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How observation of a distant galaxy changes when travelling towards it

by doudou1910
Tags: distant, galaxy, observation, travelling
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ghwellsjr
#19
Nov23-12, 04:06 AM
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I did not have time to respond to your train analogy but I also don't understand what you are trying to say:
Quote Quote by stu dent View Post
well, i would consider that semantics, that's like saying that if i'm looking out the window of a train, and i say, look the train is slowing down. and you say, well you can't see the train slowing down, that's the trees slowing down. i mean, it's nearly the same thing, except in the dilatation case, i would look outside and see the world speed up and exclaim how i am seeing the train slow down.
I can't tell when you say "speed up" and "slow down" whether you mean the velocity of the train/world or time for the train/world. If you could use terminology that makes this clear, and you still care, then I will respond accordingly.
ghwellsjr
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Nov23-12, 04:06 AM
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Quote Quote by stu dent View Post
as for your speaker analogy, i'm not sure what you mean by what is happening at the speaker. you mean the wind blowing against the speaker itself?
Yes, I was imagining that you were outdoors at a concert and very far away from the speakers and there is a steady wind blowing in the direction from the speakers toward you so that the sound gets from the speakers to you in less time than if there was no wind.
Quote Quote by stu dent View Post
if so, i purposefully ignored that, the same way i would ignore air resistance when calculating the trajectory of a projectile. it was not pertinent to the discussion. it was necessary to isolate that variable. you were talking about how slowly or how quickly the sound reaches my ear affecting the pitch. not a specific scenario in the real world with actual wind blowing. ok, so, let's say then, a speaker sends sound through an ultra high powered dyson typed fan, that is pumping out a perfectly steady stream of air towards me, now would that not affect the pitch of the sound? from what i read of your post, you believe that it would. therefore, is that not the equivalent of the speed at which sound reaches you, that affects the pitch?
In the situation that I just described, you will not hear any difference in the music even though the sound gets to your ears in less time than if there was no wind.
Quote Quote by stu dent View Post
imagine a wave, a sine wave, made out of a solid piece of plastic, the period is pitch, the amplitude is volume, as obviously you know, now, imagine a laser beam, imagine that this laser beam is attached to an audio recording machine. this audio recording machine, records at a given beat rate, a sound wave. it does this, when something interrupts it. the location along the laser beam where it gets interrupted records that amplitude of volume.

now, you can take this plastic sine wave and pass it through the laser beam at some speed, this will record a tone, if you pass it through at another speed, it will record another tone. the faster you pass it through, the higher the pitch.

now, correct me if i'm wrong, but that's a perfect analogy as to humans perceive sound, and is representative of the fact, that yes, indeed, the speed at which sound passes by you, affects what pitch it is.
That's a good analogy and the speed of the air (the wind) that is bringing you the sound does affect the pitch. However, as I pointed out before, you have to also consider what happens at the speakers. If a wind is blowing there, it will change the wavelength of the sound compared to still air. And the two effects cancel each other out so that you will hear the same thing whether there is a wind or not. (We're assuming a steady wind because if it is changing, then you will hear differences in the pitch in some complicated way that we don't want to consider.)
Quote Quote by stu dent View Post
now, i re-read your post, and maybe you're saying what matters is the difference of speed of the sound passing by you, versus the speed of the sound produced by the source, them being equal produces the correct pitch, and them being different producing each a different pitch, if you know what i mean.
Yes, that is what I was trying to say.
Quote Quote by stu dent View Post
now, a light source moving very quickly away from me, will appear a different color than if it were moving toward me very quickly, given the same source of light.

right?
Right.
Quote Quote by stu dent View Post
i do not see, why, it would be any different, from a physics standpoint, whether the source is moving towards me quickly, or away from me quickly, versus, it being still, and me moving quickly towards it, or me moving quickly away from it. for sound or light.
There won't be any difference in the case of light but there will be for sound.
Quote Quote by stu dent View Post
these two scenarios, aside from the absolute speed of i or the light in relation to the constant c, should be exactly the same, and i do not see how either of our speeds relative to the constant c, would have any bearing on the doppler effect.
Here is the formula from the wikipedia article for the Doppler Effect:
the relationship between observed frequency f and emitted frequency f0 is given by:


where

c is the velocity of waves in the medium;
vr is the velocity of the receiver relative to the medium; positive if the receiver is moving towards the source;
vs is the velocity of the source relative to the medium; positive if the source is moving away from the receiver.

