Traveling at 99.99..% c with respect to what? Are they both moving at the same speed?hubble_bubble said:Spaceship A and spaceship b are traveling at 99.99... % the speed of light.
Doc Al said:Traveling at 99.99..% c with respect to what? Are they both moving at the same speed?
OK. So why wouldn't ship A's missile hit ship B? The fact that they happen to be moving with respect to something else doesn't matter.hubble_bubble said:Both moving at the same speed and in the same direction.
Doc Al said:OK. So why wouldn't ship A's missile hit ship B? The fact that they happen to be moving with respect to something else doesn't matter.
The missiles are firing at some speed with respect to the ships. Say I am in ship A and you are in ship B. You and I are at rest with respect to each other--we are moving together. If I fire a missile at you, then it will eventually hit you regardless of its speed. The fact that we happen to be moving with respect to something else is irrelevant.hubble_bubble said:If they are traveling at 99.99...% the speed of light of course it matters. Say you fired your missile at 5% the speed of light (yes I know it is impractical) then you have a problem unless time dilation has skewed yourview of the universe enough to make you think the wrong velocity is 5% c.
Right. The speed of light (in vacuum) is the same for all observers.hubble_bubble said:Let's put it another way. In all frames of reference light is at a constant speed.
No. Something moving at less than the speed of light will appear to move at a different speed in different frames.So anything measured with regard to the speed of light should be constant.
Say A and B are traveling at 0.99c with respect to the earth. A fires a missile at B at 0.05c with respect to the ships. As far as A and B are concerned, that missile is moving at 0.05c.So if spaceship A fires a missile at 5% the speed of light then that is the speed it should travel at from the observers perspective unless his measurement of the speed of light is wrong.
Doc Al said:Right. The speed of light (in vacuum) is the same for all observers.
No. Something moving at less than the speed of light will appear to move at a different speed in different frames.
Say A and B are traveling at 0.99c with respect to the earth. A fires a missile at B at 0.05c with respect to the ships. As far as A and B are concerned, that missile is moving at 0.05c.
Of course, as seen by Earth observers, the speed of the missile is much greater, a bit faster than 0.99c. So Earth observers will see the missile eventually catch up to B.
An observer on A would measure the speed of the photon as c, as would an observer on B (obviously), an observer on the missile, and any other observers (e.g. on earth). Similarly with photons fired from B, the missile, the earth, or anything else.hubble_bubble said:OK. If I Were to fire a photon from ship A to a target on ship B and measure its speed in relation to the missile would I get correct results?
Please show your work. You made a mistake somewhere, but it is not clear what that mistake is from this description.hubble_bubble said:It would mean that the first photon would have to slow dramatically and be time dilated so its speed appeared right to the observer in spaceship B. Otherwise it would simply not be detected at all as time dilation changes spaceship Bs view of the universe.
DaleSpam said:An observer on A would measure the speed of the photon as c, as would an observer on B (obviously), an observer on the missile, and any other observers (e.g. on earth). Similarly with photons fired from B, the missile, the earth, or anything else.
Please show your work. You made a mistake somewhere, but it is not clear what that mistake is from this description.
hubble_bubble said:Sorry I was being a little flippant. Of course there is no time dilation of the photon. This however still raises the question as to how that individual photon would be perceived. Would it be seen as in a lower energy state? Something has to change due to the time dilation.
phinds said:WHAT time dilation? The people on the ships aren't experiencing any time dilation. Do you understand that or not?
The fact the someone in a DIFFERENT frame of reference sees them time dilated is irrelevant to them.
You do realize, I assume, that you personally are moving at 99.9% of the speed of light from some frame of reference and that frame of reference sees you as time dilated. Do you notice anything?
What paradoxes?hubble_bubble said:Frames of reference are useful tools but the paradoxes abound.
A second frame had to be introduced to give meaning to your statement that the ships were moving at 0.99c. (Moving at 0.99c with respect to what?) The fact that Earth observers will need to consider time dilation (and other relativistic effects) in interpreting the observations of the ships is irrelevant.I am speaking with reference to the earlier poster who included Earth into the scenario so from the point of view of an Earth observer there would be time dilation.
