# OK Relavists

Discussion in 'Physics & Math' started by MacM, May 6, 2003.

1. ### MacMRegistered Senior Member

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10,104
I have a question for you.

We all know that the velocity of light is always c in a vacuum (barring mute or irrelavant side issues to avoid the question):

I am an observer. However, I happen to be riding in a craft powered by a solar sail. We all know that light from a point source has an inverse square dispersal and that the further we go from the source the less flux you have per area.

But for this question you are to assume that we are sufficiently far away from the source that the decrease in dispersal is minimal and can be neglected since the angle of the rays intercepting my sail are virtually parallel, yet my craft is small enough and light enough that at the start of my question my craft has an acceleration of 1 g.

Just to avoid the arguement of energy source loss my craft is equipped with an expandable sail and for the minor change in flux I enlarge my collection surface.

WHAT IS MY ULTIMATE TERMINAL VELOCITY TO THE SOURCE?

3. ### Martin BRegistered Member

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Easy

Provided you continue accelerating relative to your origin (which appears to be your setup) you will approach the speed of light relative to your origin asymptotically from below.

There will be no terminal velocity. Your velocity will be bounded by, but never reach, the speed of light.

5. ### MacMRegistered Senior Member

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10,104
OK Easy

Martin B,

Care to explain your reasoning. I hope it isn't mass change relative to the source since the source isn't driving the ship. It is the relative velocity and momentum transfer of the photon to the sail. which has remained unchanged.

If you choose to argue the relative velocity to the source magically restrains the physics of the ship I want to know why we ever got past 0.1%c since there just happened to be a few atoms coming at us from a distant quasar at 99.999999%c.

7. ### Martin BRegistered Member

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Well if I wanted to quibble I would point out that "momentum transfer of the photon" is certainly not constant. As the light source accelerates away from the craft, the light emitted from it will become increasingly red-shifted. The momentum carried by each photon, as seen by the craft, will diminish.

However that is not really the point here. You are asking what velocity will be observed between two bodies under conditions of constant acceleration. We know how to work this out and the answer is as I gave you.

Naughty! That would only be the case if we could directly add our 0.1% to their 99.999999% to get a final relative velocity of 100.999999%c - and you know that isn't the case.

Your argument is equivalent to asking what is the 'magic' that stops Newtonian mechanics from working in Newtonian space-time. It's no magic - it's just the way the Universe is.

Last edited: May 6, 2003
8. ### MacMRegistered Senior Member

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10,104
Quibble

Martin B,

May you might want to "Quibble" with UCLA.

*******************************************
http://www.astro.ucla.edu/~wright/doppler.htm

Thus for the largest known redshift of z=6.3, the recession velocity is not 6.3*c = 1,890,000 km/sec. It is also not the 285,254 km/sec given by the special relativistic Doppler formula 1+z = sqrt((1+v/c)/(1-v/c)). The actual recession velocity for this object depends on the cosmological parameters, but for an OmegaM=0.3 vacuum-dominated flat model the velocity is 585,611 km/sec. This is faster than light.

*******************************************

1.952c to be more precise.

Mac

9. ### Martin BRegistered Member

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That's a second naughty! Cosmological redshift is not motion through local space-time and hence apparent recession velocities are not restricted to c.

I'm sure you knew that as well. :bugeye:

10. ### MacMRegistered Senior Member

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10,104
Several

Martin B,

Yes there are several purported causes for red shift.

1 - Distance

2 - Gravity.

3 - Velocity

4 - Time Dilation

5 - Quantum Red Shift.

I like this last one because it says your velocity induced red shift doesn't even exist.

You are looking for anyway out of facing the undesireable conclusion that photons impinging the sail maintain v = c and that no such limit on velocity exists.

#5 above (which is a bonified theory in the scientific community) happens to makes much more sense than a lot of the older stuff.

I'll be back with more specifics.

PS: I know that you already know that red shift doesn't follow the same relavistic curve.

Last edited: May 6, 2003
11. ### Martin BRegistered Member

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Re: Several

This makes no sense.

Firstly it is a fundamental of relativity (even the ol' fashioned Galilean kind) that you have to say what velocity is relative to.

Secondly, the precise mechanism for acceleration is ultimately irrelevant to the question involved. You are asking "if two bodies continually accelerate away from each other, what will be the terminal velocity relative to each other?" and I have given you the answer that relativity - a theory with abundant observational confirmation - gives.

You have certainly not suggested why photn sails are any different to say, a rocket.

You have given no reason at all - except that something or other "makes sense" as if the Universe is constrianed by your powers of intuition. This is a quaint way of doing science that has been found to be unproductive for around 500 years now.

12. ### MacMRegistered Senior Member

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10,104
QUAINT

Martin B,

Let me suggest the only thing quaint here is the double talk invoking a theory.

Lets talk about your rocket. I've used that one before and the defenders of old theory seem to want to make the propulsion a factor of gas being ejected at v =c which is nonsense.

The propulsion of a rocket is a function of the gas being accelerated relative to the rocket and ejected at some velocity relative to the rocket.

So where is its velocity limit created?

Damn sure not Houston nor Alpha Centuri. There is no mass change between the rocket load, its fuel or the thrust engines.

So if my sail is no different than a rocket, just answer about the terminal "absolute" velocity limit of the rocket.

I understand all velocity is relative but you seem to miss the point. Relative to what. If you want to apply Relativity and limit its relative velocity to earth or some other feature have at it because you can't support that arguement in absense of mass change, which is non-existant for the rocket case.

Unless you are going to be one of those that claim observers affect the physics of independant inertial systems.

