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Maybe a little early...
The Dime
wilowisp
But I know there's someone out there who can fill in my blanks on this. Here are my assumptions:

So a car going 50 mph passes another car, going in the exact opposite direction, also going 50 mph. From the point of reference in the car, the other car is approaching and retreating at 100 mph. Is this first assumption correct?

Light travels at a constant in a vaccuum, and there is nothing faster. I'm pretty sure that assumption is correct, but feel free to condition it.

A 'unit' of light is a photon. I'm pretty shakey on this one, but we'll just use that name for the sake of the post. (For Gods' sake, correct me!)

So if two photons are traveling in opposite directions, from the point of reference of one photon, wouldn't the other one be traveling twice the speed of light? Maybe the answer is "yes, but what the fuck does that matter, it's just a relative, not an actual speed", but I thought relativity had something to do with all this time-space-light craziness.

Please, anyone and everyone, enlighten me. While I decide if the pun was intended or not.

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I'm pretty sure that relativity handles all of that nicely, but it's been so long since I took the requisite physics that I've forgotten all of it.

If two photons are travelling in opposite directions, from the point of reference of either photon, the other one would be still travelling at the speed of light. That's what they mean when they say the speed of light is an "absolute speed limit," and that's why people thought Einstein was totally fucked up when he first thought of it.

So a car going 50 mph passes another car, going in the exact opposite direction, also going 50 mph. From the point of reference in the car, the other car is approaching and retreating at 100 mph. Is this first assumption correct?

Yes, this is correct. This is because the concept of an inertial reference frame (or "frame of reference", if you prefer) makes sense at these speeds.

Light travels at a constant in a vaccuum, and there is nothing faster. I'm pretty sure that assumption is correct, but feel free to condition it.

This is also correct - for the purposes of most of these gedanken experiments, it is assumed that the vacuum is perfect (contains no particles at all), although this is almost never the case in nature.

A 'unit' of light is a photon. I'm pretty shakey on this one, but we'll just use that name for the sake of the post. (For Gods' sake, correct me!)

The most basic bundle of energy that we call light is a photon. It can be either a wave or a particle depending on how you're looking at it, but that's not important right now.

So if two photons are traveling in opposite directions, from the point of reference of one photon, wouldn't the other one be traveling twice the speed of light? Maybe the answer is "yes, but what the fuck does that matter, it's just a relative, not an actual speed", but I thought relativity had something to do with all this time-space-light craziness.

This is the part where the thought experiment breaks down, since the only way that photon A can see photon B is by receiving a smaller photon that is given off by B. Most thought experiments assume there are two spaceships traveling parallel to each other, but in opposite directions (there can be some space stations, as well). Then the spaceships flash the current time to each other, or signal lights, and the universe gets amazingly funky. It's special relativity if they are each within the speed of light (say, 0.8c), and this is where time gets messed up. Here's an example of this kind of experiment, and the answer:

"Question: Out in deep intergalactic space, with no significant motion with respect to the nearest galaxies, are two space stations 4 light years apart. They are stationary with respect to each other. Their distance is determined periodically when one station sends a signal to the other which is immediately returned. The round trip light time is consistently 8 years.

"A space ship leaves station A and goes some distance away from A and B. It accelerates to 0.8c and then passes near A on its way to B. As it passes A, it sends a message giving the time and date on its atomic clock, and A sends it date and time to the ship. As the ship flies past B, it sends a signal to B giving its date and time. B sends a signal to the ship giving its date and time.

"Will the space ship give B a date that is 3 years later than that given to A? Will B give the space ship a date that is 5 yrs later than the date given by A?."

Answer: S will give B a date 3 years later than that given to A. B gives S a date 5 years later than that given by A. "

(ganked from here)

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