PDA

View Full Version : Speed of Light from Distant Sources

Eric12407
2006-Jan-03, 11:34 PM
Hi there Experts ...

My name is Joe Average .... I love reading about astronomy, physics etc ..
Almost all of it is very hard to grasp ...

Anyway i had this thought ... I'm sure one of you can set me straight on this.

The story goes that light travels at 186,000 miles per second ... no matter how fast you are travelling ...

The light from distant galaxies has been travelling that speed and therefore has taken a long time to get to us ...

Okay .. so a photon takes off from a distant galaxy towards the earth ..
I also live in that distant galaxy and i have a space ship that travels at half the speed of light ....

If i chase that photon headed for your earth ... will the act of me chasing it cause the photon to speed up relative to you?

Thanks for any enlightenment .....

grant hutchison
2006-Jan-03, 11:54 PM
If i chase that photon headed for your earth ... will the act of me chasing it cause the photon to speed up relative to you?No, it won't. Photons travel at a fixed speed. But the Universe appears to be put together in such a way that whenever you measure the speed of a light in a vacuum, you'll always get the same answer, no matter how fast or slow you happen to be moving. It's a weird result, since you'd think that from your spaceship, the light would appear to be moving at half the speed of light (or if it was measured at light-speed on your spaceship, it would be light-speed-and-a-half on Earth).
Nevertheless, it's been verified experimentally over and over again: both you and the Earth observers would measure light as moving at the same speed.
The fact that the Universe does behave like this is what made Einstein come up with Special Relativity, and all the odd things that predicts about shrinking length and slowing time-rate in rapidly moving objects.

Grant Hutchison

Fortunate
2006-Jan-04, 12:59 AM
The photon does not speed up. The apparent paradoxes are resolved by the fact that the observer on the spaceship measures time and distance differently from an observer on Earth.

Assume that your spaceship travels at a constant velocity (i.e., that you are in an "inertial frame of reference"). Suppose that the light originates from a point 1 light year from Earth (there are no galaxies at that distance, but this assumption keeps the numbers simple). Ignore the fact that the universe is expanding. An observer on Earth will think that the photon traveled a distance of 1 light year in a time equal to 1 year.

The theory of special relativity says that you, in your spaceship, will think that the photon went a distance of about .866 light years in a time equal to about .866 years*. Dividing the distance by the time to calculate the speed, you will compute the speed of the photon to be 186,000 miles per second, the same speed observed by the person on Earth.

This may all sound counterintuitive, but the differences in measurements of length and time predicted by special relativity have been verified in many different experiments.

*Length and time both get multiplied by a factor of SQRT(1-v2/c2), Where c is the speed of light and v is the radial speed of one reference frame with respect to the other.

Eric12407
2006-Jan-04, 01:13 AM
Hello and thank you for your response ...

I sort of understand the paradoxes of time, space and speed of light and such ..

In my scenario though ... if I continue to follow the photon to its destination (earth) won't it arrive there ahead of schedule? I'll be behind it twice as slow but it will be getting to you according to my position .. half the speed of light sooner than if I wasn't there.

Is there any way for the speed of light to be instant in some way .. and that it only slows down on our perception of it? It seems to me that gravity and light need to be a fundamental aspect of reality inherent to existence .. permeating and underlying all creation and communicating instantly and effortlessly ...

I have absolutely no evidence to back this up .... just pondering ...

Thanks again ....

Fortunate
2006-Jan-04, 01:27 AM
In my scenario though ... if I continue to follow the photon to its destination (earth) won't it arrive there ahead of schedule?

I'm not sure what you mean here. According to an observer on Earth, the photon will have traveled a distance of 1 light year in a time of 1 year, arriving exactly on schedule.

Is there any way for the speed of light to be instant in some way .. and that it only slows down on our perception of it?

Interestingly enough, in the photon's own frame of reference, the time taken by the trip to Earth is 0, so the "journey" is instantaneous. In the frame of reference of the photon, the distance from its point of origin to Earth was also 0. So from its own point of view, the photon went nowhere in no time.

I'm going out now, so I will not add any more comments for a while:).

Eric12407
2006-Jan-04, 01:50 AM
Hi there fortunate ...

