Seeing things in their past? You are full of beans!

[TrueBeliever37]

Yes, and you can use this formula in every valid frame (not a limiting one because such limiting ones are not valid).



We can apply it in every frame. If you go *between* frames, you will get nasty inconsistencies.



I mentioned the limiting 'frame' and what happens in that limit. But, again, there is no valid frame moving at the speed of light.

So let's first get things straight for the frames that *are* valid. Different frames will measure different distances and different time intervals. But when they watch light move, it always has the same distance/time.

Hi Polymath,

What I am trying to find out is why it is ok to use C=distance/time to determine time from sun to earth, but the same equation is not allowed to be used to determine time from star to another planet.

 

[Polymath257]

How is it ok to use the formula to determine the time it takes to get from the sun to earth. But using it to determine how long it takes to get from a star to another planet any different?

Both are looking at time from a star to a planet.

And nobody has said any other formula is required. But the distance and the time involved have to be measured in the same reference frame.

It's just using a constant and a distance and determining a time. Why is it ok in one instance, but then off limits in the other? The speed of light is involved in both cases, what makes one limiting and the other not?

Because you are wanting to know what the 'light experiences'. That would involve looking at a reference frame moving at the speed of light. There is no real reference frame that does so. The only way to make sense of the question is to take the limit of frames going slower than light as the speed goes to that of light.

 

[Polymath257]

Hi Polymath,

What I am trying to find out is why it is ok to use C=distance/time to determine time from sun to earth, but the same equation is not allowed to be used to determine time from star to another planet.

Who said it is not allowed? You have to use the distance and the time *as measured in the same frame*. When you do that, the speed of light will always come out to be the same.

 

[Polymath257]

No they can actually take a ruler and measure the exact specific physical length. They might be able to use that information while using the other frame of reference, but it is not the true physical length. I think you know that, but it conflicts with your theory so you can't acknowledge it.

No, I don't 'acknowledge' it because it is wrong.

So, suppose that you see something moving past you at 50% of the speed of light. Are you saying you cannot determine its exact physical length? Are you saying we have to be right next to something in order to determine its exact physical length?

You also get to use your ruler to find an exact physical length. For example, you can watch the ruler and the pen as you go past and take a picture exactly when they are lined up.

The point is that we can and do perform exactly these types of measurement all the time. And, for ordinary speeds that are small compared to light, we can and do find the same results as when the object is at rest.

As the relative speeds approach that of light, however, the exact physical length depends on the relative motion.

 

[TrueBeliever37]

No, I don't 'acknowledge' it because it is wrong.

So, suppose that you see something moving past you at 50% of the speed of light. Are you saying you cannot determine its exact physical length? Are you saying we have to be right next to something in order to determine its exact physical length?

You also get to use your ruler to find an exact physical length. For example, you can watch the ruler and the pen as you go past and take a picture exactly when they are lined up.

The point is that we can and do perform exactly these types of measurement all the time. And, for ordinary speeds that are small compared to light, we can and do find the same results as when the object is at rest.

As the relative speeds approach that of light, however, the exact physical length depends on the relative motion.

In all your examples with the pen. The ships were not able to measure the true 5 1/2" length unless the pen was right beside them in the ship.

They would know the true length, because they would be able to measure it before they took off on the trip.

 

[TrueBeliever37]

Who said it is not allowed? You have to use the distance and the time *as measured in the same frame*. When you do that, the speed of light will always come out to be the same.

Ok - so please use the formula (C=distance/time) to determine the time for light to move from a star to a planet 30 million light years away from it.

Should be similar to using it to determine the distance for light to move from a star (the sun) to a planet (the earth) some number of light years away from it.

 

[TrueBeliever37]

It is useless to ask for a distance in a situation where distance doesn't exist. Why is that so hard to understand?

If I told you that photons have no mass, would you keep asking how much 10,000 photons weigh?

It is hard to understand because in one breath you guys say it is a distance of 30 million light years, but say it is 0 distance in the next.

Could an alien wanting to measure the time for light to get from a star (the sun) to a planet (the earth) be able to use the formula C=distance/time?

Why is that same alien not able to use the formula to measure the time for light to get from another star to another planet?

