# Approaching c

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as you approach c (the speed of light) your mass decreases...

but where does the mass go?

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Your mass increases. And it doesn't 'go' anywhere, it's an energy thing.

Mass is not conserved.

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ah, oops, oh well... i'm learning

i know that at c your mass is infinite... dunno what got into me today!

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ok, so as you nearer c your mass increases... where does this mass come from?

i know it's today with energy, but the energy of what? (or from what?)

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5614 maybe somebody here at SFN knows how to give a clear satisfactory explanation in just a few words

I dont, but I will introduce a piece or two of the puzzle

you have to know the concept KINETIC ENERGY which there are two formulas for (the simple newton one and the correct 1905 one)

kinetic energy measures how much whallop something delivers when it crashes into something-----and also it measures how much WORK it took to get it going that speed in the first place.

the work of shoving is a combination of force and distance that the force is applied.

THE POINT IS THAT AS THE THING ABSORBS MORE AND MORE KINETIC ENERGY by being shoved faster and faster, it GAINS INERTIA

so it gets harder and harder to accelerate it that next bit.

The KINETIC energy that something absorbs adds to its TOTAL energy (which includes the mass energy it originally had at rest) and it is the total energy which determines the things (forwards) INERTIA which is the sluggish stubbornness with which it resists further acceleration.

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BTW "mass" is one of the more tricky concepts in physics. everybody thinks they know intuitively what it means. but there are actually at least two different concepts called mass, and the most common idea is actually based on INERTIA

as mass is measured, ultimately, it has to do more with inertia of of something than with some vague idea of "quantity of material"

so there is no reason that mass has to be conserved! if it grows it doesnt have to "come from somewhere"

in the case of something being accelerated, the increased inertia comes from the ENERGY of the work put into the thing to make it accelerate.

and that increased the total energy

and that gave the thing more inertia (because the closer it gets to the speed of light the more reluctant it becomes to take on additional speed)

and that increased inertia translates into more mass (because mass IS inertia)

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many people find it better to restrict the idea of mass to the REST mass,

that is the inertia something has at rest

and then they dont talk about the mass increasing

they just say the total energy increases, and do the calculation with total energy taking the role of mass.

then they say that the mass does NOT increase because for them the mass is always the rest mass, the mass it WOULD HAVE if it were stopped.

so you get some arguments and confusions that simply result from people using words differently

but this should not affect one's intuitive grasp of what is going on, which is what I'v tried to convey.

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there was a lot of discussion about the different types of masses in the blackholes vs. massless photons thread, so i am aware of them.

otherwise thanks for the mass, soz, i mean thanks for the post!!!

this thread is what i'd call a 'knowledge patcher'... it'll make sure i never make the same mistake i did in the orignal post and we all know mass increases as you nearer c, now we know why, cheers guys!

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Ok, I'm still trying to clarify this. Inertial mass increases as you approach c, meaning the object will have more resistance to acceleration. Does this cause the gravity of the object to increase as well, or does that only depend on the rest mass value?

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'']Ok, I'm still trying to clarify this. Inertial mass increases as you approach c, meaning the object will have more resistance to acceleration. Does this cause the gravity of the object to increase as well, or does that only depend on the rest mass value?

the gravity of an object, by which I mean its gravitational attractiveness, increases with the total energy of the object

if you have a tank of air

and you heat it

so the molecules are whizzing around faster

then each one of them has more total energy (rest plus kinetic)

and all that contributes extra gravitational attractiveness

so the whole tank of air, being warmer, is more attractive gravitationally

the gravitational mass goes up in step with the inertial mass

(however i am still uneasy with any idea of mass except rest mass, in my heart of hearts I would rather we kept track of a moving object by its rest mass (which doesnt change) and its total energy. because the inertia of a moving object is tricky to define operationally and may have unexpected pitfalls)

the hot tank of air has more inertia, and more weight, and more gravitational attractiveness------but you will never be able to measure it because scales are not that sensitive

lot to say on the topic isnt there?

-----I will edit in a graphic way of putting it-----

What holds the earth in orbit, and keeps us circling the sun at this speed and distance, is the sun's gravitational attractiveness. Part of that is simply due to the rest masses of all the protons and neutrons and electrons in the sun. Part of it also is due to the LIGHT energy deep inside the sun.

the equation of Gen Rel says that gravity does not depend on mass but on the stress energy tensor, essentially the cause of gravity is not strictly or merely mass but more generally the ENERGY DENSITY and where the density of energy is high, spacetime will be highly curved accordingly.

OK so what pulls us to the sun is not just the restmasses of the particles and it is not just the radiation energy it is also the KINETIC ENERGY of the particles moving around inside the sun. That kinetic energy has gravitational attractiveness and it contributes a PART of the gravity-pull of the sun.

things dont have to move near the speed of light in order to acquire extra gravitational attractiveness. Like the air molecules in the tank which are going on the order of 1000 feet per second, a modest speed, even that modest speed contributes-----only the effect is too small to measure.

Do you want to know what percent of the suns gravity is due to the kinetic energy of the motion of its particles? It might be in some handbook where one could look it up.

I am trying to make this as concrete as possible.

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Gravity definitely does NOT increase as the relative speed of an object nears lightspeed. And mass does not increase in any real absolute sense either. And a rocket-propelled craft that appears to be travelling at near lightspeed does NOT become harder and harder for its own propulsion system to accelerate. Mark my words.

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Actually, mass doesn't "increase" - it's just measured differently depending on your frame of reference. If observer A sees you travelling at a constant velocity, he will measure your mass as being greater than you will measure your own mass. And since, according to relativity, you can just as well regard yourself as being at rest, and A as moving at a constant velocity in the opposite direction, you will measure his mass as greater than he will measure his own mass. In other words, mass - like length and time - is a relativistic quantity.

