Why Can't Objects Accelerate to Light Speed?

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Discussion Overview

The discussion centers around the question of why objects cannot be accelerated to the speed of light, exploring concepts related to mass, energy, and the laws of motion as they approach relativistic speeds. The scope includes theoretical explanations and conceptual clarifications regarding relativistic physics.

Discussion Character

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

Main Points Raised

  • One participant suggests that an increase in velocity does not result in an increase in mass, questioning why objects cannot reach light speed.
  • Another participant explains that an object's energy increases without limit as its velocity approaches the speed of light, requiring infinite energy to reach light speed.
  • A participant raises a question about the nature of resistance or counter forces that might affect acceleration as speed approaches light speed.
  • Another participant clarifies that the decrease in acceleration as speed approaches light speed is not due to a counterforce, but rather the way speeds add in relativistic physics, contrasting classical and relativistic velocity addition.

Areas of Agreement / Disagreement

Participants express differing views on the nature of mass and acceleration, with some proposing that no counterforce is needed while others inquire about the implications of approaching light speed. The discussion does not reach a consensus on these points.

Contextual Notes

Participants reference classical and relativistic laws of motion, indicating that assumptions about velocity addition may differ significantly at relativistic speeds. There is an acknowledgment of the limitations of classical physics in explaining phenomena at high velocities.

Who May Find This Useful

This discussion may be of interest to those exploring concepts in relativistic physics, including students and enthusiasts seeking to understand the implications of speed on mass and energy.

trigonatus
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From a quick search of the forum is appears that an increase in velocity does not result in an increase in mass. If this is correct why can't objects be accelerated to light speed and beyond?

If every action causes an equal and opposite reaction, what force stops an object from reaching light speed?

Thank you in advance.
 
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An object's energy increases without limit (##E \rightarrow \infty##) as ##v \rightarrow c##.

totalenergy2.gif


As you give an object more and more energy, by continuing to "push" on it somehow, its speed continues to increase, but more and more "slowly" as it gets closer to c. It can never actually reach a speed of c, because that would require an infinite amount of energy, which is impossible.
 
Thank you both.

Why more and more slowly as it gets closer to c? Is there some type of additional resistance or counter force that causes the decrease in acceleration as c is approached?
 
trigonatus said:
Thank you both.

Why more and more slowly as it gets closer to c? Is there some type of additional resistance or counter force that causes the decrease in acceleration as c is approached?

There is no counterforce needed - it follows from the way that speeds add.

You are almost certainly thinking in terms of the classical law of velocity addition, which says that if A is moving at speed ##u## relative to B, and C is moving at speed ##v## relative to B, then C is moving at speed ##(u+v)## relative to A. Using that law, a spaceship moving at .99c relative to A and at rest relative to B could fire its engines and accelerate to .02c relative to B - and then it would be moving at 1.01c relative to A.

But that's not really how speeds add - the classical ##(u+v)## is an approximation that only works for speeds that are small compared with the speed of light. The more accurate law that you have to use for speeds near that of light is ##(u+v)/(1+uv)##; use this and you'll see that even though nothing prevents the ship from accelerating just as you would expect, it still can't get above c relative to any observer.
 
Last edited:
Now, I understand.
Thank you.
 

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