Does the moon exists when we don’t look at it ?


This question may puzzle you at first but you are not alone. Even the greatest minds of physics in the 20th century were always at loggerheads whenever this particular topic arose. Whether or not anything exists depends whether or not we experience its effect?? Yes!! That is until and unless I observe any physical object, it simply doesn’t exist. In other words, its our measurement process which forces the physical properties of any object to come into existence.

Now, Let us consider the Moon itself.  Moon looks beautiful in the night sky and it’s always been part of many mythical stories around the world but right now we are least concerned about its beauty and bedtime stories related to it.  Is the moon not there when we don’t look at it??  Before quantum mechanics it was obvious that the moon is always there whether we look at it or not but after the introduction of quantum mechanics nothing remained obvious. When we talk about “looking” at moon, we refer to making any observation on the moon.

According to quantum mechanics if you are not able to observe any properties of any particle that particle simply doesn’t exists  and  In  case of the moon we know it causes tides in the ocean , we know moon being a heavenly body exerts some gravitational pull on us which we can  calculate . We know it lights up the night, we know it causes eclipses.  There is one more thing which we don’t often hear about.  It’s about the axis of rotation of the earth. We know that the axis of rotation of the earth is a bit tilted. This tilt of the axis causes seasons and since earth remains always tilted at roughly same angle we always experience the same seasons overtime. Have you ever wondered why is this tilt of the axis of rotation of the earth not changing over time???  Its the moon!!  Had there been no moon we would have never experienced seasons the way we do now.

I am trying to make a point that we are always experiencing its effect.  Hence , even if we choose not to literally “look” at it we are observing it indirectly by experiencing its effect. This implies whether we realize it or not, we are always “looking” at the moon. Unknowingly , we are always doing measurements on the moon. Had there been no measurable effects associated with it we could have said that the moon doesn’t exists if we don’t literally look at it . So in general , moon doesn’t exist when we don’t “look” at it.

Let us take one more example. An electron, before we measure its location we can only talk about probabilities of finding an electron at a certain location but we can never precisely predict its trajectory. Its only after we start the measurement process, we get to know about its location i.e. by performing measurement we are forcing that electron to take a definite position. The above examples disqualify the notion of objective world that “the world is as it is irrespective of our action” . It turns out that the physical world depends on our action and is subjective in nature.

You really don’t need to worry if you refuse to believe this because even Albert Einstein opposed this idea of Quantum mechanics. He always believed that world is predictable and that there must be some lack of information which is why we aren’t able to predict the future precisely but eventually he surrendered on this issue. There is one famous experiment known as “Bells inequality” which proves the above assumption of quantum mechanics. I won’t be explaining “Bells inequality “here but I mentioned this experiment just in case you might want to check the validity of the statement. So, next time someone asks you the above question, you know how to reply at least to keep the conversation alive.

This article was first published on March 12, 2016 but due to some technical issue it has been republished

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Surya Kiran Yadav

Delhi University Physics graduate .
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Last modified: July 18, 2019

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