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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Post by big_kev »

joe wrote: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.
I will have to think about that one !
GeorgeC
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Post by GeorgeC »

big_kev wrote: 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.
The acceleration of any object depends solely on its mass and the force applied. As you said, F=ma. The speed of the first object is not important, it is only the force applied that counts. The ability of the first object to apply a force could alter depending on its state of motion, but the acceleration of the second object will not be 'aware' of the speed of the first.
big_kev
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Post by big_kev »

GeorgeC wrote: , but the acceleration of the second object will not be 'aware' of the speed of the first.
Don't get this ?

Would I be aware of the speed of a moving car if I stepped out in front of it ?
Earthshine
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Post by Earthshine »

big_kev wrote:
GeorgeC wrote: , but the acceleration of the second object will not be 'aware' of the speed of the first.
Don't get this ?

Would I be aware of the speed of a moving car if I stepped out in front of it ?
Yes, in Newtonian physics this is true, but you are attempting here, to question arguably the best physicist that ever lived, Einstein. At higher speeds (approaching the speed of light) relativity takes a noticeable effect. Please read up on General and Special Relativity, then come back with your argument.
big_kev
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Post by big_kev »

big_kev wrote:
joe wrote: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.
I will have to think about that one !
I've thought about it !

The fuel would only have for its reference the spaceship.

All the fuel would "know" would be that the spaceship was accelerating away from it at 10m/s/s ( yeh I know it would be a bit more than this ) however it would not know the speed of either itself or the spaceship relative to anything else.

Therefore this would not prohibit constant acceleration to greater than 300,000km/s.
ajb
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Post by ajb »

One good reason is that if you travel faster than light, then you will have issues with causality. In some frame you will be travelling backwards in time!

To understand (without causality arguments) just think about what happens to the Lorentz factor as you approach v -> c. As pointed out earlier, as measured from the moving frame (so not your rest frame) your energy and (3) momentum tend to infinity. Also, as rightly stated as far as you are concerned it stays the same.

Anything anyone measures should be finite or it is not physical. And as inertial observer's measurements are equally valid your energy and momentum at v=c cannot be physical. Thus, v=c cannot be allowed.

This is so important I will say it again. Any inertial observers measurements are equally valid. Thus, you cannot really say things like "length contraction is an apparent effect". Such things are real and been observed.

More mathematically the Lorentz group is non-compact. Meaning one can Lorentz transform into any frame with comoving speed v for 0 <= v < c. (Interestingly v>c is also ok, but you can never slow down to c!)
big_kev
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Post by big_kev »

Earthshine wrote:
Please read up on General and Special Relativity, then come back with your argument.
"the laws of physics are the same for all observers" .( Einstein )

Basically it is impossible to determine your speed without an outside reference, you would not know if you were moving slowly or travelling at near the speed of light...there would be no difference.

Again this is from Einstein, although not a direct quote.

The first statement you made would equally apply to Einstein when he questioned Newton who was certainly the best physicist before Einstein ( and arguably the best ever).

Was Einstein wrong to question Newton ?

Not for one minute do I place myself anywhere near the level of these people, however almost certainly there are people out there who are of that level.
Should all physicists just give up "because Einstein was clever and therefore must be right" ?

I will be most happy if someone proves me wrong with this basic assumption about constant acceleration....it would teach me the error of my ways and hopefully expand my knowledge of Astrophysics, which is the reason I am on this forum....to learn and discuss.
ajb
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Post by ajb »

big_kev wrote: Basically it is impossible to determine your speed without an outside reference, you would not know if you were moving slowly or travelling at near the speed of light...there would be no difference.
Maybe in complete isolation as special relativity really talks about things passing or heading towards each other. So in that respect you can have speed faster than light, but these would not be as measured in any inertial reference frame.

