Einstein once postulated that time was like a river, a current which seems to carry us forward. And just like a river, Einstein believed it was theoretically possible for the current to meander in odd ways, forming eddies and oxbows, and perhaps to even flow backwards at times.
Our perception of time is certainly faulty. You can perform the following experiment easily yourself: lie down on a table and close your eyes. Have a partner touch your face and your toe at the exact same time. You will experience these touches as happening simultaneously. But this is impossible, because it takes a perceptible amount of time for information to travel across your nerves.
The touch on your face happens close to your brain, and therefore has only a slight distance to travel. But the touch on your toe happens far from your brain. It takes a perceptible amount of time to travel the distance to your brain for processing. You ought to feel a delay, but you don't, because your brain processes your perception of time to account for the lag. Crazy, right?
According to the "time slip" theory, certain places are thin spots in time where leakage can seep through between the ages. This leakage is most often auditory - bangs, knocks, footsteps, and shuffling are the most commonly reported ghost phenomena. Perhaps sound waves travel better through time than other things, for reasons we don't fully explain.
This theory is most appealing in ancient locations like hotels, castles, and military forts that are hundreds of years old. Across the centuries, various sounds and occurrences pass back and forth, building evidence that a place is haunted.
It's also fun to think of cases where ghosts seem to respond to questioning. If you have ever looked across a crowd of people and had the feeling that everyone was talking on their cell phones to everyone else, you can see how this would work. It's intriguing to think that ghost hunters throughout history are basically talking to each other through the time rift.
I had a similar experience when I was a little girl. One evening I accidentally rapped my knuckles against the wall in my bedroom, and heard an answering knock. I knocked back, out of curiosity. Sure enough, the "ghost" replied in kind.
Months later I met the girl who lived in the other half of the duplex apartment. Funny thing; she had a ghost story about knocks in the wall, too! Sheepishly, we quickly realized that our bedrooms shared a wall, and that we had been accidentally scaring the dickens out of each other.
The time slip theory can't account for a lot of physical actions which occur. Unless you believe that some guy in the past suddenly saw a modern lamp materialize on his table, and was moved to turn it on before it vanished back into the ether. (I'm sure we would have heard some of those stories!) But it's an interesting thought experiment, at the least!
Photo credit: Flickr/hpk