1. we know that the farther away the objects are from us the faster they are travelling and by this we know that neutron stars are moving at a great velocity but only relative to us but if we suppose that we measure the distance of milky way galaxy from neutron stars (or whatever thing which is far away and travelling at a very high speed), wouldnt it also show that we are moving at really great speed relative to neutron star . which one of this is correct are we moving very fast or the neutron stars (or some thing which is really far away and travelling at a very high speed.)

2.

3. which one of this is correct are we moving very fast or the neutron stars (or some thing which is really far away and travelling at a very high speed.)
It's all relative. If you were on the neutron star, you could say that you were stationary and the earth was moving. If you were on the earth, you can say that the earth is motionless and the neutron star was moving.

All motion is relative to something else. There is no absolute frame of reference to judge motion by.

4. Thats right i understand it completely. If i measure earths speed relative to neutron stars i will find the speed to be very great (maybe at 90%of speed of light) that means the mass of our earth must be greater than the original one, and all the effects which takes place when we nearly reach the speed of light must also take place but we do not find any of these effects existing. Why is that so

Thats right i understand it completely. If i measure earths speed relative to neutron stars i will find the speed to be very great (maybe at 90%of speed of light) that means the mass of our earth must be greater than the original one, and all the effects which takes place when we nearly reach the speed of light must also take place but we do not find any of these effects existing. Why is that so
No, apparently you don't understand it.

First of all, the idea of relativistic mass is misleading and for that reason, isn't used anymore. It's not mass that increases, it's energy.

Second, all motion is relative. When you say the earth moving at 90% of c, that's as measured from the neutron star. Effects such as time dilation and length contraction and the increase in energy affecting the earth are only measured from the neutron star. From the Earth's frame of reference, we are stationary, and there are no relativistic effects. OTOTH, we would see the neutron star as moving at 90%c, and the concurrent relativistic effects would be seen by us on the star.

There is no absolute frame of reference to measure velocity in, it's all relative to something else.

Thats right i understand it completely. If i measure earths speed relative to neutron stars i will find the speed to be very great (maybe at 90%of speed of light) that means the mass of our earth must be greater than the original one, and all the effects which takes place when we nearly reach the speed of light must also take place but we do not find any of these effects existing. Why is that so
Any such relativistic effects are only seen from the point of view (frame of reference) of the "stationary" observer.

So from the PoV of that neutron star the earth would be more massive (with the caveat that AlexG makes), length contracted, time dilated, etc.

Form the PoV of the Earth that neutron star would appear more massive (with the caveat that AlexG makes), length contracted, time dilated, etc.

7. Thats right but i do still have doubts.

Now its not about neutron stars. Assume a galaxy which is very far away from earth. If one measures the speed the far away galaxy is travelling with, we find it to be very great,but the problem is if one is to measure the speed of our earth from that faraway galaxy they find it to be great too,but we know thats false,it shows that either their prediction of the speed of earth is false ,or our prediction of that galaxy's speed is false, which one of these is true.

Assume a galaxy which is very far away from earth. If one measures the speed the far away galaxy is travelling with, we find it to be very great,but the problem is if one is to measure the speed of our earth from that faraway galaxy they find it to be great too,but we know thats false,it shows that either their prediction of the speed of earth is false ,or our prediction of that galaxy's speed is false, which one of these is true.
I am not sure what you mean "we know that's false". We know that all motion is relative.

Speed can only be defined relative to something else.

Therefore we know that we can either consider them stationary and us moving, or we can consider us stationary and them moving away, or we can consider some point in between as stationary and both of us moving away from that point.

These are all equivalent. They all produce the same results. There is no way to tell them apart. They are all "true".

9. we know the speed with which earth is moving . But relative to faraway galaxy we find the speed to be very great . for example we can show that the speed of a galaxy which is faraway to be very great ,but if one measures the speed of earth from that galaxy , we find it also to be great . suppose i measure the speed of our earth from that galaxy and i find it to be half of speed of light( but you surely know that my measurements are false) and it also shows that our measurement on speed of that faraway thing might be false also, so i think assuming that, faraway thing's are at great speed is inconsistent or probably false.

we know the speed with which earth is moving .
Relative to what? There is no single, true, absolute speed of the Earth. Do you mean the speed relative to the Sun? Or the speed relative to the center of our galaxy? Or the speed relative to Andromeda? Or the speed relative to a distant galaxy? There are all different and all equally true.

suppose i measure the speed of our earth from that galaxy and i find it to be half of speed of light, but you surely know that my measurements are false
Why are they false? You don't seem to understand the basic idea that speed is relative. You can only define speed relative to something else. From our perspective on Earth, it is not moving at all; it looks like the sun and moon go round the Earth.