The frequency is decreased if either is moving away from the other.
Now let's use a speed of 80% the speed of sound and plug it into the formula for your two comparison scenarios. In your first scenario, you are comparing the source moving towards you quickly versus you moving quickly towards it. In the first case, the Doppler factor evaluates like this:

c/(c-0.8c) = c/0.2c = 5

In the second case we get:

(c+0.8c)/c = 1.8c/c = 1.8

There is a big difference.

In your second scenario, you are comparing the source moving away from you quickly versus you moving quickly away from it. In the first case, the Doppler factor evaluates like this:

c/(c+0.8c) = c/1.8c = 0.556

In the second case we get:

(c-0.8c)/c = 0.2c/c = 0.2

Again, there is a big difference.

Now let's do the same evaluation for light. We will use the formula that Ben gave you in your referenced post at the end of your post #17. Note that the formula does not have two different terms for velocity like the one for normal Doppler had. There is just one speed to consider and that is the relative speed between the source and the receiver. In this case, we'll use 80%c.

So for your first scenario, where you and the source are rapidly approaching, we have to use the reciprocal form of the formula and we get:

√((1+0.8)/(1-0.8)) = √((1.8)/(0.2)) = √9 = 3

Note that this number is "half way" between the two different numbers we got for sound. In fact, it is the geometric mean of those two numbers:

√(5*1.8) = √(9) = 3

And for your second scenario, where you and the source are rapidly diverging, the formula gives us:

√((1-0.8)/(1+0.8)) = √((0.2)/(1.8)) = √0.111 = 0.333

And this is the geometric mean of the two numbers we got for sound:

√(0.556*0.2) = √(0.111) = 0.333

So the bottom line is that Doppler for light is different than Doppler for sound. For light it is called Relativistic Doppler and for sound it is just the normal Doppler Effect. You can look these up on wikipedia or anywhere else if you want more explanation.
Quote Quote by stu dent View Post
i'm not sure what you mean that we cannot measure how long it takes for light to reach us. if a light source is a light year away, then can't we just say that the light takes a year to reach us?
Only if we have previously defined light to take that long to traverse that distance which is what we do in Special Relativity but it's important to realize that it's a definition and not a measurement. I challenge you to come up with a way to measure how long it takes for light to propagate any given distance without previously defining how long it takes. This is what Einstein's second postulate does and is the basis for the concept of a Frame of Reference. When you specify a frame, then you are defining how long it takes for light to propagate any distance according to the coordinates of that frame. Notice how I did this in post #5:
Quote Quote by ghwellsjr View Post
Or another way to put it is in the rest frame of the distant object before you start out, you are seeing it in the past by the number of years equal to its distance in light years. If you could travel to it instantly, you would see it age by that number of years but since it will take at least that same number of years for you to get to that object (in your original rest frame), you will see it age by another of that same number of years for a total of twice the number of light years away.
And notice how the OP's question was worded that avoided the need to specify a frame:
Quote Quote by doudou1910 View Post
If the image we see from a distant galaxy shows how it looked 1 billion year ago, what would that galaxy appear if we are travelling towards it at the speed of light?
Do you see the difference? He did not mention any distance and so no frame was required to unambiguously interpret the question or to answer it. His question was about what he would see and is independent of any frame or defining the time it takes for light to propagate a given distance. His question, whether he realized it or not, was purely about Relativistic Doppler which has nothing to do with time dilation which requires the coordinates of a specified frame. That's what I'm trying to get you to understand.
stu dent
#21
Nov23-12, 06:37 PM
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Quote Quote by ghwellsjr View Post
I did not have time to respond to your train analogy but I also don't understand what you are trying to say:

I can't tell when you say "speed up" and "slow down" whether you mean the velocity of the train/world or time for the train/world. If you could use terminology that makes this clear, and you still care, then I will respond accordingly.
you are in a train. the train slows down, you look outside, notice the outside world slowing down in the direction opposite of the train's movement, and you exclaim, "look! the train is slowing down!" and then the guy next to you says: "ummm you can't see the train slowing down by looking out the window."

this is what you did. you were the guy sitting next to me saying that i couldn't see time dilatations. or maybe not, idk, it seemed to be that, when you explained to me why i couldn't.

but although you cannot see the train you're in, slowing down by looking out the window you can see the effect that the train slowing down has on the world that you perceive outside the window, i.e. the terrain passes you by less quickly.

in the galaxy example, you would notice, i was postualting, the events of the galaxy speed up, as your time slowed down, it would be an effect of the time dilatation. i would refer to that as seeing the time dilatation occur, just as i would refer to watching the terrain pass by my train window at a slower rate, seeing the train i'm in slowing down.
ghwellsjr
#22
Nov23-12, 07:56 PM
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The problem is that in the case of the train, you can actually see something that correlates to the claim that is being made, namely, you can see the trees coming toward you at a slower rate.

