Just two things

[Science] fiction reflecting life

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Keith S. Taber

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I imagine the physicist Henri Poincaré was entirely serious when he suggested,

"the principle of relative motion, which forces itself upon us for two reasons:

first, the commonest experience confirms it, and

second, the contrary hypothesis is singularly repugnant to the mind."

Henri Poincaré (mathematician, physicist, philosopher)

Perhaps Poincaré was reflecting how two opposing schools of philosophical thought had disagreed on wherever the primary source of human knowledge was experience (the empiricists) or pure reasoning (the rationalists), but elsewhere in the same text Poincairé (1902/1913/2015) dismisses the idea that the laws of physics can be obtained by simple reflection on human intuitions. Such intuitions can lead us astray.

If he is being consistent then, surely "the contrary hypothesis is [only] singularly repugnant to the mind" because "the commonest experience confirms…the principle of relative motion". That is, suggestions that are clearly contrary to our common experience – such as, perhaps, the earth is moving? – are readily rejected as being nonsensical and ridiculous.

If that is so, then Poincaré was not really offering two independent lines of argument as his second reason was dependent upon his first.

This put me in mind of some comments of Kryten, a character in the sci-fi series 'Red Drawf',

{responding to a crew suggestion "Why don't we drop the defensive shields?"}

"A superlative suggestion, sir, with just two minor flaws.

One, we don't have any defensive shields, and

two, we don't have any defensive shields.

Now I realise that, technically speaking, that's only one flaw but I thought it was such a big one it was worth mentioning twice."

Kryten (mechanoid assigned to the mining spaceship Red Dwarf)

or alternatively,

{responding to the crew suggestion "I got it! We laser our way through [the 53 doors from here to the science deck]!"}

Ah, an excellent plan, sir, with only two minor drawbacks.

One, we don't have a power source for the lasers; and

two, we don't have any lasers.

Kryten

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The principle of relative motion

What Poincairé meant by 'the principle of relative motion' was that

"The motion of any system must obey the same laws, whether it be referred to fixed axes, or to moveable axes carried along in a rectilinear and uniform motion."

the principle of relative motion

In other words, imagine a train passing a station at 10 ms^-1, in which a naughty physics student throws a pencil eraser of mass m with a force of F at another passenger sitting in front on him; while a model physics student observes this from the stationary station [sic] platform.

The student on the train would consider the eraser to be at rest before being thrown, and can explore its motion by taking u=0 ms^-1 and applying some laws summarised by

• F=ma,
• v=u+at,
• v²=u²+2as,
• s=ut +¹/₂at²…

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The student on the platform would observe the eraser to initially be moving at 10 ms^-1, but could calculate what would happen using the same set of equations, but taking u=10 ms^-1

Any values of v calculated would be consistent across the two frames (when allowing for the 10 ms^-1 discrepancy) and other values calculated (s, t) would be the same.

This reflects the relativity principle of Galileo which suggests that there is no absolute way of determining whether a body is moving at constant velocity or stationary: rather what appears to be the case depends on one's frame of reference.

We might think that obviously it is the platform which is really stationary, as our intuition is that the earth under our feet is stationary ground. Surely we could tell if the ground moves?

We can directly feel acceleration, and we can sometimes feel the resistance to motion (the air on our face if we cycle, even at a constant velocity), but the idea that we can directly tell whether or not we are moving is an alternative conception.

For centuries the idea of a moving earth was largely considered ridiculous as experience clearly indicated otherwise. But if someone was kidnapped whilst asleep (please note, this would be illegal and is not being encouraged) and awoke in a carriage that had been set up to look like a hotel bedroom, on a train moving with constant velocity, they would not feel they were in motion. Indeed anyone who as travelled on a train at night when nothing is visible outside the carriage might well have experienced the impression that the train is stationary whilst it moves at a steady rate.

Science has shown us that there are good reasons to think that the earth is spinning, and orbiting the sun, as part of the solar system which moves through the galaxy, so who is to say what is really stationary? We cannot tell (and the question may be meaningless).

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Source cited:

• Poincaré, H. (1902/1913/2015). Science and Hypothesis (G. B. Halstead, Trans.). In The Foundations of Science. Cambridge University Press. {I give three dates because Poincaré published his book in French in 1902, and it was later published in an English translation in 1913, but I have a 2015 edition.}