The words passed, past sound the same but have different meanings and spellings. Why do passed, past sound the same even though they are completely different words?
The answer is simple: passed, past are homophones of the English language.
Simple past tense and past participle of pass.
That has passed beyond a certain point (chiefly in set collocations).
That has passed a given qualification or examination; qualified.
No longer current; gone by; over: His youth is past.
Having existed or occurred in an earlier time; bygone: past events; in years past.
Earlier than the present time; ago: 40 years past.
Just gone by or elapsed: in the past few days.
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License, from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.