But in the case of time dilation, there is nothing that you can see that correlates to time dilation. When you are traveling at a high speed towards a distant clock, you see the clock ticking faster than yours, not slower, and time dilation means that a clock is ticking slower than the coordinate time, not faster. In Special Relativity, with inertial reference frames, moving clocks always tick slower than the coordinate time, never faster.

As an example, if you are stationary in a frame of reference and a clock is traveling at 60% of the speed of light, it will be ticking at 80% of the coordinate time (the same as your own clock). Then if that clock is traveling toward you, you will see it ticking at double the rate of your clock and if it is traveling away from you, you will see it ticking a one-half the rate of your clock.

What are you saying that you can see that looks like time dilation?
stu dent
#23
Nov26-12, 02:48 PM
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Quote Quote by ghwellsjr View Post
The problem is that in the case of the train, you can actually see something that correlates to the claim that is being made, namely, you can see the trees coming toward you at a slower rate.

But in the case of time dilation, there is nothing that you can see that correlates to time dilation. When you are traveling at a high speed towards a distant clock, you see the clock ticking faster than yours, not slower, and time dilation means that a clock is ticking slower than the coordinate time, not faster. In Special Relativity, with inertial reference frames, moving clocks always tick slower than the coordinate time, never faster.

As an example, if you are stationary in a frame of reference and a clock is traveling at 60% of the speed of light, it will be ticking at 80% of the coordinate time (the same as your own clock). Then if that clock is traveling toward you, you will see it ticking at double the rate of your clock and if it is traveling away from you, you will see it ticking a one-half the rate of your clock.

What are you saying that you can see that looks like time dilation?
i feel like you're just confusing things. i mean, when i look out the window and see trees flying past me, i can deduce that i am moving forward, and i can look at the window and say, look, i am moving forward, even though the trees are moving in the opposite direction. this is because by my moving forward, the trees move in the opposite direction as compared to me.

in the case of time dilatation, if i see the clock speeding up, then i can say, look, i see time dilation, even though, time dilatation is my frame of reference slowing in comparison to other frames of reference. watching the clock speed up, to me, is seeing time dilatation, it is seeing the direct effect of it, just like looking out the window seeing the trees go by in the opposite direction, is looking out the window and seeing that my train is moving forwards.

yes, of course, it must be slower. there is motion and there is still. there is no opposite of motion. entropy must increase. you cannot put energy into yourself to move slower than another object. if you reference another 3rd object, or coordinate system, then yes. but 2 objects relative to each other, may be still or may move at ever increasing speeds in comparison to each other. for which time dilatation is a result.

but, if i look out a window and i have a telescope typed thing device, that compensates for all my motion save the dilatation effect, and i view a distant planet, and i accelerate to relativistic speed, i will see the time of the planet increase, as a direct result of my time in my coordinate system slowing. so, to me, that is seeing time dilatation.


i think the op in his question, was not thinking time dilation, and was not thinking doppler effect.

he was thinking more simplistically.

i get the feeling though that in your response, you were thinking the same sort of way actually, and didn't take time dilation into effect. it seems to me at any rate.

if the closer you get to the speed of light, the faster everything else develops around you.

how far into the future you get to when you reach the other planet, must necessarily depend on exactly how close to the speed of light you got.

i'm not certain exactly of the formula you would use to calculate the time dilatation in comparison to your speed relative to the speed of light, and although, as v→c i would expect time to tend to zero, i.e. you do not age, i would expect that this would have a non converging effect on the speed at which other things occur around you.

iow, i would expect that as time dilates for you, it increases for others, and while the formula would dictate for you a converging number, 0, as your speed approaches c, i would expect that opposite would occur for time of other frames of reference, meaning they would be divergent, which would therefore indicate that the actual precise speed at which you travel would be important in defining what it is you would see happen to the galaxy as you approach it, and to stipulate moving at the speed of light, which is impossible, you would need to conclude that the universe would be over, since all things around you aged at an infinite rate, given you aged at a rate of 0.

otherwise, there is a maximum rate at which time can pass for a reference frame, and that would be the speed at which it may pass as you go at the speed of c. and this would imply that there is a slowest possible speed as compared to you, whereas this would not sit nicely with relativity, and lack of any reference other than c.

now, some of what i said might be wrong. it is speculation. i don't know the math.