If he can, then please tell me what he determines for the time it takes light to get from another star to a planet 30 million light years away.

Polymath257, please respond to this post also. Thanks

 

[TrueBeliever37]

Which frame are you measuring in? In your frame (the one where the galaxy is 30 million ly away) it will take 30 million years.

In the 'frame' of the photon, it will be instantaneous. But the distance in *that* frame is 0.

The answer depends on the frame. As we have said many times. Same physical situation, different description.

Ok - you said in the 'frame' of the photon, it gets 30 million light years instantly. (I know, you said the distance is 0 in that frame.)

Now, in that same 'frame' of the photon, does it get from the sun to the earth instantly also?

 

[TrueBeliever37]

And nobody has said any other formula is required. But the distance and the time involved have to be measured in the same reference frame.



Because you are wanting to know what the 'light experiences'. That would involve looking at a reference frame moving at the speed of light. There is no real reference frame that does so. The only way to make sense of the question is to take the limit of frames going slower than light as the speed goes to that of light.

But I am wanting to know what the light experiences when I am determining time from the sun to earth. And you are able to use the formula there. How is that not an example of using a reference frame moving at the speed of light?

Why is calculating time from sun to planet earth not using a frame moving at C?
But calculating time from a different star to a different planet, is suddenly using a frame moving at C?

 

[Polymath257]

Cn

In all your examples with the pen. The ships were not able to measure the true 5 1/2" length unless the pen was right beside them in the ship.

They would know the true length, because they would be able to measure it before they took off on the trip.

Why would they not be able to measure the 'true length' as they move past? Suppose they are going past that pen at a distance of less than 1/10" and they have the rule positioned and they take an ultra-fast photo. Are you saying there is NO WAY they can measure the 'true length' as they are moving?

Suppose they didn't know ahead of time how long that pen was before they took the trip. Is there *any* way they can determine it for themselves as they move past?

 

[Polymath257]

Ok - so please use the formula (C=distance/time) to determine the time for light to move from a star to a planet 30 million light years away from it.

Should be similar to using it to determine the distance for light to move from a star (the sun) to a planet (the earth) some number of light years away from it.

OK, let's be clear about our assumptions. I am on the Earth (or any planet, or whatever) and that star is *at rest* with respect to me. It is not moving closer of farther away from be while I am waiting for the light to get there.

Next assumption: I measuring the distance to that galaxy in my reference frame and I measure the time it takes also in my reference frame.

Are you OK with these assumptions?

If so, then the distance is 30 million light years and the time it takes for light to go that distance is 30 million years. And 30 million ly/30 million years = c.

And, in general, if there is uniform motion, we can always use distance/time = speed.

So, let's change things just a bit. Suppose that star is moving towards us at 50% of the speed of light. NOW how long will it take for light to travel from the star to us? I'm not going to give you the answer here. I want you to figure it out. And yes, you will use speed = dist/time.

 

[Polymath257]

It is hard to understand because in one breath you guys say it is a distance of 30 million light years, but say it is 0 distance in the next.

It is 30 million in one frame and 0 in a limiting frame.

Could an alien wanting to measure the time for light to get from a star (the sun) to a planet (the earth) be able to use the formula C=distance/time?

Yes, of course. That alien would measure distances in its frame and times in its frame and do that calculation to get the speed. And it would get the speed of light if it was light moving.

Why is that same alien not able to use the formula to measure the time for light to get from another star to another planet?

The alien can, in fact, do so.

If he can, then please tell me what he determines for the time it takes light to get from another star to a planet 30 million light years away.

Polymath257, please respond to this post also. Thanks

Again, I am assuming that star is at rest (mot moving towards or away from us) and that all measurements are made in the frame where that star and us are at rest. In that case, distance = 30 million light years and time = 30 million years and dist/time = c.

 

[Polymath257]

But I am wanting to know what the light experiences when I am determining time from the sun to earth. And you are able to use the formula there. How is that not an example of using a reference frame moving at the speed of light?

Because the distances and times are measured from the Earth. The Earth is not moving at the speed of light. You are confused what it means to be a reference frame.

Why is calculating time from sun to planet earth not using a frame moving at C?
But calculating time from a different star to a different planet, is suddenly using a frame moving at C?

