# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jkwaters2 on 01/10/2012 04:25:55

Title: If an object drifts in space without stopping, what stops acceleration in space?
Post by: jkwaters2 on 01/10/2012 04:25:55
I have long pondered the speed of light and how it is used to define so much.  I was once more of a student but I grew up and had to get a job.  But now with the news of pulse fusion engines on the horizon I think again on two things that always baffled me.
1.  Why is the speed of a light wave the maximum speed attainable; could it be just the fastest naturally occurring phenomenon?
2.  Why is speed in space a problem (not withstanding collisions/radiation/gravity of nearby object/etc.) when an object will drift forever once in motion?  What slows acceleration in space assuming gravity and light hit equally from all sides? Formulas aside, if there can be acceleration at all, and a mass in movement continues to move, why would there be any limit in speed as we define it?  Is there a simple answer out there so I can sleep at night?
Title: Re: If an object drifts in space without stopping, what stops acceleration in space?
Post by: CliffordK on 01/10/2012 07:52:55
The problem is ...  Say an object is travelling 3/4 the speed of light.
Then one accelerates it again, say another 3/4 the speed of light.
Isn't the object now travelling 1.5 x the speed of light?
Is there a directionality?  So, accelerating the object in one direction is different than accelerating it in the other direction?

For a universal speed limit, one needs to define some kind of a "fabric of space" or "fabric of space-time" within which everything is embedded.

There was a theory of Luminiferous Æther (http://en.wikipedia.org/wiki/Luminiferous_aether), largely superseded by modern physics since no such matrix or grid has yet been discovered.  But, it seems as if one must have some kind of a matrix to relate the movement of bodies, particles, photons, and etc, or the "fabric of space".  Without it, one could not define the motion of one object (for example Earth) with respect to another object (the sun).
Title: Re: If an object drifts in space without stopping, what stops acceleration in space?
Post by: yor_on on 01/10/2012 10:32:27
What you're considering is why a object stops accelerating in space, right? If there is no 'resistance'? It's a principle of the universe it seems, you can't get more energy out of a system than what you put in. For the rocket to continue a acceleration it needs energy and burning that energy in 'action and reaction', as in a propulsion, it will accelerate but once you're out of energy action and reaction stops. And that goes for turning of the engine too.
=

If you mean why we can't reach 'c', the speed of light in a vacuum? Another principle of the universe in where the energy you would need never can be enough for matter. Another way to see it is to look at it as it was gaining (relativistic) mass, getting heavier to propel for the engine, needing to burn a 'infinite energy' before ever reaching 'c'. It's a barrier of sorts where you meet infinity's mathematically. And then we have the Higgs way in where I might presume the space to become 'clogged' with Higgs Bosons, as you start to gain relativistic speeds :)
Title: Re: If an object drifts in space without stopping, what stops acceleration in space?
Post by: syhprum on 01/10/2012 12:29:04
There have been many substitutes for the æther , the CMBR, the sea of Neutrinos, Dark matter, Machs, universal gravitational field now we have a new one the clogging effect of all the Higgs particles.
it was nice and simple when we just had the luminiferous stuff.
Title: Re: If an object drifts in space without stopping, what stops acceleration in space?
Post by: yor_on on 01/10/2012 13:38:33
Yes Clifford, I'm wondering about that one too. How do we define a relativistic motion? I find blueshift as the measure there, and a blueshift is defined as belonging to another frame of reference. But it becomes a philosophical dilemma to in that the light you see then can be defined as only needing to be one Plank length from you, as I see it :) to become another frame of reference. If you do it my way 'locality' becomes a very narrow definition, and what we describe by light/radiation must constantly be infinitesimally time dilated as well as LorentzFitzGerald contracted. Macroscopically it has to do with how we define frames of reference as something 'being at rest', but introducing gravity that definition must 'shrink' as I think. And gravity is still the metric of Space, wonder how the Higgs solve that?
Title: Re: If an object drifts in space without stopping, what stops acceleration in space?
Post by: lightarrow on 01/10/2012 15:31:51
1.  Why is the speed of a light wave the maximum speed attainable; could it be just the fastest naturally occurring phenomenon?
It is the "maximum speed of causal connection between two events in spacetime".
Roughly speaking it means: you want a rose lamp to switch on to your girlfriend's home "when you push a button" to your home; in the sense that you want the event "pushing the button" be the one and only cause of the event "rose lamp switches on to your girlfriend's home".

