Spacetravel...I still don't get it.

The non amateur stuff. Hawking, black holes, that sort of thing

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big_kev
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Spacetravel...I still don't get it.

Post by big_kev »

I am having trouble with the issue about not being able to travel to the nearest stars because of the distance involved.

Ok it takes light ( allegedly ! ) a bit over 4 years to get from there to us.
However I still see no real problems in travelling to it in a lot less than this.....ignoring for the moment fuel, food etc.

I know light ( again allegedly ! ) travels at around 300,000km/s but I do not see any difficulties in exceeding this.

Please note however I am referring to travelling faster than 300,000km/s not faster than light (there is a big difference ).
I would still measure light to be travelling at 300,000km/s faster than myself whatever speed I was doing.

Consider if my mate Fred and I got in a spaceship and left the Earth constantly accelerating at approx. 10m/s.

What would we notice ?

Nothing !

We would feel an force approx equal to the earths gravity and after about a year we would have exceeded 300,000km/s.
After 2 years we would have doubled this.

At no time would Fred or I have noticed any relativistic effects either in mass, length or time also at all time we would measure light as travelling 300,000km/s faster than ourselves ( Note: there are no windows in this spaceship ).

I still can see no problem with this...

Where have I gone wrong in my misunderstanding.
Zeke
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Post by Zeke »

Mainly technology.
We simply can't do it, yet.
Come on you brainy ones give me some help here!
Never worry!
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Cliff
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Post by Cliff »

Dear Big_Kev
You mention that light allegedly travels at around 300,000 km\sec.
The speed of light was first reasonably well observed\ascertained by Ole Rome back in the 17th century. He realised that the apparent time irregularities in predictions related to the actual positions of Jupiters satellites were caused by the changes in distance of Earth observers relative to Jupiter as the two planets travel around their orbit.
As I think Zeke suggests even if we are wrong to think that the speed of light cannot be exceeded, we certainly do not seem capable of travelling that fast just yet.
Best wishes from Cliff
big_kev
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Post by big_kev »

These first 2 replies seem to have missed the point.

Maybe I can rephrase it better.

Assuming that we have the resources and the technology to accellerate a space craft at 10m/s ( or slightly faster if needed, this figure is not important ) for a period of a year or greater if my maths is wrong.
Then would there be anything to stop it travelling at speeds in excess of 300,000km/s and reaching the nearest star in less than 4 years relative to an observor on the earth.
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Post by RL Astro »

Won't go into the maths (mainly 'cause I can't) but Einstein said that the faster you get the more your mass increases and so the more energy required to accelerate it. Eventually the energy required to accelerate it more becomes infinite and as there isn't infinite energy, it can't be done.
So your hypothetical craft could never be made
big_kev
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Post by big_kev »

RL Astro wrote:Won't go into the maths (mainly 'cause I can't) but Einstein said that the faster you get the more your mass increases and so the more energy required to accelerate it. Eventually the energy required to accelerate it more becomes infinite and as there isn't infinite energy, it can't be done.
So your hypothetical craft could never be made
As I said neither myself nor my travelling companion would notice any increase in mass.

Any increase in mass could be measured and from this we could determine our absolute speed ( we know the previous results of going down this road ) which cannot be measured unless you subscribe to the aether theories ( which personally i still like the sound of ).

Anyhow the basic issue with this increase in mass is that we are currently travelling at almost the speed of light away from some remote galaxies and as such relativistic mass increase should be obvious.
It isn't........there is none.

Or alternatively you could look at the speed we are currently orbiting the centre of the galaxy....easily measurable according to relativity...again there is no increase in mass.

Relativity is only relavent to an observer.
On earth as I accelerate to near 300,000km/s you may appear to see my mass increase, I as the traveller would notice no effect whatsever.
Neither would me passenger, my spaceship or my fuel supply.

My energy requirements would only be those required to accelerate me at 10m/s regardless of the speed I was travelling away from the earth.....ignoring any gravitational effects whcih are not relevant as I could make the same scenario in inter galactic space.

I am afraid without using absolute speed relative to the aether as a factor in this, then relativistic mass increase is not an issue.

Again have a think on this and let me know if I am wrong.
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Post by 12dstring »

Relativity is all about reference frames that are moving relative to each other.

If two people are in the same reference frame (moving with the same speed and direction) then no relativistic effects are noticed.

However here we have two, you and Fred in your spaceship, and the destination star.

One way to picture it would be as it would appear to you, you being stationary - and the star appearing to accelerate towards you. Then the star would appear to gain mass, and eventually and insufficient amount of energy would be required to keep accelerating it. Of course actually the star is stationary, but the same applies (it makes no difference, it's all relative :P ) - to an observer near the star, you are accelerating towards them, thus gaining mass....etc
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big_kev
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Post by big_kev »

12dstring wrote:Relativity is all about reference frames that are moving relative to each other.

If two people are in the same reference frame (moving with the same speed and direction) then no relativistic effects are noticed.

However here we have two, you and Fred in your spaceship, and the destination star.

One way to picture it would be as it would appear to you, you being stationary - and the star appearing to accelerate towards you. Then the star would appear to gain mass, and eventually and insufficient amount of energy would be required to keep accelerating it. Of course actually the star is stationary, but the same applies (it makes no difference, it's all relative :P ) - to an observer near the star, you are accelerating towards them, thus gaining mass....etc
As you say this is just an observational effect.

The star will only gain mass if I observe it and I will only gain mass if an observer looks at me.....I think there is a flaw in here somewhere.

I think the flaw here is in the measuring tool involved....basically light.

