Traveling outside of the Earth's atmosphere

[sideshow]

TL;DR Summary

If space outside of earths atmosphere is a vacuum there should be no friction, therefore constant thrust should produce constant increase in speed eventually moving faster than light.am I crazy for thinking this,or is it possible?

If outer space is a vacuum there is no force in a vacuum to create friction, therefore if a vehicle i.e. spacecraft space shuttle is it a vacuum and produces any amount of thrust it should move a constant through space continuing to add thrust will only make the craft move faster until it eventually moves at a speed greater than anything humanly known even faster Then the speed of light. This is something I have been thinking about for a very long time over a decade but I’ve never had anyone to talk to you about it so I’m posting here

 

[russ_watters]

If outer space is a vacuum there is no force in a vacuum to create friction, therefore if a vehicle i.e. spacecraft space shuttle is it a vacuum and produces any amount of thrust it should move a constant through space continuing to add thrust will only make the craft move faster until it eventually moves at a speed greater than anything humanly known even faster Then the speed of light.

The premise is correct, but the conclusion is not. As it turns out, the universe isn't constructed in a Newtonian way, and while you can feel that acceleration for as long as you can maintain the thrust you'll never measure your speed to exceed the speed of light. This is part of what Special Relativity is about.

 

[Ibix]

continuing to add thrust will only make the craft move faster until it eventually moves at a speed greater than anything humanly known even faster Then the speed of light.

Unfortunately, it doesn't work that way. A constant thrust will, relative to the starting point of the rocket, produce less and less increase in velocity as the speed increases. This effect is almost undetectable at everyday speeds, which is why it went unnoticed for centuries and why we can usually neglect it, but it becomes more and more significant as you approach the speed of light.

 

[Dale]

Summary:: If space outside of Earth's atmosphere is a vacuum there should be no friction, therefore constant thrust should produce constant increase in speed eventually moving faster than light.am I crazy for thinking this,or is it possible?

continuing to add thrust will only make the craft move faster until it eventually moves at a speed greater than anything humanly known even faster Then the speed of light.

There are two types of acceleration: proper acceleration and coordinate acceleration. Proper acceleration is the physical acceleration that you feel, measured by an accelerometer. Coordinate acceleration is the second time derivative of the position, and the first time derivative of the velocity.

As you, on your rocket, continue to produce constant thrust you will continually produce constant proper acceleration. A constant proper acceleration, in turn produces coordinate acceleration, but not constant coordinate acceleration. Your speed will always increase, but not without limit. Your coordinate acceleration approaches zero as your velocity approaches c in any inertial frame. Thus you will not exceed the speed of light, but instead you will asymptotically approach c.

 

[sophiecentaur]

Thus you will not exceed the speed of light, but instead you will asymptotically approach c.

But the light traveling around your cabin will still be going at c. So your speed needs to be quoted relative to something. (Just stirrin' things up a bit but we need to keep this stuff in mind.)

 

[jbriggs444]

The eternal question for PF: Should I have compassion for the horse? Or get my two licks in?

 

[Mister T]

constant thrust should produce constant increase in speed

No, it turns out that when you test this idea it doesn't work out to be true. At speeds that are small compared to $c$ it's a very good approximation.

When researchers first encountered this phenomenon they tended to attribute it to an increase in the mass of the moving object. But that was over 100 years ago. It's now understood to be a consequence of the geometry of spacetime. It's an everyday fact of life for scientists, engineers, and technicians at thousands of places around the world.

 

[sophiecentaur]

It's now understood to be a consequence of the geometry of spacetime.

True, but the effect is very much the same as a change in mass. It's a bit like what your Physics Teacher tells you - "There's no such thing as centrifugal force" but you certainly feel it when your are in the frame of the rotating funfair machine.
As soon as you go beyond Newton, there are lots of effects that don't make sense to our bodies - just to our brains.
The (let's call it) apparent effect of mass change is described in this wiki article although it doesn't strictly follow the recent model.

