I started posting to what seemed to be a related thread here: https://www.physicsforums.com/showthread.php?p=2825641&posted=1#post2825641 After concluding I was hijacking another thread, I am posting here. The concept is I think that, in theory, one can determine absolute speed. Am I the one moving or is that other object moving past me. I wrote up a theoretical experiment to demonstrate my concept. I decided to post it on a personal web site rather than post a bunch of text here. The location is: http://www.bkelly.ws/space_time/index.htm [Broken] Click on the link about absolute velocity to view or download the essay. Ich was kind enough to respond in the first thread and my response is rather general so I will put them in this post. For context, Ich raised the issue of simultaneity and the problems it would cause my experiment. Hello Ich, Regarding the trolley and the issue of simultaneity, while the essay stated that it makes the marks as it passed directly in front of Tom, I am claiming that given the goal of the essay it really does not matter if the marks are made when the trolley is a bit to the left of Tom, right in front, a bit to the right, or what the heck, a couple of hundred meters to the right or left. So exactly where the marking event(s) occur is not really important. That should get us down to two events. However, as I think on this a bit more, we can dispense with simultaneity. Worrying about the simultaneity provides no advantage in discussing this concept. Its sort of like saying we cannot have a theoretical discussion because no one knows how to build a trolley that can move at relativistic speeds. So, given that for our discussion we have a trolley that can move at near the speed of light, the trolley is declared to be 1 meter in width when stationary and has two paint cans, one at each end and they make two marks that are one meter apart, with respect to the trolley if you will, at the appropriate time and place. The concern is not in making and conducting the experiment for real, the concern is the concept of what would happen if we found a way to conduct this experiment. Lets go down your path a bit further. Assume that the fence goes past Tom at ½ C while Tom and the trolley are stopped, and the trolley makes its two marks. Then the fence is brought back to a center position and the marks examined. While moving, the fence is length contracted. When stopped then the fence will “un-contract” and the marks will be more than 1 meter apart. On the other hand, make the fence were stationary and the trolley moves along it at 1/2 C and the marks made. The trolley will be length contracted and the fence will not. The marks will be less than one meter apart. That is in contradiction to the marks being further apart when the fence is moved for the marking then stopped. That tells me that from the perspective of the trolley, we can detect which was moving. And that is the fundamental purpose of my thoughts.