53613
Albert Einstein called this the equivalence principle , and said that only observers who feel no force at all - including the force of gravity - are justified in concluding that they are not accelerating .
 Albert Einstein
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 Albert Einstein