 

[sideshow]

No, it turns out that when you test this idea it doesn't work out to be true. At speeds that are small compared to $c$ it's a very good approximation.

When researchers first encountered this phenomenon they tended to attribute it to an increase in the mass of the moving object. But that was over 100 years ago. It's now understood to be a consequence of the geometry of spacetime. It's an everyday fact of life for scientists, engineers, and technicians at thousands of places around the world.

I know I’m going to sound like a total retard here, but what is c ?

 

[Mister T]

True, but the effect is very much the same as a change in mass.

As long as you don't mind redefining mass as $\gamma^3 m$ and accepting that that applies only when $\vec{a}$ and $\vec{v}$ are co-linear.

 

[Mister T]

, but what is c ?

Speed of light in vacuum.

 

[sideshow]

Summary:: If space outside of Earth's atmosphere is a vacuum there should be no friction, therefore constant thrust should produce constant increase in speed eventually moving faster than light.am I crazy for thinking this,or is it possible?

If outer space is a vacuum there is no force in a vacuum to create friction, therefore if a vehicle i.e. spacecraft space shuttle is it a vacuum and produces any amount of thrust it should move a constant through space continuing to add thrust will only make the craft move faster until it eventually moves at a speed greater than anything humanly known even faster Then the speed of light. This is something I have been thinking about for a very long time over a decade but I’ve never had anyone to talk to you about it so I’m posting here

I now understand, the vehicle can only move as fast as the original thrust no matter what it is because nothing humankind has produced or discovered is faster than the speed of light and the speed of light cannot be harnessed for the purpose of Propulsion

 

[bob012345]

For you I would recommend a book such as Einstein for Beginners by Joseph Schwartz as a gentle introduction to Relativity. Try your local library.

https://www.worldcat.org/title/einstein-for-beginners/oclc/4957585

 

[Dale]

I now understand, the vehicle can only move as fast as the original thrust

No. That has nothing to do with it. If your rocket has a low exhaust velocity then you will need a much larger mass of fuel. But you are not limited by your exhaust speed. The limit is not based on that, it is based on spacetime geometry.

 

[jbriggs444]

I now understand, the vehicle can only move as fast as the original thrust no matter what it is because nothing humankind has produced or discovered is faster than the speed of light and the speed of light cannot be harnessed for the purpose of Propulsion

Rocket ships can travel many times faster than the velocity of their exhaust. This is entirely normal. We routinely launch craft to escape velocity (11,000 meters per second) using engines with exhaust velocities of 4500 meters per second or less.

Light can be used for propulsion. With anything less than matter-antimatter engines, it is not very energy efficient, but it can be done. Light has the highest energy to momentum ratio of any possible exhaust. That's great if you are mass-limited but terrible if you are energy-limited.

Google for the Tsiolkovsky Rocket Equation for more information about ships traveling faster than their exhaust streams. Tack on the word "relativistic" if you want to know how relativity affects that equation.

Edit: That horse was alive! I swear I saw it twitch!

 

[sideshow]

Ever since I was a child I found space travel and exploration to be quite fascinating however I am merely a master automotive technician I was never taught physics in high school although physics is something that I’ve had an interest in for quite some time I am now trying to learn everything that I can about the subject.All of the replies to my Quandre are greatly appreciated, as well as the suggestions for reading materials

 

[sophiecentaur]

applies only when a→ and v→ are co-linear.

The effect in a Synchrocyclotron is observed when they are normal to each other (?). Also, the 'increase in mass' was described in a wooly way but, as with a ball on a string, the 'tension' required is not just proportional to v². Not such a hopelessly dodgy description of what happens as the cognoscenti would have it.
Just imagine what future Scientists are likely to think of our use of terms like Dark Matter and Dark Energy.

 

[jbriggs444]

The effect in a Synchrocyclotron is observed when they are normal to each other (?).

