My question involves faster than light travel. I'm not sure that I've ever seen a valid thought-experiment here, and I'm wondering if anyone else has, or can come up with one. Like many of you, I'm used to thinking of light speed as a constant, i.e. if you are travelling toward a beam of light that is coming toward you, the speeds don't add. What I'm wondering is whether that also applies to two masses. Supposedly it does, but like Einstein often said, "I'm convicted (by the equations) but not convinced (by intuitive understanding)". Parenthetical phrases are added by me.
Here goes: Suppose you are sitting in your spaceship out in interstellar space. Sensors detect a Federation ship approaching at .9c, and a Romulan ship approaching from the opposite direction at .9c. Yet when you talk to them ship-to-ship, they each claim the other is still going less than lightspeed. I'm willing to accept it, as long as I can gain an intuitive understanding of why it should be so. Could it be just because of the way they measure distance, using light going at c? Would their measurements be closer to c as they approached and passed by each other? My thought is, just because light can't be made to go faster than c, does that necessarily mean that nothing can? Can handle math but prefer thought experiment. Thank you.

What do you consider a "thought experiment"?


If you deny relativity, then you can claim the relative speeds of the ships are whatever you want.

If you accept relativity (however tentatively) then you have to calculate the speeds according to that:

If one ship is approaching at a speed, relative to you, of .9 c and the other is approaching from exactly 180 degrees away at a speed, relative to you of .9c, then the speed of either one relative to the other is (.9c+ .9c)/(1+ (.9c)(.9c)/c^2)= 1.8c/1.81=
.994 c.

That's about as thoughtful as it gets.

Suppose you are sitting in your spaceship out in interstellar space. Sensors detect a Federation ship approaching at .9c, and a Romulan ship approaching from the opposite direction at .9c. Yet when you talk to them ship-to-ship, they each claim the other is still going less than lightspeed.
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Interesting thought experiment. First, the light would be very highly phase shifted coming from the approaching ship. The observer would (and this is my opinion) be able to determine the relative velocity of the oncoming vessel. If my speed is known to me, and the approaching vessel's light is shifted significantly, I should be able to determine its relative velocity. I think you are mistaken about the observed phenomena.

Here's where it gets really nasty...

Each ship's radio normally emits a radio frequency of 1 Terahertz.
At .9c each ship's radio signal is time-dilated accordingly, and actually transmits to 100 Ghz. (approx.) But, If I'm approaching that ship at .9c, it reaches me at 1 Thz.

The problem is, my equipment is time-dilated. I will percieve that signal to be transmitted at ~10 Thz. I will percieve the events of the oncoming ship to be occuring at approximately 100 times the normal rate of time.

The moment we pass each other, things happen in reverse. I start 'running over' the signals transmitted by the departing ship, backwards.
The signal still reaches me at 1Thz, but because my equipment is time-dilated, and I am going the opposite direction, I recieve them at ~900Ghz (reverse orientation), and I percieve the events on the departing ship occuring in reverse order, time-dilated only 10 percent.

Understand the math, and the time dilation. What I am after is something along the lines of:
Assume the two ships measure each other's velocity to be 1.8c. This then leads to a contradiction which can be PHYSICALLY described as.....
Basically, I'm not saying the math is wrong, I'm only wondering if the measurements are less than c because of limitations in the measuring devices or not. Such limitations could be ingrained in the universe so that an object could go >c but never be observed to do so. This may seem philosophic and trivial, but a similar question in quantum mechanics, the EPR thought-experiment, let to a new understanding of how qm works (Bell's inequality). There could be such a thing hiding here, as well.