I understand that it’s approx 186000 miles per second. I got that. But 186000 miles per second in relation to…what?
Let me see if I can verbalize what it is that I can’t wrap my mind around. Let’s say I’m traveling in a plane. That plane is moving through the air at an airspeed of 300 miles per hour. However, it is flying into a strong headwind, so my speed in relation to the ground is less, say 250 MPH.
So the speed of the plane in relation to the air around it is 300 MPH and the speed of the plane in relation to the ground is 250 MPH.
But an object traveling through space at the speed of light is traveling 186000 miles per second in relation to what? The space around it? That doesn’t make sense to me. The center of the nearest galaxy? The center of the universe?
But is point A moving or is point A standing still? Let’s say point A is moving towards point B, but point B is also moving at the same speed and direction as point A. The distance between the two never changes. They are both moving at 1/2 the speed of light. Since they are 186000 miles apart, does it still take light one second to travel from A to B? If so, that would mean light would be traveling faster than 186000 miles per second, since it’s not only traveling the 186000 miles between A and B, but it would also be traveling the distance that A and B have moved during that second
And how do we know how fast point A would be moving? Is there a reference point somewhere in the vast emptiness of space?
I started writing a clear, concise explanation of how time slows down for a moving observer, but relativity was never my forte. I’m sure someone will be along soon for whom it is.
That’s kind of the significance of the constant speed of light right there. In my reference frame, I would say it’s moving at 186,000 miles per second relative to me. But in your reference frame, you’d say it’s moving a 186,000 miles per second relative to you. This is true even if you are moving at some non-zero speed relative to me.
This probably seems like it leads to some kind of paradox or contradiction, and indeed it would in the physics of Newton or Galileo. The only reason we can sensibly say that light moves at the same speed in both our reference frames is because we allow that measurements of time intervals and distances in my reference frame will differ from measurements of those same time intervals and distances in your reference frame.
Basically, Einstein started from the assumption that the speed of light is the same in every reference frame (despite the fact that we wouldn’t have traditionally expected this), and showed that as a consequence of this, other things that we would have expected to be the same in all reference frames are in fact reference frame dependent.
It all seems pretty weird, but we know from experiment that Einstein was right.
It’s not that simple . . . if you and I are in different reference frames, we may disagree on the distance between points A and B . . . but we’ll also disagree on the time it takes light to travel between them, and these two disagreements perfectly cancel out so that we do agree on the speed of light.
Seen through the lens of our everyday experience, this is all very counter-intuitive. (Of course in our every day experience we don’t have different observers moving relative to each other at a significant fraction of the speed of light.) Pduol’s question is a good one.
OK, I’m not a physicist so this might be wrong :-p, but this is the “not a physicist” version that got explained to me.
The speed is described in comparison to the point of reference to another object and must be described separately in reference to other objects, distortions in relative time occur because of this.
Imagine you have two planets, that are somehow perfectly at rest in relationship with each other.
Planet A sends a spaceship towards planet B at .75C and Planet B does likewise
both spaceship A and spaceship B are travelling at .75C with respect to both planets which is OK. However this would seem to mean that space ship A is travelling at 1.5C relative to space ship B which is not OK. Distortions in space time will prevent this from occurring and will complicate pretty much any other case in which objects are moving very fast in response to each other, at this point my brain breaks.
Exactly! That’s why relativity tells us that things like time and distance change. If the speed of light is a constant, which it is, everything else has to be flexible to make room for it.
[Maybe beating the horse’s corpse here, but]
This is actually a really good question, and led to that whole “Relativity” thing.
Around 1900 some guys tried to answer this and found out it was the same in any direction. Which led to this guy named Einstein thinking hard about it and realizing that light could travel at the same speed in relation to everything only if distances and times are very different for you than for someone moving very quickly relative to you. Turns out the equations Einstein came up with actually predict the real world (as proven by the fact that GPS works).
So, again the answer is, anyone, no matter how fast they’re moving in relation to anything else, will always measure the speed of light (in a vacuum) as the same speed. But they might not measure distances and times the same as someone moving relative to them.
Relative to the speed of light, it is. That’s the point behind the theory of relativity. You know all those physics formulas that include “v” in them? Per Einstein – and he could prove it mathematically, and physicists have demonstrated it with real-world proofs – that “v” is actually subject to a correction factor of the square root of a number that is one minus v[sup]2[/sup]/c[sup]2[/sup] – for “normal”, non-relativistic velocities, that fraction approximates zero and the resulting number pproximates one. But as “v” approaches “c”, it deviates farther and farther from “what it oughta be” by everyday experience – because everyday experience doesn’t deal with the force or weight or duration of things traveling at substantial fractions of “c”.
Although Einstein seems to have been prompted more by Maxwell’s theory of electromagnetism and the consequences thereof, rather than directly by experiments such as Michelson-Morley. I am not even sure that he was aware of that experiment at the time he started thinking about Special Relativity.
Last I saw, the evidence was that he was not aware of the Michelson-Morley experiment. In a way, it makes Maxwell’s work even more impressive, in that he managed to come up with a correct relativistic formulation of electromagnetism decades before relativity was even developed.