## Speed of light question

Message boards : Science (non-SETI) : Speed of light question
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Bob DeWoody

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Message 1718324 - Posted: 25 Aug 2015, 23:19:01 UTC

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?
Bob DeWoody

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William Rothamel

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Message 1718334 - Posted: 25 Aug 2015, 23:47:17 UTC - in response to Message 1718324.

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.
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Message 1718475 - Posted: 26 Aug 2015, 7:14:43 UTC - in response to Message 1718334.

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.
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kittyman
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Message 1718491 - Posted: 26 Aug 2015, 7:59:31 UTC

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

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

Wilder and Teri explore this very question.....
"The secret o' life is enjoying the passage of time." 1977, James Taylor
"With cats." 2018, kittyman

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William Rothamel

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Message 1718518 - Posted: 26 Aug 2015, 9:27:02 UTC - in response to Message 1718475.

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.
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William Rothamel

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Message 1718522 - Posted: 26 Aug 2015, 9:39:57 UTC

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 ?
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Message 1718814 - Posted: 26 Aug 2015, 22:32:26 UTC - in response to Message 1718518.

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.

My previous example was wrong.
Ship A and ship B are each moving at .75 C and away from each other.
1 year after departure from earth the ships are 1.5 Light-Years apart.
Ship B releases a photon in the direction of ship A.
The photon is gaining on ship A at .25 C.
It will take 6 years for the photon to close that initial 1.5 LY gap.
So 7 years earth time into the voyage ship A will see the photon from ship B as it was 6 years in the past. It will be a red shifted photon.
It's only when the space ships are far enough apart, where the expansion of the
space separating ships A and B is equal to or greater than .25 C, that they will no longer be able to see each other.
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William Rothamel

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Message 1718852 - Posted: 27 Aug 2015, 0:10:32 UTC - in response to Message 1718814.

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.
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Message 1718866 - Posted: 27 Aug 2015, 0:38:43 UTC - in response to Message 1718852.

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

You are wrong William.
1 year into flight as measured from earth, A and B are separated by 1.5 LYs.
A photon leaves ship B traveling towards ship A at speed of light C.
Ship A traveling at .75 C after 6 more years will be 7 X .75 C = 5.25 LYs from earth.
After the photon leaves ship B the photon, traveling at speed of light, will travel .75 LYs to earth and 5.25 LYs to catch up with ship A in 6 earth years.
It will seem like less time than 7 yrs has passed since they left earth for the people on ship A.

ps. I see that you changed your answer from The light from one ship would never reach the other. , to, they'll see the light as it was in the past, which is what I said all along.

It's really pretty simple William.
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Gary Charpentier
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Message 1718936 - Posted: 27 Aug 2015, 4:08:53 UTC

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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Message 1718938 - Posted: 27 Aug 2015, 4:16:58 UTC - in response to Message 1718936.

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.
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KLiK
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Message 1718992 - Posted: 27 Aug 2015, 6:58:28 UTC - in response to Message 1718475.

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! ;)

non-profit org. Play4Life in Zagreb, Croatia, EU
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Gary Charpentier
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Message 1719065 - Posted: 27 Aug 2015, 13:31:12 UTC - in response to Message 1718938.

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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Bob DeWoody

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Message 1719074 - Posted: 27 Aug 2015, 13:58:31 UTC

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.
Bob DeWoody

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Message 1719075 - Posted: 27 Aug 2015, 14:01:29 UTC - in response to Message 1719065.

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

If you can explain what happens then please do so.
Saying i'm wrong and not trying to explain how i'm wrong is rather vague.

Are you saying that no light from either ship can reach the other, like William did initially?
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Message 1719108 - Posted: 27 Aug 2015, 15:08:26 UTC - in response to Message 1719074.

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.

Actually as soon as ship A and ship B are moving relative to each other there will be a red shift if moving away from each other.
It's the basis of Laser speed gun used in traffic enforcement.
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William Rothamel

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Message 1719113 - Posted: 27 Aug 2015, 15:25:34 UTC - in response to Message 1719108.

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 ?
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Message 1719115 - Posted: 27 Aug 2015, 15:35:22 UTC - in response to Message 1719113.

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.

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.

Well William you speak with authority and a great deal of vagueness.
You still haven't even tried to answer the initial question.
You seem to intimate greater understanding, but I don't see any.
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William Rothamel

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Message 1719119 - Posted: 27 Aug 2015, 15:49:09 UTC - in response to Message 1719115.

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.
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Message 1719125 - Posted: 27 Aug 2015, 16:05:51 UTC - in response to Message 1718334.

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.

So you still stand by your statement in bold in the above quote?
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Message boards : Science (non-SETI) : Speed of light question