Immutable

The large clock in the back of the auditorium showed exactly three o'clock as the bell rang and the din of voices and shuffling of books and papers began to wind down. The famous Professor Amos Sandoval known for his brilliant lectures delivered seemingly with little to no preparation began to speak.

"As you can see on the board behind me, the topic for today is the immutability of time. What does this mean? Any ideas?"

"OK, Ms. Andrews, what do you think?"

"It means that time cannot be changed, although I think Einstein proved otherwise."

"Yes, excellent Ms. Andrews. And you are quite right about Einstein showing that time does indeed change depending on the speed of a body relative to the observer. However, today I want to discuss the immutability of the direction of time. Does time move toward the future only or is travel to the past possible? Yes, Mr. Hansen?"

"Well, I've heard of a theoretical particle called a tachyon that travels backward through time, and I've heard that Einstein's equations don't forbid backward time travel. Besides it happens in Star Trek all the time!"

There were laughs throughout the room but then the professor continued.

"But, other than theoreticals and science fiction, is it possible? I say no. First, there are paradoxes, like the "grandfather paradox". If you went back in time and killed your grandfather before your mother was conceived, then you would never exist and could not have gone back in time to kill your grandfather. Wait. I forgot the next one."

Without another word, the professor reached under the lectern and pressed a button. There was a bright flash and...

The large clock in the back of the auditorium showed exactly three o'clock as the bell rang.