Okay then ... in comparing me in the spaceship to you ... in my reference of movement ... the photon is moving at 186,000 miles per second ahead of me ... but in your reference ... the light has slowed down by half that amount? It looks like we are living in 2 different worlds! (Tried to get a smiley in here but didn't know how to do it)

In my world on the spaceship ...I should see the photon hitting you 186,000 miles ahead of me ... but that would mean it got there too soon if the speed of light is constant ..

If the photon travels no distance in no time ... then wouldn't that mean instantaneous?

Is the photon in a different state because I am watching it and all of space time adjusts to my movement? If this is the case doesn't that say more about me than world I am trying to explain?

Sam5
2006-Jan-04, 02:17 AM
Hi there Experts ...

My name is Joe Average .... I love reading about astronomy, physics etc ..
Almost all of it is very hard to grasp ...

Anyway i had this thought ... I'm sure one of you can set me straight on this.

The story goes that light travels at 186,000 miles per second ... no matter how fast you are travelling ...

The light from distant galaxies has been travelling that speed and therefore has taken a long time to get to us ...

Okay .. so a photon takes off from a distant galaxy towards the earth ..
I also live in that distant galaxy and i have a space ship that travels at half the speed of light ....

If i chase that photon headed for your earth ... will the act of me chasing it cause the photon to speed up relative to you?

Thanks for any enlightenment .....

Davis and Lineweaver wrote an interesting paper about this a few years ago. Basically they said that galaxies moving away from us at or faster than light speed, emit light at “c” relative to those galaxies, but at less than “c” relative to us. In fact, they said that the photons from superluminal galaxies actually can move away from us at first, when they are first emitted in our direction. They say that specifically in this sentence: “When a X y > c the distance between us and the photon increases.” That sentence is in the third paragraph from the end of their paper.

That means the photon is initially moving away from us when it is first emitted by a high speed or “superluminal” galaxy. The photon’s speed gradually changes relative to us and eventually it begins to move toward us and we finally receive it traveling at “c” relative to us. The SR rule that is not violated in their theory is that in each individual area of space through which each photon moves, the photon always moves in/through those areas at the local-space-speed of “c”.

Here is their important paper:

http://arxiv.org/PS_cache/astro-ph/pdf/0011/0011070.pdf

Here is their latest article in Scientific American:

page 1:
http://www.scientificamerican.com/article.cfm?chanID=sa006&articleID=0009F0CA-C523-1213-852383414B7F0147&pageNumber=1&catID=2

Regarding your question about you in a rocket, I don’t think that would have any effect on the steady local-space speed or the changing earth-relative speed of the photon.

Fortunate
2006-Jan-04, 05:05 AM
Hi there fortunate ...

Hi, Eric. Eric, I am trying to learn and clarify my own thoughts on this topic. I no longer believe the analysis I gave in my post #3 above. I do not doubt the conclusion: each observer thinks that the photon is moving at the speed of light. But I do doubt the statement that from the frame of reference of the spaceship that the photon travels a distance of .866 light years in a time of .866 years. I will have to think about this some more.

Okay then ... in comparing me in the spaceship to you ... in my reference of movement ... the photon is moving at 186,000 miles per second ahead of me

Yes. As I understand it, in your frame of reference you are stationary, and the light is moving away from you at a speed of 186,000 miles per second, and the Earth is moving toward you at a speed of 93,000 miles per second. In my analysis in post #3, I had considered you to perceive yourself to be moving, and the Earth to be stationary, but now I think I need to think from your point of view that you are stationary. So, then, I need to rethink my analysis. But I don't have time to do that now. I'll finish this post and retire for the night.

but in your reference ... the light has slowed down by half that amount?

In my frame of reference, the photon is also moving at 186,000 miles per second, and I see you moving at 93,000 miles per second.

It looks like we are living in 2 different worlds! (Tried to get a smiley in here but didn't know how to do it)

We are living in different worlds. We measure different distances and times, and we also measure many speeds differently. But there is one thing that we both get the same value for: the speed of light in a vacuum. You can make a yellow smiley by typing a colon followed by a right parenthesis.

I am through for the night. Thank you for the interesting posts.

