Speed of light, propellent incease in mass?

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Discussion Overview

The discussion revolves around the implications of approaching the speed of light in a spaceship, particularly focusing on the concepts of mass, energy, and propulsion. Participants explore theoretical aspects of relativistic physics, the nature of mass, and the challenges of achieving light-speed travel.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that as a spaceship approaches the speed of light, the mass of the fuel increases, potentially leading to increased thrust and energy output.
  • Others argue that from the spaceship's perspective, there is no increase in mass, and the relativistic effects are only observed by outside observers.
  • A participant questions what limits the spaceship from reaching the speed of light, suggesting that mass increases to infinity.
  • Another participant mentions that the rest mass of the spaceship does not change, and physicists prefer to discuss energy rather than "relativistic mass."
  • One participant suggests exploiting time dilation by traveling close to the speed of light to increase fuel availability upon return, which is met with skepticism from others.
  • There is a discussion about conservation laws, with one participant asserting that mass is conserved and that energy can increase due to external forces.
  • A participant expresses interest in the idea of a spaceship collecting antimatter fuel while traveling close to light speed, raising a hypothetical scenario about future space missions.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the nature of mass and energy as one approaches the speed of light. There is no consensus on the implications of relativistic effects or the feasibility of proposed propulsion methods.

Contextual Notes

Some statements reflect unresolved assumptions about mass and energy conservation, as well as the definitions of relativistic mass versus rest mass. The discussion includes speculative scenarios that may not align with established physics principles.

ThoughtProces
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So i just came across some information that states that if you were in a spaceship trying to reach the speed of light, since the reacting force is being produced on the spaceship and all mass traveling towards the speed of light increase. This means that fuel mass increases and so does the thrust produced due in part to the increase in fuel mass. So does this mean that as you move towards the speed of light then your energy output would increase proportionally to your velocity?
 
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As far as the spaceship is concerned there is no increase in mass. The relativistic effect is seen by outside observers going at a much different speed.
 
ThoughtProces said:
So i just came across some information that states that if you were in a spaceship trying to reach the speed of light, since the reacting force is being produced on the spaceship and all mass traveling towards the speed of light increase. This means that fuel mass increases and so does the thrust produced due in part to the increase in fuel mass. So does this mean that as you move towards the speed of light then your energy output would increase proportionally to your velocity?

No.
Remember that the laws of physics do not change with speed. The fuel will work the same, regardless of your speed with respect to any reference point.
 
So then as you reach the speed of light what is it that limits you from reaching c and or over coming it...doesn't your mass increase to infinity?
 
If the observer is on the spacescraft, the spacecraft will probably have to lose mass for acceleration. then the only propellants you have available are a) light photons, created using only energy, or b) throwing mass out the back, i.e., using energy to accelerate ions, or chemical propellants
to produce thrust.
If you cannot afford to lose mass, then maybe a laser beam will have to do.

If you want to use a nuclear reactor, then maybe Project Rover should be revived. If you want to use nuclear explosions, then ?
 
so then its more a matter of the amount of fuel needed to actually get there, increase...
 
ThoughtProces said:
So then as you reach the speed of light what is it that limits you from reaching c and or over coming it...doesn't your mass increase to infinity?
Your rest mass never changes at all, and physicists these days usually prefer to avoid the concept of "relativistic mass" and just talk about energy instead. In the frame of reference where you're approaching the speed of light, your energy does approach infinity as your approach c, so you will never reach c as long as only a finite amount of energy is being used to do work on you and increase your kinetic energy.
 
I kind of get it, now the actual constant of the speed of light, do physicists know what gives us this universal constant?
 
sry to correct, i guess its not a universal constant
 
  • #10
What you can do is exploit time dilation. If you have a fuel factory on Earth that has some fixed output per unit time, then you can take some of the fuel, travel close to the speed of light and then when you return, you'll have a huge amount of fuel available, allowing you to travel even closer to light and with the larger time dilation you will have even more fuel available when you return.
 
  • #11
Post 10 is just plain silly...
 
  • #12
Naty1 said:
Post 10 is just plain silly...


Not silly at all. Suppose you have an antimatter factory that works on solar power that is in orbit around the Sun. You have a photon rocket that is powered by antimatter. Your goal is to travel to some distant location in the least possible proper time.


Mass of photon rocket without fuel is M

Define tau to be the time it takes for the factory to produce an amount of M of matter/antimatter fuel for the rocket.

We want to travel to a location that is at distance L = lambda c tau.

What is the optimal strategy that will allow the traveler to arrive at the location in the least proper time?
 
  • #13
ThoughtProces said:
So then as you reach the speed of light what is it that limits you from reaching c and or over coming it...
If you use a inertial guidance system not programmed to deal with relativity (it senses acceleration and multiplies by time to get velocity), you'll think you have accelerated beyond C. But if you look in your rearview mirror, Earth will be closer than your navigation system says it should be.
 
  • #14
Conservation

That is wrong. The term "relativistic mass" has been died out. Because just multiplying a constant "c2" we get the energy E= mc2/(1-u2/c2). It can be called " relativistic energy" .
If the mass increases then conservation law will be broken. Mass is conserved. Energy is also conserved but your system's energy can increase because external force works on it. But your system itself does not increase energy.

So, in every closed system, the total relativistic energy and momentum are conserved.

But rest mass is not conserved. For example think of a nuclear bomb where mass is transformed to energy following E=mc2
 
Last edited:
  • #15
Post #12 is unrelated to post #10...
 
  • #16
I really like the idea of the spaceship traveling close to the speed of light trying to pick up a huge stockpile of antimatter fuel at rest. Some of the resulting little bits of spaceship should definitely get even closer to the speed of light.

If the mission could be done by the year 2109 do you think we will still have the "Darwin Awards"? It seems like a winner to me.
 
  • #17
Naty1 said:
Post #12 is unrelated to post #10...


Just solve the problem explained in post 12. What does the solution look like in the limit of large lambda?

You assume that you start with a photon rocket without any fuel.
 

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