but the original question i think is missing key information. i think it matters exactly at what speed you go. and that can't be c. i would expect that the formula for time dilation might yield even a exponential increase in time dilation as you approach the speed of light, meaning every tiny fraction of the speed light that you increase upon, makes the world around you seem to accelerate much faster, as your time frame slows in comparison to theirs.

so, i think this question must be answered using the formula for time dilation.

after a quick check on wikipedia, this seems to at least not be in disagreement with what i was saying.

http://en.wikipedia.org/wiki/Time_di...ative_velocity
phinds
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Nov26-12, 02:59 PM
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Quote Quote by stu dent View Post
i feel like you're just confusing things. i mean, when i look out the window and see trees flying past me, i can deduce that i am moving forward ...
If you are moving at a uniform rate, then you most certainly can NOT make any such claim. That would be giving yourself a preferential frame of reference and there is no such thing in special relativity.
Nugatory
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Nov26-12, 03:14 PM
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Quote Quote by stu dent View Post
when i look out the window and see trees flying past me, i can deduce that i am moving forward, and i can look at the window and say, look, i am moving forward, even though the trees are moving in the opposite direction. this is because by my moving forward, the trees move in the opposite direction as compared to me.
I say that you and the train are at rest and the trees are moving. Why is this not an equally reasonable deduction?
nitsuj
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Nov26-12, 03:28 PM
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Quote Quote by Nugatory View Post
I say that you and the train are at rest and the trees are moving. Why is this not an equally reasonable deduction?
That is a blatantly symmetrical scenario, Stu Dent describes how he/she "sees time dilation" as the net of Doppler & dilated time (if I understood the post correctly, still symmetrical but you didn't jump on that one so... ).

In both situations you have to assume along with Stu Dent the moving observer knows they are the one moving. (though ignore the impossible deduction mentioned)

I don't think he/she is disputing motion is relative, although the opening paragraph reads that way, the context follows.
stu dent
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Nov26-12, 04:48 PM
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Quote Quote by phinds View Post
If you are moving at a uniform rate, then you most certainly can NOT make any such claim. That would be giving yourself a preferential frame of reference and there is no such thing in special relativity.
Quote Quote by Nugatory View Post
I say that you and the train are at rest and the trees are moving. Why is this not an equally reasonable deduction?
yes, right, i am not saying differently.

you could then say, that time dilation, is that all the rest of the universe's time increases by the same ratio.

my point is that i would say that seeing the universe outside of you speed up, would be indeed seeing time dilation. your time has slowed, having the effect of the outside world increasing in speed which is, imo, seeing time dilation in effect, even though you aren't seeing time slow down.

just like, if i see trees pass me by, i could say i am seeing the train i am in zipping along.

yes, it is correct to say, that the trees are moving past me, and i am still. i am not disputing that.

i used it as an example only to explain something specific.
PAllen
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Nov26-12, 04:55 PM
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Quote Quote by stu dent View Post

my point is that i would say that seeing the universe outside of you speed up, would be indeed seeing time dilation. your time has slowed, having the effect of the outside world increasing in speed which is, imo, seeing time dilation in effect, even though you aren't seeing time slow down.
1) Time dilation is never observed. Doppler is what is observed. Time dilation is computed from Doppler + assumptions/definitions. Even when you pass right by a clock, what you directly observe is transverse Doppler.

2) Computing time dilation from observations, train would compute earth clocks are slow. Earth would compute that train clocks are slow.
PeterDonis
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Nov26-12, 05:11 PM
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Quote Quote by stu dent View Post
when i look out the window and see trees flying past me, i can deduce that i am moving forward
Relative to the trees, yes.

Quote Quote by stu dent View Post
, and i can look at the window and say, look, i am moving forward, even though the trees are moving in the opposite direction. this is because by my moving forward, the trees move in the opposite direction as compared to me.
Yes, you are moving forward relative to the trees, so the trees are moving backward relative to you.

Quote Quote by stu dent View Post
in the case of time dilatation, if i see the clock speeding up, then i can say, look, i see time dilation, even though, time dilatation is my frame of reference slowing in comparison to other frames of reference. watching the clock speed up, to me, is seeing time dilatation, it is seeing the direct effect of it, just like looking out the window seeing the trees go by in the opposite direction, is looking out the window and seeing that my train is moving forwards.
If you insist on using the term "time dilation" this way, nobody can stop you; however, you should understand that this is *not* the way the term "time dilation" is standardly used in relativity, and it's not the way ghwellsjr has been using the term. That seems to me to be a major source of confusion between the two of you.

The standard usage of the term "time dilation" in relativity refers to whatever effect is left over *after* you have corrected for the relativistic Doppler effect and the effect of light travel time delay. With that usage of the term, ghwellsjr is correct that you can't directly "see" time dilation; you calculate what it is by taking what you directly "see" and correcting for those other effects.