11. you dont seem to understand my question.

Here i will put it this way so you can understand it clearly.

Suppose i am standing on a road and a vehicle passes by the direction that i am facing on, now assume another vehicle with different speed from the first one passes simultaniously with the first one but in opposite direction. Now if the first vehicle guy measures my speed relative to him he gets a particular value and this value will not be the same when the other guy measures my speed relative to him. So my question is how can one person have two different velocity (when we know that he is still). I clearly understand that its all relative but then how can you be sure that neutron stars are travelling at 90 percent of c.For some one farther away from us and travelling at different speed measure the speed of neutron stars to be different . This shows that no one can predict the actual velocity of a particle.Then how can one be sure that where the particle is and with what speed is it travelling if everything is measured relative to something else.

12. You say you understand that it's relative but you still appear to assume that there is some absolute reference frame.

you dont seem to understand my question.
You don't seem to understand my answers.

Suppose i am standing on a road and a vehicle passes by the direction that i am facing on, now assume another vehicle with different speed from the first one passes simultaniously with the first one but in opposite direction. Now if the first vehicle guy measures my speed relative to him he gets a particular value and this value will not be the same when the other guy measures my speed relative to him. So my question is how can one person have two different velocity (when we know that he is still).
This is good, because this is just "Galilean relativity" which is much simpler.

You say: "when we know that he is still". But he is only still relative to the road. He is not still in any absolute sense. He is rotating with the Earth at nearly 1,000 MPH. He is moving rund the Sun at enormous velocity. You must measure the velocity relative to something.

Relative to the ground he is still.

Relative to the first car he is moving at 30MPH (for example)

Relative to the other car he is moving at 45MPH (for example)

I clearly understand that its all relative
Clearly not, or you wouldn't have asked that question.

Look at it this way. You are in a spaceship travelling away from the Earth at a constant velocity of 1,000 miles per hour. Now nothing you do inside that spaceship can tell you your velocity. The only way you can measure your velocity is to look at the Earth and see how fast you are moving away from it. Because you can't tell that you are moving, it will appear that the Earth is moving away from you at 1,000 mph.

Nothing you do on the spaceship can distinguish between you mving away from the Earth or the earth moving away from you.

Your velocity can only be measured relative to something else.

but then how can you be sure that neutron stars are travelling at 90 percent of c.
It depends what you measure the speed relative to. (I don't think there are any neutron stars that we can observer that are travelling that fast relative to us, but I could be wrong).

For some one farther away from us and travelling at different speed measure the speed of neutron stars to be different .
Yes. Because speed is relative (as you "clearly understand").

This shows that no one can predict the actual velocity of a particle.
You can if you say what that actual velocity is relative to (do you begin to see a pattern here?)

Then how can one be sure that where the particle is and with what speed is it travelling if everything is measured relative to something else.
You can do that, if you know the speed and position of that "something else".

14. but the best thing to do would be to measure the speed of that particular body with respect to speed of light. Since light speed is constant it can be used to measure the velocity of an object very accurately.

but the best thing to do would be to measure the speed of that particular body with respect to speed of light. Since light speed is constant it can be used to measure the velocity of an object very accurately.
The speed of light is the same for all observers. So I don't know how you can define speed relative to it.

In other words, if your spaceship was flying away from earth at 99.99% c (relative to Earth) you would not see light moving at 0.01% c; you would see light moving at c.

16. But theory of relativity predicts that every observer irrespective of his speed measures the speed of light to be same. So it is common sense to use light as constant and measure the speed with which a body is travelling with respect to speed of light. But light measured with respect to light is zero since both velocities are equal to zero.

But theory of relativity predicts that every observer irrespective of his speed measures the speed of light to be same. So it is common sense to use light as constant and measure the speed with which a body is travelling with respect to speed of light. But light measured with respect to light is zero since both velocities are equal to zero.
Which is why it makes no sense to do that. Everyone who measured their speed relative to light would get the same answer because, by definition, the speed of light is the same for all of them.

You can only measure speed relative to some other object.

18. which is why we can measure a body's speed with respect to light. That would be treated as absolute speed which is better than relative speed. Relative speed is never correct it changes with change in speed of a body but by measuring the speed with respect to speed of light this conflict can be resolved.

19. How do you suggest you measure speed relative to light?

Suppose A and B and moving apart at 50% of the speed of light.

Person A measures the speed of light: it is c.

Person B measures the speed of light: it is c.

What does this tell you about their speeds?

