# Subjecteve speed always Newtonian?

• August 5th, 2011, 01:38 PM
neilxt
Subjecteve speed always Newtonian?
I'm not entirely sure but it seems to me that from the POV of the traveler at close to light speed all the C2's cancel out and the journey appears to operate using Newtonian math? So if you are near light speed and accelerate you travel hardly any faster, but the universe shrinks by the same amount as if you had gone that much faster. You cannot get to e.g. alpha centauri faster than light, but you can get there in a subjective time frame that would subjectively seem like you did.
• August 6th, 2011, 04:35 PM
mathman
If you traveled to Alpha Centaurus at close to the speed of light, special relativity says you would get there (in your frame) in a very short time. If you then turned around and came back at the same speed you will have aged slightly, while people on earth would have aged 8.6+ years. This is the twin paradox explained.
• August 7th, 2011, 10:53 AM
neilxt
Quote:

Originally Posted by mathman
If you traveled to Alpha Centaurus at close to the speed of light, special relativity says you would get there (in your frame) in a very short time. If you then turned around and came back at the same speed you will have aged slightly, while people on earth would have aged 8.6+ years. This is the twin paradox explained.

That almost, but not quite, answers the question. You'd get there faster, but could your subjective time to get there be calculated using Newtonian physics based on how you accelerated? If you had an engine that could accelerate you at .1C/day all the way could you use that to calculate your subjective travel time without resorting to Einsteinian physics? And would you have to start the deceleration at the halfway point, or 10 (subjective) days out?
• August 7th, 2011, 11:59 AM
Janus
Quote:

Originally Posted by neilxt
Quote:

Originally Posted by mathman
If you traveled to Alpha Centaurus at close to the speed of light, special relativity says you would get there (in your frame) in a very short time. If you then turned around and came back at the same speed you will have aged slightly, while people on earth would have aged 8.6+ years. This is the twin paradox explained.

That almost, but not quite, answers the question. You'd get there faster, but could your subjective time to get there be calculated using Newtonian physics based on how you accelerated? If you had an engine that could accelerate you at .1C/day all the way could you use that to calculate your subjective travel time without resorting to Einsteinian physics? And would you have to start the deceleration at the halfway point, or 10 (subjective) days out?

No. Relativity will give you a greater distance traveled. The longer you accelerate or the greater the acceleration , the greater the difference. Even if you were only accelerating at .1g (.98/m/sec^2) for 10 days subjective time, you will find that by Relativity, you will have traveled ~837 km further.