Thought Experiment about light speed

In summary, it will take about thirteen minutes for the light to come on, but it will remain on for about thirty seconds.
  • #1
Clay Gillespie
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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?
 
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  • #2
What do you think? How do you arrive at that thought?
 
  • #3
You’ll see the light come on in thirteen minutes. Light doesn’t have to travel from there to here to be seen.
 
  • #4
Clay Gillespie said:
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.
 
  • #5
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?
 
  • #6
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.
 
  • #7
Clay Gillespie said:
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.
 
  • #8
gmax137 said:
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.
 
  • #9
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
 
  • #10
Clay Gillespie said:
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.
 
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  • #11
Clay Gillespie said:
Easy, We’re seeing what is there not what light is reporting.
This is just wrong.
 
  • #12
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?
 
  • #13
Clay Gillespie said:
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.
 
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  • #14
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
 
  • #15
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.
 
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  • #16
Clay Gillespie said:
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.
 
  • #17
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.
Ibix said:
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.
 
  • #18
Clay Gillespie said:
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.
 
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  • #19
Clay Gillespie said:
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.
 
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  • #20
Nugatory said:
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.
 
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  • #21
Clay Gillespie said:
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?
 
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  • #22
This thread is closed
 
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  • #23
Clay Gillespie said:
That’s actually not reality.
Yes it is. We've done it. You are wrong.
 
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1. What is a thought experiment about light speed?

A thought experiment about light speed is a hypothetical scenario that explores the implications of objects moving at the speed of light. It is a mental exercise used to understand the fundamental principles of physics and to test theories about the nature of light and its behavior.

2. Can objects actually reach the speed of light?

According to Einstein's theory of relativity, objects with mass cannot reach the speed of light. As an object approaches the speed of light, its mass increases infinitely and it would require an infinite amount of energy to accelerate it to that speed. This is why the speed of light is considered to be the universal speed limit.

3. How does time dilation affect objects moving at the speed of light?

Time dilation is a phenomenon that occurs when an object is moving at high speeds. As an object approaches the speed of light, time slows down for that object relative to an observer. This means that time would appear to pass slower for an object moving at the speed of light compared to an object at rest.

4. What would happen to the mass of an object if it were to reach the speed of light?

As mentioned earlier, the mass of an object increases infinitely as it approaches the speed of light. This is due to the relationship between mass, energy, and the speed of light in Einstein's famous equation, E=mc². Therefore, it is impossible for an object with mass to reach the speed of light.

5. How does the concept of space-time come into play in a thought experiment about light speed?

Space-time is the four-dimensional framework in which all physical events occur. In a thought experiment about light speed, the concept of space-time is used to understand how objects moving at the speed of light would affect the fabric of space and time. It also helps explain phenomena such as time dilation and length contraction that occur at high speeds.

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