A search is now under way to confirm the suspected existence of particles of matter that move at a speed greater than light, and therefore, might serve as our passports to the past.
出自-2011年6月听力原文
柯林斯高阶英汉双解学习词典释义
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Your passport is an official document containing your name, photograph, and personal details, which you need to show when you enter or leave a country.