Last edited: May 6, 2003
13. ### ryansCome to see me about a dog heyRegistered Senior Member

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995
Hey MacM you ****, you didn't even read the whole of the link, as it gives a perfect reason for this phenomona and I quote;

And antway it is clear to see why your arguement is invalid. How would the sail maintain a velocity greater than c if it was traveling at c. At that instant, no photons would be incident on the sail and a terminal velocity of c would be reached, AFTER AN INFINITE AMOUNT OF TIME.

You are **** trying to change physics without picking up a pen. And do you know why? Because you are **** and won't get **** out the chair in front of your computer and read a book or journal. Websites aren't peer reviewed, and so they can stick any shit up there they want and you will believe it.

Moderator edit: Personal insults add nothing useful to the discussion.

Last edited by a moderator: May 6, 2003
14. ### ProsoothusRegistered Senior Member

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Macm,

From a classical point of view, your light sail will never reach the speed of light for two reasons:

1) If we assume that the principle of invariance of light isn't valid, then the number of photons impacting the light sail per second would decrease since the light sail is moving away from the light source. The higher the speed of the light sail, the lower the number of photon impacts per second on the sail. If the ship is travelling at the speed of c, the number of photon impacts per second would be 0.

2) As Martin pointed out, the faster the ship is travelling, the greater the light is redshifted. If the ship is travelling at c, the frequency of the light would be redshifted to zero, and therefore, the light would have zero momentum.

The combination of these two effects would restrict the ship of ever reaching c because the total force created at any single moment from the photon impacts per second AND the decreased momentum of the photons, would not be sufficient to bridge the gap between the current speed and c.

Of course, to a layman, it would appear that the ship is gaining mass as it's travelling faster.

As you can see, even if relativity was wrong (no time dilation or relative mass), the ship would still not be able to reach the speed of c.

Tom

Last edited: May 6, 2003
15. ### ProsoothusRegistered Senior Member

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ryans,

Only an uncivilized person would use the language that you did in your previous post. What gives you the right to call someone a ****?

None of us are impressed by your personal insults. They only show how insecure you really are. GROW UP.

Tom

Last edited by a moderator: May 6, 2003
16. ### ryansCome to see me about a dog heyRegistered Senior Member

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995
I have been trying give to all you non-relativists guidance in seeing the light (speed is constant. Joke), mostly to have my valid arguements ignored when they bring down any one of your arguements. Thus as valid scientific reasoning isn't working, insultive language is my last resort. If this was a genuine dilemma someone had, I would not have used that language, and the only reason I can see that you and MacM still fail to comprehend SR is because you are ****. So there is no reason

And Tom, you are the layman, and since you do not comprehend the language of physics (mathematics) your contribution is limited to the realm of philosophy. It is not my fault that you cannot understand SR in laymans terms.

Last edited by a moderator: May 6, 2003
17. ### ProsoothusRegistered Senior Member

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ryans,

Now, translate that into English. You sound like George Bush Jr. on a bad day.

Why don't you shut up while your still ahead. You should stick to physics, cause you're obviously having problems analysing people.

Tom

18. ### ryansCome to see me about a dog heyRegistered Senior Member

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Here we go Tom

If this was a genuine dilemma someone had, I would not have used that language. The only reason I can see that you and MacM still fail to comprehend SR is because you are ****.

And that's another thing Tom. If I put forward a valid arguement, you start to question my english as obviously you have no legitimate reply.

Last edited by a moderator: May 6, 2003
19. ### ProsoothusRegistered Senior Member

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ryans,

There is never a reason to use that language (at least not on sciforums

). If you get frustrated at MacM or me, simply ignore us. There is no benefit for anyone if a debate is dragged down to the personal level.

First, let me say that I've tried to respond to all your arguements in a civilized manner using logical counter-arguement. As you know, I've never responded to any of your posts in a personal level until this thread.

Secondly, I was only questioning your English to stress my point. Everybody has weaknesses, so everyone can be attacked. You have to remember, just because someone doesn't know or understand something in physics, doesn't make him/her a worthless individual. A person is more than just the knowledge they possess in physics. And because you don't know the whole person, you should treat everyone on sciforums with respect.

Tom

Last edited by a moderator: May 6, 2003
20. ### James RJust this guy, you know?Staff Member

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31,288
Last edited: May 7, 2003
21. ### MacMRegistered Senior Member

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10,104
Summary

James R,

I will indeed look up the relavistic rockets you mention. Thanks

However, the only potentially correct answer I have seen is the red shift arguement.

Last edited: May 6, 2003
22. ### PersolI am the great and mighty Zo.Registered Senior Member

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MacM,
Is your singular goal in life to attempt to convince someone on sciforums that relativity is incorrect? You find a a report and read a piece of it, then post a question in the hopes that you stump everyone on the site. Then you ignore the rest of the paper you quoted from. You'll only goal that I can see is to disprove relativity. You could just as easily learn about it by spending a day or two reading a physics book instead of the rest of your remaining life trying to disprove it.

23. ### MacMRegistered Senior Member

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10,104
Not True

Persol,

Proving Relativity wrong is not a specific goal. But what you failed to notice is that some of the answers given were absolute bunk, passed of as fact that a 1st grader should know.

My goal is and will be to show some of the know it alls that they don't know it all. I accept the red shift answer even though I suspect in the long term it will be found inappropriate since quantum red shift currently seems to have the leading edge on red shift function. It voids velocity red shift.

But that is another issue. I particularily like it when the know it all gets smart mouthed and then turns around and says things that are assinine.