Lets say that hypothetically we develope space vehicles that can accelerate to 75% of the speed of light. Two are built and are launched in opposite directions and after achieving their top speed will the crews be able to see each other? Does their apparent speed relative to each other exceed the speed of light?

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Bob DeWoody

My motto: Never do today what you can put off until tomorrow as it may not be required. This no longer applies in light of current events.

Yes of course it does but an observer on each ship would not perceive this since the measurement of position travels at the speed of light. The light from one ship would never reach the other. An observer on Earth would see each one moving away ay 3/4 light speed.

Good.................question.
The speed of light is...........................................

Oh, I'll let the scene speak for itself.

Wilder and Teri explore this very question.....

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"Time is simply the mechanism that keeps everything from happening all at once."

When the light reaches point A the spaceship at that point will be long gone. You do realize that we cannot see a great part of our universe because the light will never reach us. The universe is said to be expanding at faster than the speed of light.

The reason that we can see as much as we can is that we are not moving away at an appreciable part of the speed of light relative to these distant objects.

Try this--I think that it is attributable to Einstein.

Imagine a very very long electric fence with a cow touching it every 100,000 miles or so. If the fence is activated the cows would immediately drop of the fence. Now put an observer at either end of the fence and let one of them throw the switch. One observer would see the cows drop off every second or so and the other would see them all drop off at once. Do you agree ?

By that time the ship at point A will have moved on. at some more years (I'll let you figure it out) they will see the ship as it was some years in the past.

Well, that's just wrong. If it were correct Ship A would measure Ship B's speed to be 1.5C. Ship A can't measure ship B's speed at greater than C.

Your mistake is only measuring the time dilation. You missed the spatial dilation. Ship A and Ship B's yardsticks aren't a yard long anymore.

Also you have this special not relative place called earth in you example. Ditch it. Same result is A stays put and B flys away, or B stays put and A flys away, or they both fly away from each other.

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Yes of course it does but an observer on each ship would not perceive this since the measurement of position travels at the speed of light. The light from one ship would never reach the other. An observer on Earth would see each one moving away ay 3/4 light speed.

I thinks you're wrong.
Spaceship A or B would see the other ship B or A, but they would see them from further back in time, and red shifted.
Taking a point 4 years from earth, ship A would be 3 LYs from earth and would see
light from ship B that traveled 4 LY's , so ship B would have sent that light
4/3 years from it's departure from earth, or 8/3 years before the light reaches point A.
Distance and time measured relative to Earth.
Space/time dilation is relatively small at 3/4 speed of light.

correct, but only if we r talking about Earth time...

if we talk about traveler A or B time, then we have 2 put some Lorentz equations in formulas! ;)

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non-profit org. Play4Life in Zagreb, Croatia, EU

Yeah. Like i said, William is wrong.


To be honest i have no idea what you're talking about.
You're as vague as William is.

You are wrong as well. You shifted from reference frame of earth to reference frame of ship B without doing a Lorentz transformation.

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OK, one ship can't see the other further back in time than when they leave earth. It seems to me that when they have both accelerated beyond 0.5 of the speed eventually they would red shift and eventually vanish from each other's point of view.

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Bob DeWoody

My motto: Never do today what you can put off until tomorrow as it may not be required. This no longer applies in light of current events.

Yes: time to come clean on the question. You may not be able to calculate the correct answer if you have not had a graduate course in theoretical physics or at the minimum a very thorough high school advanced placement course in Physics (mechanics of relativity), which I think is unlikely.

The trick here is that velocities do not add arithmetically at speeds that are an appreciable percentage of light. Time, distance, velocity: which do you have to treat in a relativistic fashion to answer the question?

I will leave it to those who want a definitive answer to google this question and try to follow the math. You will also need some good skill in algebra and possibly calculus. Also it is essential for understanding that you are clear about which inertial frame of reference you are referring to (ship A, ship B or Earth). In other words: Where is the observer ?

You seem to intimate greater understanding, but I don't see any

As a teacher of some success I find that guiding the student to uncover the answer is far more effective than if I work the problem for them. I am not sure that they would practice enough to secure profound knowledge--if the student works the problem from authoritative equations and hints then he will be truly informed.

Others have laid out the math on this topic and I find trying to use an equation editor on these boards to be burdensome. Rather than me pasting in the math--I will let those who seek to truly understand this question and it's answer to follow the Google search to these topics.