Quote Quote by stu dent View Post
if the closer you get to the speed of light, the faster everything else develops around you.
As I said, nobody can stop you from using terms in your own way; however, you do need to make sure that you reason correctly from whatever definitions you use for terms, and you have to use the terms consistently.

If you are moving close to the speed of light relative to all the rest of the stuff in the universe (which is what you seem to mean here by "the closer you get to the speed of light"), then things in front of you will appear to "develop faster", but things behind you will appear to "develop slower". So by the definition of "time dilation" you were using in what I quoted above, you are only seeing time dilation relative to the things in front of you.

To figure out that, once you reach your destination and stop, everything else in the universe will have aged a lot more than you did--including the stuff behind you and the stuff in front of you--you have to correct for the relativistic doppler effect. So to say that *everything* around you "develops faster"--i.e., to say that you experience "time dilation" relative to everything else, not just the stuff in front of you--you need to use the term "time dilation" in the standard way, to refer to what's left after you correct for the relativistic doppler effect. So you appear to be using the term two different ways, and this may also be a source of confusion.

I have found in cases like this that the best way to avoid confusion is to avoid terms like "time dilation" that are ambiguous, and to try to describe everything in terms of direct observables--instead of saying "I observe time dilation", say "I see the galaxy ahead of me appearing to evolve much faster than I am evolving" or something like that.

Quote Quote by stu dent View Post
i'm not certain exactly of the formula you would use to calculate the time dilatation in comparison to your speed relative to the speed of light, and although, as v→c i would expect time to tend to zero, i.e. you do not age, i would expect that this would have a non converging effect on the speed at which other things occur around you.
You first need to settle on a consistent definition of "the speed at which other things occur around you". See above.

Quote Quote by stu dent View Post
now, some of what i said might be wrong. it is speculation. i don't know the math.
Yes, quite a bit of it is wrong. I would strongly advise you to learn some of the math; a good introductory text is Taylor and Wheeler's Spacetime Physics.

Quote Quote by stu dent View Post
but the original question i think is missing key information. i think it matters exactly at what speed you go. and that can't be c.
You're right that it can't be c; that was pointed out early on in the thread. The exact speed doesn't matter too much as long as it's close enough to c that the gamma factor is much greater than 1; the exact quantitative details will change, but the general qualitative behavior is basically the same once you reach that regime.

Quote Quote by stu dent View Post
i would expect that the formula for time dilation might yield even a exponential increase in time dilation as you approach the speed of light, meaning every tiny fraction of the speed light that you increase upon, makes the world around you seem to accelerate much faster, as your time frame slows in comparison to theirs.
Not really. As ghwellsjr pointed out, the distance to the galaxy is finite, and that limits how much "history" of the galaxy can elapse before you reach it. For example, say the galaxy is a billion light years away. You accelerate to .9999999999c and travel towards it. You will reach the galaxy in a little over a billion years (by the galaxy's clock); plus, when you started the trip the light you were seeing from the galaxy was a billion years old. So during your trip you will see the light from two billion years' worth of "galaxy events" reach you; you won't see any more than that.

Quote Quote by stu dent View Post
so, i think this question must be answered using the formula for time dilation.
derek101
#30
Nov26-12, 05:11 PM
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relativistic aberration would change your view of the universe.
stu dent
#31
Nov26-12, 07:43 PM
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Quote Quote by PeterDonis View Post

If you insist on using the term "time dilation" this way, nobody can stop you; however, you should understand that this is *not* the way the term "time dilation" is standardly used in relativity, and it's not the way ghwellsjr has been using the term. That seems to me to be a major source of confusion between the two of you.

The standard usage of the term "time dilation" in relativity refers to whatever effect is left over *after* you have corrected for the relativistic Doppler effect and the effect of light travel time delay. With that usage of the term, ghwellsjr is correct that you can't directly "see" time dilation; you calculate what it is by taking what you directly "see" and correcting for those other effects.
ooooh ok, i get it. that makes sense. then, what's the word i should be using to mean what i mean then? 'the warping of the passage of time one frame relative to another'

[quote]
As I said, nobody can stop you from using terms in your own way; however, you do need to make sure that you reason correctly from whatever definitions you use for terms, and you have to use the terms consistently.

