# Rethinking what is clearly bogus

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The worst myth ever about relativity is that a spaceship can never accelerate to light speed because it is steadily getting heavier, making it ever harder to accelerate. That is rubbish, and I'll convince you right here and now. Under relativity, the speeding space craft can rightfully consider itself to be stock still in space, and so cannot attribute causal distortions of any kind to itself... it's other folks who are in motion, duh.

However, in a collision, then there is relativistic mass increase ascribed to the speeding projectile, and it becomes a factor in how much of an impact will result. In that case, it is the collidee's perspective that what impacts it is extra heavy.

Similarly, if you were using a stationary force field to control a relatively speeding entity, then relativistic mass increase becomes pertinent. While using an electromagnetic field to alter the course of a speeding charged particle, it will become ever harder to divert the particle's momentum as its relativistic mass increases nearing light speed... because this all happens according to the perspective of the lab frame, whereon the EM field producer is mounted.

So that begs the question then, "What is the real reason why a spaceship can't accelerate to light speed?" The reason is because of the limitations imposed by relativity's Addition of Velocities formula. Let's say that you, a stationary observer, see the craft going 1000 MPH slower than lightspeed, and then it accelerates. The pilot of the craft might discern that his recent engine thrust boosted his speed by 5000 MPH (confirmed by his observation of something he left behind, eg. a sister ship). But to you, when the speed 5000 is added to the speed (c-1000), the sum does not surpass c. You have to use the velocity addition formula, (u+v)/(1+(uv/c^2)).

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that still doesn`t answer HOW he might be able to accelerate from c-1000 to c+4000 )or go 5000 mph faster, without becoming criticaly massive?).

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Lightsword: you didn't convince him

The most important staement you made was the first one: relativity does not have objects increasing in mass as they move faster. That is just a legacy of bad high school physics teachers who don't understand what they are talking about.

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Severian: Then it sounds as if you cannot agree with my subsequent paragraphs describing instances where relativistic mass increase IS pertinent. I'm all ears.

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YT2085: Where do you get your (c+4000) figure?? (c-1000mph) + 5000mph does not equal c+4000mph .. that was my whole essential point.

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You are right - I don't agree. Mass doesn't change - it is just that momentum and velocity have a non-linear relationship:

$p=\frac{mv}{\sqrt{1-\frac{v^2}{c^2}}}$

Old text books tend to define mass as p/v, but this is somewhat bogus since mass is a property of the body and should not change with reference frame.

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Thank you! I now get that you agree with the two paragraphs in effect, but not in terminology. Very good! The (charged) particle moving at relative speed .99c DOES make disproportionately more impact in a collision, and IS disproportionately harder to divert with EM fields... 'that right? One just shouldn't go about ascribing those two effects to "relativistic mass", which is -- yeah I knew -- outmoded terminology. A property of the body shouldn't change with reference frame, aye.

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YT2095: Okay, okay. I guess there's more to your point than just that c+4000. How can he do it?? Everything that facilitates his acceleration is found onboard, and everything onboard is in the native observatory (frame) and hence cannot be said to be in motion. The *other* guy is the one doing any moving -- that's quintessential relativity.

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but what about in the case where matter is being expelled out the back of the craft to produce thrust. I suppose it would increase in relativistic mass and produce more thrust when it is shot out the back right?

maybe someone who knows a bit more about it than me can figure it out.

(the example assumes that you had a really powerfull engine that allowed you to get that fast in the first place)

(my point in the example is that you can have different reference frames on the same ship)

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Thank you! I now get that you agree with the two paragraphs in effect, but not in terminology. Very good! The (charged) particle moving at relative speed .99c DOES make disproportionately more impact in a collision, and IS disproportionately harder to divert with EM fields... 'that right? One just shouldn't go about ascribing those two effects to "relativistic mass", which is -- yeah I knew -- outmoded terminology. A property of the body shouldn't change with reference frame, aye.

Yes, I agree. The impact is really how much momentum it transfers, and since the momentum/velocity relationship is non-linear the higher velocities have disproportionately more more momentum (and thus more 'impact').

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Hmm, I thought that mass really did increase with velocity. I guess I learned something new today.

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• 2 months later...
T

So that begs the question then' date=' [i']"What is the real reason why a spaceship can't accelerate to light speed?"[/i] The reason is because of the limitations imposed by relativity's Addition of Velocities formula. Let's say that you, a stationary observer, see the craft going 1000 MPH slower than lightspeed, and then it accelerates. The pilot of the craft might discern that his recent engine thrust boosted his speed by 5000 MPH (confirmed by his observation of something he left behind, eg. a sister ship). But to you, when the speed 5000 is added to the speed (c-1000), the sum does not surpass c. You have to use the velocity addition formula, (u+v)/(1+(uv/c^2)).

So it's Einstein's fault? Nobody blamed Newton when they fell down.

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