20. it is always c. it must be clear that irrespective of the observer's speed he measures the speed of light to be c. Consider that i am moving at certain speed now if i measure my speed relative to other objects i will get many answer's . But if you measure your speed with respect to speed of light you'll get only one answer because of its consistent velocity. Light can be treated as an absolute reference frame (which is ofcourse not still but it is constant).

it is always c. it must be clear that irrespective of the observer's speed he measures the speed of light to be c. Consider that i am moving at certain speed now if i measure my speed relative to other objects i will get many answer's . But if you measure your speed with respect to speed of light you'll get only one answer because of its consistent velocity.
You might get only one answer. But everybody will get the same answer. Therefore everybody is moving at the same speed? Does that make sense to you?

22. every body gets the same answer if they measure the speed of light at whatever the velocity they are travelling with . But not their speed with respect to speed of light. It completely differ's .You are only concentrated in measuring the speed of light . But i am saying that we can measure our speed with respect to speed of light. How can every one get the same answer if they are travelling at different speed's .How can you think that every body is moving at same speed. Your case is possible only if we measure the speed of earth with respect to light.

every body gets the same answer if they measure the speed of light at whatever the velocity they are travelling with . But not their speed with respect to speed of light.
Can you explain how I would measure my speed with respect to the speed of light. I am really struggling to understand what this means.

I start by measuring the speed of light. (Maybe in three different directions?) And then ... what do I do?

But theory of relativity predicts that every observer irrespective of his speed measures the speed of light to be same.
You left out an important part of that: as measured relative to themselves.

For example, if you have two observers, each moving at 1/2c relative to each other. at the moment they are next to each other they each turn on a flashlight. Each of them measure the both light beams as traveling neck and neck and both will measure the beams as traveling at c with respect to themselves. IOW, after one microsecond after the lights turn on, observer A will note the both beams have extended 300 m from him and after one microsecond, observer B will note that the beams have extended 300 m from him

If you are in the spaceship traveling at 1/2c away from the Earth and tried to perform an experiment to measure your velocity with respect to the speed of light, you would get the same answer as if you did it while at rest with respect to the Earth. If you perform the experiment, accelerate your ship and perform it again, you get the same result both before and after acceleration.

In fact, back in the 19th century, when it was thought that one could measure your speed in such a way, a famous experiment was performed to measure the Earth's own speed. The fact that this experiment came up with a null result, even though it was sensitive enough to detect Earth's motion around the Sun, was one of the first inklings we got that eventually led to Relativity. This same experiment has been done many times since then with more and more sensitive equipment with the same result.

every body gets the same answer if they measure the speed of light at whatever the velocity they are travelling with . But not their speed with respect to speed of light. It completely differ's .You are only concentrated in measuring the speed of light . But i am saying that we can measure our speed with respect to speed of light. How can every one get the same answer if they are travelling at different speed's .
Because that is what Relativity predicts happens and that's what happens in actual real life experiments.

26. Janus you have not understood my point of view. Yes its ofcourse true that every body gets the same answer if they measure the speed of light, irrespective of their speed.But i am saying how can one get the different answer if one measure's one's speed with respect to speed of light. Be wary that speed of light is alway's constant. The speed will only differ if we measure it relative to different object's which are travelling at different speed . But this is not the case here I am only saying to measure the speed with respect to light which is always constant.

But this is not the case here I am only saying to measure the speed with respect to light which is always constant.
Can you please explain how you think this is possible. How, exactly, do I measure my "speed relative to the speed of light"? Please explain.

And, because you "clearly understand" that speed is relative, please explain what the difference is between "measuring the speed of light relative to me" and "measuring my speed relative to the speed of light"

28. Why are you changing the discussing topic.First of all i need to know how do you measure a body's speed relative to other.

Why are you changing the discussing topic.
I am not changing the topic.

You said:
which is why we can measure a body's speed with respect to light.
I am asking how you measure a a body's speed with respect to light. Can you please, please explain this.

First of all i need to know how do you measure a body's speed relative to other.
Many ways: bounce radar signals off it, measure Doppler shift, count how many milestones it passes, ...

30. Then how do you know that neutron star's are at 90 percent of c.

Then how do you know that neutron star's are at 90 percent of c.
Which neutron star? I do not know of any neutron stars that are moving at this speed relative to Earth. Could you provide a reference so we know what you are talking about.

which is why we can measure a body's speed with respect to light.
Can you please explain how you measure a a body's speed with respect to light.

32. This article reports on some fast moving neutron stars: Neutron star clocked at mind-boggling velocity - space - 11 August 2006 - New Scientist

These are moving at much less than 1% of the speed of light. The speed is determined by measuring the angular distance they move in a given time. This is obviously speed relative to earth (as that is where we are measuring it from). The same way you could work out the speed of a car going past.

