It can be argued that the definition of simulatneity in SR is artificial and only a matter of calculation and reconstruction. You can never now be aware of simultaneous events at other locations, according to the standard definition. Wouldn't it then be more natural to consider those events you can see now as simultaneous to your present? For example, assume that the Earth and Mars are 5 light minutes apart from each other, and have synchronized clocks, according to Einstein's convention. We also assume that the Earth and Mars are effectively at rest relative to each other and that their common reference frame is an inertial frame. Also, assume that: At 8.00, I, on Earth, finish my breakfast. At 8.05, I recieve a radio message from Mark on Mars saying "Hello Erland". I immediately reply and send the radio message "Hello Mark" to Mark. At 8.10, Mark is beginning his breakfast. Now, when I receive Mark's message at 8.05, I can make a calculation and infer that Mark sent his message at 8.00, just when I finished my breakfast. But I was totally unaware of that when it happened. I finished my breakfast without knowing that Mark at that very moment sent a message to me. Isn't it quite artificial then, to say that these two events are simultaneous relative to me? Wouldn't it be more natural for me to say that the events when Mark sent the message and when I received it are simultaneous? The idea is, then, to consider all events I can see now, or more general, can get knowledge about now from light speed signals, as simultaneous to my present. In this case, we could for example follow the development of a supernova, say, 10 000 light years away, from eruption to extinction, and say that this development is going on now, and not during some weeks 10 000 ago, when noone of Earth could have the faintest idea of what was going on far away in the sky. One might argue that if we define simultaneity in this way, it becomes dependent upon the observer. I can consider the two events when Mark sent the message and when I recieved it and sent the reply as simultaneous, but if Mark adopts the same definition of simultaneity, these two events are not simultaneous for him, but 10 minutes apart. He receives my reply at the moment when he begins his breakfast, and that is 10 minutes after he sent his message, according to him. But in SR, simultaneity is dependent upon the observer even if we use the standard definition. Two observers who move relative to each other with a high constant velocity will not agree about which events are simultaneous, using the standard definition. Also, if we use this alternative definition of simultaneity described here, it may be easier to find out what you actually see according to SR. For example, a length contracted object does not at all look contracted according to the standard formula, which in turn is connected to the standard simultaneity definition. So, why not reformulate SR in terms of this alternative definition of simultaneity? Surely, someone must have got the same idea before. Does anyone know if there is anything written about this? I tried to make some calculatons myself, but the math is more complicated than with the standard definition. With this alternative definition, simultaneity becomes dependent, not only upon the reference frame, but also upon every observer at every location. Mark and I have different opinions of which events are simultaneous, despite that we belong to the same reference frame (at rest relative to each other), while we agree about which events are simultaneous with the standard definition. Maybe this is a decisive disadvantage compared to the standard definition. What do you think? And do know any articles (or similar) written about this earlier?