Since the dawn of humanity time machines have existed and they will continue to exist as long as there are sentient, thinking creatures in the universe. These time machines are not the cobbed together pieces of scientific apparatus invented by some professor in his garage but the inventions of people who wish to record and preserve the past. The energy requirements for machines that can physically transport a person through time are so demanding that it is unlikely they will ever be constructed, but there are ways for people to see and know the past despite the fact they cannot interact there.
The very first time machine to exist, and probably the least reliable, is the time machine of the human mind. The ability for a person to recall past events and to anticipate future ones should probably be considered the first true time machine. The ability of a person to relay stories about past events to other people was a way for a small number of people to experience those prior events and to learn from them but then the reliability of such mouth to ear history becomes uncertain. The reason that human memory is the least reliable, however, is because the human mind does possess an uncanny ability to distort such memories by clouding them with a wide range of both positive and negative emotional responses to those past events making the accuracy of such memories highly questionable. Furthermore, the outcomes of anticipated future events have a tendancy of not working out the way a person expects.
The second great time machine came about with the invention of writing and provided a means for many more people to experience the wonders of the past. Though writing is a slightly better means of preserving the past for future generations because the words don't change for each successive telling the fact is that those words are clouded by the opinions and beliefs of the author. Unless the author is purposely making an attempt to relay only the facts of the past events there is always the possibility that those past events will be distorted by the readers interpretation. Writing was a genuine improvement over mouth to ear time travel but further advances in technology were to come that would take the experience of exploring the past to a different level.
The time machine known as the camera forever changed the way people see the past (no pun intended). Cameras and the pictures produced by them, for the first time, actually allowed people to see into the past and not just hear or read about such events. People could now see how the world was changing and would know how the past looked to their predecessors. With careful record keeping historians could preserve the past indefinitely for the future generations of the world. Movie/video cameras and audio recorders further enhanced this time travel experience by allowing a person to see and hear how people in the past interacted within their environment. Important events were recorded on movie reels or video tape and could almost be felt by people who watch these windows to the past. Though cameras and audio recorders have proved to be the most important time machines to date, current technology now threatens the history of humanity.
The recent advances in computer, photographic, and video technology are threatening the accuracy of compelling visual evidence of the past. Paint programs and photoshop software have made it much to simple to alter pictures and CGI technology allows for the generation of video that is indiscernible from real life. The camera was a genuine time machine into the past but computer and CGI technology is proving to be a portal for exploring alternate realities. Unfortunately, the imagined alternate realities are more compelling to a lot of people than the real life past.
It is doubtful that true time machines that allow people to physically travel into the past will ever be invented and psychic time travel is highly unlikely. Audio/visual records are the best means of traveling into the past and even those are threatened by computer and CGI technology. What does the future hold for time travel machines? Nobody knows the answer to that, but if people aren't careful history could be changed forever.