# Thread: Observation at Light Speed

1. I hope you all don't mind these newbie questions...I'm probably going to be posting some kind of question here day to day as I read this book, if that's ok. It's not that I haven't read anything, it's just that sometimes you need someone to explain something differently than the author, or at least specifically.

I'm getting a little more comfortable with the idea of time permeability with relation to the state of motion of the observer. But, so far anyway, this book hasn't talked about how the observer that is in motion perceives the stationary.

I understand now, that if I'm stationary and I observe someone in motion (close to the speed of light, let's say) that they will appear to be living in slow motion, their clock slows down.

But, how about if I'm the one traveling close to the speed of light? How will I perceive those that are stationary, or at least not traveling very quickly?

2.

3. Hey ParanoiA

You would not notice anything real special about an observer who was stationary to yourself (you dont observe anything special about anyone you walk past in the street and realtive to a far away galaxy you are travelling close to the speed of light). Remember stationary is relative to you, so someone stationary relative to yourself means they are moving with you.

4. Originally Posted by ParanoiA
I But, so far anyway, this book hasn't talked about how the observer that is in motion perceives the stationary.

I understand now, that if I'm stationary and I observe someone in motion (close to the speed of light, let's say) that they will appear to be living in slow motion, their clock slows down.

But, how about if I'm the one traveling close to the speed of light? How will I perceive those that are stationary, or at least not traveling very quickly?
You will see their clock run faster than yours

5. Originally Posted by river_rat
Hey ParanoiA

You would not notice anything real special about an observer who was stationary to yourself (you dont observe anything special about anyone you walk past in the street and realtive to a far away galaxy you are travelling close to the speed of light). Remember stationary is relative to you, so someone stationary relative to yourself means they are moving with you.
Forgive me, but it sounds like you're splitting hairs with the verbiage. I mean for academic understanding, if I can supposedly "observe" someone traveling at light speed, relative to me, then I can "observe" someone stationary relative to my speed of light travel.

Maybe I should set it up different. If "Dave" is standing on a platform and I zoom past him at close to light speed - he will perceive my clock moving slower. So, my question is, how will I see Dave's clock? Slower or faster?

6. Originally Posted by Megabrain
Originally Posted by ParanoiA
I But, so far anyway, this book hasn't talked about how the observer that is in motion perceives the stationary.

I understand now, that if I'm stationary and I observe someone in motion (close to the speed of light, let's say) that they will appear to be living in slow motion, their clock slows down.

But, how about if I'm the one traveling close to the speed of light? How will I perceive those that are stationary, or at least not traveling very quickly?
You will see their clock run faster than yours
Hmmm...I was afraid of that. Intuitively, that's the answer I expected. But it doesn't jive with the example of relative motion used in the book, and how both observers think the other's clock is moving slower ( or at least, I'm not seeing the difference).

George and Gracie are floating in space with big clocks strapped to them, neither knows which one is in motion. As they pass each other at 99.5 percent of light speed, George perceives Gracie's clock moving slower, while Gracie perceives George's clock moving slower.

This has really got me hung up at the moment.

7. THe international space station exhibits time dilation at a rate of about 3-4 microseconds per day. THink about it..... ie what each would see..

Their time dilation is due to their velocity....

makes my brain hurt...

8. Originally Posted by Megabrain
Originally Posted by ParanoiA
I But, so far anyway, this book hasn't talked about how the observer that is in motion perceives the stationary.

I understand now, that if I'm stationary and I observe someone in motion (close to the speed of light, let's say) that they will appear to be living in slow motion, their clock slows down.

But, how about if I'm the one traveling close to the speed of light? How will I perceive those that are stationary, or at least not traveling very quickly?
You will see their clock run faster than yours
Actually, I think I found the answer and I believe it is that you will see their clock run slower.

In this example, Slim is driving a car at 120 mph. Jim is stationary and is going to measure the length of Slim's car by using a stopwatch and recording the time Slim's front bumber reaches him and when the back bumper passes him. Then Jim can use the elapsed time and the speed of the car to determine the length.

Straight out of the book:

"...Using our newfound appreciation of the subtleties of time, we realize that from Slim's perspective he is stationary while Jim is moving, and hence Slim sees Jim's clock as running slow. As a result, Slim realizes that Jim's indirect measurement (stopwatch method) of the car's length will yeild a shorter result..."

So, it appears that it doesn't matter who is in motion, as it's all "relative".

In my example, I said I was traveling at close to light speed past "Dave" who is standing on a platform. So, using relativity, this can also be seen as "Dave" traveling at close to light speed past me.

Oddly enough, I think I'm starting to grasp this better now.

9. I think I must have got mine off the back of a breakfast ceral box...

10. Originally Posted by ParanoiA
Forgive me, but it sounds like you're splitting hairs with the verbiage. I mean for academic understanding, if I can supposedly "observe" someone traveling at light speed, relative to me, then I can "observe" someone stationary relative to my speed of light travel.

Maybe I should set it up different. If "Dave" is standing on a platform and I zoom past him at close to light speed - he will perceive my clock moving slower. So, my question is, how will I see Dave's clock? Slower or faster?
Not splitting hairs with needless talk, just clarrifying what you are trying to say, be it rather badly. Dave is not stationary w.r.t your reference frame, he is moving at close to the speed of light relative to you. The word stationary implies zero velocity relative to you, "dave" is definitly not stationary!

All moving clocks appear slower in your reference frame btw.

11. Originally Posted by river_rat
Not splitting hairs with needless talk, just clarrifying what you are trying to say, be it rather badly. Dave is not stationary w.r.t your reference frame, he is moving at close to the speed of light relative to you. The word stationary implies zero velocity relative to you, "dave" is definitly not stationary!

All moving clocks appear slower in your reference frame btw.
Cool. So, I guess the mathematics would include angle or trajectory in figuring the difference in time from two people in relative motion?

I ask, because direction really hasn't been discussed specifically just yet. I'm assuming that if Dave and I are going the same direction at the same speed, then our clocks are in sync. But if Dave steers just a degree or two to the right, then we're on different paths and we both perceive the other's clock to be moving slower - albeit very small. Is this a true assumption?

12. mmm, now you have introduced a third observer. Relative to the third observer, both your clocks would be running equally slow. To work out what you would observe is a bit more difficult to work out as you would have to transform velocities using the more general lorentz transforms and not the usual 1D transforms - will have to sit and work that one out.

13. Ok, well I thought myself into another conundrum....

Ok, if I leave earth in a spaceship traveling 80% of the speed of light and come back in 5 years my time, then I have aged 5 years, while the earth and everyone on it has aged several decades. ( I don't know the math, but I'd like to know the formula ).

So, here's my question: If motion is all relative, then why do I age less and everyone else ages more?

We both see our clocks running slower, however I'm the only one that time slows down for. If motion is relative, then shouldn't this be the same as the earth traveling 80% of the speed of light and coming back to me in 5 years their time?

14. Formula.. Easy,

td = ts sqr((1-v^2)/(c^2)

c = speed of light
td dilated time
ts stationary time
v = velocity
c = speed of light.

Remember this is for two travellers in 'free space' It's different if one is subject to gravity.

http://en.wikipedia.org/wiki/Gravita..._time_dilation

Save me writing it all out.....

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