# A small problem for legendary JamesR on Relativity

Discussion in 'Physics & Math' started by RawThinkTank, Sep 27, 2004.

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1. ### RawThinkTankBannedBanned

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If a train is at 99.999...% speed of light and a rocket is launched in the same direction the train is moving , for the rocket the train is relatively stationary then would the rocket achieve 99.999...% speed of light relative to the train ???

3. ### Janus58Valued Senior Member

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First off, do you mean 99.999...% or 99.999...9%?

Because if you mean the first, it is equal to 100%, and since physical objects cannot travel at 100% c, your question is meaningless.

Though just the way it is phrased, even with sub-light velocities, it sounds pretty meaningless. What is the rockets velocity with respect to what?. If it has the same (less than c) velocity as the train as measured with respect to the same frame as the train's velocity is measured, As you seem to mean when you say that the train is stationary with respect to the rocket as seen from the rocket, then the rocket will be stationary with respect to the train as seen from the train.

5. ### FacialValued Senior Member

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The rocket would appear to be a normal rocket, accelerating to however fast you want it to go. If somehow you intend to question whether or not the rocket seems to accelerate more easily, it does not.

7. ### Quantum QuackLife's a tease...Valued Senior Member

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interesting.......as light is invariant to the train and the rocket is travelling at 99.999.....% of 'c', then the answer to the question is yes I think.
99% for the train and teh rocket is the same velocity relative to the light......hmmmmmm

8. ### HallsofIvyRegistered Senior Member

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I'm guessing that it was intended that the rocket was launched FROM the train- that wasn't said.

Yes, the rocket could reach 99.999% of c relative to the train. If the train is moving at 99.999% of c relative to the ground, the rocket would then have a speed of 99.99999999995% of c relative to the ground.

9. ### Quantum QuackLife's a tease...Valued Senior Member

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I am not sure either but I think RTT was talking about how to the train what velocity is the rocket. The ole relative frames question again.

If teh train has measured it's own speed as 99% 'c', and the rocket is traveling like wise at 99%'c' then the speed of the rocket to the train would intuitively be zero both sharing the same velocity relative to Earth. But the question is leading to the answer being that the rocket is also 99% 'c' relative to the trains frame as well. Due to the invariance of light for both frames.
As there is no velocity addition allowed. the train would see the rocket at what measurable velocity even though it would appear to be at rest with the train?
I think this is what RTT was wanting to explore.....but maybe he could enlighten us all himself?

10. ### RawThinkTankBannedBanned

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Let me rephrase the question for humans;

The train is 99% speed of light when measured on earth, a rocket is launched from it, the rocket has no idea what the earth is, hence it should achive 99% speed of light relative to the train.

My point is , Relative to what in the universe will speed of the rocket be decided.
ie. there should be a universal zero speed, that opens up too many questions.

11. ### Janus58Valued Senior Member

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Relative to the train, as measured from the train or rocket, the rocket will have a velocity of 99% of c

Relative to the Earth, as measured from Earth or rocket, the rocket will have a velocity of 99.99495% of c.

Velocity of separation of the rocket and the train, as measured from the Earth, 0.99495% of c.

Velocity of separation of the rocket and the Earth, as measured from the train, 198% of c.

No universal zero speed, and all velocity additions are subject to the Lorentz transformations.

12. ### RawThinkTankBannedBanned

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429
Let me rephrase the question so that Janus58 can understand it.

The question is ....

and the question is

Will the rocket achive 99% speed of light relative to the train in the same direction the train is moving 99% speed of light relative to the earth ?

Bingo ,
James R , U have successfully ignored this question.

13. ### 2inquisitiveThe Devil is in the detailsRegistered Senior Member

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RTT, Janus58 DID answer your question, completely, according to SR. What is it that
you think is missing?

14. ### MacMRegistered Senior Member

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Actually you have a valid point. Probably more so than you realize. I have mentioned something like this before and that is "Why do we ot se different energies reqired to accelerate particles. Considering the 1E22 or whatever the number of stars etc., in the universe and the fact that there have been dozens of sightings of material that is approaching us near the speed of light, how is it that we are not more restricted. ?

That is v is always relative to something and frankly I don't think it cares if we claim it is relative to the moon or Alpha Centuri, OR SOME UNKNOWN QUASAR material already going 95%c. We should see that it is more difficult to accelerate when the direction of motion just happened to be in the direction of such material.

I find it more than just coincidence that we have never seen any restraint on our acceleration in any direction which suggests that the only limit on acceleration and velocity has to do with relative velocity between the fuel, thrust engines and the rocket or the stationary magnets and particles and not the relative motion to any other particles or masses in motion in the universe.

Clearly if Relativity were being properly interpreted and being at a relative velocity of 0.95 c to ejected Quasar mass coming at us then it should require that we would have to apply 3.2 times as much energy to accelerate in that direction.

Hmmmm. See no such affects do we?

That is the basis for my conclusion that what we claim is changing mass (energy required to accelerate) is due to a decrease in energy transfer efficiency and not any mass affect.

Last edited: Oct 16, 2004

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16. ### Paul TRegistered Senior Member

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This is enough, mister. Please go to sleep and come back tomorrow. You are talking a lot of BS.