Anyway, your post is about a rocket ship heading towards a planet. You can measure you velocity with respect to the planet.
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Post by Cliff »

Dear BigKev
If you are travelling at 30,000km\s light is only travelling at 270,000km\s faster than you. Well that is your way of looking at it.
I think you "believe" you can continue acelerating a some\whatever rate you might specify for whatever period you might like without running into fuel problems. Personally I doubt it.
Einstein may be wrong but as far as I am concerned he is usually more right than most of us so I personally rarely disagree with him.
Best of luck from Cliff
big_kev
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Post by big_kev »

Fron "ajb"

"One good reason is that if you travel faster than light, then you will have issues with causality. In some frame you will be travelling backwards in time!"

I don't believe causality would be an issue as if you were travelling in excess of 300,000km/s you "may" be outside of the "cone of influence" of other observers.....yes I know this is a bit vague but I think you get the idea....you would be travelling in a different "dimension" ( for want of a better word).

"To understand (without causality arguments) just think about what happens to the Lorentz factor as you approach v -> c. As pointed out earlier, as measured from the moving frame (so not your rest frame) your energy and (3) momentum tend to infinity. Also, as rightly stated as far as you are concerned it stays the same."

Three issues with this the first one would be obvious from my previous posts ...define v without using a reference ( unless you want to use "the aether" ,c or an infinite number of reference points ) none of which I see as being valid. Secondly energy and momentum are what I am doubting with the jist of my arguement.
And thirdly ( yes I know I shouldn't start a sentence with "and" ) the origin of the Lorentz factor, in my mind throws potential doubt on it's validity. It may be correct but remember it was "invented" to "tidy up" some mathematical problems it was not deduced by reasoning or transposition of other formulas. As such although it does fit quite well it may not be entirely correct.

"Anything anyone measures should be finite or it is not physical. And as inertial observer's measurements are equally valid your energy and momentum at v=c cannot be physical. Thus, v=c cannot be allowed."

Again this is the same as the previous statement about energy and momentum and my response is the same in that I still doubt the validity of observer's measurements being equally valid.

"This is so important I will say it again. Any inertial observers measurements are equally valid. Thus, you cannot really say things like "length contraction is an apparent effect". Such things are real and been observed."

I still have an issue with the traveller who observes no effects, surely his viewpoint is as valid as anyone elses ( I would think more so).

"More mathematically the Lorentz group is non-compact. Meaning one can Lorentz transform into any frame with comoving speed v for 0 <= v < c. (Interestingly v>c is also ok, but you can never slow down to c!)"

Again you are assuming that the Lorentz group is correct.
If it is valid it may only apply to a set of velocities below that of c and may not apply to velocities equal or greater than c.

As far as I understand you can never actually detect yourself moving.
You would have no way of knowing whether you were moving away from something or whether it was moving away from you.
Obviously in everyday use you can tell from experience and "common sense" you drive down the road and you "know " it is the car moving not the road......however if you had no means of outside observation you could just as easily be on a rolling road...there would be no difference.

Again I doubt the validity of the mass and length contractions as they are reliant of the measuring tool involved....light.
This gives rise to the observational appearance of mass increase and length contraction when in actuality neither takes place.

The bit that throws it for me is the infinite values of v and length, mass and time that are all "equally valid".
Sorry but I'm not having it, this must be wrong.

Or is it just a trick of the light.
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Post by joe »

big_kev wrote:All the fuel would "know" would be that the spaceship was accelerating away from it at 10m/s/s
Isn't that enough for there to be a problem? It's an acceleration not constant speed therefore equivalence doesn't hold.
to ajb you wrote:I still have an issue with the traveller who observes no effects,
He does observe an effect, the force of the acceleration. Other frames don't feel this.
200mm Newtonian, OMC140, ETX90, 15x70 Binoculars.
RL Astro
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Re: Spacetravel...I still don't get it.

