Is It Possible to Reach the Speed of Light with a Spacecraft?

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Discussion Overview

The discussion revolves around the theoretical possibility of reaching the speed of light with a spacecraft, particularly focusing on the implications of relativistic speeds and the behavior of objects moving at such velocities. Participants explore concepts from Einstein's special theory of relativity, including velocity addition and mass increase at high speeds.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether a bullet shot from a spaceship traveling at half the speed of light would appear to travel at the speed of light to an outside observer.
  • Another participant asserts that the bullet would not appear to reach the speed of light, providing the relativistic velocity addition formula to calculate the bullet's speed relative to an outside observer.
  • A participant clarifies that the "outside observer" could have different relative speeds to the spaceship, affecting their perception of the bullet's speed.
  • There is a discussion about how objects within a black hole might appear to exceed the speed of light, though the implications of this are questioned due to the inability to observe such objects.
  • One participant emphasizes that no observer, regardless of their frame of reference, would measure the bullet's speed as equal to the speed of light.
  • A participant explains that according to special relativity, as an object approaches the speed of light, its mass increases, requiring an infinite amount of force to reach light speed, which is deemed impossible with any rocket engine.

Areas of Agreement / Disagreement

Participants generally agree that no object can reach the speed of light, but there are competing views regarding the implications of relativistic speeds and the nature of observations related to objects near or within black holes. The discussion remains unresolved on some points, particularly regarding the interpretation of observations in extreme gravitational fields.

Contextual Notes

Limitations include assumptions about the observer's frame of reference and the hypothetical nature of the scenarios discussed, particularly regarding black holes and the behavior of objects at relativistic speeds.

Andrew Buren
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If you had a spaceship travleing at half the speed of light then shot a bulit at half the speed of light again, would the bulit appear to go the speed of light relative to an outside obverver?
(All preposed in theory, of cource.)
 
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If not, why?
 
Andrew Buren said:
If you had a spaceship travleing at half the speed of light then shot a bulit at half the speed of light again, would the bulit appear to go the speed of light relative to an outside obverver?
(All preposed in theory, of cource.)
No. To find the speed of the bullet with respect to that outside observer, you must use the relativistic addition of velocity formula:

V_{a/c} = \frac{V_{a/b} + V_{b/c}}{1 + (V_{a/b} V_{b/c})/c^2}

Which gives: (0.5c + 0.5c)/(1 + (.5)^2) = 0.8 c
 
Last edited:
The "outside observor" being assumed to be one relative to which the spaceship is moving at (1/2)c, of course. There can exist "outside observors" with other speeds relative to the spaceship!
 
Good point. :wink:
 
So this is like the idea that objects within a black hole can appear to be exceeding c, even though they aren't?
 
Hi there,

But no matter what, the bullet would never have c for any observer, in any relative frame.

Cheers
 
NWH said:
So this is like the idea that objects within a black hole can appear to be exceeding c, even though they aren't?

Since we cannot observe an object within a black hole, I have no idea what "appear" can mean here.
 
HallsofIvy said:
Since we cannot observe an object within a black hole, I have no idea what "appear" can mean here.
Hypothetically speaking, of course...
 
  • #10
ccording to Einstein's special theory of relativity, objects gain mass as they accelerate to greater and greater speeds. Now, to get an object to move faster, you need to give it some sort of push. An object that has more mass needs a bigger push than an object with less mass. If an object reached the speed of light, it would have an infinite amount of mass and need an infinite amount of push, or acceleration, to keep it moving. No rocket engine, no matter how powerful, could do tthis
 

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