My question is an extension of the celebrated question on the moon’s existence if unobserved.

“do we still have tides on earth if the moon is unobserved?”

  • 5
    $\begingroup$ I'm voting to close this question as off-topic because it is philosophy not physics $\endgroup$ Aug 28, 2015 at 6:25
  • 3
    $\begingroup$ it's the beginning of an interesting physics'question, not only on correlations. But stated like that, the answer is trivial since there is a contradiction between getting tides and saying that moon is unobserved. $\endgroup$
    – user46925
    Aug 28, 2015 at 6:53
  • $\begingroup$ The interesting thing about physics is, it does not rely on believe. Having never heard about Newton or Einsteins theory of gravity, an elephant still knows (or experiences) that if he were to climb up that tree he'd eventually fall down. $\endgroup$
    – Clever
    Aug 28, 2015 at 6:55
  • $\begingroup$ @clever I understood 'measure' ... it's just a 'bad measure' or something which happened before Sir Newton or after Mr Bohr $\endgroup$
    – user46925
    Aug 28, 2015 at 6:59
  • $\begingroup$ I withdraw the question and sorry for wasting everybody’s time. Just to clarify, though. It’s a general relativistic extension of Einstein’s famous question in his debate with Bohr on the meaning of Quantum Mechanics. With the collapse of the wave function, which "actualizes" a particle’s position, will that also "actualize" the surrounding gravitational field (for a particle with mass) and also distort the surrounding space? If so, how can it be? But, I am sure you are right: the question is off-topic. $\endgroup$
    – Fosco
    Aug 29, 2015 at 5:47

2 Answers 2


What's an observation? I think your question delves into the nature of "consciousness", a term which has never to my knowledge been satisfactorily defined.

The seas observe the moon, and therefore there are tides. Whether or not a "conscious" being observes the seas is of no consequence in physics.


To add to Ernie's Answer:

Actually, tides on Earth are tantamount to an observation of a Moon-like object. Bernard Schutz in his book "A First Course in General Relativity" imagines an instance of your scenario whereby Earthlings lived under skies permanently shrouded completely and impenetrably by clouds:

"The true measure of gravity on the Earth are its tides. These are nonlocal effects, because they arise from the difference of the Moon’s Newtonian gravitational acceleration across the Earth, or in other words from the geodesic deviation near the Earth. If the Earth were permanently cloudy, an Earthling would not know about the Moon from its overall gravitational acceleration, since the Earth falls freely: we don’t feel the Moon locally. But Earthlings could in principle discover the Moon even without seeing it, by observing and understanding the tides. Tidal forces are the only measurable aspect of gravity."

So you can't have tides and "not observe" the Moon. We would know, by observing the tides, that the Earth must span a region of spacetime that is big enough for curvature to be measured. We could measure the period of the tidal bulge, i.e. it's period of progression around the Earth. Then, by quantifying Coriolis effects (e.g. through a Foucault pendulum) we could measure the Earth's rotation period and subtract this from the total progression of the bulge, thence infer that there must be a periodic motion of whatever it is that was making the tidal bulge.


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