Question: spaceflight, relativity, etc...


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I have not yet studied all that relativity stuff. I have a couple of question about it all.

First. I have often heard it said that continually increasing your velocity, in spaceflight for example, requires an ever-larger supply of energy. That the amount of energy required increases the faster you go. However, what science has so far acheived, in repeatable experiments and real-life applications, demonstrates clearly that such is not the case. The Deep Space One spacecraft has been accelerating for about two year, with an ever-diminishing supply of energy. It is only logical. If there is no resistence, and thrust is continually applied, then the spacecraft will continually accelerate. While velocity continually increases, the energy requirements do NOT increase. So why on Earth do people often claim otherwise?

Second. As that Deep Space One probe accelerates, it's mass is NOT increasing. In fact its total mass is decreasing, due to its slow fuel consumption. There is only ONE thing about that spacecraft which is increasing: it's potential kinetic energy. And that only matters if it hits something or thrust is applied in another direction. So, real-life applications and repeatable work demonstrate quite clearly that mass DOES NOT increase as things go faster. So why do people say otherwise?

As I've mentioned many times, I am a beginner at all this. But this particular matter has bothered me for a while.

<i>First. I have often heard it said that continually increasing your velocity, in spaceflight for example, requires an ever-larger supply of energy.</i>

That's true.

<i>The Deep Space One spacecraft has been accelerating for about two year, with an ever-diminishing supply of energy.</i>

To continuously accelerate, fuel must be used, which uses energy. Some of the energy of the fuel goes to speeding up the spacecraft, and some is lost as heat. According to relativity, some of the energy provided by the fuel goes to increasing the relativistic mass, too.

<i>While velocity continually increases, the energy requirements do NOT increase. So why on Earth do people often claim otherwise?</i>

The energy requirements do increase. To accelerate your spaceship from zero to 1 km/hr requires a certain amount of energy, supplied by the fuel. To accelerate it from 200,000 km/hr to 200,001 km/hr requires MORE energy, as dictated by relativity.

<i>As that Deep Space One probe accelerates, it's mass is NOT increasing. In fact its total mass is decreasing, due to its slow fuel consumption.</i>

The spacecraft is throwing away some of its mass as it goes, to provide thrust. The relativistic mass increase refers to the mass of the components which are not thrown out the back. As the spacecraft accelerates, the relativistic mass of those parts gradually increases.
But this spacecraft is continually accelerating. It is not increasing its power output at all. There is no resistence, it is maintaining a steady power output, and it is accelerating. And it is losing mass. I know that continually accelerating on Earth, for example, will require ever-more energy and you get to that diminishing returns. But that's due to the friction, the resistence, gravity. Nonce of that occurs in space. As I said, that craft is maintaining a steady power output while continuously accelerating.
James R ...

Could not the 'slingshotting' which permitted Voyager 2 to make the
'Grand Tour' in the mid-80's permit a space craft to accelerate to near
light speed prior to leaving our Solar system?

Take care ;)
If I may jump in here Chagur.

I don't believe a slingshot around any planets would provide sufficient momentum to accelerate to near light speed. If it did, the acceleration would have to be very rapid, in order to go from a hundred meters per second or so to almost 300,000 meters per second in the length of about 1/4 to 1/2 of an orbit. The G forces on the spacecraft would be incredible.
Newton. If you apply thrust in a vacuum (or in space, near enough) you go. You don't stop. Apply more thrust, you go faster. Apply more thrust (not a greater amount of thrust, just more of the same) and you go faster.

You're still in space. You fire your engine. You move. Stop firing your engine. Still going forward. Fire again, exactly the same thrust as before, and you gain speed.

No slingashot tricks. No increasing fuel consumption.

Fuel consumption and power output remain constant, speed increases. Mass decreases due to fuel consumed.
Hello,Sorry for being offshot.

James R,

If you happen to pass by,can you just answer a question for me?
what if nothing is in motion,everything is stationery and only thing that moves is the space around us.i mean is that possible that we merely bend or Warp(to be precise)the space around Us so as to move.i mean we merely apply a force which releases loads of antigravitons from the CG of our bodies and that causes reverse pair production and produces such energy so as to bend the space around us to move?

As Xelios implied, gravity mediated momentum transfers at high speed would crush ship passengers due to “tidal” effects. However other limits likely preclude that scenario.

One limit is how close the spacecraft may approach a planet. Too close and the ship is in the atmosphere (or hits the planet). Not close enough and there is insufficient change in momentum.

Another limit is the “escape” velocity of the solar system. The ship couldn’t depend on system gravity to allow multiple passes. At high speeds the spacecraft would have to fly from planet to planet while accumulating speed. However one runs into the first limit that the momentum change during each planetary encounter is limited. At extremely high speed there is insufficient “steering” to go from planet to planet.