Fortunate
2006-Jan-04, 07:43 AM
Hi, Eric. Here is my latest formulation. Imagine that a photon is emitted from a galaxy 1 light year from Earth (even though there are actually no galaxies at that distance). Ignore the expansion of the universe, which would be trivial at that distance, and suppose that the galaxy is stationary with respect to the Earth. You are traveling in a spaceship moving at half the speed of light (in other words at 93,000 miles per second) toward the Earth at the moment that the photon is emitted from the galaxy.

An observer on Earth would consider himself to be stationary. He would see the galaxy as stationary and think that it was 1 light year away. He would think that the photon was moving toward him at 186,000 miles per second, and that you were moving toward him at 93,000 miles per second. He would think that the photon took 1 year to travel from the galaxy to Earth. He would also think that at the moment the photon reached Earth that you were .5 light years form Earth and .5 light years from the galaxy.

In your frame of reference, you would consider yourself to be stationary. You would think that the galaxy was moving away from you and the Earth was moving toward you and that the distance from the Earth to the galaxy was remaining constant at .866 light years (because of the Lorentz contraction). You would see the photon moving toward the Earth at 186,000 miles per second and the Earth moving toward the photon at 93,000 miles per second, the two destined to meet somewhere in between their present positions. Since the photon seems to you to be moving twice as fast as the Earth, it should cover 2/3 of the original gap (2/3 of .866 light years equals .577 light years), and the Earth should cover the other 1/3 of that gap. Thus, you would think that the photon had traveled about .577 light years from the galaxy to the Earth. You would think that the trip took .577 years. You would think that the photon reached the Earth when the Earth was .577 light years from you but hurtling toward you at 93,000 miles per second. You would think that at the time the photon reached Earth, that the galaxy was .288 light years away from you and moving away from you at 93,000 miles per second.

This seems right now, but I may feel differently tomorrow. I'm just trying understand this myself. The contention that the speed of light is the same in every inertial reference frame is not a cosequence of special relativity, it is an underlying assumption on which the theory is based. Thus, we are not calculating the speed of light from the values of the other variables; we are calculating the values that the other variables have to have to be consistent with the assumption that the speed of light is the same in every inertial reference frame.

grant hutchison
2006-Jan-04, 01:52 PM
Okay then ... in comparing me in the spaceship to you ... in my reference of movement ... the photon is moving at 186,000 miles per second ahead of me ...Bear in mind that there's no way you can actually observe that photon ahead of you. It's moving at lightspeed away from you! And there's no way you can tell the exact moment it arrives at Earth, unless someone there sends a message to you (which will need to travel back to you at lightspeed or slower).
So maybe a better "thought experiment" would be to imagine your spaceship, travelling at half light-speed towards Earth, emitting a flash of intense red light when it's one light year out. Then you watch through a telescope to see when the Earth "lights up" red with the reflected light. Earth-bound observers, meanwhile can watch for the time-lag between your initial flash and the time at which your spacecraft lights up with light reflected from the Earth.
This has all the components of your original problem, but doesn't require us to make an impossible measurement of the speed of a receding photon.

Both you and the Earth-bound observers will see light make the double journey in a time that is compatible with light moving at 186,000 miles per second in your own frame of reference. So you'll wonder how on earth the Earth-bound observers didn't measure the speed of light as light-speed-plus-a-half. But if you check what they've done, it will seem to you that they have made distorted measurements: with a short measuring stick, a slow clock and a faulty perception of what events are simultaneous with each other.
Likewise, they'll wonder how you managed to find that light moved at lightspeed relative to your spaceship: but again, to them it will appear as if your measuring tools are distorted, in exactly the way that produces a consistent measurement of lightspeed.
That's Special Relativity.

Grant Hutchison

Eric12407
2006-Jan-04, 03:31 PM
Yes .... space and time distort in my reference so that the speed of light remains constant ... I realize that ... but it seems it has more to do with me than light. I don't know if that has any meaning to anyone ... just a thought.

Okay here's another question ... suppose there is only one photon in the whole universe and it originates from a distant source ... and heads for me on earth and I have my eyes closed ... If I open my eyes light must recede from me at light speed .. therefore the photon would never arrive ... but in the photons world it is already there .. as soon as I open my eye.. somehow I think me and the light need each other ....

grant hutchison
2006-Jan-04, 05:53 PM
Yes .... space and time distort in my reference so that the speed of light remains constant ... I realize that ... but it seems it has more to do with me than light.Not just you. It has to do with the whole Universe: it's just the way the Universe seems to work.