The full understanding and a definitive, numeric answer will only come from the
mathematical physics involved. You may find summaries of these results. It depends on one's desire for profound knowledge or simply an informed understanding to decide how much time and brainpower to invest in this quest.

Hint: the velocities between these two ships can never exceed the speed of light--that is a fundamental belief in the cosmic speed limit. So does that answer the question or do you want to know why ? You may want to argue and challenge this on the basis of universe expansion constants. (Bait for a new question and go-round ?)

My original comment back a few posts ago was obviously false to test the level of physics maturity out there--much like I might do if I were still teaching Physics.

Yeah. Like i said, William is wrong.


To be honest i have no idea what you're talking about.
You're as vague as William is.

You are wrong as well. You shifted from reference frame of earth to reference frame of ship B without doing a Lorentz transformation.

Actually i never shifted to ship B reference point.
C distance and time all relative to earth/starting-point reference frame

You thought you didn't shift, but when you asked when B sees the light, that is in B's reference frame. You didn't say what time is it on earth when B should see the light.

The problem is more interesting when you realize that A somehow has to know when to emit this light. As A's clock and yardstick have changed relative to earth's, if he uses his clock to time the year he won't emit the pulse when or where the earth observer expects. Same also applies to B and earth, they don't agree on the time or place when the pulse gets there. Why not also include that A encodes the time of his clock on the pulse of light. (The problem gets more interesting if you do it in reality and these ships don't break another of Einstein's laws and have to accelerate at less than infinity, bringing general relativity and gravity into the picture.)

Are you saying that no light from either ship can reach the other, like William did initially?

Of course not. A and B do not measure each other at 1.5c. Work out the transformations to find out what speed they see each other moving at.

Here is a little hint to all. B doesn't stop moving when A emits the light. Where is B when the light finally gets to him? Oh and how much farther does A travel waiting for B to see the light?

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If that's inaccurate do tell me why.

As I said, velocities do not add arithmetically when the speeds are an appreciable part of the speed of light. Their relative speeds to one another have to be less than the speed of light since nothing can travel faster than the speed of light.

Their relative speeds would probably be about .95 or so of lightspeed in free space.

You're thinking of the relativistic velocity-addition formula.
That's not needed to come up with the right answer.

Take your pick:

velocity addition
length Contraction
Time dilitation

http://www.aljazeera.com/video/europe/2011/09/201192421516923404.html

I don't know if this is real.

Steve

Never mind. This was from 2011, and has since been proved false.

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Warning, addicted to SETI crunching!
Crunching as a member of GPU Users Group.
GPUUG Website

If that's inaccurate do tell me why.

As I said, velocities do not add arithmetically when the speeds are an appreciable part of the speed of light. Their relative speeds to one another have to be less than the speed of light since nothing can travel faster than the speed of light.

Their relative speeds would probably be about .95 or so of lightspeed in free space.

NO, they can!

1 speed over another can't be over 1LS...

so if A goes 0,75LS from reference (R)...& B goes 0,75LS from R...than A moves from B in a FTL speed...
'cause A moves from B in a FTL speed, so A can only C past of ship B...in other words in a 1Y time A & B will be 0,75LY from R - but A will C that B has traveled (roughly) 0,25LY...& it will shift 2 red...& d ship B will look "shortened by factor 0,75c in Lorentz transformation"...
also, same this is 4 observer on ship B...

if u don't believe me...u don't!
just ask yourself, what is the speed of expansion of this Universe?
it's also happening in BOTH (or all) directions!
;)

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non-profit org. Play4Life in Zagreb, Croatia, EU

http://www.aljazeera.com/video/europe/2011/09/201192421516923404.html

I don't know if this is real.

Steve

Never mind. This was from 2011, and has since been proved false.

Still waiting for the hypothetical tachyons to be discovered..

Most physicists think that faster-than-light particles cannot exist because they are not consistent with the known laws of physics. If such particles did exist, they could be used to build a tachyonic antitelephone and send signals faster than light, which (according to special relativity) would lead to violations of causality. Potentially consistent theories that allow faster-than-light particles include those that break Lorentz invariance, the symmetry underlying special relativity, so that the speed of light is not a barrier.

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rOZZ
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Still waiting for the hypothetical tachyons to be discovered..

Still waiting for the tachyon drive to be discovered.........

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"Time is simply the mechanism that keeps everything from happening all at once."

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rOZZ
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