Right. That's transverse relativistic mass. One difficulty with relativistic mass is that you can't just convert all the non-relativistic formulas with a single factor of gamma. You end up with $F=m\gamma a$ for transverse acceleration and $F=m\gamma^3a$ for co-linear acceleration. So your value for "relativistic mass" depends on which direction you are accelerating. It's no longer a simple scalar. Instead, it is a concept that complicates as much as it simplifies.

 

[sophiecentaur]

Right. That's transverse relativistic mass. One difficulty with relativistic mass is that you can't just convert all the non-relativistic formulas with a single factor of gamma. You end up with $F=m\gamma a$ for transverse acceleration and $F=m\gamma^3a$ for co-linear acceleration. So your value for "relativistic mass" depends on which direction you are accelerating. It's no longer a simple scalar. Instead, it is a concept that complicates as much as it simplifies.

Yebbut how would you even begin to put that over to someone who had just grasped the Ds in a klystron and needs to realize there’s an upper limit to the energy?
I’m saying it’s a bit like the duck argument: “If it quacks like a duck” etc. when it could be a goose. Same on your Sunday lunch plate.
Over simplistic perhaps but there are levels to everything.

 

[cjl]

Light can be used for propulsion. With anything less than matter-antimatter engines, it is not very energy efficient, but it can be done.

Worth noting that this isn't unique to light, this is always a tradeoff with rockets or momentum thrusters in general. You get the highest energy efficiency (delta v per unit energy) with the lowest exhaust velocity, but that requires an extremely high mass of fuel. You get the best mass efficiency (delta v per unit mass of propellant) with the highest possible exhaust velocity, but that requires far more energy.

Light obviously gives the highest possible exhaust velocity, so it's the limiting case of the second half of my statement above.

 

[anorlunda]

Light obviously gives the highest possible exhaust velocity, so it's the limiting case of the second half of my statement above.

It doesn't have to be rockets. Light sails are another way.

Sailors versus power boaters.

 

[cjl]

Sure, my statement above was specific to momentum thrusters (devices that generate thrust by throwing something out the back that contains momentum).

Light sails are cool though - you get twice the delta v for the same amount of light since it's reflected 180 degrees rather than just generated and emitted.

 

[bob012345]

Sure, my statement above was specific to momentum thrusters (devices that generate thrust by throwing something out the back that contains momentum).

Light sails are cool though - you get twice the delta v for the same amount of light since it's reflected 180 degrees rather than just generated and emitted.

If you have a mirrored surface to bounce laser light between you can get thousands of times the $\Delta{v}$.

It's been done in the lab

 

[jbriggs444]

If you have a mirrored surface to bounce laser light between you can get thousands of times the $\Delta{v}$.

There is, of course, a limitation based on red shift. You cannot harvest more kinetic energy from the light than went into the light in the first place.

In the limit, you are effectively pushing off from the mirror.

 

[anorlunda]

Light sails are cool though - you get twice the delta v for the same amount of light since it's reflected 180 degrees rather than just generated and emitted.

Even better when you pump them using a remote laser in orbit around the Sun. That makes the energy source not part of the mass accelerated.

 

[sideshow]

If you have a mirrored surface to bounce laser light between you can get thousands of times the $\Delta{v}$.

View attachment 297917

It's been done in the lab

How can a laser move an object like that?

 

[bob012345]

There is, of course, a limitation based on red shift. You cannot harvest more kinetic energy from the light than went into the light in the first place.

Of course, but if you can convert a few percent of the beam energy into useful kinetic energy rather than the miniscule ~$\frac{1}{c}$ you get normally , you have something useful.

In the limit, you are effectively pushing off from the mirror.

That's the desired goal. To make it as close as possible.

How can a laser move an object like that?

It can if it bounces over a thousand times.

 

[cjl]

How can a laser move an object like that?

Light has momentum, it's just usually too small to notice.