Whatever other arguments that one make for or against time travel, the one that limits us to the greatest degree is one of composition. Each of us is made up of cells made up of physical elements from the air we breathe, the food we eat and even have a brain that is shaped by the experiences and thoughts that we’ve had. The air we have breathed is dependent on the earths weather patterns. The food we have eaten has come from all over the country and the world and is dependent on the soil, rain and CO2 content of the atmosphere and any other number of variables that change on a continuous basis. Each molecule of our body has come from the various reaches of the world or universe. Any point in time, any NOW, can only be described by fixing the positions of a particle or selection of particles in comparison to other particles or selection of particles. To physically move from now to any time in the past would mean that all of the molecules that make up our being would have to return to a previous position in the world or universe. Therefore we could only move exactly backward in time, basically like rewinding a video tape. We would have no knowledge of our trip because the molecules in our brain would move to a former position and our memories would be the same as at a former time. To physically move to any point other than a point in our own former lives would not be to move back in time but would be to move to another reality.