The maximum speed between the two events is c.

Quote
2.  Why is speed in space a problem (not withstanding collisions/radiation/gravity of nearby object/etc.) when an object will drift forever once in motion?  What slows acceleration in space assuming gravity and light hit equally from all sides? Formulas aside, if there can be acceleration at all, and a mass in movement continues to move, why would there be any limit in speed as we define it?  Is there a simple answer out there so I can sleep at night?
At low speeds it happens as you think: keeping accelerating with the same engine's power results in constantly increasing of speed: 10 m/s, then after 1s: 20 m/s, after another second 30 m/s and so on.
But at high speeds this simple law is not true anylonger.

Note, however, what would happen for you inside a spaceship which travels keeping the engine on at the same power: for your friends at home on Earth you will reach a maximum speed (light speed) but for you it is as if your speed would keep increasing indefinitely because near c you would visit almost all universe in just a few seconds of your time (and of course you would be aged of those few seconds as well).

Edit:
So, from the viewpoint of the astronaut, c doesn't mean a real limit.
Title: Re: If an object drifts in space without stopping, what stops acceleration in space?
Post by: jkwaters2 on 01/10/2012 17:22:28
Alright, I have read your responses and I understand everyone's reasoning.  But I am trying to stand back and think more simplistically.  We see a falcon and know the fastest animal is 200MPH in a dive however we easily overcome this with engines and such.  Light has a speed like the falcon, it is finite and known.  I like the analogy of the lamp turning on/off and light being "instantaneous" (i.e. nothing can be faster or the light would turn on before the switch is flipped) however clearly this is only a function of distance.  Move that lamp to a mountain top (or a candle) and you can now measure a time between turning it on and seeing the light.  Spread it out in space and now it is not unusual at all to ask if something was faster than the switch of a lamp on/off.  I think the same way; energy aside for the moment, speed is relative to what we know; relative to the movement of the Earth as an origin, relative to the movement of the galaxy.  Acceleration is by defintion a limitless increase.  Why would it stop at light speed if acceleration could be kept constant within a vacuum? (Makes me wonder with speed as a mesurable value, is an object faster moving away form the movement of the big bang or toward the origin if both object are technically clocked the same speed, i.e. speed of light (I am sure the scientific answer is they can't be but why)?

As for energy and mass, I understand the limits based on formula, but my contention or confusion I suppose is that bodies in space, without significant resistance, should continue to accelerate not just continue at a steady speed, with little energy.  And as with savings, compounded acceleration adds up fast; double our current speeds with acceleration and it isn't long before we are knocking on the speed of light (24,000 to 48,000 to 96,000 to 192,000 to 384,000 to 768,000 etc.).

So what slows down objects in motion from acceleration to deceleration out in space?  I know of the different evolution of vacnat space to dark matter and Gibbs particles but then lets assume we add aerodynamics to our imaginary projectile to compensate.  It seems we are pitting several laws of physics against each other to define a limit.  Why not instead consider infinite accelleration might be possible, beyond light and beyond the speed of light as we know it.  Maybe we have to consider some type of yet discovered cold-fusion energy and some type of rocket wthout moving parts to break down, etc. but if we assume energy is not the problem, what else limits the projectile?  I am not satisfied with the answers yet and I hope you all can indulge me a bit longer.

