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，如果这一切成为可能的话，那么将会出现一些非常令人困惑的逻辑混乱……比如，如果某人或某物逆时间旅行并改变随之发生的情况，那会发生什么情况呢？又比如说，你是否曾听说过这样的事情——一位时间旅行者在时间隧道中穿梭往返，利用各种医学技术使自己成为自己的父亲和母亲？还有，如果时间旅行成为可能，未来的所有人都会在哪里呢？他们肯定想来见见我们这些滞留在21世纪的可怜人。

注释：

present vt. 引起（问题），造成（困难）（+to/with）
All this presented new safety problems.
所有这些都造成了新的安全问题。

knotty adj. 难解决的，复杂的，棘手的
She met with a lot of knotty problems.
她碰上了许多棘手的问题。

paradox n. 似非而是的论点，悖论
This paradox cannot be explained, you have to experience it.
这种悖论是不能解释的，你必须经历它。

ensure vt. 保证，担保
I can't ensure his being on time.
我不能保证他的准时。
I can't ensure that he will be there on time.
我不能担保他会及时到那儿。
This medicine will ensure you a good night's sleep.
这药将保证你一晚睡眠良好。

standard n. 标准，水平，规格
Your recent work has been below standard.
你最近的工作一直低于标准。
Your work is not up to (the) standard.
你的工作未达到标准。

being n. 生物；人
Has anybody ever seen any beings from outer space?
有人见到过外星人吗?？