Post by RL Astro »

big_kev wrote:I know light ( again allegedly ! ) travels at around 300,000km/s
big_kev wrote:"the laws of physics are the same for all observers" .( Einstein )

Basically it is impossible to determine your speed without an outside reference, you would not know if you were moving slowly or travelling at near the speed of light...there would be no difference.
big_kev wrote:I don't believe causality would be an issue
big_kev wrote:the origin of the Lorentz factor, in my mind throws potential doubt on it's validity.
---
Again you are assuming that the Lorentz group is correct.
Hi big_kev.
It seems to me like what you're saying is "if these laws of physics weren't true, would my idea work?". So far, for your argument, you've questioned the speed of light, relativity, causality and the Lorentz factor. These are all observationally / mathematically sound so I reckon you need to keep in line with them.
Unless I've completely misinterpreted your logic? :)
GeorgeC
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Post by GeorgeC »

Big_Kev says - This gives rise to the observational appearance of mass increase and length contraction when in actuality neither takes place.

It seems as if you are saying that time, mass and length changes are purely observational phenomena, and do not actually take place?
There are many experimental results that show that they really do change. The change is visible to both the observer and the object travelling at speed. The conclusion from this is that faster than light speeds is impossible.

Have a look at this http://demonstrations.wolfram.com/Relat ... MuonDecay/
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Post by ajb »

First thing we must accept is that special relativity has been experimentally verified. Probably the best evidence comes from particle colliders where particles are accelerated to speeds comparable to c.

The Lorentz transformations were not just "cooked up", but came from understanding the symmetries of Maxwell's equations. Again, these have been tested to a high degree.

Only later was the geometric significance of the Lorentz transformations really appreciated. The important thing is that there is a "conversion factor" c, which allows us to mix space and time. In particlular, ct now has the units of length. The important thing is that this factor c is the same in all inertial reference frames. Again, this has been experimentally verified.

The Lorentz transformations are defined as linear transformations that preserve the space-time interval

ds^{2}= -(cdt)^{2} + dx^{2} (there are different conventions in use)

Now by working this through you will get the standard representation of the Lorentz transformations.

The point being is that the derivation does not assume that c is the ultimate speed, but rather that it is a corollary.

Again, I'll stress this. The factor c (which turns out to be the speed of light) is the same in all frames. Using this and the invariance of the space-time interval you get that c is the ultimate speed.

If you think about it, all we are saying is that it is impossible to "rotate" into a frame where you are comoving at v=c. If you have something like a separation speed of v>c then, as you said they are outside of any causal influence of each other. Either observer cannot measure his speed against the other! Again, this means you cannot Lorentz boost between the frames.

The biggest "point" of special relativity is that the only physical observables are Lorentz invariant. By this we mean that any thing that is frame dependant cannot have any deep meaning. As length and duration of time are not Lorentz invariant, they have no great meaning. The same applies to energy and 3-momentum. They have no deep intrinsic meaning.

Any "theory" you are constructing that allows v >c is not special relativity. The space-time interval (or a modification of it) would be invariant under some other group of transformations other than the Poincare group. This group should contain in some limit the Poincare group.

Trying to unite quantum mechanics and gravity may suggest that Lorentz symmetry is not exact in nature, but rather it is broken on the Planck scale. I should stress there is no experimental evidence for this, only theoretical evidence.
big_kev
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Post by big_kev »

joe wrote: Isn't that enough for there to be a problem? It's an acceleration not constant speed therefore equivalence doesn't hold.

He does observe an effect, the force of the acceleration. Other frames don't feel this.
Ok maybe I should have made it clearer.

The issue is related to speed therefore I was looking at the instantaneous speed during the acceleration.

Perhaps if I had added that dyring the periods of acceleration the spaceship is coasting how then would any relativistic effects be felt.

It cab be assumed that it is theoretically possible to reach speeds of 90% of 300,000km/s now according to most of the posts on this thread the effects of the changes in mass, length etc should be apperent to the traveller.
I am saying they would not and as such you could accelerate again, have a little coast, observe no effects, accelerate again and so on for ever and ever ( or as is practical).
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