I assume you meant "as C passes ship B".Spaceship C is traveling in exactly the opposite direction to A and B but not on a collision course. Spaceship C is traveling at the same speed as A and B but in the opposite direction. As C passes spaceship A, A fires one photon at ship B and one photon at ship C. Both ships B and C are at equal distances from A at that time.
At the same time according to whom?Will both photons be detected at the same time?
That is given by the relativistic Doppler equation.hubble_bubble said:Would it be seen as in a lower energy state?
Once again, all observers will measure the speed of that fired photon as being c with respect to their own frame. To understand how those observations can all be consistent, you must apply several relativistic effects, not just time dilation. You must include length contraction and the relativity of simultaneity as well.hubble_bubble said:OK. If I Were to fire a photon from ship A to a target on ship B and measure its speed in relation to the missile would I get correct results? If at the same time I fired a photon at a target that was stationary with respect to the Earth and measured its speed would that result agree with spaceship Bs result. If so how? It would mean that the first photon would have to slow dramatically and be time dilated so its speed appeared right to the observer in spaceship B. Otherwise it would simply not be detected at all as time dilation changes spaceship Bs view of the universe.
hubble_bubble said:Spaceship A and spaceship b are traveling at 99.99... % the speed of light. Spaceship A is behind spaceship B. Spaceship A wants to destroy spaceship B and fires a missile. Will spaceship A succeed for any velocity the rocket is launched at and at a variable distance?
DaleSpam said:That is given by the relativistic Doppler equation.
http://en.wikipedia.org/wiki/Relativistic_Doppler_effect
Paradoxes abound only in the sense that there are a lot of things which are confusing to students of relativity. None of the things which confuse students are actual paradoxes representing logical inconsistencies of the theory.
Doc Al said:I assume you meant "as C passes ship B".
It's certainly true that clocks (and other processes) on the fast moving ship will be observed to run much more slowly according to Earth observers, but that's not really a problem.hubble_bubble said:As time dilation basically slows down all activity in the two spaceships with reference to an Earth based frame of reference then that also means that propulsion is also slowed down. How does this square with theoretically being able to produce near light propulsion? If all processes slow with relation to the surrounding universe the propulsion would weaken in effect the nearer to light speed one got.
OK, A fires the photon when C is the same distance from A as is B, as determined by A. (Since C is moving, and simultaneity is frame dependent, C would disagree as to when the photon was fired.)hubble_bubble said:When C has moved past A and is at the same distance from A as B is then the photon is fired.
Yes: Since the source (A) is not moving with respect to B, B would observe the normal wavelength. But C is moving away from the source, so it would see a red shift. (No need for any elaborate set up if all you care about is the observed frequency.)At the time the two photons are fired both C and B will be traveling at the same velocity in opposite directions. As these opposite trajectories are mirror images of each other the interest would be in determining how each photon would be perceived. As A is traveling in the same direction as B we would see the photon at a steady wavelength? Would we see a red shift from the perspective of C?
hubble_bubble said:Spaceship A and spaceship b are traveling at 99.99... % the speed of light. Spaceship A is behind spaceship B. Spaceship A wants to destroy spaceship B and fires a missile. Will spaceship A succeed for any velocity the rocket is launched at and at a variable distance?
ExecNight said:I think i understand what you are thinking. Well you have 3 reference points in your post.
But i will guess you are the 4th reference point that is at rest while the spaceships are %99c..
What happens is this;
Spaceship A fires a Rocket towards Spaceship B..
In their frame Spaceship B gets hit within the moment..
In your frame rocket can't go faster than c
With respect to that and the speed of the spaceships it will take a very very long amount of time, before you see that the spaceship B is hit with the rocket.
To my understanding that's what happens.
Again, you are making a mistake somewhere. Please post your calculations.hubble_bubble said:And of course the propellant with respect to my frame of reference will also take an incredibly long time to expel itself. This is paradoxical. In fact the amount of energy and the rate of expulsion needed to maintain velocity/acceleration appears to be inversely proportional to the slowdown due to time dilation. The increase in mass and rate of expulsion should be exponential and directly proportional to the velocity with respect to c. This should approach infinity rapidly as we near light speed.
