How are light-years measured?

[Jacob]

Radar measurements of the distance of the Moon don't require reflectors. Same goes for measuring the distance to Venus and some asteroids. The speed of light has also been measured since victorian times, and is very well known.

I have to say, none of your objections so far encourage me to think you have read any primers on the subject: are you really interested in finding out, or are you just pushing a preconceived notion that it's all rubbish?

I'm challenging the assumption that we know how far away distant galaxies are in terms of light years, since light does not carry with it a quality that can be measured to determine the distance it has traveled.

As for objections, I don't know what objections you might think that I have? Objections to what?

 

[gcthomas]

I'm challenging the assumption that we know how far away distant galaxies are in terms of light years, since light does not carry with it a quality that can be measured to determine the distance it has traveled.

As for objections, I don't know what objections you might think that I have? Objections to what?

Light gets dimmer as it travels, following an inverse square law. There are a variety of events we know the absolute luminosity of, so we can determine a distance by measuring the apparent brightness by the time the light reaches its.

But this has been mentioned before and you still make the same erroneous claim. So why do you feel the need to challenge well evidenced and theoretically strong ideas?

 

[PureX]

Light gets dimmer as it travels, following an inverse square law. There are a variety of events we know the absolute luminosity of, so we can determine a distance by measuring the apparent brightness by the time the light reaches us.

I went back through the thread, and I have found your explanations very … illuminating … , seriously. Thank you for taking the time.

 

[Jacob]

Light gets dimmer as it travels, following an inverse square law. There are a variety of events we know the absolute luminosity of, so we can determine a distance by measuring the apparent brightness by the time the light reaches its.

Would you differentiate between the light of a lazer and the light of a distant star here?

But this has been mentioned before and you still make the same erroneous claim. So why do you feel the need to challenge well evidenced and theoretically strong ideas?

Such as what? What do you think I am challenging?

I don't know what you are saying has been mentioned before and I don't know what you think I am making a claim about, erroneous or not.

 

[CabinetMaker]

Would you say that light can be modeled in terms of a wave, such that a width or length of period can be determined?

Yes, light has wavelengths which is why there are different colors of light. Light behaves like a wave and wave theory can be used to model it. But it also acts like a particle.

 

[Jacob]

Yes, light has wavelengths which is why there are different colors of light. Light behaves like a wave and wave theory can be used to model it. But it also acts like a particle.

What is the difference in the wave between a sound wave and a light wave?

When light behaves like a wave, how is this modeled? Would the light wave have a width perpendicular to the direction the light is traveling?

If light acts like a particle, what does this mean? Does it mean light is not a wave at the same time it is a particle? Is it that particles are in a chain from one location to the next, in a straight line?

I'm having trouble understanding both of these ideas, obviously.

 

[Stripe]

I'm saying theoretically it sounds great but I don't know what would be involved to make the determination accurate practically, or if the experiment that presumably has been set up has actually been done. Or, the existence of a set-up indicates a result has been determined (at least once or multiple times). Has this experiment been completed and verified?

All of this was described in Res' post.

 

[Jacob]

All of this was described in Res' post.

You commented on the set-up, and that is what I know of your quote.

 

[Stripe]

You commented on the set-up, and that is what I know of your quote.

You should go read post No. 50. :thumb:

 

[Jacob]

You should go read post No. 50. :thumb:

Here is post No. 50:

The universe looks isotropic so is assumed homogeneous, and the evidence is that it is expanding at an accelerating rate.

It can be both.

 

[Stripe]

:chuckle: OK, now try post 100.

 

[CabinetMaker]

What is the difference in the wave between a sound wave and a light wave?

When light behaves like a wave, how is this modeled? Would the light wave have a width perpendicular to the direction the light is traveling?

If light acts like a particle, what does this mean? Does it mean light is not a wave at the same time it is a particle? Is it that particles are in a chain from one location to the next, in a straight line?

I'm having trouble understanding both of these ideas, obviously.

Spend some time reading the Wiki link I provided above regarding the dual nature of light. It is far more complex than can be summed up in a few lines on a forum.

What it means is that the mathematical models used yo study light follow the rules of wave functions and particle theory.

 

[Jacob]

Spend some time reading the Wiki link I provided above regarding the dual nature of light. It is far more complex than can be summed up in a few lines on a forum.

What it means is that the mathematical models used yo study light follow the rules of wave functions and particle theory.

I'm asking about what I have been taught concerning light and how it can be modeled as a wave (or particle). New information not directed to answer a personal old information problem, when I am posing it as a question, does not solve it. It has to do with how we think about it.

When I first learned about light waves, I learned they are variable depending on the type of light. I also learned they are not like sound waves or pulse waves in water. They are more like stretching a slinky and watching the wave produced travel down the slinky and back again after hitting the slinky on its side near one of the ends that is being held. But is this only how it appears (like a sine wave?)?

 

[Jacob]

:chuckle: OK, now try post 100.

Here is what you posted in post 100.

The first successful entirely earthbound measurement of the speed of light was carried out by Hippolyte Fizeau in 1849. Fizeau's experiment was conceptually similar to those proposed by Beeckman and Galileo. A beam of light was directed at a mirror 8 km away. On the way from the source to the mirror, the beam passed through a rotating cog wheel. At a certain rate of rotation, the beam could pass through one gap on the way out and another on the way back. But at slightly higher or lower rates, the beam would strike a tooth and not pass through the wheel. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, the speed of light could be calculated. Fizeau reported the speed of light as 313,000 kilometres per second. Leon Foucault improved on Fizeau's method by replacing the cogwheel with a rotating mirror. Foucault's estimate, published in 1862, was 298,000 kilometres per second.

 

[CabinetMaker]

I'm asking about what I have been taught concerning light and how it can be modeled as a wave (or particle). New information not directed to answer a personal old information problem, when I am posing it as a question, does not solve it. It has to do with how we think about it.

When I first learned about light waves, I learned they are variable depending on the type of light. I also learned they are not like sound waves or pulse waves in water. They are more like stretching a slinky and watching the wave produced travel down the slinky and back again after hitting the slinky on its side near one of the ends that is being held. But is this only how it appears (like a sine wave?)?

They do act a bit sound waves or water waves in that, when passed through a double slit, the waves will add and cancel. On the other, when passed through a single slit, the light that strikes the screen displays the dispersion expected from a particle, not a wave.

 

[Jacob]

They do act a bit sound waves or water waves in that, when passed through a double slit, the waves will add and cancel. On the other, when passed through a single slit, the light that strikes the screen displays the dispersion expected from a particle, not a wave.

Does this have anything to do with "breaking light apart" as with a prism?

 

[Stripe]

Here is what you posted in post 100.

Now you know how the speed of light can be measured and therefore you should know to calculate how far a light year is.

 

[Jacob]

Now you know how the speed of light can be measured and therefore you should know to calculate how far a light year is.

But I don't know how far away a distant galaxy is, even if your method is the best method.

 

[Stripe]

But I don't know how far away a distant galaxy is, even if your method is the best method.

That is an entirely different matter to what you asked in OP and how I have been responding to your thread title.

 

[Jacob]

That is an entirely different matter to what you asked in OP and how I have been responding to your thread title.

Yes but the concern with a light year is in actually measuring something with it, or in its inability to measure these things.