33. A neutron star has been clocked travelling at more than 1500 kilometres per second. It joins the ranks of other fast moving neutron stars, deepening the puzzle over how these dense stellar corpses are accelerated to such astonishing velocities.
Neutron stars are the city-sized spheres that remain after stars are destroyed in supernova explosions. They are incredibly dense - a teaspoonful of neutron star material would weigh a billion tonnes.
Many neutron stars are now known to travel at speeds of hundreds of kilometres per second, with one shown in 2005 to be moving at 1100 km/s (see Fastest pulsar set to escape the Milky Way). Some others have been estimated as travelling faster than 1500 km/s but with less certain measurements: their speeds were measured in an indirect way, based on observations of their effect on the gaseous medium that they move through.
Astronomers have had a hard time figuring out how neutron stars get accelerated to such blistering speeds. Their theoretical models can produce speeds of a few hundred kilometres per second, but these suggest that neutron stars should rarely, if ever, reach more than 1000 kilometres per second.
The neutron star now found to be zipping along at 1500 km/s is providing an even bigger challenge for the models. Its speed was measured by Frank Winkler of Middlebury College in Vermont and Robert Petre of NASA's Goddard Space Flight Centre in Greenbelt, Maryland, both in the US.

34. The fact above is not my opinion its every cosmologist's opinion.

A neutron star has been clocked travelling at more than 1500 kilometres per second. It joins the ranks of other fast moving neutron stars, deepening the puzzle over how these dense stellar corpses are accelerated to such astonishing velocities.
Neutron stars are the city-sized spheres that remain after stars are destroyed in supernova explosions. They are incredibly dense - a teaspoonful of neutron star material would weigh a billion tonnes.
Many neutron stars are now known to travel at speeds of hundreds of kilometres per second, with one shown in 2005 to be moving at 1100 km/s (see Fastest pulsar set to escape the Milky Way). Some others have been estimated as travelling faster than 1500 km/s but with less certain measurements: their speeds were measured in an indirect way, based on observations of their effect on the gaseous medium that they move through.
Astronomers have had a hard time figuring out how neutron stars get accelerated to such blistering speeds. Their theoretical models can produce speeds of a few hundred kilometres per second, but these suggest that neutron stars should rarely, if ever, reach more than 1000 kilometres per second.
The neutron star now found to be zipping along at 1500 km/s is providing an even bigger challenge for the models. Its speed was measured by Frank Winkler of Middlebury College in Vermont and Robert Petre of NASA's Goddard Space Flight Centre in Greenbelt, Maryland, both in the US.
That appears to be from this article: Neutron star clocked at mind-boggling velocity - space - 11 August 2006 - New Scientist

These are moving at about 0.5% of the speed of light (not 90%). As the article says, the speed is determined by measuring the angular distance they move in a given time. This is obviously speed relative to earth (as that is where we are measuring it from).

36. No matter how you are moving through space, when you measure the speed of light, you will always get the same answer. So that measurement will tell you nothing about how fast you are moving.

First of all i need to know how do you measure a body's speed relative to other.
Let us imagine that we have two spacecraft. Both ships are moving in the same direction. We are in one of them. The other ship is moving faster than we are, and as it passes us from behind, we use a stop watch to time how long it takes to pass us. Since we know how long our ship is, knowing how much time it takes to completely pass us will give use the speed of the other ship. This gives us the speed of the other ship relative to our ship. Now let us say that our ship is moving twice as fast as it was moving before, but the other ship, moving at the same speed as before, is still moving faster than us. When we use the stopwatch to time the other ship passing us, it will take longer to pass us. so RELATIVE TO OUR SHIP the other ship is moving slower than before.

Now let us imagine that on the other ship, they try to measure their speed, relative to the speed of light. They do this by measuring the speed of light, thinking "since the speed of light is constant, if we measure it and find it is 1/2 c, that must mean that we are moving at 1/2 c". So they measure it, and the result they get is not 1/2 c, but c.
Now we, on our ship, which is moving slower than their ship (it passed us, so we know it's moving faster) also measure the speed of light, and even though we're moving slower than they are, we get the same answer, the speed of light is c.

Since regardless of how the ships are moving, they both get the same result when measuring the speed of light, it tells them nothing about their own speed.

Now let us imagine that both ships are moving in space, at the same speed, side by side. How do you measure the speed of the ships? You can't measure the speed of light to find out how fast you are going, since no matter what your speed, you will always get the same answer. You can only measure your speed by comparing it to something else, and since both ships are motionless with respect to one another, their relative speed is zero.

