There was a thread and it seems the major question came up from a person who did not start the thread (reference: Is Force Instantaneous). So I am starting a new thread to discuss Simultaneity and Special Relativity. So I am starting this to post my understanding on the subject open to errors and improvements in description. The focus I want to bring to this thread is “The difference between Newtonian Physics, Special Relativity, why they differ on simultaneity, and where Quantum Mechanics comes into play.” Understanding the deeper nature of Special Relativity (SR) much less General Relativity (GR) can hinge, from my perspective, on understanding this issue. Without getting into issues of philosophy I prefer to keep the discussion in the thread to, “Is there a mathematical description?” and does it at least have a level of accuracy according to experiment and observation. Observation here means things in the universe which we can observe and measure. The “laboratory of the universe” is fast becoming the place where the latest advances in physics are being tested. This is an investigation into how we know what we know from our observations. We must therefore be careful in precisely defining our terms according to what we can observe. “Instantaneous” means, “It happened at the same time”. However, what does “same time” mean? It is used in consideration for events at two different points. It is not used for events at the same point. Events at one point always have the same time order. If you see event A and a point and then event B at a point, then all observers will see the same order of event. If two events happen at distant points then the order of events may differ for different observers in SR. The concept of time is that of change. We measure time by using for example of the number of cycles that a cesium atom has gone through for the start and stop points of an experiment or observation period. That is how much has it changed. The concept of distance needs a more precise definition. Distance is not a basic value. By that I mean it is not a unit we directly measure. It is a ratio of the time, number of oscillations for cesium, to the speed of light (d=vt) where v is known and t is known. The length is equal to the time, number of cycles, for light to go from point A to point B and then back again divided by two for the round trip. Knowing that this is round trip can be important for understanding some theories and thoughts on the matter. The concept of velocity is always “with respect to something”. If you are in an aircraft and say “I am moving at 200 kph” the question will be “with respect to what?” Is that air speed or is it ground speed?” Also, velocity is a vector in mathematical terms. It has a magnitude and a direction. One more concept is “expected observed velocity”. By this I mean, “If I am moving at 10 kph with respect to a ground point on the earth and I measure another traveler moving at 20 kph with respect to me then what do I expect that observer to measure for a velocity with respect to the ground? This is where Newtonian Physics and SR differ. Newtonian Physics has simultaneity. It works for most all our simple observable experience. In Newtonian Physics expected observed velocity is simple vector addition. From the previous example, my velocity with respect to the ground gives a magnitude of speed and my direction with respect to the ground gives a direction for the vector, v1. My observation of the other traveler gives me a speed from myself and a direction from myself, v2. If I add the two vectors I get what I expect will be the traveler’s observation for ground velocity according to Newtonian Physics. This little mathematical method for determining what another observer will see has built into it the essence of it simultaneity. Special Relativity on the other hand has one basic principle “all observers will observe light moving at the same speed”. This breaks the simple vector summation of Newtonian Mechanics. By breaking and reformulating the math a wider variety of implications is changed. That is where the length and time dilation effects of SR become implied. It is also where its lack of simultaneity becomes an implied conclusion also. You can look up wiki simultaneity for more explanation for this. However, the explanation given points to the basis reason that Newtonian physics has simultaneity and SR does not. SR also has the concept that what an observer sees is correct. There is no absolute inertial frame which sees correctly where other observers have a skewed observation. This is important for understanding some of the deeper philosophical and mathematical implications of SR. The math of SR has been shown to accurately predict the outcome every of experiment where GR does not come into play. Newtonian physics has not. This is why SR is a main stream of modern day physics. Quantum Mechanics (QM) has the concept of entanglement. What this means is that two objects, photons, electrons, etc, may have been created with a relationship such that if one is measure with a particular property then the other object will be observed to have a direct relational property. For example, one experiment is with two photons with polarity entanglement. If one photon (1) is measured to have a horizontal polarity another photon (2) will be measured to have vertical polarity. Look up Bell’s theorem for more explanation on this. According to QM both objects are in a related but undetermined state until measured. At the point in time one is measured to have a particular property then the other will have the related property. From the previous description, the photon is not horizontal until measured horizontal. The other photon is not vertical until it is either measured vertical or the other is measured horizontal. This effect is immediate. This effect has been measured to be faster than the speed of light. Now there is one more thing extremely important point to make. QM does not care about time for this. What does that mean? The question can be posed “was photon (1) measured as vertical polarity done before or after photon (2) was measured with horizontal? It makes no difference in QM which was done first. It may have been photon (1) with vertical or it may have been done with photon (2) with horizontal. It makes no difference in QM theory. Therefore QM cannot be used to determine which happened first. QM cannot, by mathematical proof, be used for determining the simultaneity of events. QM cannot be used to create an instantaneous communication device. That is to say, I want to instantly sense when photon (1) has been changed to vertical polarity. That cannot be done with QM. QM cannot be used for an instantaneous causal communication device. So SR relativity in it prediction of no-simultaneity is safe from QM. If an instantaneous causal communication device could be constructed then the “return path” used in observation would be broken and SR would either fail or an even deeper mystery of physics would be revealed. Well actually in either case an even deeper mystery of physics would be revealed. No such device can be constructed using modern Physics. There is no known way to make such a device. So this is how QM and SR work together.