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Question
Hi there,  so much has been said about "Space-time", but some may ask, does it [really] exist?  Is it nothing more than a mathematical construct written on paper, or a chalkboard, or does it actually exist in the physical universe?  I believe that the latter is most likely true..right?    --  it must be.  How else can thise individuals who believe, or theroize  that space-time doesn't really exist  explain things like gravity & black holes, planetary systems..etc..? All things that are, of course,  associated with space-time.  However, my real question is, what exactly is it, that is, what's it made out of? I've heard  that it's  some kind of electromagnetic energy field.

Answer
Dan, first you need tom know what a frame of reference is.  In ordinary space of three dimensions, a frame of reference is like a coordinate system. It leys you represent the position pf a speck of dust in terms of three numbers, the x, y, and z coordinates. That is as old as Descartes.

To solve certain problems of motion in time and space, we use four-dimensional coordinates and instead of a point, we discuss events, which have not only positions but also a time. This would be trivial if the time of an event is independent of any spatial consideration, but what blew people's mind was Einstein's theory that the time of an event depends on its velocity, which obviously depends on what frame of reference you choose if you allow moving frames of reference.

Consider two simple frames of reference which are the same except for the fact that one frame is moving with respect to the other along their x axes at constant relative speed v. You would think that the explosion of a firecracker at point x1 with respect to one frame might be described also with respect to the other by x2 = x1 - vt, if the point is fixed in one frame and moving in the other.

But Einstein says no. The time coordinate is not the same in the two frames, even if they start out the same at the beginning. You have to use t1 and t2 as the time of the event, and there must be a relation

x2 = gx1 - ht1
t2 = ix1 - jt2

where g,h,i, and j are constants that depend on v and c, the speed of light. You can find them in relativity textbooks.

So you see that the time of the event in one frame depends not only on the time in the other frame but also on its position in the other frame. That is what space-time is all about.

Don't expect to "understand" this. Nobody understands it in any intuitive way. But experiment shows it to be true whether we understand it or not!


Uncle Ben  

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Uncle Ben

Expertise

I can answer questions regarding Einstein's Theory of Relativity, particularly in Special Relativity. I will not answer homework questions or mathematical problems that require special symbols.

Experience

I have taught physics at the college level, undergraduate and graduate, for many years including Special Relativity. I have taught at Johns Hopkins, Case-Western, and MIT. I have also served as a staff member of the Commission on College Physics, which was supported by the National Science Foundation to recommend improvements in the curriculum of college physics departments in the US. I am also the author of a textbook titled Vector Calculus, which was used at MIT in the teaching of electromagnetic theory and relativity. My research interests were mainly in solid state physics, especially the properties of metals at low temperatures. I am listed in the publication known as American Men of Science.

Publications
I have dozens of papers published in the Physical Review and in the American Journal of Physics.

Education/Credentials
I hold a Ph.D. degree in physics from the Johns Hopkins University.

Past/Present Clients
Johns Hopkins University, Case-Western Reserve University, Massachusetts Institute of Technology, Empire State College, Georgetown University, Commission on College Physics, and UNESCO.

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