If you are moving close to the speed of light relative to all the rest of the stuff in the universe (which is what you seem to mean here by "the closer you get to the speed of light"), then things in front of you will appear to "develop faster", but things behind you will appear to "develop slower". So by the definition of "time dilation" you were using in what I quoted above, you are only seeing time dilation relative to the things in front of you.[quote]

ok, i don't fully understand why things behind would appear slower, and things in front would appear faster, and also, what might be confusing, is that, when i refer to appearance, i may often be omitting some things, like for instance, the doppler effect, where at a certain speed, i would assume stuff goes into a non visible spectrum, and i see nothing.

so when i say appearance, sometimes i mean, not really actually if i was there looking out the window, but instead, if i could be there in the ship, looking out the window, and seeing a real representation of what is occurring on that planet. or whatever. i mean, there are a large number of things that come into play as to what you would actually see, and to decide really what you'd actually see, we'd need to be precise with our situation and do some math.

so, do you mean, it would appear that way to me, and yet, in actuality time for me is slower than for them? or do you mean, it would appear that way to me, and that's because their relative time is actually slower than mine.

and also, whichever it is, if you could please explain, cause either way, i'm missing something.

To figure out that, once you reach your destination and stop, everything else in the universe will have aged a lot more than you did--including the stuff behind you and the stuff in front of you--you have to correct for the relativistic doppler effect. So to say that *everything* around you "develops faster"--i.e., to say that you experience "time dilation" relative to everything else, not just the stuff in front of you--you need to use the term "time dilation" in the standard way, to refer to what's left after you correct for the relativistic doppler effect. So you appear to be using the term two different ways, and this may also be a source of confusion.
yes, this is because, as i mentionned earlier, i am assuming that what i see is an accurate representation of what is actually going on, and is not subject to other influences.

I have found in cases like this that the best way to avoid confusion is to avoid terms like "time dilation" that are ambiguous, and to try to describe everything in terms of direct observables--instead of saying "I observe time dilation", say "I see the galaxy ahead of me appearing to evolve much faster than I am evolving" or something like that.
that's sensible. yet on the other hand, then i wouldn't learn the proper meanings of the words either. and that would suck for someone like you to have to not use accurate words, and to keep in mind which ones not to use, because that works both ways.



You first need to settle on a consistent definition of "the speed at which other things occur around you". See above.
I was working on the principle that regardless of the direction i would be travelling in, and i think i am not mistaken on this point, that my time in my reference will be moving slower than all the rest of the universe as my speed approaches c. i was working of the principle that this would be observable to me, i mean, i knew many other things occurred and i knew i was omitting some of those, but i am not fully aware of them all, and was ignoring most of them. which i guess was actually stupid, since those must be important, but whatever, that's what i was doing.



Not really. As ghwellsjr pointed out, the distance to the galaxy is finite, and that limits how much "history" of the galaxy can elapse before you reach it. For example, say the galaxy is a billion light years away. You accelerate to .9999999999c and travel towards it. You will reach the galaxy in a little over a billion years (by the galaxy's clock); plus, when you started the trip the light you were seeing from the galaxy was a billion years old. So during your trip you will see the light from two billion years' worth of "galaxy events" reach you; you won't see any more than that.
hmmm.. that's what i would have originally intuitively thought, but it seemed like too simple. as though i must have been missing something.

what about watching the trip from earth? you'd see the spaceship travelling at or rather, very near, the speed of light for one billion years roughly, to get there, if the speed of light were instantaneous, and given the fact that it isn't, it would appear to slow down, at a rate that would end up doubling the travel time?
UglyNakedGuy
#32
Nov26-12, 07:59 PM
P: 11
hi all, quite a discuss, may I ask a question here...about time dilatation.

I am just thinking, we all know (approximately) how long one second is, according to our day to day life, and ( please correct me if im wrong, sorry :) ) if something moves at v=c, for itself, time wont move at all, right?

if we travel at a speed v near c, our watch will slow down lot, so why cant I "observe" the time dilatation?

I mean i just assume myself as a photon traveling in vaccum at c, it should be the outside observer to say "1000 lightyears or ..." for the photon itself the time should be stopped?

once again, I am just interesting in these area, plz dont laugh and help me to clarify :)
UglyNakedGuy
#33
Nov26-12, 08:29 PM
P: 11
[QUOTE=ghwellsjr;4167802]
A good way to think about this is if you are traveling toward something at a very high speed, not only do you watch its past history unfold before your eyes, you also watch its future unfold by the same amount.

