Observers and the speed of light

[Alex S]

Imagine there are two beings, Evan and John. Evan is in a spaceship, and the spaceship's instruments tell him that he's not moving. John, who is on a faraway planet, observes that Evan is moving at the speed of light. Is this possible? If so, is it possible for Evan to accelerate until he observes that he's going the speed of light? If that's possible, what would John observe?

[Nick]

How does his spaceship's instruments determine that he is not moving? In space, everything's relative. Also, it is possible; if Evan is perfectly still (somehow), John could be moving the speed of light, and observing it as Evan moving the speed of light.

[Alex S]

If John is moving, and Evan is not, if Evan accelerated, John would perceive Evan slowing down and eventually stopping. If the ship's instruments operate on a different principle than our current ones, then it can be possible. Nothing is certain until it's measured, and velocity is not an exception. It may be possible for two different velocity measurements to be made simultaneously, and since reality (and measurement) is relative to the observer, Evan may be able to accelerate if he is already moving at the speed of light.

[Alex S]

Um, could I get an answer on this?

[Keiji]

If two spaceships were moving away from a point at the speed of light as observed from the point in opposite directions, I guess the spaceships would appear invisible to each other.

[hosj]

No. Light always reaches every observer at exactly 186,282 miles per second. No matter how fast you are moving away from it.

That is why time dilation occurs. Space and time adjust so the speed of light is 186,282 miles per second for all observers.

Unless you buy the variable speed of light theory, but that gets complicated fast.(especially for someone who hasn't gotten past algebra 1 yet)

And no, no object with mass can accelerate to the speed of light.

[hosj]

However, they would both be severely red shifted to the point of invisibility.

[Prashantkrishnan]

It is impossible for John to measure Evan's speed as equal to the speed of light. It might be 99% of the speed of light. Evan can accelerate further and try to increase his speed by an amount equal to the speed of light, but he will not succeed. He might reach 99.9% of the speed of light. Even the speed of Evan as measured by John cannot reach the speed of light.

[anderscolingustafson]

Reaching the speed of light for any object with rest mass would require an infinite amount of energy and momentum and in order to reach that speed a space ship would either need to accelerate for an infinite amount of time or have an infinite acceleration. If you moved at the speed of light then relative to a stationary observer or any observer moving slower than light your length would be zero and all clocks on your ship would stop. You would not notice this effect as from your frame of reference everything else would be moving at the speed of light and from your frame of reference the Universe would have zero length along the direction it was moving relative to you and all clocks in the rest of the Universe would stop. Also if you were moving at the speed of light all light moving in the opposite direction from you would get infinitely blue shifted and all light moving in the same direction as you would get infinitely red shifted.