Neither uses such a frame by necessity. A reference frame is the way that the distances and times are measured. So, when we measure the distance to the sun or a star, we use the reference frame where the Earth is at rest. Someone moving past us at 50% of c would measure those distances and times to have different values. They would be in a different reference frame. But, when they take *their* values for distance and time for the travel of the light, they also use dist/time=speed and get the same answer.

So, when you ask what someone on a spaceship going past Earth at 50% of the speed of light experiences, you are going to use *their* reference frame: their measurements of distances and times. For the experience of someone on the spacecraft, you use the distances and times as measured by those in the spacecraft. That is the reference frame of the spacecraft.

 

[Polymath257]

Maybe it's best to start over. Let's do some very simple calculations and go over some basic concepts and see what happens. A new start for clarity?

So, when we say a star is 4 light years from Earth, we generally are using the distance as measured from Earth. When we say a process on that star takes 10 years, we are using clocks on Earth to make that measurement (taking into account things light the time it takes for light to get from the star to us---4 years). Because the measurements are made from the earth, we say they are the values in the reference frame of the earth.

So, if light stars from a star, planet, whatever that is 4 light years from earth, then that light will take 4 years to reach the earth. All of these distances and times are measured in the frame of the earth. This is dist/time=speed.

Now, suppose that a spaceship (let's call it spaceship A for convenience) goes past earth on the way to that star. That spaceship is moving at 50% of c. How long does it take that spaceship to reach that star according to measurements on the earth? Well, in this case, dist=4ly and speed = .5c and so time = dist/speed = 4/.5 = 8 years.

Now, another spaceship also goes past earth on the way to that same star. it is going past earth at the rate of 90% of c. How long does it take for that spaceship to reach that star according to measurements on earth? Well, again, we do time = dist/speed and find time = 4/.9 = 4.44 years.

-----------------------------------------------------
Now suppose there is a different star. Star B is currently 4ly from earth, but is moving towards the earth at 50% of c. How long does it take for that star to reach the earth (according to measurements from the earth)? Well, once again, time = dist/speed, so time = 4ly/.5c = 8 years.

Next, suppose that a spacecraft is going past earth to star B and is going at 80% of c. How long does it take that spacecraft to reach star B? now, we have to combine the speed of the star and the speed of the spacecraft. So, the speed used in the time calculation is .5c+.8c = 1.3c. So the time it takes for that spaceship to reach the star is now time = dist/speed = 4ly/1.3c = 3.08 years.

Finally, suppose that the earth sends a light signal to star B. How long does it take the light to reach star B? Once again, the speed of star B needs to be combined with the speed of light in the time calculation. The speed used is going to be .5c + 1c = 1.5c. So now, the time it takes for the light to reach the star is going to be 4ly/1.5c = 2.67 years.

Before I go on, I want to make sure you have these examples and understand them.

Please let me know if you agree so far.

 

[TrueBeliever37]

Cn


Why would they not be able to measure the 'true length' as they move past? Suppose they are going past that pen at a distance of less than 1/10" and they have the rule positioned and they take an ultra-fast photo. Are you saying there is NO WAY they can measure the 'true length' as they are moving?

Suppose they didn't know ahead of time how long that pen was before they took the trip. Is there *any* way they can determine it for themselves as they move past?

Why did they never come up with the pen being 5 1/2" when they moved past it? That was the length they would have measured one of the pens at before they took off.

 

[TrueBeliever37]

It is 30 million in one frame and 0 in a limiting frame.



Yes, of course. That alien would measure distances in its frame and times in its frame and do that calculation to get the speed. And it would get the speed of light if it was light moving.



The alien can, in fact, do so.



Again, I am assuming that star is at rest (mot moving towards or away from us) and that all measurements are made in the frame where that star and us are at rest. In that case, distance = 30 million light years and time = 30 million years and dist/time = c.

Ok now, please tell me, the time that same alien calculates for light to get from the sun to the earth.

Or show me how he comes up with the speed of light like you did for the previous calculation.

I don't want to start over yet, because I think I have a point here, and in post 1035 that I don't want neglected.

 

[Subduction Zone]

Why did they never come up with the pen being 5 1/2" when they moved past it? That was the length they would have measured one of the pens at before they took off.

