natural to assume there was no rotation?

Bill Wallace said...

With no evidence of movement, in this case rotation, why would anyone assume movement?

Edward Ockham said...

Because it looks as though the universe is moving?

Bill Wallace said...


Gio Ster said...

Came across this as a search result when I couldn’t recall Wittgenstein’s name and now am compelled to comment.
Wow! Way to miss his point: It isn’t sufficient to find an explanation of the phenomenon that is consistent with the observations. There may be others and one of these others may have the distinct advantage of actually being true. The path of the sun across the sky is consistent with earth going around the sun; however, it is also consistent the other way around. Neither one should be preferred without additional information.

The quote has nothing at all do with how human (or any other animal) vision works. Also the final question isn’t a quiz, it is a rhetorical device used to make his point (above).

Edward Ockham said...

@Gio - I took W's point to be that it wouldn't look any different either way. My point was that it conceivably might look different.

Brian Schafer said...

I've always taken the point of this exchange to be that it does NOT, in fact, look as though the sun is revolving around the earth. It merely ALMOST looks that way, but it EXACTLY looks like the earth is rotating.

Similarly, one could ask, "why was it natural to assume the earth orbits around a stationary sun, rather than the sun and earth both move under the influence of each other's gravity"

Answer: "because it LOOKS as if the earth orbits a stationary sun"

Reply: "what would it have looked like, if it had looked as if the earth and sun move under the influence of each other's gravity?"

The answer is the same, it merely ALMOST looks as if the earth orbits a stationary sun. It EXACTLY looks as if the earth and sun move under the influence of each other's gravity.

susan varney said...

I read this quotation as Wittgenstein's allusion to a fundamental egocentric predicament; that we never fail to put ourselves at the centre of the universe. We "see" the sun moving around the earth because we always put ourselves (and our understanding of ourselves) at the centre of the universe. To "see" the earth rotating around the sun requires, in fact, many fewer astronomical calculations but such a solution is less offensive to our egocentrism.