3 ms·

I do not know where I first saw it, but I saw someone observe that the shocking thing about Einstein's theory of relativity is not that everything is relative;

by jerf 2y ago

I do not know where I first saw it, but I saw someone observe that the shocking thing about Einstein's theory of relativity is not that everything is relative; that was understood reasonably well for a long time before him. What is shocking is that the speed of light in a vacuum, or the speed of causality, is a constant. You can derive huge swathes of relativity from that fact alone. As a result, of all the speeds, when discussing the speed of light (which conventionally means "in a vacuum" unless otherwise mentioned), you can in fact ignore the question of reference frame, with the exception of you don't want to use a reference frame itself moving at the speed of light. But other than that, an article exclusively confining itself to the discussion of the speed of light in fact doesn't need to worry itself about the relativity of reference frames. For that speed alone, you can't level the complaint against it that it ignores the issue of different frames, because it uniquely doesn't matter. (Relatedly: It is frequently given as the reason you can't reach or exceed the speed of light is some stuff about masses rising. While mathematically true in its own way, I think there's a cleaner reason to explain why you can't reach or exceed it, which is that you can't even get closer to it. No matter what you do, the speed of light is c. You accelerate to a thousand miles a second in some direction, and how much closer are you to c? The answer is, none. Light continues fleeing from you, in all directions, at c. There is no "get really close to c somehow and then just push yourself over really hard" because there is no "get close to c" in the first place. No matter how hard you accelerate, in what direction, in what order, in what manner, you not only can't get "close" to c, you can't even get closer. For similar reasons, in this paragraph, I don't need to qualify in which reference frame you go a thousand miles per second different than before, because it doesn't matter for c. You can't even get slightly closer to it, let alone "exceed" it somehow. It is an absolute.)

thfuran 2y ago
That motion is relative was understood. That even notions like simultaneity also depend on reference frame I think was shocking. And I'm not really sure what it would mean for light to not be at rest in any reference frame but for it to not have the same speed in all reference frames. I think that would be even more shocking than light having a constant speed.
the_sleaze_ 2y ago
Please bear with me, but how is this true? If we're in an airplane going 250 m/s - it also takes me 2 hours to fly to NYC. The air around the plane is windy, the air inside is still. Now we're in a spaceship headed from the earth to the moon & it's going to take us 3 seconds. We've agreed ahead of time it takes 1.5 seconds for light to go from the earth to the moon. C is the speed at which light moves, and we're taking 2x the amount of time light would. The air inside is still. But we're still going fast. How could it be that we are not approaching the speed of light? What do you make of time dilation at higher speeds if we aren't approaching the speed of light at all, ever?
- JdeBP 2y ago
  It's the stumbling block that everyone has grasping this stuff, and it's rooted in the old faulty kind of speed calculations. Speeds don't add like you think. There are many ways to grasp this, and one has to try several of them to find an intuitive explanation that works for onesself; but one way is to consider that you, in your spaceship (as long as you are coasting along with no rockets firing, and aren't performing orbit/deorbit burns) are, in your frame of reference, at rest. Your speed is zero. You haven't approached anything at all, and light is still whizzing away from you at c. Indeed, it's Terra and Luna that are experiencing time dilation as far as you are concerned, because they are the ones with the high speeds. * https://www.youtube.com/watch?v=Zkv8sW6y3sY https://www.youtube.com/watch?v=Zkv8sW6y3sY (FloatHeadPhysics addressing this in another way: there are a other approaches still) Accelerate at the beginning and end of your trip over to Luna, and of course general relativity comes into play and things get more complicated. What many gedankenexperiments get wrong is that usually there's only a short period of burning the rockets, in the real world. So for most of your trip in the rocket you aren't burning propellant and are in an inertial frame of reference. No inertial frame of reference approaches the speed of light/causality, by postulate 2 of special relativity, and one is always at speed zero in one's own inertial frame of reference.
dboreham 2y ago
iirc Maxwell had already shown that the speed of light was constant. Einstein's quest was to explain htf that could be the case. The word Maxwell appears in the first sentence of the 1905 paper, and the first couple of pages are about what today we would call "causality".
- jerf 2y ago
  In the modern formulation and understanding of Maxwell's equations, yes, they do show that the speed of light is a constant. However, in the context of the time they were written, it was not known whether or not that was a true fact of the universe or if that meant the equations were incorrect. In the historical context I'd classify it as a "clue" but not a proof. Proof really came from observations.

by jerf 2y ago

thfuran 2y ago

That motion is relative was understood. That even notions like simultaneity also depend on reference frame I think was shocking. And I'm not really sure what it would mean for light to not be at rest in any reference frame but for it to not have the same speed in all reference frames. I think that would be even more shocking than light having a constant speed.

the_sleaze_ 2y ago

Please bear with me, but how is this true? If we're in an airplane going 250 m/s - it also takes me 2 hours to fly to NYC. The air around the plane is windy, the air inside is still. Now we're in a spaceship headed from the earth to the moon & it's going to take us 3 seconds. We've agreed ahead of time it takes 1.5 seconds for light to go from the earth to the moon. C is the speed at which light moves, and we're taking 2x the amount of time light would. The air inside is still. But we're still going fast. How could it be that we are not approaching the speed of light? What do you make of time dilation at higher speeds if we aren't approaching the speed of light at all, ever?

JdeBP 2y ago
It's the stumbling block that everyone has grasping this stuff, and it's rooted in the old faulty kind of speed calculations. Speeds don't add like you think. There are many ways to grasp this, and one has to try several of them to find an intuitive explanation that works for onesself; but one way is to consider that you, in your spaceship (as long as you are coasting along with no rockets firing, and aren't performing orbit/deorbit burns) are, in your frame of reference, at rest. Your speed is zero. You haven't approached anything at all, and light is still whizzing away from you at c. Indeed, it's Terra and Luna that are experiencing time dilation as far as you are concerned, because they are the ones with the high speeds. * https://www.youtube.com/watch?v=Zkv8sW6y3sY https://www.youtube.com/watch?v=Zkv8sW6y3sY (FloatHeadPhysics addressing this in another way: there are a other approaches still) Accelerate at the beginning and end of your trip over to Luna, and of course general relativity comes into play and things get more complicated. What many gedankenexperiments get wrong is that usually there's only a short period of burning the rockets, in the real world. So for most of your trip in the rocket you aren't burning propellant and are in an inertial frame of reference. No inertial frame of reference approaches the speed of light/causality, by postulate 2 of special relativity, and one is always at speed zero in one's own inertial frame of reference.

dboreham 2y ago

iirc Maxwell had already shown that the speed of light was constant. Einstein's quest was to explain htf that could be the case. The word Maxwell appears in the first sentence of the 1905 paper, and the first couple of pages are about what today we would call "causality".

jerf 2y ago
In the modern formulation and understanding of Maxwell's equations, yes, they do show that the speed of light is a constant. However, in the context of the time they were written, it was not known whether or not that was a true fact of the universe or if that meant the equations were incorrect. In the historical context I'd classify it as a "clue" but not a proof. Proof really came from observations.