Title: Re: If an object drifts in space without stopping, what stops acceleration in space?
Post by: jkwaters2 on 01/10/2012 17:28:55
Excuse me, "Higgs particles"! Ouch that was poor.
Title: Re: If an object drifts in space without stopping, what stops acceleration in space?
Post by: imatfaal on 01/10/2012 19:10:32
The main reason why you cannot get to the speed of light is Einstein's famous equivalence E=mc2.  The energy in that equation includes both the Kinetic Energy and the rest mass energy.  This allows us to link the energy (and thus the mass) of a particle with its kinetic energy (and thus its velocity).

Very simplisitically, and a little dangerously, we can consider what is called the relativistic mass

Now you can see from this as the speed nears the speed of light then the mass gets bigger

From Newton's second law Force = mass x acceleration rearranged to Acceleration = Force / mass .  So you can see that as the velocity gets higher then the relativistic mass gets higher and the force required for acceleration gets higher (cos it is divided by the mass).  This is a really shonky explanation - we should really deal with energies and momenta, but this is simpler than getting into momentum, force and energy.

I would recommend Hyperphysics - it has nice step by step explanations.
Title: Re: If an object drifts in space without stopping, what stops acceleration in space?
Post by: imatfaal on 01/10/2012 19:18:43
And Guys - knock it off with the offtopic ramblings about the Aether
Title: Re: If an object drifts in space without stopping, what stops acceleration in space?
Post by: jkwaters2 on 01/10/2012 20:48:25
It seems that in space, no matter how much effort is is takes to get to a specific velocity that once there you can coast with little to no energy.  From that arguement I guess it just seems that more acceleration is no more difficult than initially accelerating since the source of the force (the engine) is going the same velocity as the object it is pushing.  I do understand and have known that mass incresaaes to a point too large to propell as you approach c however when I fall back and look at it logically it doesn't make sense to me in space.  I am obviously destined to question the establishment!  I will think on it more until I find some peace!  Obviously I got the wrong doctorate for understanding space travel.

Title: Re: If an object drifts in space without stopping, what stops acceleration in space?
Post by: evan_au on 02/10/2012 11:41:31
Perhaps we can address the question by starting with the Michelson-Morley experiment: http://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment

Regardless of the Earth's daily rotation on its axis (about 1000 MPH), or its annual rotation around the sun (which is even faster at 70,000MPH), or its steady rotation around the galaxy (which is even faster still, at around 500,000MPH), it is found that the speed of light in a vacuum is always exactly the same, which we call "c".

Physicists, starting with Einstein, extrapolate that result to predict that no matter how long, or how hard you accelerate, you will still see light in a vacuum passing you at "c".

This means that you cannot exceed the speed of light - in fact, you even cannot go fast enough to make light go a bit slower than "c" in a vacuum.

Conclusion: There is nothing that stops your acceleration; it's just that light is always going faster than you are, by exactly "c".

[Of course, if you are going fast enough, running into a grain of dust would release the energy of an atomic bomb - that might upset your acceleration schedule for that day!]
Title: Re: If an object drifts in space without stopping, what stops acceleration in space?
Post by: yor_on on 02/10/2012 18:45:08
You do not like the idea of  'c' becoming a barrier for matter as Space is without resistance, right? The barrier does not have to do with resistance but with the energy required. It simply becomes a impossibility to spend that infinite energy required, and you can't even count on how much of that 'infinite energy' it theoretically would take as it in fact is 'infinite'. Assuming a resistance in space won't change this either, only add to energy you need to spend.

Now, if we had 'negative resistance' though :) But no, it is a whole concept SpaceTime, you push some parameters for it and some other adjust to your pushing, but you won't get out of the bag.
Title: Re: If an object drifts in space without stopping, what stops acceleration in space?
Post by: butchmurray on 05/10/2012 07:01:04
As the devil’s advocate only:

The mass of the fuel will also increase. If the energy output from the chemical or nuclear reaction is a function of the mass of the fuel there is a positive feedback loop. Until the explosion!