NASASpaceFlight.com Forum
General Discussion => Q&A Section => Topic started by: cube on 02/02/2022 02:38 pm
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Hello I was wondering this: if a spacecraft located 1 light year from the earth and moving away from it at 99% of the speed of light sends a laser signal to the earth this signal would take how long to to go down to earth? (Does the speed of the object of its emission influence its speed relative to the earth?)
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No. Light always travels at lightspeed. So a signal emitted at 1LY distance (assuming it's short enough to be considered to have been 'sent' all at once) will take exactly one year to reach Earth, regardless of spacecraft direction of travel.
Other effects do happen, but it's not something you're going to get a good grasp on by simple Q&A. You really need a good primer that walks you through the original formulation of Special Relativity starting from Maxwell's Equations.
When I was doing my undergrad work, we used Ohanian
https://www.amazon.com/Special-Relativity-Introduction-Hans-Ohanian/dp/0971313415
which I thought was pretty good at the time (still is, really). Obviously, you're free to use any other existing resource, but a good, cheap paperback book will beat 99% of websites/youtube videos out there. Good luck!
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And for the same situation except that the signal is sent from Earth to the ship traveling at 0.99 c? would the signal's travel time still be 1 year or more (its relative speed to the spacecraft would be 0.01 c ) but I doubt it's that simple.
*travel time relative to Earth (I know that time on a ship traveling at these speeds does not flow at the same speed as time for example on Earth)
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No, the speed of light is always lightspeed no matter what reference frame you're in. The message will travel at lightspeed and take a year to get back to earth from earth's perspective, but it will also be traveling at lightspeed from the spaceship's perspective.
I do recommend reading up on special relativity, reference frames can be counterintuitive and I don't trust myself to explain it properly.
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The only difference is doppler effect, i.e. wavelength shift.
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No, the speed of light is always lightspeed no matter what reference frame you're in. The message will travel at lightspeed and take a year to get back to earth from earth's perspective, but it will also be traveling at lightspeed from the spaceship's perspective.
I do recommend reading up on special relativity, reference frames can be counterintuitive and I don't trust myself to explain it properly.
Cube, (just quoting jdon to use his post as a starting point)
Here’s a bit of how I think of it:
It is the absolute truth of the statement that the speed of light as measured by any observer is always the same, regardless of relative motion between source and observer, that defines relativity. The whole of special relativity flows from this fact, and the rest of a correct understanding of space and time wraps around it. All of the weird things like time dilation, etc, flow from figuring out what this fact implies about the rest of the universe. It’s *super* confusing - that’s part of why relativity was such a big deal and controversial among *some* physicists for a long time.
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Thank you all, I just have to go learn more on the subject!
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And for the same situation except that the signal is sent from Earth to the ship traveling at 0.99 c? would the signal's travel time still be 1 year or more (its relative speed to the spacecraft would be 0.01 c ) but I doubt it's that simple.
No. The signal from Earth to the ship will take 100 years.
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And for the same situation except that the signal is sent from Earth to the ship traveling at 0.99 c? would the signal's travel time still be 1 year or more (its relative speed to the spacecraft would be 0.01 c ) but I doubt it's that simple.
No. The signal from Earth to the ship will take 100 years.
Careful ... it depends on your frame of reference. As seen from Earth, your right, it'll take 100 years to catch up. As seen from the ship, it will still take only one year.
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And for the same situation except that the signal is sent from Earth to the ship traveling at 0.99 c? would the signal's travel time still be 1 year or more (its relative speed to the spacecraft would be 0.01 c ) but I doubt it's that simple.
No. The signal from Earth to the ship will take 100 years.
Careful ... it depends on your frame of reference. As seen from Earth, your right, it'll take 100 years to catch up. As seen from the ship, it will still take only one year.
The bigger issue is would the crew get paid by ship time or Earth time?
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And for the same situation except that the signal is sent from Earth to the ship traveling at 0.99 c? would the signal's travel time still be 1 year or more (its relative speed to the spacecraft would be 0.01 c ) but I doubt it's that simple.
No. The signal from Earth to the ship will take 100 years.
Careful ... it depends on your frame of reference. As seen from Earth, your right, it'll take 100 years to catch up. As seen from the ship, it will still take only one year.
Careful. In the ships frame of reference, it will not take one year. .99c gives a gamma of about 7 so it should be about 14 years shiptime. In the ships frame of reference, the initial distance is not one light year.
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My head hurts
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My head hurts
It actually sort of gives you a way around travelling faster than light, in a way but not really.
If you accelerated at a constant 1G. you could reach the far side of the galaxy, 70,000 light years away, in about 26 years. But a little over 70,000 years would have passed back home.
Fun with relativity.
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My head hurts
It actually sort of gives you a way around travelling faster than light, in a way but not really.
If you accelerated at a constant 1G. you could reach the far side of the galaxy, 70,000 light years away, in about 26 years. But a little over 70,000 years would have passed back home.
Fun with relativity.
If you like Science Fiction, the classic story on this theme is Tau Zero, by Poul Anderson
https://en.wikipedia.org/wiki/Tau_Zero