# Hypothetical spacecrafts traveling at light speed.

## Main Question or Discussion Point

Ignoring the mass problem, imagine 2 spacecrafts are traveling at exactly the speed of light. One spacecraft directly behind the other, with only a hundred feet or so between them.

If the speed of light is always the same for any observer, is it safe to say that when the trailing spacecraft flashes a beam of light at the leading spacecraft, that an observer WOULD see the leading spacecraft as having been "illuminated"???

## Answers and Replies

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Ignoring the mass problem, imagine 2 spacecrafts are traveling at exactly the speed of light.
You are having a question about a situation that cannot physically exist, not even in principle. So how do you expect a sensible answer?

You are having a question about a situation that cannot physically exist, not even in principle. So how do you expect a sensible answer?
My apologies.

Agreed. Unless you're willing to rephrase your question to "Just under the speed of light" there's no way to answer. If you rephrased the question so, the answer would be 'yes'.

Mentz114
Gold Member
Ignoring the mass problem, imagine 2 spacecrafts are traveling at exactly the speed of light.
Also you have not said in what frame of reference they are measuring the speed. Any statement 'a is travelling at speed v' is meaningless unless it is specified what observer measures v.

I think that it would not be actually possible to see them as they both would have 0 size to any third observer. But I am not sure that the rule is exactly such, and if we ignore that I would say that to any third observer both travellers would seem to be completely halted. So a third observer would not be able to see the one behind doing anything to light the flash. Again, if we ignore that then anyway a third observer would not see the spacecraft in front being iluminated. The third observer would see that they both manage to catch up with all the light beams that get lighted out of their spacecrafts in the same direction that they are travelling. I.e. you yourself can not see that you catch up with a light beam when running after it but any other observer that moves with a different speed can see you doing more or less well.

I'm not an expert though.

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I think I may get what you are asking and there is purpose for exploration. It is not that nothing can go faster than the speed of light ( save things with mass) which light does not. It is that INFORMATION can not travel faster than light, usually making any discussion of this type a moot point seeing as their would be no way to prove or even know that something has broken the lightspeed mark. With that said if you do want to ignore the spaceship problem and strictly discuss the behavior of light then this is a very interesting question and it is disappointing to see snappy responses to your question. If your situation were to occur the rear spaceship would impart its relative motion onto the light causing it to shift frequencies. If the space ship in front were going slightly slower than the spaceship in the rear I believe the front ship would receive some sort of notification

If the speed of light is always the same for any observer, is it safe to say that when the trailing spacecraft flashes a beam of light at the leading spacecraft, that an observer WOULD see the leading spacecraft as having been "illuminated"???
This, not "the mass problem," is the reason nothing can travel at c. The speed of light is the same for everyone, so there is no reference frame which travels at the same speed as light rays do, for in that frame, photons would be at rest, not traveling at c. Mass has nothing to do with it.

No mass has nothing to do with it and I hope Holocene understands it is only through relativity that you cannot break the speed barrier. In short light is always a constant speed and if you were driving forward and turn on your headlights they would come out, as they should. Clearly if you are traveling at x speed and your headlights are running out in front of you AND we know that light never goes faster than C It is by default or simple reasoning that you cannot break the speed barrier. Einstein says the speed of light is always the same and never faster and anytime you try light will out run you. 2nd place means you cant break the speed barrier ;)