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^^ Mark his words too ^^

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If inertia increases as you get closer to C, then is it just coincedence that C is the boundary that we can't reach, or what? Makes even those unreligious people like me wonder if there was a plan in this, since the significant figures happen to be equal.

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Religion or no religion, it wouldn't be RIGHT for a material object to be able to move faster than light itself. As light is the quintessential mediator of all that exists, her unique speed needs to be inviolable, and expressly tantamount to infinite speed.

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I think the formula he was looking for is

E=GMC^2

Where G=1/Sqrt 1-V^2/C^2

any help to you?

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If inertia increases as you get closer to C, then is it just coincedence that C is the boundary that we can't reach, or what? Makes even those unreligious people like me wonder if there was a plan in this, since the significant figures happen to be equal.

It's not a coincidence, one is the justification for the other (sort of). They're both parts of special relativity, which works on the basis that the speed of light is invariant for all observers.

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JaKiri, they're both parts of special relativity, but special relativity was thought up by Eintstein (well, relativity was, but I dunno the differences, so I assumed). That means that it wasn't originally part of special relativity, it was part of the way things worked.

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JaKiri, they're both parts of special relativity, but special relativity was thought up by Eintstein (well, relativity was, but I dunno the differences, so I assumed). That means that it wasn't originally part of special relativity, it was part of the way things worked.

What?

The ('best') reason you can't travel at light speed is because it would take infinite energy to reach it. This is because mass increases to infinity as light speed is approached.

Special Relativity was a theory published by Einstein in 1905, building on the work of Michaelson, Moreley and others.

General Relativity is to do with gravity, and was published by Einstein in 1916.

There is no 'Einstein's Relativity' per se, just these two theories.

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...mass increases to infinity as light speed is approached
...Relative to a presumed still observer, but that would have no bearing whatsoever on the ability of a rocket ship's local, ie. onboard, propulsion engine to give further boosts.
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...Relative to a presumed still observer, but that would have no bearing whatsoever on the ability of a rocket ship's local, ie. onboard[/i'], propulsion engine to give further boosts.

surely then the conversion of the entire ship into Energy would be required then?

and it would have to be a VERY efficient conversion leaving even the smallest of particle(s) massless, and thus rendering the excersize futile, and Jakari point Valid?

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surely then the conversion of the entire ship into Energy would be required then?

and it would have to be a VERY efficient conversion leaving even the smallest of particle(s) massless' date=' and thus rendering the excersize futile, and Jakari point Valid?[/quote']

HUH?? You misunderstand it wholly. Huh??

Let me put what I previously stated in the simplest most unequivocal terms: If you see a rocket ship moving at 299,000 km/sec (1000 km/sec slower than light), that very same rocket ship can then fire its propulsion engine to (locally) gain an additional 50,000 km/sec in speed, with NO inordinate difficulty whatsoever. So the fact that it was already moving at near lightspeed to you, a random observer, has no bearing whatsoever on the ships ability to accelerate itself.

Where you came up with "conversion to pure energy" is beyond my ken. Of course, energy is proportionate to velocity squared, as always, but let's not mucky up the waters with fruitless asides.

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...Relative to a presumed still observer, but that would have no bearing whatsoever on the ability of a rocket ship's local, ie. onboard[/i'], propulsion engine to give further boosts.

Incorrect, actually, but only by coincidence. If there was an absolute reference frame, and you observed your mass increasing as you approached lightspeed, you could still accelerate all you want and never hit lightspeed, because your mass would approach to infinity, and therefore require infinite energy.

Furthermore, I never said anything about the ability to accelerate; merely that you couldn't hit lightspeed.

And finally, it doesn't matter what the ship can do; if it would require infinite energy for you to travel at that speed relative to a stationary observer, it would break energy conservation and thus must be invalid for all cases.

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HUH?? You misunderstand it wholly. Huh??

Let me put what I previously stated in the simplest most unequivocal terms: If you see a rocket ship moving at 299' date='000 km/sec (1000 km/sec slower than light), that very same rocket ship can then fire its propulsion engine to (locally) gain an additional 50,000 km/sec in speed, with [u']NO[/u] inordinate difficulty whatsoever. So the fact that it was already moving at near lightspeed to you, a random observer, has no bearing whatsoever on the ships ability to accelerate itself.

Where you came up with "conversion to pure energy" is beyond my ken. Of course, energy is proportionate to velocity squared, as always, but let's not mucky up the waters with fruitless asides.

simply because it wouldnt work that way, the faster you go the more massive you become, and the more massive you become the more fuel youll need to go faster, and then youll reach a max zenith of energy to speed, so you think AHA lets take more Fuel, but more fuel would weigh more

AHA! how about the same mass of fuel but more efficient! well that would work to a point again, but reach a plateau again. until the only way you could do it would be to convert the entire rocket to pure enery, THEN!!! you may do it, but at the sacrafice of your rocket having to be at Zero mass as all was needed to be converted to pure energy to make it possible.

thats what I meant

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simply because it wouldnt work that way, the faster you go the more massive you become, and the more massive you become the more fuel youll ... thats what I meant
Okay, so now I at least know what you meant. But your assertions are not accurate. One does not 'become' more massive the faster one 'goes'. Going, ie. moving, is all relative; there is NO SUCH THING as a body's ABSOLUTE UNEQUIVOCAL velocity (through space), so there is no such thing as a body ABSOLUTELY becoming more massive. Get it??

I AM sorry, but I decline to lead your hand further until you make some earnest effort to research the subject of relativity. Hey, you OWE it to yourself.

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then you shall wait a long time, as this I have already tried to no avail.

do you REALLY think Ide ask if I knew ????

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Of course, energy is proportionate to velocity squared

Only to first order, and certainly not as v approaches c.

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