You can only measure velocity by comparing it to something else. That's what is meant by all speed is relative.

37. Originally Posted by Strange
Then how do you know that neutron star's are at 90 percent of c.
Which neutron star? I do not know of any neutron stars that are moving at this speed relative to Earth. Could you provide a reference so we know what you are talking about.
The neutron star moving at 90% c is an imaginary object, used only for this discussion. He is not claiming that there is an actual neutron star moving at that relative velocity.

38. If i have claimed that the neutron stars are moving at 90 percent of the speed of light i wouldnt be so dumb to post that article. It is all good even with 0.5 percent of c . My question is how do you measure the speed of neutron star to be 0.5 percent of c. Your answer will probably be "I measure the speed of neutron star relative to earth since thats the only way " but for some one who is measuring the speed from pluto he will measure the speed(relative to pluto) to be even more than or less than 0.5 percent of c which depends on its distance. How are you sure that its surely 0.5 of c. Its correct because when the speed of neutron star is compared with speed of light we find it to be half of it ,thats the way we find its speed .

39. but 0.5 percent of c also makes sense. I didnt say that these guy's are cosmologist's please do read it once more ,and i meant the real cosmologist's. Now i think some one need's to understand LANGUAGE properly before commenting.

Janus you have not understood my point of view. Yes its ofcourse true that every body gets the same answer if they measure the speed of light, irrespective of their speed.But i am saying how can one get the different answer if one measure's one's speed with respect to speed of light. Be wary that speed of light is alway's constant. The speed will only differ if we measure it relative to different object's which are travelling at different speed . But this is not the case here I am only saying to measure the speed with respect to light which is always constant.
It is the speed of light with respect to yourself that is always constant or conversely, it your speed with respect to light that is always constant. You seem to be insisting that there is some absolute frame that light has a constant speed with respect to and thus by measuring our speed with respect to light, we can determine our speed with that frame. This is not the case.

It is not that I and others don't understand your point of view, it is that we are trying to explain to you that it is mistaken and based on a false premise.

If, light speed was constant in the manner you are suggesting, then yes, you could determine your speed by measuring your speed with respect to it. However, light speed is not constant in this manner, as has been demonstrated in real life by actual experiment.

It is that very fact that light always travels at the same speed with respect to you that leads to the effects predicted by Relativity.

41. with due respect
phdemon

0.5 percent of c also makes sense. I didnt say that these guy's are cosmologist's please do read it once more ,and i meant the real cosmologist's. Now i think some one need's to understand LANGUAGE properly before commenting ,and for your kind information no one till now in this forum have excepted what i think .So i think its not right on your part to think that they are cosmologists.

42. dont take me wrong janus the last message was not meant to you . I am only pinpointing my view's and objecting the view's that are against mine, since i am not convinced properly.If afterall my view's are false i would gladly accept them after i have my doubt's cleared.

If i have claimed that the neutron stars are moving at 90 percent of the speed of light i wouldnt be so dumb to post that article. It is all good even with 0.5 percent of c . My question is how do you measure the speed of neutron star to be 0.5 percent of c. Your answer will probably be "I measure the speed of neutron star relative to earth since thats the only way " but for some one who is measuring the speed from pluto he will measure the speed(relative to pluto) to be even more than or less than 0.5 percent of c which depends on its distance. How are you sure that its surely 0.5 of c. Its correct because when the speed of neutron star is compared with speed of light we find it to be half of it ,thats the way we find its speed .
But if you measured its speed with respect light from Earth, and measure its speed with respect light from Pluto, you still get two different answers.

If that Neutron star is heading away from the Earth at 0.5c and we measure its speed with respect to light we get an answer of 0.5c

However, if we were in a spaceship chasing after that neutron star traveling at 0.25c with respect to the Earth and measured the neutron stars speed with respect to the same light, we would get an answer of ~0.2857c, not 0.5c. (we don't get 0.25c either because of the way velocities add and subtract under Relativity.)

That's simply the way the universe works.

44. The Earth moves around the Sun at ~30 km/s.
The Sun orbits the galaxy at ~ 220 km/s.
Our galaxy is moving relative to the local CMBR rest frame at ~600 km/s.
Our galaxy is moving relative to a galaxy on the other side of the observable universe at more than twice the speed of light!

There is no absolute frame of reference against which speeds can be measured, as speed is a relative concept.

45. with due respect
phdemon
if i am mistaken i would gladly accept them as said earlier. But please do allow me to point my view's with out misunderstanding.

