A radio beacon @ the speed of light.

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Discussion Overview

The discussion revolves around the implications of sending a spacecraft near the speed of light and the behavior of signals sent from such a craft. Participants explore concepts related to relativity, signal transmission, and the effects of high-speed travel on communication with Earth.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant proposes the idea of using gravitational slingshots to accelerate a spacecraft to near-light speeds, suggesting it could send back images from distant locations.
  • Another participant clarifies that the speed of light is constant for all observers, challenging the initial analogy of a truck and a ball, and stating that signals sent from the spacecraft will take a fixed time to reach Earth regardless of the spacecraft's speed.
  • Some participants express confusion about the implications of sending signals from a craft traveling at or near the speed of light, noting that signals may not reach the craft if it were traveling at light speed.
  • One participant emphasizes that nothing can travel at the speed of light and discusses the complexities of time and perception from different reference frames.
  • Another participant suggests that if a signal were sent to a spacecraft traveling just under the speed of light, it would take a significant amount of time for the signal to reach it from Earth's perspective.

Areas of Agreement / Disagreement

Participants express differing views on the implications of sending signals to a spacecraft traveling at high speeds, particularly regarding the speed of light and the nature of signal transmission. There is no consensus on the nuances of these concepts, and confusion remains evident among participants.

Contextual Notes

Some limitations include the dependence on the definitions of speed and reference frames, as well as unresolved questions about the nature of signals at relativistic speeds.

deckart
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I just read of an idea to send a craft outside our solar system using the various stellar bodies to sling shot it and also create a way for it to continually accelerate. In which case it would eventually near the speed of light. So in fifty years it could send us pictures and what not of faraway places. Which, of course, one would think it would take fifty to get back.

But then it occurred to me that if it is traveling close to the speed of light and it sends signals toward the direction from which it is traveling, near (or at) the speed of light, the signal would actually be traveling, relative to us, VERY slow. Is this accurate? Or am I missing something? If I'm traveling in the back of a pick-up truck at 55mph and I throw a ball behind the truck at 40mph, is not the ball traveling in the direction of the truck 15mph?

Thank you in advance for your response.
 
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Welcome aboard.

Yes, you are missing something: the speed of light is constant to all observers regardless of inertial frame of reference. Speeds do not add and subtract in the same way as your truck-ball example. That only works at very low speed.

So it doesn't matter what speed the probe is going: when it passes alpha centuari, for example (4.36 light years distant), the signal it sends will take exactly 4.36 years to get back to earth.
 
Wow, ok, that's a mind boggler.
 
yeah especially since from the frame of the craft the signal would have gotten to Earth sooner.
 
The other question is.

If you sent a signal to the spacecraft that is traveling at the speed of light away from the Earth will it ever get there?
 
4Newton said:
The other question is.

If you sent a signal to the spacecraft that is traveling at the speed of light away from the Earth will it ever get there?

Nothing can travel the speed of light. Period.

Even if it could, it's time would be mixed up, and everything will just crash. What they see will be both the end and the start of the universe, and everything else.

From the reference frame of the Earth, however, we will observe that the light beam never reaches the spacecraft .
I wish that helps.
 
4Newton said:
The other question is.

If you sent a signal to the spacecraft that is traveling at the speed of light away from the Earth will it ever get there?
To make it physically possible, I'll change it a little: if you sent a signal to a spacecraft traveling just under the speed of light, it would take a long time (from Earth's point of view) for the signal to get there.
 
deckart said:
Wow, ok, that's a mind boggler.

Yup, it is boggling for everyone who first learns about it. And that's why Einstein is nearly worshiped (although he is sometimes overrated, others were quite close to solving the riddle at the time.). It's what relativity is all about.
 
kuenmao said:
Nothing can travel the speed of light. Period.

Is that so? :rolleyes:
Think about what you just wrote.

-Ray.
 

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