If I open my eyes light must recede from me at light speed .. .Or approach you at lightspeed, or go past you at lightspeed: as long as it's moving at the speed of light, everything's fine. So the photon will arrive at your eye, whether or not you have your eyes open. (I wonder if you're not mixing together the "observer effect" from quantum mechanics with special relativity, here.)

Grant Hutchison

rahuldandekar
2006-Jan-06, 03:12 AM
Eric, if there were only one photon, you wouldn't see it until it came to your eye. So, even if you had your eyes open, you wouldn't see anything until *boom* a flash and then nothing again. that flash was the one photon in the universe. You wouldn't see it until it came to your eye.

It has not much to do with you as it has to do with the constancy of the speed of light. For more, you should read smething on special relativity. Here's one good book (free to download) : http://www.lightandmatter.com/area1book6.html. Download only the first 50 pages, that ought to be enough. The rest contain Elementary Quantum Mechanics.

Eric12407
2006-Jan-06, 06:58 PM
Thank you everybody for your replies ..

Would you allow me to ask some other questions?

1. Okay .. so to light ... the transmission across any distance takes place in no time over no distance ...

So what would happen to an entity that went faster than light ...Does it go back in time? What kind of world does it live in?

If we can achieve light speed ... then we could travel anywhere in the universe ...correct? in no time ....

2. Just say for instance there was no big bang and we live in a solid state universe that is infinite and that has existed for infinity ....
Would that have any ramifications on our concept of light from distant sources and light itself?

3. In our big bang universe model ... galaxies are receding away from us at faster than the speed of light ... isn't that supposed to be impossible?

Thanks for any info

grant hutchison
2006-Jan-06, 07:37 PM
Quick responses:

So what would happen to an entity that went faster than light ...Does it go back in time?Its length and time rate would be measured in imaginary numbers. I can't imagine what that means. :neutral:

If we can achieve light speed ... then we could travel anywhere in the universe ...correct? in no time ....No time in our own reference frame. Outside, time would elapse at one year per lightyear travelled. And it would require infinite energy to get up to lightspeed, if we had a non-zero mass. (I certainly have ...)

2. Just say for instance there was no big bang and we live in a solid state universe that is infinite and that has existed for infinity ....
Would that have any ramifications on our concept of light from distant sources and light itself? I think you mean "steady state": "solid state" is an electronics term. I don't know of any ramifications, but I'm swimming well outside my depth at that point.
Ooo. (Edit) If the Universe were infinite and had existed for an infinite period of time, then Olber's Paradox would apply: the sky would be everywhere as bright as the surface of a star. So that's a significant ramification ...

3. In our big bang universe model ... galaxies are receding away from us at faster than the speed of light ... isn't that supposed to be impossible?These galaxies are at rest in their own reference frame: the space between us and them is expanding at more than the speed of light.

Grant Hutchison

Eric12407
2006-Jan-06, 08:17 PM
Hi Grant ..

Thanks for putting up with me ... solid state .. that's pretty funny ...
Yes .. I meant steady state ...

Question #3 ... you say space is expanding at faster than the speed of light in this model

But there are objects in this space so relative to us aren't they travelling faster than light too .. especially the ones at the supposed ridge of the universe? Or is it just empty space being created? How does a galaxy create space?

If it's empty space that is being created what's pushing it out faster? Where is all the energy coming from to move something faster than the speed of light?

Does this mean eventually everything we see is going to disappear?

Since light moves at the speed that takes an infinite amount of energy
to attain ... does one photon of light contain that much energy?

Ken G
2006-Jan-06, 08:23 PM
If the Universe were infinite and had existed for an infinite period of time, then Olber's Paradox would apply: the sky would be everywhere as bright as the surface of a star.
Interesting nitpick here-- I don't know where this idea came from that Olbers' paradox says the sky would be as bright as the surface of a star, perhaps that's how Olbers put it, but it's wrong. Stars don't swallow the energy that falls upon them, they reradiate it. So if the stars could magically keep pumping out light for an eternity (say by constantly having new mass appear, in violation of the conservation of energy) then the sky would be infinitely bright, not as bright as the surface of today's stars. Not that the distinction is terribly important, but it makes me wonder why anyone could have believed in an infinite steady-state universe. They must have thought the universe was infinite but the mass was finite and the light could just keep escaping, but then there's no way of knowing how bright the night sky would be without knowing how many stars there were.

