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Message: <blockquote data-quote="Umbran" data-source="post: 6689095" data-attributes="member: 177">We know that it does.  The effect is measurable, and we have a model that can calculate how it changes, to high degrees of accuracy.  We can describe, as a sort of physical interpretation of that model, what is happening.But *why*?  I don't think we can actually answer that question yet.  To answer a why, we must understand the universe one level down from the thing we are describing.  Perhaps a string theory, or whatever quantum gravity we come up with (if we ever come up with one) may answer the why.  For now, we can more talk about what happens...Yes.  #5 is alive!Special Relativity, and by extension, General Relativity, are the inevitable fallout of one simple basic observation - to all observers, everywhere, light travels at the same speed.Say I am sitting on a train that is moving at 50 mph.  Say I pick up a baseball, and throw it to the front of the car.  And I measure the throw to be going at 50 MPH, relative to me and the car.My friend, sitting beside the track as the car and I go by, would say the ball was moving at 100 mph, relative to her and the ground outside.  for her, the ball was moving at 50 MPH before I even threw it, after all, so my arm only adds to that speed.That's not how light works.  If I fired a laser toward the front of the car, I'd measure it at 186K miles per second, and my friend would measure it going the *same* speed.  The speed of the train doesn't add to the speed of the light.How is that possible?  How can we both see the same speed?  There's only one way to do that.  Speed is the distance something covers over some period of time.  If the speed is immutable, then distance and time must not be!  Time and space with bend, fold, and mutilate such that everyone, everywhere, sees light moving at the same speed.  Time and space measurements are not absolute and objective for all, but are *relative* to the observer - thus "Relativity".So, one answer to "Why?" is "Because light moves at the same speed for all observers."   But, why does light travel at the same sped for all observers?  Um, well, it just does!  <img src="https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f61b.png" class="smilie smilie--emoji" loading="lazy" width="64" height="64" alt=":P" title="Stick out tongue    :P"  data-smilie="7"data-shortname=":P" />(And people tell us quantum mechanics is weird.  Piffle!)Anyway.  So, Special Relativity is called that because it is the special case - describing the alteration of time and space in frames that are not accelerating with respect to each other.  They can be in motion - like my friend says she's sitting still on the ground, and I'm moving by on the train, but they aren't accelerating.General Relativity is the general case, more complicated, that also handles frames that are accelerating with respect to each other.Newton tells us that Force = Mass * Acceleration.  Therefore, if you feel a force, you are under an acceleration.  So, if you feel your weight, you are under acceleration.  So, on the planet, you are under acceleration.  If the ground wasn't under you, you'd start to speed up going downwards, right?  Thus, General Relativity gives us the answers here.  Unfortunately, to really do General Relativity requires some really heavy math - tensor calculus - which I can't even write properly in plain text of these boards.  So, I'll go to being descriptive....You, on the ground, and the satellite in orbit, are under different accelerations - you are accelerating more than the satellite is.  So, for both of you to see light move at the same constant speed, you must have different clocks and rulers.Note that I mentioned clocks *and* rulers.  It isn't just that time bends - space bends too.  In relativity, space and time are dealt with in largely the same way, which is why you hear us refer to "spacetime", as a unit, inseparable.  We can think of a duration as merely a distance traveled through the time dimension, just as a separation between two points is just a distance though a space dimension.So, you and the satellite are both accelerating.   If you compare clocks and rulers, they'll be different.  So, somewhere between you and them, space is bent.  It is bent because each of you feels the force of gravity.  The force of gravity is there because the Earth has mass.  Ergo, mass bends spacetime!    Really big masses bend spacetime so much that it curves back in on itself - this is a black hole.  Things fall in, and never come out, because all the paths out are bent back around to be paths in!  Smaller masses bend spacetime less.  So, the clocks on Earth, on the Moon, and on Mars will all be different, as the masses of the bodies they are on are different.</blockquote>

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