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

[Polymath257]

Still, when I ask how long it takes for a photon to get from a star to a planet, 30 million light years away. The same formula should work, that we use to determine how long the photon takes to get from the sun to this planet. What difference does it make about frame of reference? The photon is not in a ship, it is just traveling at the speed of light in both cases.
We have a constant speed of light in both cases , and an actual physical distance.

The measurement that the star is 30 million light years away is from a particular reference frame.

In a reference frame of a spacecraft moving toward the star from the Earth at 86% of c, the measured distance between the Earth and that star will be 15 million light years. And in *that* frame, the light will take 15 million years to travel that distance.

The distances and the times depend on the specific frame of reference in which the measurements are done.

But, in all frames of reference, the speed of light will have the same value: c.

 

[Subduction Zone]

Even worse. If you measured how long it took for a photon to get from the sun to a point on the surface of the Earth with a very accurate clock a person that was on top of a mountain with a very accurate clock would get a very slightly different answer than you did. Who is right, you or the person on top of a mountain?

 

[TrueBeliever37]

What about a spaceship that is speeding away from the Sun at 0.5 c. How fast do you think the light from the Sun is travelling relative to the spaceship? Will people on the spaceship see light going by at 0.5 c, or at 1.0 c?

No because they will constantly see light if it going by them. It would be light all the time as far as they are concerned, unless something blocks the light.

But yes the light would be going by them at a difference of 0.5C faster.

 

[Thermos aquaticus]

No because they will constantly see light if it going by them. It would be light all the time as far as they are concerned, unless something blocks the light.

But how fast is the light from the Sun going as measured on the spaceship that is speeding away from the Sun at 0.5 c?

 

[Subduction Zone]

No because they will constantly see light if it going by them. It would be light all the time as far as they are concerned, unless something blocks the light.

.

The question was not whether it is going by them, it is how fast the light is moving relative to them. If they measured the velocity of that light what value would they get?

EDIT: I see that @Thermos aquaticus is right on the ball. Carry on.

 

[Thermos aquaticus]

But yes the light would be going by them at a difference of 0.5C faster.

Sorry, I missed this part in my previous reply.

The people on the spaceship will not see light going by them at 0.5 c. The measured speed of that light will be 1 c on the spaceship. The speed of light is the same for all frames of reference.

So how can this be? It is because high relative velocities shortens lengths and slows down time.

 

[TrueBeliever37]

The measurement that the star is 30 million light years away is from a particular reference frame.

In a reference frame of a spacecraft moving toward the star from the Earth at 86% of c, the measured distance between the Earth and that star will be 15 million light years. And in *that* frame, the light will take 15 million years to travel that distance.

The distances and the times depend on the specific frame of reference in which the measurements are done.

But, in all frames of reference, the speed of light will have the same value: c.

If you could take a tape measure, and measure the actual physical distance from the Sun to the Earth, and then measure the actual physical distance from a star to a distant planet 30 million light years away.

Then you measured how long it took a photon to travel in both of those cases. You should find that the formula Speed of light =C = a Constant = Distance/Time should work in both cases.

 

[TrueBeliever37]

Sorry, I missed this part in my previous reply.

The people on the spaceship will not see light going by them at 0.5 c. The measured speed of that light will be 1 c on the spaceship. The speed of light is the same for all frames of reference.

So how can this be? It is because high relative velocities shortens lengths and slows down time.

They won't actually see light going by at all. The light is constantly there unless something blocks it's path as far as what they see.

 

[Thermos aquaticus]

If you could take a tape measure, and measure the actual physical distance from the Sun to the Earth, and then measure the actual physical distance from a star to a distant planet 30 million light years away.

That depends on how fast you are going when you make those measurements. If you are stationary with respect to the Sun then you will measure the distance between the Sun and the Earth to be ~1 AU (Earth doesn't have a perfectly circular orbit). If you are travelling 0.86 c as you fly over the north pole of the Sun towards the Earth, then you will measure the distance between the Sun and the Earth to be 0.5 AU. Both are correct measurements.

