Yes. If you’re an observer moving along with point A or point B, you would see light take one second to travel between them.
However, if you’re an observer at point C watching A & B whiz past you at half the speed of light, the beam from A to B will take a longer or shorter amount of time to make the trip (depending on whether it’s traveling A-to-B or B-to-A).
There’s a decent explanation incorporating what everyone has said using the train thought experiment here. Basically, a woman standing on the middle of a train is passing a station with a man standing on the station. Lightning strikes the train–one bolt in the front of the train, one bolt in the back, and leaves marks on the front and back of the train, as well as the ground. To the man, it (for this thought experiment) happens at the same time. He can go out and measure the marks on the ground and they’ll be the exact same distance to where he was standing on the platform. But the woman will have seen the bolt that hit the front of the train strike first, since she’s moving toward the light from that bolt, and away from the light from the bolt that hit the rear of the train. So time is different for her. Not only that, but length is different, too–the train, while moving, will for her be a different length than it is for the man on the platform. The link explains it better than I can.
The Michelson-Morley among others experiments tried to solve the same dilemma as the OP. They measured the speed of light from a star when the earth was moving toward it and away from it in orbit and found the speed of light the same.
They used the interferometer on a spinning platform to measuure the speed of light at different angles to the motion of the light source and the earth; same speed. (Interferometer sends light down two arms at right angles, then puts the light beams back together. As you increase the length of one arm, you can see by wave interference that you are increasing it.
If light is dependent on some underlying medium, it will travel faster in some directions than others. Set up your interferometer, rotate it and see if it has the same effect as making the arms longer in one direction. It did not. The speed of light therefore is constant in any direction.
What color was her dress?
I think this says it all.
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If she’s approaching fast enough, gamma-ray-colored.
The last thing he ever saw before his retinas were seared away was his love racing toward him at nearly the speed of light … .
Yes and no. An observer on one of the planets does see the distance changing between the spaceships at a rate of 1.5c. But that observer does not see any object moving faster than c relative to him, i.e. in his frame of reference. And because of relativity, an observer on one of the spaceships measures his speed relative to the other one as less than c.
That all depends on your frame of reference.
Giggity!
Zero in relation to what?
(There is no absolute frame of reference for linear motion, so nothing can be guaranteed to be moving, or not)
They aren’t moving relative to what?
How different would the universe look if light travelled at half the speed it does? What about a quarter of the speed?
That didn’t seem right to me at first, then I realized that once the beam is “released” you can ignore A completely. B, however, is moving away from the beam, so the speed of the beam is constant but the distance it has to travel is longer since B is moving away (from the perspective of the observer). Is that right?
To the traffic cop you just went by.
It’s a complicated set-up.
Since A and B are moving relative to C, an observer at C will see the distance between them shortened. So they’re less than 186,000 miles apart from his point of view. (If they’re moving at .5, my back of the envelope calculations say they’d be about 160,000 miles apart.)
So a beam of light traveling from A to B only takes .85 seconds to get to where B was when it was emitted. But at that point B is ~70,000 miles further ahead. The beam will eventually catch up to B, but it will take much longer than a second to get there, even though the distance between A and B is less than 186,000 miles. (From the perspective of C.)
But if, at the same moment, you shoot a beam from B to A, it arrives much faster. It only has 160,000 miles to travel and A is closing with it at the same time.
So, from the perspective of an observer at C, the B-to-A beam arrives about a second before the A-to-B beam. But from the perspective of observers at either A or B, they arrive simultaneously.
There has been some (I think) good answers in this thread. But I’m still befuddled about the whole thing. My mind just won’t wrap around it. Does anyone have any links that can, as simply as possible, explain the whole relativity and speed of light thing to a non-scientist, non-physicist, layman type person?
Here’s another example that might help:
Alan is on a rocket traveling at 99% of the speed of light relative to Becky, who is watching him from a nearby space station. Alan turns on a flashlight and shines it straight ahead. From his perspective it looks like the beam of light is traveling at 186,000 miles per second. It flashes out ahead of his rocket very, very quickly. However, from Becky’s perspective the beam is also traveling 186,000. That means to her it seems as though the beam is slowly widening the gap as the rocket chases after it.
How can these two perspectives be reconciled?
One of the things that relativity says is that time flows more slowly for fast-moving objects. And that fast-moving objects will be contracted along the direction of movement. From Becky’s perspective, Alan’s rocket is stubby, and his clocks are running slow. But from Alan’s perspective his rocket is the normal length and his clocks are running normally. So Alan and Becky, by virtue of being in different reference frames, have different measuring tools. Alan’s yardstick isn’t the same length as Becky’s yardstick, and Alan’s clock doesn’t run at the same speed as Becky’s clock. Light can travel at 186,000 miles per second relative to both of them, because each has their own definition of what a “mile” and a “second” is.
I remember reading a description of what things would be like if light moved at one metre per second or something. If you run, objects behnd you appear to move in front and bunch up there.
I think there’s a limit to how much you can simplify it, because much of it is non-intuitive. But, as a layman myself, I found Einstein for Everyone to be a pretty thorough and, as far as I can tell, authoritative introduction to the topic.
Here’s another perspective that might help. Everything in the universe moves at the speed of light. Moves through space-time, that is.
When two objects are not moving relative to each other, they each perceive the other as moving through time as the same rate as themselves, and what one perceives as distance will match up with the other’s measurement. When they are moving relative to each other, what one object perceives as the time axis is not the same as the other, so they perceive time flowing at different rates, and what they measure as spatial distance will also differ.