DaleSpam said:Again, you are making a mistake somewhere. Please post your calculations.
hubble_bubble said:The hope that one day mankind will be able to travel at near-to-speed-of-light velocities seems farfetched, because of the incredible amounts of energy needed to accelerate a spacecraft to these speeds. The forces are likely to destroy any vehicle before it comes even close to the required speed."
PAllen said:Are you aware that a thrust that you experience as 1 g, the same force you feel standing on earth, will quite quickly accelerate you to 99.99999% the speed of light relative to earth? The fuel needed would depend on your elapsed time. At any 'moment', the requirements to accelerate at 1g relative to an instantly comoving inertial frame are the same as when you started from earth.
hubble_bubble said:That sounds reasonable. How will that relate to the increase in dilation? What happens when we get into the 90% plus range as we approach c? From the external observer's point of view time is almost standing still. However we are still using the same amount of fuel at the same rate in our reference frame. How can we resolve this? Is it the compaction of space that matters in this case?
The page does not demonstrate nor even claim to demonstrate any paradox related to fuel consumption or power. Please post your derivation of the paradox you mention in post 25.hubble_bubble said:See the graph here. http://www.thebigview.com/spacetime/timedilation.html
"As it can be seen from the above function, the effect of time dilation is negligible for common speeds, such as that of a car or even a jet plane, but it increases dramatically when one gets close to the speed of light. Very close to c, time virtually stands still for the outside observer."
The outside observer see no change close to light speed. This applies to the propulsion mechanism if the mass dos not increase.
"Can we travel at the speed of light?
The hope that one day mankind will be able to travel at near-to-speed-of-light velocities seems farfetched, because of the incredible amounts of energy needed to accelerate a spacecraft to these speeds. The forces are likely to destroy any vehicle before it comes even close to the required speed."
PAllen said:I don't see much to resolve. Let's say you accelerate at 1 g (your experience) for 1 year of your time. You use up 1 million tons (rest mass) of fuel in one year to accomplish this. To an Earth observer, you have still used up exactly 1 million tons (rest mass - invariant) of fuel, but you have done so over, say, 1000 years (this isn't the real number - I haven't computed it, but it is some large multiple), and traveled nearly 1000 light years.
Yes.hubble_bubble said:Also, if we take the moving spaceship's frame to be stationary and the universe moving past at speed, can the spaceship ever launch a missile that from his perspective would be traveling away from him at 99.99% the speed of light?
Again, the fact that the spaceship is moving (with respect to something) is irrelevant to its being able to launch a missile at any particular speed.hubble_bubble said:Also, if we take the moving spaceship's frame to be stationary and the universe moving past at speed, can the spaceship ever launch a missile that from his perspective would be traveling away from him at 99.99% the speed of light?
hubble_bubble said:First thanks for all the replies. Take the time dilation to be one thousand fold, then would the space compaction be to this same percentage? Also, if we take the moving spaceship's frame to be stationary and the universe moving past at speed, can the spaceship ever launch a missile that from his perspective would be traveling away from him at 99.99% the speed of light?
DaleSpam said:The page does not demonstrate nor even claim to demonstrate any paradox related to fuel consumption or power. Please post your derivation of the paradox you mention in post 25.
PAllen said:Yes, the universe going by would appear to have short distances in the direction of motion. The rocket would consider they had traveled only 1 light year in one year. To an observer stationary relative to the starts, the rocket seems compressed in its direction of motion. The length/distance contraction factor is exactly the same as the time dilation factor:
γ = 1/(√(1-v^2/c^c)
For an accelerating rocket, γ would by time varying, but still only depends on current relative speed.
understood. Just don't be so ready with the word paradox or contradiction. There aren't any. Also, IMO, the best way to learn relativity is to actually work some problems, draw some spacetime diagrams, solve some equations, etc.hubble_bubble said:Yes sorry I am thinking about several things at once. Just trying to make sense of the concepts.
HallsofIvy said:Hubble bubble, you posted this under "relativity" and Doc Al's first question was "99% the speed of light relative to what?" which you never answered. You don't seem to be clear that speed always has to be measured relative to something and that is what the physics depends upon. In your first post you set up a situation in which spaceships A and B were motionless relative to each other. The situation is exactly the same as if you had said that A and B had speed 0 relative to whatever frame of reference you intended in your first post.