A multi-stage “tether” ship might be possible. Use planetary slingshots to accelerate a spinning “tether” ship. At the proper moment sever the tether sending the payload on it way while sending the “booster” back to the solar system where it may be reused to launch another payload. The passengers in a “tether” ship would experience “G” forces so the momentum transfer at each stage would be limited. The final speed won’t be "near light speed".
Xelios, ImaHamster2 ...

'Liquid breathing' using perflubron could possibly be used to offset the high
G forces by allowing total submersion during acceleration or decceleration.

Hmmm ... multiple passes using Sol and a highly elliptical orbit?

Just thoughts ;)
This hamster thought the maximal gravity assist was twice the orbital speed of the assisting body. In the solar system frame of reference the sun is nearly at rest so the sun should provide a negligible gravity assist. (The sun would be useful for changing spacecraft direction.)

Multiple passes through the solar system could accelerate a spacecraft until the solar system escape velocity was reached. Voyager used Jupiter’s 13.1 kmps orbital velocity to boost its speed to 36.2 kmps which was greater than solar system escape velocity at that distance from the sun. Far, far from the speed of light.

(This hamster is no expert in this area and would appreciate corrections.)
ImaHamster2 ...

Good points, particularly the second.

Oh well, so much for slingshotting.


Take care :(

You need to distinguish between the mass of the spaceship and the the mass of the spaceship+fuel system. The relativistic mass of the spaceship alone increases (as seen from Earth) as the spaceship accelerates. At the same time, the mass of the spaceship+fuel system is decreasing because fuel is being used (thrown out the back). There's no relativity problem there.


I'm not sure how your slingshot scenario is relevant here. One thing to bear in mind is that slingshotting transfers energy from the planet's orbital motion to the spaceship doing the slingshotting. Total energy is conserved in the process.


It seems you are thinking of something similar to a "warp" drive. Such a thing maybe possible, but would still require an energy input to do the warping of spacetime.
I still have this slight problem. Mass is mass. It still looks to me like the only thing increasing is the craft's force, its potential kinetic energy, its mass times velocity. How do you say that instead of "mass times velocity" increasing, it increases its "mass" alone? Since we know its mass its decreasing. That, as you may see, is a major sticking point for me.

Look ONLY at the parts of the spacecraft which stay with the craft at all times. Ignore the fuel which goes out the exhaust. What happens to those parts?

Answer: Their relativistic momentum increases:

p = <font face="symbol">g</font>mv

where <font face="symbol">g</font> is the relativistic factor:

<font face="symbol">g</font> = 1/sqrt(1 - (v/c)<sup>2</sup>),

v is the speed, m is the rest mass and c is the speed of light. Note: m is constant for the spaceship, excluding fuel, which we are ignoring.

The relativistic energy also increases:

E = <font face="symbol">g</font>mc<sup>2</sup>

Now, notice that when the speed is zero, <font face="symbol">g</font> equals 1. In that case p=mv and E=mc<sup>2</sup>. Let's look at this second equation more closely. As the spaceship speeds up, we have m -> <font face="symbol">g</font>m. One way of looking at that is to call the quantity <font face="symbol">g</font>m the "relativistic mass". Hence, we say that the relativistic mass increases as the spaceship speeds up.
Hi James R,

I was talking about our general life regular warping.what i meant was that when we lets say walk we apply some force,now this force applied leads to warping of space around us so as to reduce the distance we want to that possible?
also,if that is the case then do you think everything is just applying force and that there is no movement,and the only thing that moves is the space around (becasue of warping)travelling higher distances would then require more Warping,isnt it?so in short is all motion just warping the space around us?

Secondly,is it possible that magnetic lines of force are not imaginary motion of separate poles,instead it is motion of gravitons of exactly opposite polarity?

waiting for your response terribly...:confused:

JamesR, thanks for the maths, it gives me something to play around with.
Hi zion,

I don't think all movement involves warping of space and time. There are other forces at work - mainly electromagnetic in our daily lives. What stops you falling through the ground to the centre of the Earth? Answer: electrostatic repulsion between your feet and the ground.

Sorry, but I don't understand your second question.
Hi James R,

the question was say i have a magnet,i draw magnetic lines of force around it on a sheet using a compass detecting change in its orientations,now these lines of force are imaginay lines of force along which(MONO POLES,that is if they would have existed)attracted toward each other(opposite poles attract)so these lines are merely tracing of that.but do you think can be like oppositely charged gravitons attracted to each other??:confused:

Gravitons are, at present, hypothetical particles. If they exist, they will have no electrical charge. They will not be affected by magnetic fields.
I think what Zion's trying to ask is could gravity be like magnatism in that the gravitons attract eachother in some way which causes objects to move together. Could be wrong though. ;)