My question is how do you measure the speed of neutron star to be 0.5 percent of c.
The article explains how the speed was measured:
Originally Posted by New Scientist
they took snapshots five years apart of the neutron star ... They found it to be moving 44 millionths of a degree per year. Given the estimated distance of 6500 light years to the supernova remnant, this translates into about 1500 kilometres per second.
OK?

Obviously, if you measured the speed from a different place (Pluto, Alpha Centauri, whatever) you would get a different answer.

Imagine someone walking along in a moving train. For the people on the train he will be going at 3 mph. For people on the ground, who see the train pass at 40mph, the man will be moving at 43 mph. Speed is relative.

47. k ,Then why does one use light years as an unit to measure the distance faraway and why does'nt one go with the actual method by using miles.

48. Do you agree this statement "every person irrespective of his speed has to measure the speed of light to be same." There need not be any reference frame that light is constant with when it is clearly stated that "with respect to any frame of reference the light speed is always constant" doesnt this make it absolute . Moreover one might have used miles instead of light years to measure distance's faraway. The above statement clearly states the speed of light is constant every where and every time and that it doesnt need to be calculated with respect to something else.

k ,Then why does one use light years as an unit to measure the distance faraway and why does'nt one go with the actual method by using miles.
Because it's more convenient to calculate using 1 light year than it is using 5,900,000,000,000 miles.

50. The above statement clearly states the speed of light is constant every where and every time and that it doesnt need to be calculated with respect to something else.
That is correct, but it applies only to light. All other speed of mass is relative to some other mass.

51. but why only light.

but why only light.
Because light is massless. All massless particles (i.e. photons) travel at the speed of light and only at the speed of light. The invariance of the speed of light in all frames is shown mathematically in Maxwell's Equations.

53. You are wrong its not because 5,900,000,000,000 is complicated (but i do agree that it is a bit) it is measured in light years because light speed is constant with respect to any frame of reference . This shows that indirectly we are measuring speed with respect to speed of light.Suppose you are travelling in a spaceship at c ,then how will you come to know that you are travelling at c.

54. First of all do you agree that speed of light is the distance travelled by it divided by the time taken.It seems that you do agree on the distance travelled by light and time taken, but not on its speed.

You are wrong its not because 5,900,000,000,000 is complicated (but i do agree that it is a bit) it is measured in light years because light speed is constant with respect to any frame of reference . This shows that indirectly we are measuring speed with respect to speed of light.Suppose you are travelling in a spaceship at c ,then how will you come to know that you are travelling at c.
No, you're wrong. We measure the distance by calculating how far light will travel in 1 earth year. We use the speed of light, 299,792,458 km/sec and 31,557,600 seconds. A light year is really an arbitrary measurement, that's simply easy to use, given the distances we deal with. it's easier to say that one parsec is 3.26 light years instead of 3085677580000000 km. It's easier to use 1 ly as opposed to 946073047258080 km.

We do not measure light with respect to light, we measure light with respect to us.

First of all do you agree that speed of light is the distance travelled by it divided by the time taken.It seems that you do agree on the distance travelled by light and time taken, but not on its speed.
I'm not sure how you conclude this. The speed of light is 299,792,458 km/sec in all frames of reference. I've said that several times.

57. then what's the wrong in saying that i am travelling at 0.00000000000576 percent of speed of light.

58. you are right on measuring how much light will travel in one year, but why do you only use light, you can also use the velocity of sun and conclude that an object is 100 sun years faraway where one sun year will be the distance travelled by the sun in one year. My question is why only light year, why not something else as stated above.

My question is why only light year, why not something else as stated above.
You can use any measure of distance you like. There is nothing special about "light year" as a unit of distance. You can use miles or kilometers or furlongs or astronomical units. It makes no difference.

For very large scales, the light-year is inconveniently small and you might use megaparsecs or something else. The units are just chosen for convenience.

then what's the wrong in saying that i am travelling at 0.00000000000576 percent of speed of light.
Absolutely nothing. You could mark the dial in you car in units of fractions of the speed of light if you want.

But you would still have the same effect you mentioned in post #10. If you are standing by the road and two cars measure your speed relative to them, then they will get different results, whether they use mph or percent of c:

Relative to the ground you are is still.

Relative to the first car you are moving at 30MPH or 0.000004% c (for example)

Relative to the other car you are moving at 45MPH or 0.000007% c (for example)

The choice of units makes no difference. Speed is still relative, not absolute.

60. nope , you are wrong . In practice time can be measured more accurately than distance ,so the unit light year has come to use. The fact that light travels at a constant speed also contributes ,and hence light year is used as an unit to measure distance. As said earlier sun year cannot be used because it changes with the frame of reference,but this is not the case here since light speed is constant with respect to every frame of reference in the universe.

