[Lars Larsen (OP)]

If it would be possible to completely eliminate an object's - say a spaceship's - mass, would that spaceship immediatly accelerate to c (let us assume the spaceship operates within a perfect vacuum)?

This implies further questions, e.g.:
- If the answer is Yes, wouldn't that be contradictory to the law of conservation of energy (perpetuum mobile))? or
- How much energy do you need to accelerate an object without mass? or
- If within this Bezugssystem no time elapses, how could such a spaceship be navigated or stopped?

any ideas?

[MOD EDIT - PLEASE PHRASE THE TITLES OF YOUR POSTS AS QUESTIONS, IN LINE WITH FORUM POLICY; THANKS, CHRIS]

[graham.d]

The answer to the first question is "no" so it is not easy to hypothesise about the other questions. A rocket, for example, accelerates by squirting mass out of its backend so needs mass (at least initially) to work. This spacecraft would have to have external means to propel it. I'm afraid I can't get past the concept of a massless vehicle.

[lightarrow]

If it would be possible to completely eliminate an object's - say a spaceship's - mass, would that spaceship immediatly accelerate to c (let us assume the spaceship operates within a perfect vacuum)?

Only if the object keeps some non-zero energy (it doesn't matter how much).

[Lars Larsen (OP)]

well, the headline is "Gedankenexperiment" - let us just assume it is possible...

[lightarrow]

Every massless object which has non-zero energy must move at c.

[graham.d]

well, the headline is "Gedankenexperiment" - let us just assume it is possible...

Well, lets imagine there are ghosts. How heavy would they be? Its just a thought experiment :-)

[Turveyd]

If you change a objects mass to zero in a vacuum it's inertia will remain unchanged.

A object with no mass could accelerate to C very quickly it would also stop very quickly,  but to accelerate it to C would need something capable of moving at C which could affect this object with zero mass.

A space ship can only accelerate to the speed of the mass it's ejecting after all.

Sorry incapable of imagining there are ghosts that just defys all logic!!

[Lars Larsen (OP)]

If you change a objects mass to zero in a vacuum it's inertia will remain unchanged.

A object with no mass could accelerate to C very quickly it would also stop very quickly,  but to accelerate it to C would need something capable of moving at C which could affect this object with zero mass.

A space ship can only accelerate to the speed of the mass it's ejecting after all.

Sorry incapable of imagining there are ghosts that just defys all logic!!

Sorry, I was talking about (inertial) mass, so the assumption was that there is no inertia.

If you assume that objects with zero mass move at the speed of light, the question is, how the transition from the original speed to c takes place. Is there an acceleration, a certain time tha can be measured, or does it happen "immediatly"?

A further question is, whether this might be contradictory to the law of conservation of energy. I remember an essay by Arthur C. Clarke, where he argues against the possibility of "removing" gravitational mass by just "blocking" gravitons (or whatsoever) with the argument, that you get a perpetuum mobile (remove mass, bring the object in a position high above the ground, "turn on" mass and so on - would be possible e.g. to generate electricity with such a device). This could be resolved of course if you assume that you need a lot of energy to "remove" mass.

[lightarrow]

A space ship can only accelerate to the speed of the mass it's ejecting after all.

But it doesn't need to eject mass because it can eject photons (for example).

[JP]

A space ship can only accelerate to the speed of the mass it's ejecting after all.

But it doesn't need to eject mass because it can eject photons (for example).

Also, that's not true even in the case of mass.  Spaceships can go faster than the mass they eject.  It's conservation of momentum, not of velocity.

[LeeE]

Every massless object which has non-zero energy must move at c.

I strongly agree.  If the object has no mass, but does have energy, it must move at 'c'.

Immediate acceleration is a bit of an oxymoron; acceleration means that something takes a non-zero period of time to achieve the specified speed but immediate implies that no time is taken.

Many might interpret this as infinite acceleration.  They'd be wrong though, because no matter how infinitely small a period of time is considered for the period of acceleration it needs to be > 0.  The massless object, then, will not accelerate to 'c' but will instantly start traveling at 'c', at the cost of no (additional) energy.

There's another current thread in this section of the forum that discusses the rate of time for objects moving at 'c', so I'll not go into it again here.  Suffice to say though, that if no time passes for the craft then nothing can change within it, so stopping and navigation would be a problem if it were possible for the situation to arise in the first place, which I strongly doubt.