QUOTE]

Dear Ghwellsjr,

I understand the first half of your explanation, (the past history part) since if I am at rest, and the light i receive from the galaxy should be "sent" by it in (say) 10000 years ago.
Am I right?

but i dont get the "future unfold" part, as I move toward it, it is like i unfold its history, and at the same time the galaxy itself is still evolving .so the evolution image I observed should be twice faster as I am at rest?

is this correct?

thank you for your help!!!
nitsuj
#34
Nov26-12, 10:48 PM
P: 1,098
Quote Quote by stu dent View Post

ok, i don't fully understand why things behind would appear slower, and things in front would appear faster, and also, what might be confusing, is that, when i refer to appearance, i may often be omitting some things, like for instance, the doppler effect, where at a certain speed, i would assume stuff goes into a non visible spectrum, and i see nothing.
Your tree analogy can actually work fairly well to describe the difference between the Doppler effect & relativistic effects and is how I thought you were describing it...so...

To use your trees passing by the window of a car analogy.

While traveling at a meager 100mph, directly to your right you see the trees pass by at a rate of once per second (just inches away from you and the car).

You engage hyper-drive to half the speed of light and as you would expect the trees now pass by at a much greater rate than once per second. ("DOPPLER")

Being a bright hyper-drive car driver you work the calculation "backwards" to ensure that at 100mph the trees would still pass by at a rate of once per second. But something isn't adding up...for a speed of 100mph you would calculate the trees would pass by at a rate of more than one tree per second! (TIME DILATION, observation orthogonal to direction of motion your time is dilated and in turn would observe more trees because your second is now longer than it was before according to calculations)

Another way to look at the scenario is once you engage hyper-drive to half the speed of light you count how many trees pass in one second, in comparison to the observation of one tree per second at 100 mph you will calculate too many trees are passing each second while traveling at half the speed of light. ("contraction" and nobody moved the trees closer together )

I am by no means an expert (let alone "educated") and merely thought through it, so....
PeterDonis
#35
Nov26-12, 11:52 PM
Physics
Sci Advisor
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P: 6,148
Quote Quote by stu dent View Post
ooooh ok, i get it. that makes sense. then, what's the word i should be using to mean what i mean then? 'the warping of the passage of time one frame relative to another'
It's not so much a matter of what words you use, as keeping distinct the several different things that are going on: Doppler effect, light travel time delay, and time dilation. "Warping of the passage of time one frame relative to another" is an OK description of time dilation, but only *after* you've corrected for the Doppler effect and light travel time delay; you can't directly observe time dilation in this sense.

Quote Quote by stu dent View Post
ok, i don't fully understand why things behind would appear slower, and things in front would appear faster
Here I was using "appear" to mean "the way the incoming light actually appears to you directly", which is primarily driven by a combination of the Doppler effect and light travel time delay. You are moving towards things in front of you, so the light signals from them will appear speeded up, because of the Doppler blueshift plus the fact that as you get closer, the light travel time delay decreases. You are moving away from things behind you, so the light signals from them will appear slowed down, because of the Doppler redshift plus the fact that as you get farther away, the light travel time delay increases.

You can *calculate* that, after you've corrected for those effects, both the things in front of you and the things behind you are time dilated compared to you--they are aging more *slowly* than you are, as seen in your rest frame. See below for further comments on this.

Quote Quote by stu dent View Post
and also, what might be confusing, is that, when i refer to appearance, i may often be omitting some things, like for instance, the doppler effect, where at a certain speed, i would assume stuff goes into a non visible spectrum, and i see nothing.
In an actual situation, yes, this would likely happen. In discussions like this, though, we can assume that we can ignore this effect; basically we are assuming that there is always enough radiation coming from the object that will get redshifted or blueshifted into a range we can detect, so we can get enough information to construct what you call a "real representation of what is occurring".

Quote Quote by stu dent View Post
so, do you mean, it would appear that way to me, and yet, in actuality time for me is slower than for them? or do you mean, it would appear that way to me, and that's because their relative time is actually slower than mine.
This brings up another good point, which I briefly referred to above. "Time dilation" by itself is not enough to tell you what some other object's clock will read when you reach it. For example, suppose I fly at a speed very close to the speed of light from Earth to some galaxy a billion light years away. Suppose I know, somehow, that clocks on my destination planet in that galaxy are exactly synchronized with Earth clocks (and suppose also that the planet is exactly at rest relative to Earth). Then I expect that, if I leave Earth at time t = 0 by Earth clocks, I will arrive at t = 1 billion years (plus a little bit because I'm not quite traveling at the speed of light) by the destination planet's clocks, whereas a much shorter time (say a year) will have elapsed on my clock (and I will only have aged a year, etc.).

