Energy, Mass, Speed of Light: Can We Reach It?

[LawH]

TL;DR Summary

I am not a physics student of any kind, but not knowing the answer to this has bothered me for some time

Hello everyone!

Let's say that you were to attempt to go as fast as possible on a spaceship with the mass of an average car in an absolute perfect vacuum. What I am wondering is, that if you were to reach a certain speed, and stop applying energy to this imagined spaceship, would the spaceship start slowing down by itself?

What I want to understand is, that reaching the speed of light in this spaceship would be impossible, because it would require a jolt of infinite energy, or that to keep it going at the speed of light, it would require a constant infinite input of energy to the spaceships movement to keep it moving? If you were to travel close to the speed of light in this spaceship, would you have to keep applying energy to it's movement, even if you had reached the speed that you wished to travel into keep it going, or, if you would get your ship to travel a certain speed, it would remain traveling at this speed and the problem is actually getting the ship to reach that speed, but once reached, the ship would carry on traveling in the speed it was pushed to travel in?

I hope you understood my question, and I hope I can understand your answers! :D

 

[russ_watters]

Let's say that you were to attempt to go as fast as possible on a spaceship with the mass of an average car in an absolute perfect vacuum. What I am wondering is, that if you were to reach a certain speed, and stop applying energy to this imagined spaceship, would the spaceship start slowing down by itself?

At one time people believed yes, but now we know the answer is no, as per Newton's 1st law.

 

[Orodruin]

Let me just underline and boldface the fact that this is nothing peculiar for relativity. This was already part of classical mechanics, as formulated by Newton through the first law.

 

[LawH]

Alright, so theoretically anything with mass could travel at almost the speed of light forever without any thrust applied to it after it had reached a certain speed.

This has bothered me for a while, and I wasn't sure if something other than Newtonian physics was going on at this speed, when it comes to movement. For some reason it had always been unclear to me whether reaching light speed was a problem due to the need for continued application of thrust, or whether it was actually getting to that speed. Thanks!

 

[LawH]

Yes, I have understood the classical mechanics, but was wondering about this particular fact. Using an infinite energy baseball bat, and striking a ball with it, would keep the ball moving forever at light speed in a vacuum, so to speak.

 

[DaveC426913]

Yes, I have understood the classical mechanics, but was wondering about this particular fact. Using an infinite energy baseball bat, and striking a ball with it, would keep the ball moving forever at light speed in a vacuum, so to speak.

Nigh-infinite energy in the bat. Can't have infinite energy.
The ball would keep moving (effectively) forever at nearly the speed of light. Can't reach c.

 

[Mister T]

What I am wondering is, that if you were to reach a certain speed, and stop applying energy to this imagined spaceship, would the spaceship start slowing down by itself?

First of all, you have to say what it's moving with respect to. As I sit here and type this I'm moving at near light speed relative to an electron in a particle accelerator. There is no experiment that you can do, even in principle, that will distinguish between a state of rest and a state of uniform motion. So, no, your spaceship will not start slowing down, relative to, say, planet Earth, when you shut the engines off.

What I want to understand is, that reaching the speed of light in this spaceship would be impossible, because it would require a jolt of infinite energy

There's no such thing as infinite energy. What that trite comment means is the following. Starting with the postulate that the speed of a beam of light in a vacuum is $c$ relative to you, regardless of how fast you chase after it, you can conclude that no matter how fast you move you will never catch it. Thus you can never reach a speed of $c$. You don't need to look at the concept of energy to reach that conclusion.

But if you throw in as another postulate the notion I described above in my first paragraph, you can come up with a relation between the energy of a particle and its speed. As that speed approaches $c$ the energy increases beyond all bounds. Or using the jargon, the energy approaches infinity.

 

[LawH]

There is no infinite energy baseball bat I think we can all agree on that. But this particular question is answered now I feel, and I can move on to understanding how it's possible that light travels at the speed of light regardless of the observers' relative speed to each other and light.

 

[PeroK]

There is no infinite energy baseball bat I think we can all agree on that. But this particular question is answered now I feel, and I can move on to understanding how it's possible that light travels at the speed of light regardless of the observers' relative speed to each other and light.

Yes, that's a more interesting question.

 

[Ibix]

Just to reiterate something @Mister T said - you cannot talk about speed without saying what it is you are regarding as stationary. This is part of the "principle of relativity", which says that all physical laws take the same form in all inertial frames of reference. This tells you immediately that you won't start slowing down above a certain speed, because that would apply to everybody. You'd have to start speeding up so that someone moving past you sees you slowing down from their reference frame, and two people moving past you in opposite directions would need you to speed up in both directions at the same time in order for you to be slowing down according to both of them. This is obviously inconsistent. As others have noted, the principle of relativity has been a feature of physics since Galileo and no violation of it has ever been detected.

Regarding the invariance of the speed of light, that is also a postulate. That is, it is something for which the only answer to "why" is "because physical models make accurate predictions when we assume that". Investigating how this can be consistent is essentially "learning special relativity". It doesn't require maths any more complicated than Pythagoras' Theorem, but letting go of the notion of a single universal notion of time does seem to give people trouble.