61. No, distances can be measured down to nanometers, every bit as accurate as our time measurements.

I'm getting the feeling that we've seen these same arguments, and this same inability or unwillingness to understand relative velocities in another poster.

nope , you are wrong . In practice time can be measured more accurately than distance ,so the unit light year has come to use.
Even if time can be measured more accurately than distance (is that true?) how is that relevant to the choice of light year? It would be hard to measure the distance to your local shop accurately using light years.

But why do you keep jumping all over the place with this thread?

Can we just summarise where we are so far. We have all agreed on the following::

1. There are no neutron stars that travel at 90% of c.

2. There are neutron stars that have been seen travelling at 1500 km/s or 0.5% c. This has been measured directly by looking at the distance moved over some period of time.

3. The speed of an object is purely relative. Different observers may measure different speeds of something. This is just Gallilean relativity.

4. There is no way for someone to determine their own speed in isolation. It can only be measured relative to something else.

5. Everybody who measures the speed of light will get the same result.

6. We can use any suitable units for measuring distance (or speed): light years, miles, nanometers, furlongs, astronomical units, megaparsecs, etc. They can all be converted into one another by multiplying by the appropriate scale. We just choose the scale most relevant to the problem we are working on.

Is that everything?

63. You edited the large bold font into your post?! Really?

nope , you are wrong . In practice time can be measured more accurately than distance ,so the unit light year has come to use. The fact that light travels at a constant speed also contributes ,and hence light year is used as an unit to measure distance. As said earlier sun year cannot be used because it changes with the frame of reference,but this is not the case here since light speed is constant with respect to every frame of reference in the universe.
The "sun year" is a measurement of time, not distance.

The light year is a measure of distance. It will change with frame of reference. For example, we measure the distance to Alpha Centauri as 4.3 light years. But someone passing by the Earth at 97.26% c would measure the distance as 1 light year (I think).

So the light year does not give a magic, absolute measure of distance.

Edit: maybe that should be 88% c (I need a coffee)

nope , you are wrong .
How do you support this?

66. never will i do something so shameful. I just typed down my point and made it bold. Even if i had edited , the discussion wouldnt have been so long.

67. light year is the distance travelled by the light in one year.If an object is said to be 5 light years away it is the distance travelled by light in one year multiplied by 5. The same explanation goes for sun year . It would just be the distance travelled by sun in one year multiplied by 5. What did you mean by saying sun year is the measurement of time and not distance.

light year is the distance travelled by the light in one year.If an object is said to be 5 light years away it is the distance travelled by light in one year multiplied by 5. The same explanation goes for sun year . It would just be the distance travelled by sun in one year multiplied by 5. What did you mean by saying sun year is the measurement of time and not distance.
Sorry, I thought you meant solar year (365 days).

never will i do something so shameful. I just typed down my point and made it bold. Even if i had edited , the discussion wouldnt have been so long.
Huh? I saw your post before you editied and after. Whatever man. And the claim about discussion length has no relevance to the bold large font you put in. You make no sense.

Sun Year:

light year is the distance travelled by the light in one year.If an object is said to be 5 light years away it is the distance travelled by light in one year multiplied by 5.
Or 0.00000153297423 megaparsecs.
Or 47302642000000 km.
Or 235139993000000 furlongs.
Or 1862308740000000000 inches.

So what? We just use the most convenient unit.

71. do you think that saying something is travelling at 0.00000000000056776679 percent of speed of light is wrong.

73. no thats not it. yeah i do agree your point but ,if i say that i am at the speed of 0.00000000000009877 percent of light wouldnt it also mean that i am comparing my speed with the speed of light.

do you think that saying something is travelling at 0.00000000000056776679 percent of speed of light is wrong.
Of course not. That is about 1700 nm/s. Pretty slow.

But of course you still need to say what that speed is relative to. For example, if that is the speed of a (very slow) snail in my garden, then relative to the sun, that snail (and the Earth) is moving at 0.009% c (100000 km/h).

no thats not it. yeah i do agree your point but ,if i say that i am at the speed of 0.00000000000009877 percent of light wouldnt it also mean that i am comparing my speed with the speed of light.
Yes, but it is still measured relative to something else.

76. now suppose that space is empty (completely) except a big chunk of metal in it,now how would you measure its speed and which will you choose as frame of reference to measure metal's speed.

now suppose that space is empty (completely) except a big chunk of metal in it,now how would you measure its speed and which will you choose as frame of reference to measure metal's speed.
You can't. There is nothing to compare its speed to.

You could consider it to be stationary. You could consider it moving at 5 mph or 99.99% of the speed of light. It makes no difference.