17. ### geistkieselValued Senior Member

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I agree, 100%. Consider the energy storing process that must occur in all acclerations, all energy exchanges, where the accelerated particle becomes ever more violenty oscillating, while ever attempting to use the energy to acceas lerate velocity, the inefficiency of energy transfer ends up mesurably proportional to the apparent disjointed energy vs. velocity curve; Observe where the particle is not subject to the rapid energy tansfer where all energy get used as velocty and momentum increases at low velocities (the energy is alammed into the ap[rticle by sheer brute force) and when the particle approaches the velocity of the incoming energy at higher velocities. where more time is required to load the energy on board the accelerated platform..

18. ### alaindu hast michRegistered Senior Member

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rawthinktank. are you sayign that a train going almost at the speed of light shoots a missile forwards, the missile should break the speed of light?

if so, then it obviously wouldn't break the speed of light, but i have no idea why that is so

19. ### PersolI am the great and mighty Zo.Registered Senior Member

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Most of this is just techno-babble.

At the start train is already moving at .99c. The rocket is also moving at .99c.

The rocket can then be launched from the train at .99c (we're assuming enough energy) with reference to the train. However, the train is time dilated with reference to the ground. This causes the velocity to be measured by the train and the ground to be different.... hence why it doesn't 'take more energy to move towards something that is fast'.

20. ### Paul TRegistered Senior Member

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This guy just had his ideas mixed up. See no such effects? Depending on relative to which reference frames this acceleration referring to, 1) fixed stars that has negligible velocity relative to earth or 2) the material ejected by quasar (call it material Q) that moves toward earth at velocity 0.95c.

Relative to fixed stars

Of course that material ejected by quasar does not affect the spacecraft acceleration with respect to the star (or the earth itself) in anyway, which means that Newtonian mechanics works fine in this situation (provided that velocity involved is relatively small).

Relative to material Q

Situation is the same as the above, but now say, the spacecraft accelerates so that it will have velocity 0.951c relative to Q, an increment velocity of 0.001c. Is it really so? Yes only according to Galilean relativity (that MacM believes still hold true), but certainly not according to Einsteinian relativity. A simple calculation using SR's addition of velocities yields 0.01036c as the additional velocity (relative to fixed star) for the spacecraft to attain velocity 0.951c relative to Q. MacM doesn't believe this computation make sense as he argued before that SR's addition of velocities is not applicable for the case of rocket acceleration like this.

Let's move on. How much energy required to achieve additional velocity of 0.01036c compared to if the additional velocity is just 0.001c? It is about 3.2 times, that is (0.01036/0.001)<sup>1/2</sup>. So, this "Hmmmm. See no such affects do we?" must be a statement given by an ignorant.

Welcome to MacM's club.

21. ### MacMRegistered Senior Member

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0.95c is not small. Not is the amount of mass coming at us from such Quasars.

Relative to material Q

Situation is the same as the above, but now say, the spacecraft accelerates so that it will have velocity 0.951c relative to Q, an increment velocity of 0.001c. Is it really so? Yes only according to Galilean relativity (that MacM believes still hold true), but certainly not according to Einsteinian relativity. A simple calculation using SR's addition of velocities yields 0.01036c as the additional velocity (relative to fixed star) for the spacecraft to attain velocity 0.951c relative to Q. MacM doesn't believe this computation make sense as he argued before that SR's addition of velocities is not applicable for the case of rocket acceleration like this. [/quote]

I don't see any justification for you disregarding the 0.95c material coming at you when you attempt to accelerate toward it. According to relativity you are attempting to accelerte to >0.95c which requires you to aply 3.2 times as much energy to accelerate toward such material.

Since we see no such affect what should that tell you?

You do seem to have a problem keeping your physics straight. If your car requires 120 Hp to go from 0-60 Mph is 8 seconds then going from 0-60 Mph in 8 seconds but in the direction of such Quasar material will require 384 Hp. I think you might notice such a load. But then maybe you wouldn't but I would.

22. ### PeteIt's not rocket surgeryRegistered Senior Member

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I'm accelerating in my car toward a quasar.
Before I begin, some piece of the quasar ejecta is approaching me at 0.95c (637,085,798 mph).
My car produces 120HP for 8 seconds to reach 60 Mph relative to my initial frame (as expected).

My new speed relative to the quasar ejecta is now (if I've done the math's correctly) 637,085,804 mph... only 6mph more than before!

So, even though I'm accelerating normally relative to slow moving objects, I need more power to accelerate relative to the pulsar ejecta!

23. ### MacMRegistered Senior Member

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1 - I have forgotten nothing. I see no basis to claim Velocity Addition. It is a simple case of two masses, you in your car and the Quasar ejecta. That is the only relationship being considered.

2 - I can replace the real world case of relative motion with a thought experiment and simply ask "What is the energy ratio required for a mass to be accelerated to a relative velocity to me of 0.95c vs the amount of energy required to accelerate the same mass relative to me starting at relative rest.

You would not and do not assume Velocity Addition is such a case and it does not apply in the case where I simply trade places with existing Quasar material.

3 - The fact that this claim of Relativity is clearly false does not mean we can simply sweep it under the rug.

I await your legitimate response as to why we do not physically see pertabations in our physics regarding our ability to accelerate in different directions since there clearly exist masses having relavistic velocities relative to us in certain directions.