Uninteresting nitpick-- it's Olbers' or Olbers's paradox, but not Olber's. If he's watching, Heinrich might appreciate the correction, because let's face it, no one can remember which one it is!

Eric12407
2006-Jan-06, 08:32 PM
Originally Posted by grant hutchison
If the Universe were infinite and had existed for an infinite period of time, then Olber's Paradox would apply: the sky would be everywhere as bright as the surface of a star.

Would it? Doesn't light just dissipate after a certain distance ... depending on its intensity?

For example I couldn't see someone waving a flashlight on Mars ...
So maybe after a certain distance I can't see any galaxies either ... depending on how strong my telescopes are that is ...

Eric ....

grant hutchison
2006-Jan-06, 09:40 PM
Interesting nitpick here-- I don't know where this idea came from that Olbers' paradox says the sky would be as bright as the surface of a star ...I do. It's a first-order thing: every sightline ends on the surface of a star. It just neglects the effect of every star on every other star.:doh:

Uninteresting nitpick-- it's Olbers' or Olbers's paradox ...Hang on. Aren't we talking about Olber, the guy who shared an office with Blotzmann? :lol:
That's a horribly embarrassing slip for me, since I've not only been wittering on about apostrophes on another thread, I've just (today!) written some harsh words about an astronomy textbook that mentions "Corioli's effect".

But doesn't the fact that I wrote "Quick responses" excuse me from complete accuracy?

Oh.
It doesn't.

Grant Hutchison

grant hutchison
2006-Jan-06, 09:48 PM
Would it? Doesn't light just dissipate after a certain distance ... depending on its intensity?It does dissipate, according to the inverse square law. So the amount of light you receive from a star declines according to the square of your distance from it. But the apparent angular diameter of the star declines in proportion to your distance from it, which means its angular area also declines according to the square of your distance from it.
Things therefore get dimmer and smaller as you move away from them, in strict proportion, so their brightness per unit area of the sky stays the same. Therefore, if there were star-surface in every direction you looked (as there would be, in an infinite, infinitely old Universe), the whole sky would be as bright as the surface of a star.
Except, as Ken has so rightly pointed out, all the light from all those stars, radiated over infinite time, would heat up all the stars to an infinite temperature.

Grant Hutchison

grant hutchison
2006-Jan-07, 01:09 AM
But there are objects in this space so relative to us aren't they travelling faster than light tooThey are travelling faster than light relative to us, so their light can't reach us: such photons are propagating towards us at light speed, but the space they're travelling through is being carried away from us faster than light speed. The point about "space expanding" is that these distant objects haven't been accelerated up to light speed and beyond (which would require infinite energy); they're just at rest in their part of the Universe, which is moving away from our part of the Universe faster than the speed of light.

If it's empty space that is being created what's pushing it out faster? Where is all the energy coming from to move something faster than the speed of light?Oooo. Cosmology. Pass. (I've got myself into enough trouble today writing about stuff I thought I understood. :))

Does this mean eventually everything we see is going to disappear?It depends on what the Universe does in the future. If its expansion accelerates, then stuff we can see will eventually become invisible; if expansion slows, then more stuff will become visible. I'll leave a passing cosmologist to say which of those is likely.

Since light moves at the speed that takes an infinite amount of energy to attain ... does one photon of light contain that much energy? My head hurts .....Photons have zero rest mass, unlike you, me, spacecraft and galaxies, so they can move at light speed without requiring infinite energy to get there. (My understanding is that, in fact, their zero rest mass in some way requires them to move at the speed of light.)

Grant Hutchison

Ken G
2006-Jan-07, 03:51 AM
Oooo. Cosmology. Pass. (I've got myself into enough trouble today writing about stuff I thought I understood.)
I don't think any of us really understand this, but the party line appears to be that general relativity need not conserve energy globally, so there's no issue about where the energy comes from. I like to think of it like this: general relativity in the standard description of cosmology is just a way to cobble together different reference frames (the so-called local comoving frames), and note that changing reference frames never conserves energy and doesn't need to.

(My understanding is that, in fact, their zero rest mass in some way requires them to move at the speed of light.)

