# Simultaneity and frames

neh4pres
i recently posted a question (light and time) someone told me to look into loss of simultanety...
according to what i read the speed of light can be judged by someone or things frame of reference.(if I am traveling at .999 the speed of light the light in front of me and coming from behind me are still both passing me at 186000 mps) so unless i look at dilation of light (red or blue shift of stars) i am not moving.. i am in my frame of reference... if i release a pulse of light it takes off at the same speed in all directions... so within my frame can't i continue to accelerate? matematicaly passing the speed of light compared to someone elses frame of reference?? even though them looking at my "clock" as being slow makeing it seem as if I am still traveling under the speed of light to their frame??

Staff Emeritus
Gold Member
It's hard to understand you. It would help if you could at least end sentences with a period and start new ones with uppercase letters. You can't accelerate to the speed of light.

If you're driving your car at 0.8c (for example) and switch on the lights, you might think that the light should move faster than c relative to the ground, but it won't because in special relativity, speeds don't add up as u+v. The result is instead

$$\frac{u+v}{1+\frac{uv}{c^2}}$$

With u=0.8c and v=c, we get 1.8c/(1+0.8*1)=c.

neh4pres
i have read janus' post on rules.. i just want to state that i am not here to chalange any existing theories.. i am here to expand my understanding of the universe by use of argument and thought experiment.. i am a high school dropout.. all my knowledge has come from the science channel. therefor i know its lacking . my questions are from many hours of thought and interest. some of them i have had for many years. i thank you all for taking time to help me understand.. and if i ask something that can't be answered from a book then try to understand it with me.. maybe the question or theory is already there just worded differently..

Mentor
i recently posted a question (light and time) someone told me to look into loss of simultanety...
according to what i read the speed of light can be judged by someone or things frame of reference.(if I am traveling at .999 the speed of light the light in front of me and coming from behind me are still both passing me at 186000 mps) so unless i look at dilation of light (red or blue shift of stars) i am not moving.. i am in my frame of reference... if i release a pulse of light it takes off at the same speed in all directions... so within my frame can't i continue to accelerate? matematicaly passing the speed of light compared to someone elses frame of reference?? even though them looking at my "clock" as being slow makeing it seem as if I am still traveling under the speed of light to their frame??
Yes, in your frame you can continue to accelerate. You need to distinguish between two different kinds of acceleration "proper acceleration" which is the acceleration that you can feel or measure with an accelerometer and "coordinate acceleration" which is the time derivative of your velocity. When you say "within my frame" you are talking about proper acceleration. So you can continue to accelerate at a given proper acceleration for as long as you wish. However, your coordinate acceleration in a reference frame where you are moving will slow down because of time dilation (although you won't "feel" any less acceleration). Due to this you will never pass the speed of light in any frame.

neh4pres
Yes, in your frame you can continue to accelerate. You need to distinguish between two different kinds of acceleration "proper acceleration" which is the acceleration that you can feel or measure with an accelerometer and "coordinate acceleration" which is the time derivative of your velocity. When you say "within my frame" you are talking about proper acceleration. So you can continue to accelerate at a given proper acceleration for as long as you wish. However, your coordinate acceleration in a reference frame where you are moving will slow down because of time dilation (although you won't "feel" any less acceleration). Due to this you will never pass the speed of light in any frame.

ok so given this. If something is 10 lightyears away. Could i accelerate to the point where in my frame it only takes 5 years to get there?? This would slow my clock. Probably to the point where from someone else's frame they seen me travel for 30 years before i arrived. Is this how it works?

Mentor
If something is 10 lightyears away. Could i accelerate to the point where in my frame it only takes 5 years to get there??
Yes. See the http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html" [Broken] page.

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Mentz114