How would you measure its speed?

78. yes, now assume the big chunk to be halved unsymmetrically .Now how would you measure the speed of big piece of metal.

yes, now assume the big chunk to be halved unsymmetrically .Now how would you measure the speed of big piece of metal.
I don't really understand what you mean. Do you mean it is cut into two (unequal sized) pieces? Are they stationary with respect to one another or are they moving relative to one another?

now suppose that space is empty (completely) except a big chunk of metal in it,now how would you measure its speed and which will you choose as frame of reference to measure metal's speed.
How would you measure its speed?

80. just assume what i said in my 79th post. If a ray of light is to travel in the direction in which the big chunk is travelling then you would definitely measure its speed relative to light wouldnt you , and here you can see that the speed of light is actually the frame of reference.(totally forget my last post)

81. You would be measuring it against You.

82. only if i am present there.

only if i am present there.
Well if you're not present in some form to make the observation, you won't be making any measurement, will you?

just assume what i said in my 79th post. If a ray of light is to travel in the direction in which the big chunk is travelling then you would definitely measure its speed relative to light wouldnt you , and here you can see that the speed of light is actually the frame of reference.(totally forget my last post)
The speed of light relative to the chunk is c. So how fast is the chunk moving?

do you think that saying something is travelling at 0.00000000000056776679 percent of speed of light is wrong.
Traveling at 0.00000000000056776679 percent of speed of light Relative to what?

86. it is very clear from the statement that it is relative to light. You sound like light needs some material medium to travel. 0.5 percent of light says that the bodies speed is measured relative to light.

it is very clear from the statement that it is relative to light. You sound like light needs some material medium to travel. 0.5 percent of light says that the bodies speed is measured relative to light.
No, it just says the speed is reported as a proportion of the speed of light.

just assume what i said in my 79th post. If a ray of light is to travel in the direction in which the big chunk is travelling then you would definitely measure its speed relative to light wouldnt you , and here you can see that the speed of light is actually the frame of reference.(totally forget my last post)
The speed of light relative to the chunk is c. How fast is the chunk moving?

it is very clear from the statement that it is relative to light. You sound like light needs some material medium to travel. 0.5 percent of light says that the bodies speed is measured relative to light.
You can't measure speed relative to light because light is invarient in all frames. Since you will always measure light to have the same velocity, regardless of your motion through space, you will always measure your speed relative to light as zero.

89. The speed of light relative to the chunk is c. So how fast is the chunk moving?
Your question is like , light speed is c and i am running . So what is my speed. How can you answer it . I am only saying that it can be measured by taking light as constant and comparing my speed with it.

90. I am only saying that it can be measured by taking light as constant and comparing my speed with it.
Your speed, compared to light will always be zero.

Your question is like , light speed is c and i am running . So what is my speed. How can you answer it . I am only saying that it can be measured by taking light as constant and comparing my speed with it.
So if all you are saying is that speed can be expressed as a fraction of the speed of light then that is obviously true. And completely pointless. You can say 1500km/s or 0.5%c. It makes no difference. Either way you need to say what the speed is relative to. Can we at least agree to that?

What was the point of this then:
now suppose that space is empty (completely) except a big chunk of metal in it,now how would you measure its speed and which will you choose as frame of reference to measure metal's speed.
You asked "how would you measure its speed" so: how would you measure its speed?

92. k what ever .Your reply's say that no one of you has understood my point.

Change in the topic

As big bang is the start of every thing wouldnt it mean that time also started with big bang.

k what ever .Your reply's say that no one of you has understood my point.

Change in the topic

As big bang is the start of every thing wouldnt it mean that time also started with big bang.
Don't change topic in the middle of a thread. You've already started a thread on the Big Bang.

As for your point, everyone has understood that you're wrong. You don't seem to have understood what everyone has been telling you for the last 90 posts or so.

k what ever .Your reply's say that no one of you has understood my point.

You keep saying "speed can be measured by comparison to the speed with light".

Can you PLEASE explain how you think this can be done.

Change in the topic
NO!

95. Speed of light is 299792458 meters/second. An object moves at half the speed of light which is 149896229 meters/second relative to another moving object which is moving at half the speed of former (relative to third,[which is stationary relative to first two]) which would be 749481114.5 meters/second. In this case both the moving objects (and the third one) regardless of their different speeds relative to each other measure the speed of light as c.

If I understand this correctly then I clearly see the flaw in op's question. I think the flaw in his question can be explained to him if someone explains the reason for the speed of light always being c regardless of speed on any moving object.

96. The OP needs to go back and start again from basic principles.

I would suggest a good place to start would be "Galilean invariance".

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