The way I look at it is, if a zero rest-mass object were moving at less than the speed of light in a vacuum, then you could transform into the reference frame of the object, and what would you have? A zero energy particle that could be created willy nilly and has no real existence as a result.

Eric12407
2006-Jan-07, 04:03 AM
Hello again experts ...

Wow ... I'm just thinking of all this energy ... All the energy in all the visible matter ... all the "dark" energy ... all the energy in dark matter ..
all the energy accelerating the universe ... and on and on and on ...

If all this was a singularity ... how much energy would it take to compress and contain it and where did that energy come from?

Never mind ....

Titana
2006-Jan-07, 05:36 AM
If it's empty space that is being created what's pushing it out faster? Where is all the energy coming from to move something faster than the speed of light?

It is not known where the energy is coming from. That is why it is known as (Dark Energy).

The following link is very interesting. Take a look at it......;)

Build a rocket that accelerates steadily and has lots of fuel. Set it to accelerating. Sooner or later it will go faster than the speed of light, won't it? In fact it won't; the problem is that as objects travel as speeds close to the speed of light (relative to an observer), their clocks (and all other processes) seem to slow down. So while the rocket was shooting out fuel at a tremendous clip initially, when it nears the speed of light, it starts shooting out the fuel ever more slowly, and its acceleration correspondingly decreases, so that it can never quite reach the speed of light.

http://fixedreference.org/en/20040424/wikipedia/Faster-than-light

Titana.

Eric12407
2006-Jan-07, 01:15 PM
Hi Titana ...

Thanks for the link ... the explanations are short and sweet ... perfect for the layman such as me ...

This whole "dark" thing is interesting ...

I am going to be really short on paying my bills this month ... maybe I'll just use my "dark" money! Hopefully the bank will understand it will show up eventually .....

Thanks ... Eric

Titana
2006-Jan-07, 06:11 PM
;)

Titana.

nokton
2006-Jan-07, 07:43 PM
Hi, Eric. Here is my latest formulation. Imagine that a photon is emitted from a galaxy 1 light year from Earth (even though there are actually no galaxies at that distance). Ignore the expansion of the universe, which would be trivial at that distance, and suppose that the galaxy is stationary with respect to the Earth. You are traveling in a spaceship moving at half the speed of light (in other words at 93,000 miles per second) toward the Earth at the moment that the photon is emitted from the galaxy.

An observer on Earth would consider himself to be stationary. He would see the galaxy as stationary and think that it was 1 light year away. He would think that the photon was moving toward him at 186,000 miles per second, and that you were moving toward him at 93,000 miles per second. He would think that the photon took 1 year to travel from the galaxy to Earth. He would also think that at the moment the photon reached Earth that you were .5 light years form Earth and .5 light years from the galaxy.

In your frame of reference, you would consider yourself to be stationary. You would think that the galaxy was moving away from you and the Earth was moving toward you and that the distance from the Earth to the galaxy was remaining constant at .866 light years (because of the Lorentz contraction). You would see the photon moving toward the Earth at 186,000 miles per second and the Earth moving toward the photon at 93,000 miles per second, the two destined to meet somewhere in between their present positions. Since the photon seems to you to be moving twice as fast as the Earth, it should cover 2/3 of the original gap (2/3 of .866 light years equals .577 light years), and the Earth should cover the other 1/3 of that gap. Thus, you would think that the photon had traveled about .577 light years from the galaxy to the Earth. You would think that the trip took .577 years. You would think that the photon reached the Earth when the Earth was .577 light years from you but hurtling toward you at 93,000 miles per second. You would think that at the time the photon reached Earth, that the galaxy was .288 light years away from you and moving away from you at 93,000 miles per second.

This seems right now, but I may feel differently tomorrow. I'm just trying understand this myself. The contention that the speed of light is the same in every inertial reference frame is not a cosequence of special relativity, it is an underlying assumption on which the theory is based. Thus, we are not calculating the speed of light from the values of the other variables; we are calculating the values that the other variables have to have to be consistent with the assumption that the speed of light is the same in every inertial reference frame.
Time is dynamic, so the reference frame of lightspeed to an observer is
a variable Fortunate, by that I mean, local gravity constraints and the
the position of the observer, ie the observers speed, will influence the
observers interpretation. You base your assumptions on an observer in
stasis, interesting concept, but it challenges the contention you propose.
Nokton.

