Thought Experiment about light speed

[Clay Gillespie]

If we’re looking through a telescope at a craft we launched from Earth that is now passing Mars and send a radio signal to our craft telling it to turn on one of its lights on and it takes thirteen minutes for the radio waves to get from Earth to our craft how long will it take before we see the light come on?

 

[gmax137]

What do you think? How do you arrive at that thought?

 

[Clay Gillespie]

You’ll see the light come on in thirteen minutes. Light doesn’t have to travel from there to here to be seen.

 

[Dale]

Light doesn’t have to travel from there to here to be seen.

Yes it does. Light on Mars will not stimulate the photoreceptors in your eyes on Earth.

 

[gmax137]

Why do you think it takes 13 minutes for the craft to see the radio waves but no time for us to see the returning light?

 

[Clay Gillespie]

Let’s say I’ve make my radio waves visible and am watching them travel toward my craft and then be absorbed into its radio dish. Viola, light comes on, visible to me looking through my telescope. Or else your saying my little old craft light sends light in the form of photons in every direction, and I have to wait for the speed of that emmination to report photons to my telescope. Now let’s say in that radio transmission I instructed the light to stay on for 30 seconds. Now the source of that emanation is out. You will see that light come on thirteen minutes after broadcast the and it will remain on for 30 seconds.

 

[phyzguy]

Let’s say I’ve make my radio waves visible and am watching them travel toward my craft and then be absorbed into its radio dish. Viola, light comes on, visible to me looking through my telescope. Or else your saying my little old craft light sends light in the form of photons in every direction, and I have to wait for the speed of that emmination to report photons to my telescope. Now let’s say in that radio transmission I instructed the light to stay on for 30 seconds. Now the source of that emanation is out. You will see that light come on thirteen minutes after broadcast the and it will remain on for 30 seconds.

This is all wrong. First of all, you can't "watch" the radio waves travel. Once the radio signal you sent to the craft has left Earth, it is propagating through space and is not visible to you. 13 minutes after you sent the signal, it will arrive at your craft, the light will come on, and the light will begin propagating back towards earth. 13 minutes after this (26 minutes after you sent you radio signal), the light will arrive at your telescope and you will see the light come on.

 

[Clay Gillespie]

Why do you think it takes 13 minutes for the craft to see the radio waves but no time for us to see the returning light?

Easy, We’re seeing what is there not what light is reporting. The relatively small light from the craft will illuminate a small circle area of space, but that light will never reach earth, but it is there, and we can see it’s there.

Your standing in a dark field, you can’t see your hands in front of your face and then someone lights a lighter a quarter mile away that you see clearly. Your still in the dark and your hands are still unlit by light but you can block the source of light from your eyes by placing them in front of your eyes, but there is no light on your hands.

 

[gmax137]

If you could "watch" your radio signal, you would "see" it enter the craft antenna 26 minutes after you sent the signal. Assuming the craft is 13 light-minutes (234 million Km) away from you.

... and I have to wait for the speed of that emmination to report photons to my telescope.

That's right, you have to wait.

Now let’s say in that radio transmission I instructed the light to stay on for 30 seconds. Now the source of that emanation is out. You will see that light come on thirteen minutes after broadcast

No, you will see it come on 26 minutes after your broadcast.

the and it will remain on for 30 seconds.

Right

 

[phyzguy]

Easy, We’re seeing what is there not what light is reporting.

You're very confused. What do you think "seeing" is? It is light entering your eyes, which your brain then interprets.

 

[gmax137]

Easy, We’re seeing what is there not what light is reporting.

This is just wrong.

 

[Clay Gillespie]

Alright I try another tact, I watched my craft launch by telescope and never took my eyes off it and have been charting how long it takes radio wave broadcasts to turn lights on the craft on. I know the radio goes from here to there and can gauge how far away the craft is by measuring that. Mind you I’ve never taken my eyes off it. When do you think it slipped into the past?

 

[Dale]

Let’s say I’ve make my radio waves visible and am watching them travel toward my craft

That is just it. You cannot see light traveling toward the craft. Let's say that we are using visible lasers instead of radios, so that there is no issue about the visibility of the wavelength. Laser light traveling away from you is not visible.

Do you understand how your eyes work? Eyes work when light enters your eye goes to the retina, and stimulates photoreceptors on your retina. Light that is anywhere outside of your eye is not visible.