Again to return to the basic principle.....what is going to stop me accelerating away from the earth at a constant acceleration of 10m/s until my speed relative to the earth exceeds 300,000km/s.

Basically if I shone a torch to a very bright mirror on an planet ( for example ) 10 light years away, I would see the reflection returning to me in 20 years time.....I think we all agree on this.

In my spaceship I could set off at the same time as the torchlight, travel to the distant planet and someone on earth would see me there before they saw the returning torchlight.
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Post by GeorgeC »

Although it goes contrary to intuition, the 10m/s/s acceleration could not be maintained. We have proof of this on Earth. Imagine replacing the spaceship with a proton, as in the Atlas machine in CERN. The proton is then accelerated and sent off round and round the ring. The accelerating force is constantly applied to the proton, which should therefore keep accelerating at the same rate, but the proton gets 'sluggish' and the increase in speed does not continue. Eventually it gets to about 0.9999999 the speed of light, but it doesn't matter how much more they try to speed it up, any further increase is negligible.
What the proton is experiencing is anybodies guess!
The smart Alecs will no doubt point out the difference in that the spaceship is carrying its own fuel and providing its own acceleration, whereas the proton is being accelerated by external means, but I suspect that Albert was right and the analogy holds.
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Post by Eclipse »

You WOULD notice an increase in your mass. Using an inertial balance, your mass and that of objects with you wll have increased. At the cost of enormous amounts of energy; if your mass increases by m kg, and the energy needed to provide this extra mass is E, then
E = mc^2
applies, where c is the free space velocity of light.

You will always obtain the same value for c, whenever you measure it, but that is because lengths and time itself alter as your velocity increases, whether anyone is observing you or not.
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big_kev
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Post by big_kev »

GeorgeC wrote: The smart Alecs will no doubt point out the difference in that the spaceship is carrying its own fuel and providing its own acceleration, whereas the proton is being accelerated by external means, but I suspect that Albert was right and the analogy holds.
The analogy does not hold.

Consider....Force=mass*acceleration.

However the important bit here is acceleration.

An object is used to shove another object the acceleration of the second object is dependant on the speed of the first object.....ignore mass for the moment.....as I am assuming that both objects have a constant mass....more on this later.

If the first object is travelling faster it will accelerate the second object further.....this bit is obvious.

What happens if the second object is moving as well ?
It starts to get a bit more complicated.

The speed of the first object is dependant upon its initial acceleration.
Again obvious......however it now has to catch the first object up.
Hence it will lose speed before it shoves the second object.

Even if the force used is electo-magnetic it still travels at a finite rate which obviously will not exceed C and therefore as the second object approaches C the force applied to it is less and less.

This gives the impression that the mass of the object is increasing as it is harder to accelerate when in fact it is the force being used to acclerate it that is diminishing.

The only way to accelerate something faster than C using an external force would be to use an external force that was travelling faster than C.
big_kev
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Post by big_kev »

Eclipse wrote:You WOULD notice an increase in your mass.
According to relativity you would most definitely not notice any increase in mass, length or time.

Others might, but you most certainly would not.

Consider three people A, B and C forming a triangle.

A runs away from B ( very fast ) such that B observes A's mass to increase threefold.
Howevr the angle A is running at with respect to C allows C to observe A and deduce that A's mass has only increased twofold.

What would A measure his mass to be ?

The only thing it could possibly be is his original mass.

Ditto for length and time.

E=mc^2

In the above scenario m would equal 0 hence the additional energy required would be 0.

According to C it would be larger and according to B even larger still.
This would certainly conflict with my idea of the conservation of energy.
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Post by joe »

At no time would Fred or I have noticed any relativistic effects either in mass, length or time
One thing you would notice is that you needed more and more fuel to sustain a 10m/s/s acceleration. We're talking about Rest Mass here and at relativistic speeds your rest mass would be quite large. I'm sure there are places on the net to find the maths but I assume that an ever increasing amount of energy is needed to increase your rest mass/acceleration further.

http://scienceworld.wolfram.com/physics/RestMass.html

Good question though.
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big_kev
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Post by big_kev »

joe wrote: One thing you would notice is that you needed more and more fuel to sustain a 10m/s/s acceleration.
No I don't believe this to be the case.

Consider if I left the earth now at a constant acceleration of 10m/s/s ( remembered the 2nd s this time...sorry about previous posts ) and I stopped observing anything ( shut my eyes and didn't look out of the window) I would have no means of measuring my current speed.
In one seconds time although my speed would have increased I would again have no means of measuring it, everything would be exactly as it was before the same for the next second.....and so on.

Any changes in my fuel conspumption (and hence increase in mass) would allow me to calculate my "absolute speed" with reference to the either the "aether" or to the speed of light as these would be the only references available.

If I could measure my speed relative to the "aether" then indeed it may be possible to observe that it had increased from one second to the next....I think people however may object to the "aether" theory.

Alternativly I could measure the increase in my speed with reference to light.
One second i am doing 10% of the speed of light ( or whatever) the next second I am doing 11% the speed of light.
Hold on !

If I am doing 10% the speed of light ( 30,000km/s) then light is only travelling 270,000km/s faster than me !
I think i would always observe light travelling 300,000km/s faster than me and my speed would always be 0% of the speed of light.
ie I would always appear to be stationary.

Any relativistic effects would enable me to calculate my speed relative to either the "aether" or to light.

I therefore do not believe this to be possible and that no relativistic effects woiuld be observed.

Therefore there is nothing to stop me accelerating to any speed I choose.
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Post by joe »

Most of the fuel used to sustain the acceleration cannot be part of your accelerating reference frame, even in an idealised situation. The fuel outside "sees" a constantly increasing mass.
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