[Lars Larsen (OP)]

Every massless object which has non-zero energy must move at c.

I think this holds true only for particles. In interstellar and even interplanetary space particle concentration is low enough as to leave huge "gaps" to photons, that raely interact with particles in this environment. A macrocosmic object like a spaceship, when travelling at relativistic speed, colides with a huge number of particles each second, in  other words: it always operates in a MEDIUM.

What do yuo think, how long would a spaceship, that consists of materials with properties similar to known materials and that travels at relativistic speed, survive in interstellar space?

[lightarrow]

Every massless object which has non-zero energy must move at c.

I think this holds true only for particles.

Maybe you are right, however I don't know what to say because I don't know massless, non-zero energy macroscopic objects [].

In interstellar and even interplanetary space particle concentration is low enough as to leave huge "gaps" to photons

Alt! Strong assumption detected here! []  How do you know that photons are spatially limited objects? How do you know how big they are?

[Lars Larsen (OP)]

In interstellar and even interplanetary space particle concentration is low enough as to leave huge "gaps" to photons

Alt! Strong assumption detected here! []  How do you know that photons are spatially limited objects? How do you know how big they are?
[/quote]

Can't follow you here, lightarrow. Photons "slow down" in media like air because they intertact with particles, isn't that true? I'm not a physicist, so please excuse any stupid questions.

PS: Cosa significa "alt"? Is it coz I is German ("Halt!") [?]  [O8)]

[lightarrow]

Can't follow you here, lightarrow. Photons "slow down" in media like air because they intertact with particles, isn't that true? I'm not a physicist, so please excuse any stupid questions.

What I mean is that <<to leave huge "gaps" to photons>> is meaningless. Photons are not little bullets.

PS: Cosa significa "alt"? Is it coz I is German ("Halt!") [?]  [O8)]

No, I believed it was an international locution. I should have said "stop".

[JP]

I'm not sure that it makes sense to consider a macroscopic, massless, rigid object traveling at the speed of light.  The internal forces that make objects rigid propagate at the speed of light: i.e. bits of the object "talk to" each other at the speed of light and "tell" each other to stay rigid.  If the entire object is already moving at the speed of light, this would be a problem. 

[Lars Larsen (OP)]

I'm not sure that it makes sense to consider a macroscopic, massless, rigid object traveling at the speed of light.  The internal forces that make objects rigid propagate at the speed of light: i.e. bits of the object "talk to" each other at the speed of light and "tell" each other to stay rigid.  If the entire object is already moving at the speed of light, this would be a problem. 

I see that there are a lot more things to consider here than I thought. I guess the hole concept is stupid, but is is fun to ask the question anyway.

[JP]

It's definitely a clever question to ask, and its not immediately obvious that it won't work.  Since the site is about science education, it certainly wasn't stupid to ask it.

[Lars Larsen (OP)]

It's definitely a clever question to ask, and its not immediately obvious that it won't work.  Since the site is about science education, it certainly wasn't stupid to ask it.

Thank you. I learned a lot!

[yor_on]

If you would eliminate its invariant mass I don't think that wouldn't mean an instant 'c'. that as we already 'transform' fermions (matter) like helium-4 into bosons (Properties similar to photons) without them losing neither 'size demands' nor getting a 'instant light-speed' although I think I can guess how Lightarrow thought when he said so, as if you could, the most reasonable explanation would be that you transformed it into 'energy' which then might be defined as photons/energy quanta. But 'invariant mass' is a defined state with very clear boundaries geometrically and macroscopically, and if you 'transformed' it wouldn't be able to go back to what it once was, so?

.--.
Ah, like 'transforming' that rocket. I doubt you would be able to recreate it after that first 'transformation'.

--
In fact the idea seems to have much to do with our arrow of time/causality macroscopically.
As when you transform invariant mass also change the constraints placed on them by that arrow.
photons f ex. have a defined locality, waves don't. And your choice of observation will define the outcome there. With fermions there are other rules as they seem to be closer to our arrow of time, they have a defined geometry in time and will be 'there' between time intervals proofing themselves (macroscopically) to exist 'independently' of our observations.