Fortunate
2006-Jan-07, 08:12 PM
Time is dynamic, so the reference frame of lightspeed to an observer is
a variable Fortunate, by that I mean, local gravity constraints and the
the position of the observer, ie the observers speed, will influence the
observers interpretation. You base your assumptions on an observer in
stasis, interesting concept, but it challenges the contention you propose.
Nokton.

I am not sure what you are saying. I interpreted the original statement of the problem to mean that I was to consider the spaceship to be an inertial frame of reference. I was trying to answer the question within what I thought was the posited context. I am not confident about the details of my calculation, however - it was just a stab.

nokton
2006-Jan-08, 06:33 PM
No, it won't. Photons travel at a fixed speed. But the Universe appears to be put together in such a way that whenever you measure the speed of a light in a vacuum, you'll always get the same answer, no matter how fast or slow you happen to be moving. It's a weird result, since you'd think that from your spaceship, the light would appear to be moving at half the speed of light (or if it was measured at light-speed on your spaceship, it would be light-speed-and-a-half on Earth).
Nevertheless, it's been verified experimentally over and over again: both you and the Earth observers would measure light as moving at the same speed.
The fact that the Universe does behave like this is what made Einstein come up with Special Relativity, and all the odd things that predicts about shrinking length and slowing time-rate in rapidly moving objects.

Grant Hutchison
Grant, think about it, 'the results' you refer to above as weird, are not.
Think Grant, the way Albert thought. If you in a space ship you describe
above, travelling at half lightspeed. Your time slows down by half.
So Grant, stay with me, your measurement of lightspeed is halved,
as lightspeed is, so your measurement is valid, and lightspeed in your
time is still 186k Miles per sec. As Albert insisted, it's all relative.
Nokton

pasha582
2006-Jan-08, 09:16 PM
Quote: In my scenario though ... if I continue to follow the photon to its destination (earth) won't it arrive there ahead of schedule? I'll be behind it twice as slow but it will be getting to you according to my position .. half the speed of light sooner than if I wasn't there.

Response: Suppose the galaxy is ten million ly away (Sombrero Galaxy). From the perspective of someone on the earth, they would have no way of knowing (of course) the photo had set out on its journey until it arrived. As far as the photon is concerned, the journey was instantaneous. Light does not age.

As you chase the photon at half the speed of light, your time slows down from the perspective of someone in our galaxy. (Think of subjective time as never changing--if you observe your watch, time seems to pass for you at the same rate it always does). From the perspective of someone in our galaxy, they would have no idea you were coming until five million years into your journey. At that time the first light of your departure would reach them, and they would watch you moving fast, and doing things faster than normal, so that your entire 10 million light year voyage would appear accelerated. Your departure light would require the full 10 million years to reach our galaxy, the light of your activities half way across the gap would reach us five million years later, and you yourself would arrive at some fraction of time after the photon you were chasing (call it your "departure" photon).

From your perspective, the light of the Milky Way would appear accelerated, so that activities of the Milky Way would occur at faster than "normal". The galaxy would appear to rotate faster, for example. You would observe ten million years of time telescoped into your departure time.

Suppose you travelled much faster, so that the entire 10 million year journey takes you only 10 years. You would witness the Milky Way rotate 1/20th of a revolution (it takes 200 million years to rotate once), during your 10 year voyage. If you watch your departure photons race away from you, you would measure them racing away at the speed of light. Someone observing from the Milky Way would not notice you had left the Sombrero Galaxy until 10 years before you were about to arrive. They would notice you blast off, and move rapidly towards them. Your entire 10 million year journey would then be telescoped into a period of only 10 years.

Quote: Is there any way for the speed of light to be instant in some way .. and that it only slows down on our perception of it? It seems to me that gravity and light need to be a fundamental aspect of reality inherent to existence .. permeating and underlying all creation and communicating instantly and effortlessly ...

Response: As far as light is concerned, the universe has no dimension. It takes no time, from the perspective of a photon, to cross the breadth of the universe. To = Ts/(1 - v^2/c^2), where v=velocity, c=speed of light in a vacuum, Ts = Time, subjective and To = Time, objective. At v=c your tau factor (ratio of v to c) is 1, and To becomes undefined.