Because length is relative. That has been the point here. Motion changes things.

 

[Polymath257]

Ok now, please tell me, the time that same alien calculates for light to get from the sun to the earth.

Or show me how he comes up with the speed of light like you did for the previous calculation.

I don't want to start over yet, because I think I have a point here, and in post 1035 that I don't want neglected.

If that alien is at rest with respect to the earth, they would find the time for the light to get from the sun to the earth is 500 seconds, which is 93 million miles/ 186000 mps = dist/time.

The problems come when the alien is NOT at rest with respect to the earth.

OK< if you don't want to start over, will you at least look over that post and see if you agree?

 

[Polymath257]

Why did they never come up with the pen being 5 1/2" when they moved past it? That was the length they would have measured one of the pens at before they took off.

Yes. But because of the relative motion, they do not measure that as the length. That is what it meant to say that length is relative.

 

[TrueBeliever37]

Yes. But because of the relative motion, they do not measure that as the length. That is what it meant to say that length is relative.

Yes, But that was what I meant by, they don't come up with the actual/real/true physical length of 5 1/2". All the ships came up with the true length when it was actually on board with them. That is how we know the true length.

Maybe it's best to start over. Let's do some very simple calculations and go over some basic concepts and see what happens. A new start for clarity?

So, when we say a star is 4 light years from Earth, we generally are using the distance as measured from Earth. When we say a process on that star takes 10 years, we are using clocks on Earth to make that measurement (taking into account things light the time it takes for light to get from the star to us---4 years). Because the measurements are made from the earth, we say they are the values in the reference frame of the earth.

So, if light stars from a star, planet, whatever that is 4 light years from earth, then that light will take 4 years to reach the earth. All of these distances and times are measured in the frame of the earth. This is dist/time=speed.

Now, suppose that a spaceship (let's call it spaceship A for convenience) goes past earth on the way to that star. That spaceship is moving at 50% of c. How long does it take that spaceship to reach that star according to measurements on the earth? Well, in this case, dist=4ly and speed = .5c and so time = dist/speed = 4/.5 = 8 years.

Now, another spaceship also goes past earth on the way to that same star. it is going past earth at the rate of 90% of c. How long does it take for that spaceship to reach that star according to measurements on earth? Well, again, we do time = dist/speed and find time = 4/.9 = 4.44 years.

-----------------------------------------------------
Now suppose there is a different star. Star B is currently 4ly from earth, but is moving towards the earth at 50% of c. How long does it take for that star to reach the earth (according to measurements from the earth)? Well, once again, time = dist/speed, so time = 4ly/.5c = 8 years.

Next, suppose that a spacecraft is going past earth to star B and is going at 80% of c. How long does it take that spacecraft to reach star B? now, we have to combine the speed of the star and the speed of the spacecraft. So, the speed used in the time calculation is .5c+.8c = 1.3c. So the time it takes for that spaceship to reach the star is now time = dist/speed = 4ly/1.3c = 3.08 years.

Finally, suppose that the earth sends a light signal to star B. How long does it take the light to reach star B? Once again, the speed of star B needs to be combined with the speed of light in the time calculation. The speed used is going to be .5c + 1c = 1.5c. So now, the time it takes for the light to reach the star is going to be 4ly/1.5c = 2.67 years.

Before I go on, I want to make sure you have these examples and understand them.

Please let me know if you agree so far.

I understand and do agree with the above as far as the math is concerned.

I just don't believe that time and distance actually become zero for everything in any reference frame. That has got to be a make believe world, because as I have said so many times - a light year is an actual physical length. It's like the pen, if they all could have actually taken a tape and measured the pen on board they would have all had one value they agreed on.

I agree that a number can be different in another frame of reference, But one number wouldn't equal every other number you can come up with in the other reference frame, which is what happens with your limiting frame.

To me when you are working in the metric system , that would be like being in another frame of reference compared to our system.
And there are conversion factors you use to go back and forth between the 2 systems. And there is only one value that relates to another value in the other system.

But in one of your systems( the limiting reference frame) , one value corresponds to an almost infinite number of values in the other reference frame (system).

1 ly in my reference = 0 in the limiting frame
4ly = 0
30 million ly = 0
even 0 = 0

And the same thing happens with times.