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A photon checks into a hotel. The front desk asks ¡°Do you need help with your luggage?¡± The photon replies, ¡°I don¡¯t have any. I¡¯m travelling light.¡±  No, jokes aside, light (electromagnetic waves) are quite important here.

If you were piloting a ship with him and asked him ¡°Are we going the right way?¡± He¡¯d reply saying ¡°Off course, we are¡±. No, jokes aside, today the topic is navigation in the middle of nowhere.