If it were possible, however, it would present some pretty knotty paradoxes... For example, what if someone or something traveled back in time and changed the ensuing future? And have you heard the one about the time traveler who dots back and forward in time and by means of various medical technologies is able to be his own father and mother?! And besides, if time travel is possible, where are all the people from the future—surely they'd want to come and meet us poor stranded 21st century beings?

然而，如果這一切成為可能的話，那么將會出現一些非常令人困惑的邏輯混亂……比如，如果某人或某物逆時間旅行并改變隨之發生的情況，那會發生什么情況呢？又比如說，你是否曾聽說過這樣的事情——一位時間旅行者在時間隧道中穿梭往返，利用各種醫學技術使自己成為自己的父親和母親？還有，如果時間旅行成為可能，未來的所有人都會在哪里呢？他們肯定想來見見我們這些滯留在21世紀的可憐人。

注釋：

present vt. 引起（問題），造成（困難）（+to/with）
All this presented new safety problems.
所有這些都造成了新的安全問題。

knotty adj. 難解決的，復雜的，棘手的
She met with a lot of knotty problems.
她碰上了許多棘手的問題。

paradox n. 似非而是的論點，悖論
This paradox cannot be explained, you have to experience it.
這種悖論是不能解釋的，你必須經歷它。

ensure vt. 保證，擔保
I can't ensure his being on time.
我不能保證他的準時。
I can't ensure that he will be there on time.
我不能擔保他會及時到那兒。
This medicine will ensure you a good night's sleep.
這藥將保證你一晚睡眠良好。

standard n. 標準，水平，規格
Your recent work has been below standard.
你最近的工作一直低于標準。
Your work is not up to (the) standard.
你的工作未達到標準。

being n. 生物；人
Has anybody ever seen any beings from outer space?
有人見到過外星人嗎?？