*But*, if I look at things from my rest frame while I am traveling, it will seem to me that clocks on Earth *and* clocks on the destination planet are running much *slower* than my clocks are. That is, once I correct for the Doppler effect and light travel time delay, I can calculate from the light signals I receive from Earth and the destination planet that they will only age by a fraction of a second during my entire trip, while I age by a year. So it seems at first glance that I should predict that clocks on the destination planet will read only t = 0 + a fraction of a second when I arrive. Why, then, do I find that they actually read a t = a billion years plus a bit?

The missing piece here is called "relativity of simultaneity". In the rest frame of Earth and the destination planet, the event of my launch from Earth happens a billion years (plus a bit) before the event of my arrival at the destination planet. That means that my launch from Earth is simultaneous (in the Earth-planet rest frame) with an event on the destination planet that happens a billion years (plus a bit) before I arrive. Call this event (when clocks on the destination planet, synchronized with Earth clocks, read t = 0) event P.

However, in my rest frame while I am traveling, those two events (my launch and my arrival) are only a year apart (because that's how much time I experience during the trip). That means that, in my rest frame while I am traveling, the event of my launch from Earth is simultaneous with some *other* event on the destination planet, call it event P'. When I calculate that only a fraction of a second elapses on clocks on Earth and the destination planet during my trip, what I am really calculating is the time, on the destination planet's clocks, between event P' and the event of my arrival. But event P' doesn't happen at time t = 0 by the destination planet's clocks; it happens at t = 1 billion years, plus a bit, minus a fraction of a second.

In other words, to predict what the destination planet's clocks will read when I arrive, it's not enough just to know about time dilation; I also have to know how the destination planet's clocks are synchronized, i.e., when their "zero" of time is. If their "zero" of time occurs when I am spatially separated from them (as it does for the destination planet--when their clocks read t = 0, I am on Earth, just launching), I have to take that into account as well as time dilation in order to know what their clocks will read.

Quote Quote by stu dent View Post
I was working on the principle that regardless of the direction i would be travelling in, and i think i am not mistaken on this point, that my time in my reference will be moving slower than all the rest of the universe as my speed approaches c.
As I hope I've shown above, it's more complicated than that.

Quote Quote by stu dent View Post
what about watching the trip from earth? you'd see the spaceship travelling at or rather, very near, the speed of light for one billion years roughly, to get there, if the speed of light were instantaneous, and given the fact that it isn't, it would appear to slow down, at a rate that would end up doubling the travel time?
This is a good question; let's look at what things look like from both Earth and the destination planet.

If we're on Earth watching the trip, we see the ship leave at time t = 0. Since its destination is a billion light years away, we will see the ship's arrival at time t = 2 billion years plus a bit (1 billion years plus a bit for the ship to get there, plus 1 billion years for the light to get back to us). So the whole trip will appear "slowed down"; it will take about twice as long for us to see the trip happen, as it takes for the trip to actually happen. That doesn't double the actual travel time: the ship still arrives in 1 billion years plus a bit. It just takes twice as long for us to receive all the light signals emitted from the ship during the trip.

If we're on the destination planet watching the trip, it's more interesting. Since the launch point (Earth) is a billion light years away, it will take a billion years for the light from the launch to reach us. So we will see the launch at t = 1 billion years, and the ship will actually arrive very soon after, at t = 1 billion years plus a bit. In between those two events, we will see *all* the light signals emitted by the ship during the trip, drastically speeded up.

Note that in both cases, we can correct for the Doppler effect and light travel time delay to calculate that the ship and its crew only age by 1 year during the trip. So what we actually see can be quite different from what we calculate that the ship and its crew will experience. That's why it's so important to keep the different things that are happening distinct.
ghwellsjr
#36
Nov28-12, 03:04 AM
PF Gold
P: 4,743
Quote Quote by UglyNakedGuy View Post
Quote Quote by ghwellsjr View Post
A good way to think about this is if you are traveling toward something at a very high speed, not only do you watch its past history unfold before your eyes, you also watch its future unfold by the same amount.
Dear Ghwellsjr,

I understand the first half of your explanation, (the past history part) since if I am at rest, and the light i receive from the galaxy should be "sent" by it in (say) 10000 years ago.
Am I right?
Yes.
Quote Quote by UglyNakedGuy View Post
but i dont get the "future unfold" part, as I move toward it, it is like i unfold its history, and at the same time the galaxy itself is still evolving .so the evolution image I observed should be twice faster as I am at rest?

is this correct?
You would see at least twice as much time go by but you will see it progressing at much faster than just twice your own rate, it could be hundreds of times faster, depending on your speed relative to the earth and the galaxy
Quote Quote by UglyNakedGuy View Post
thank you for your help!!!
You're welcome.


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