 

[Clay Gillespie]

You hung up there, It’s true now, “The speed of light is relative to the force of ejection from the center of origin” Let’s say I have a little compartment on that craft that receives an instruction to open and release a lightning bug in a glass jar. That is there around Mars. Let’s say its been blinking at about one blink every two seconds. It takes our radio signal 13 minutes to get there and 5 seconds to release the jar and pull off its covering. How long after 13 min 5 sec are we likely to see a lightning bug flash with the observer position of looking through our Earth telescope. A. 13 min 5-7 sec

 

[Ibix]

If the craft is 13 light minutes away, the time between pulse emission and echo reception will be 26 minutes. We've done this experiment. It's usually called "radar" and is extremely well understood.

 

[Nugatory]

When do you think it slipped into the past?

Never. There are three events happening in the obvious chronological order:
1) First, radio signal leaves earth;
2) 13 minutes after #1, radio signal reaches ship and turns light on so light signal leaves ship;
3) 13 minutes after #2 light from ship reaches your telescope so you see it.

Both the visible light and the radio waves are electromagnetic radiation traveling at speed ##c, so it takes 13 minutes for the radio waves to travel from Earth to ship, and 13 minutes after the light turns on for the light to travel back to earth. The same thing would happen if the radio receiver and the light were in the same rom as you, just a meter or so away - the only difference is that light travels so quickly that we don’t notice the few nanoseconds of time difference between when something happens across the room and when we see it.

The situation you’re describing is equivalent to shining a laser at a mirror on the ship and timing how long it takes for the reflected light to get back to you.

 

[Clay Gillespie]

Yes, the radio wave has to go from here to there (target) and the report time tells distance, one cycle, repeat get velocity. But what your looking at is there, visible, and being bounced off.

If the craft is 13 light minutes away, the time between pulse emission and echo reception will be 26 minutes. We've done this experiment. It's usually called "radar" and is extremely well understood.

 

[phyzguy]

Alright I try another tact, I watched my craft launch by telescope and never took my eyes off it and have been charting how long it takes radio wave broadcasts to turn lights on the craft on. I know the radio goes from here to there and can gauge how far away the craft is by measuring that. Mind you I’ve never taken my eyes off it. When do you think it slipped into the past?

Your image of it was always in the past. When it was sitting on the launch pad and you were, let's say, 1km away, what you were seeing was the rocket as it was 3 microseconds ago. As it recedes from you, what you see is an image of the rocket that is increasingly delayed from your present moment.

 

[Ibix]

Yes, the radio wave has to go from here to there (target) and the report time tells distance, one cycle, repeat get velocity. But what your looking at is there, visible, and being bounced off.

Twenty six minutes after the pulse emission and thirteen minutes after the reflection you will be able to see any reaction the craft has to receiving the pulse. Not before. Again, we have done this. We've done it with visible light (since more than a century ago) and radio waves, and it does not work the way you appear to be describing.

 

[Clay Gillespie]

Never. There are three events happening in the obvious chronological order:
1) First, radio signal leaves earth;
2) 13 minutes after #1, radio signal reaches ship and turns light on so light signal leaves ship;
3) 13 minutes after #2 light from ship reaches your telescope so you see it.

Both the visible light and the radio waves are electromagnetic radiation traveling at speed ##c, so it takes 13 minutes for the radio waves to travel from Earth to ship, and 13 minutes after the light turns on for the light to travel back to earth. The same thing would happen if the radio receiver and the light were in the same rom as you, just a meter or so away - the only difference is that light travels so quickly that we don’t notice the few nanoseconds of time difference between when something happens across the room and when we see it.

The situation you’re describing is equivalent to shining a laser at a mirror on the ship and timing how long it takes for the reflected light to get back to you.

That’s actually not reality. I can right now send that signal to a craft around Mars, knowing how long my transmission takes to get from here to there, and see it come on instantly. If the transmission takes thirteen minutes and the instruction is turn on X light, it is instantly visible after thirteen minutes.

 

[phyzguy]

That’s actually not reality. I can right now send that signal to a craft around Mars, knowing how long my transmission takes to get from here to there, and see it come on instantly. If the transmission takes thirteen minutes and the instruction is turn on X light, it is instantly visible after thirteen minutes.

Why do you keep making wrong statements? What you are saying is simply wrong, and we have centuries of experience to prove it. Are you just trolling?

 

[Nugatory]

This thread is closed

 

[Ibix]

That’s actually not reality.

Yes it is. We've done it. You are wrong.