 

[Thermos aquaticus]

They won't actually see light going by at all. The light is constantly there unless something blocks it's path as far as what they see.

They will measure the speed of light coming from the Sun to be 1 c, no matter how fast they are travelling with respect to the Sun. If they are travelling towards the Sun at 0.5 c they will still measure the speed of light to be 1 c. If they are travelling away from the Sun at 0.5 c they will still measure the speed of light from the Sun to be 1 c.

 

[TrueBeliever37]

It isn't a question of accuracy here. It isn't a question of the difficulty of the measurements. It is an issue of what the *correct* measurements in each frame would give.

For me it is a question of accuracy. The truth matters.

 

[TrueBeliever37]

That depends on how fast you are going when you make those measurements. If you are stationary with respect to the Sun then you will measure the distance between the Sun and the Earth to be ~1 AU (Earth doesn't have a perfectly circular orbit). If you are travelling 0.86 c as you fly over the north pole of the Sun towards the Earth, then you will measure the distance between the Sun and the Earth to be 0.5 AU. Both are correct measurements.

If we truly have the correct number to use for C, then only one distance is actually correct. It would be the one we get when using the formula C=distance/time. Unless the concept of time doesn't mean anything to you.

 

[Thermos aquaticus]

For me it is a question of accuracy. The truth matters.

The truth is that differences in velocity change lengths and change the passage of time. The accuracy of this truth has been confirmed in many experiments.

 

[Thermos aquaticus]

If we truly have the correct number to use for C, then only one distance is actually correct. It would be the one we get when using the formula C=distance/time. Unless the concept of time doesn't mean anything to you.

If a spaceship travelling at 0.86 c flew over the north pole of the Sun and shot a laser beam at the Earth, that laser would hit the Earth 4 minutes later as measured on the ship. The clocks on a stationary ship sitting above the north pole of the Sun would measure that same laser light taking 8 minutes to reach the Earth. Both are true, and both are accurate.

 

[TrueBeliever37]

The truth is that differences in velocity change lengths and change the passage of time. The accuracy of this truth has been confirmed in many experiments.

If I was running a mile. Even if I could run at the speed of light. The distance I would run would still be 1 mile, regardless of how fast I ran. I admit it would seem like it took almost no time to run it, since I just blazed thru it. But it would have still been a mile.

But when you start getting the distances out there to 30 million light years, It would take me a little while to run that.

 

[Polymath257]

If you could take a tape measure, and measure the actual physical distance from the Sun to the Earth, and then measure the actual physical distance from a star to a distant planet 30 million light years away.

Then you measured how long it took a photon to travel in both of those cases. You should find that the formula Speed of light =C = a Constant = Distance/Time should work in both cases.

Yes. And nothing I have said contradicts that. But, and this is important, you get the distance of 93 million miles or 30 million light years *in your reference frame*. In a different reference frame, using the measuring tape of that frame, you might well get different distances in each case. And, in the other reference frame, you would get different amounts of time it takes for the light to travel those distances.

But, in each case, the speed of light will be calculated to be the same thing.

 

[james blunt]

If a spaceship travelling at 0.86 c flew over the north pole of the Sun and shot a laser beam at the Earth, that laser would hit the Earth 4 minutes later as measured on the ship. The clocks on a stationary ship sitting above the north pole of the Sun would measure that same laser light taking 8 minutes to reach the Earth. Both are true, and both are accurate.

A spaceship travelling at the speed of light was above the earth and fired a laser back , it took 4 minutes because the light was constant.

 

[Polymath257]

But yes the light would be going by them at a difference of 0.5C faster.

NO IT WILL NOT. This is the whole point. When those on the ship going past Earth at 50% of the speed of light look and see how past the light is going past them, they do NOT see it going past them at 50% of c. They see it going past them at 100% of c.

 

[Polymath257]

For me it is a question of accuracy. The truth matters.

Truth matters for me also. And it is NOT a matter of accuracy here. If we could take *perfectly accurate* measurements in the two frames, the results would be different.

 

[Polymath257]

They won't actually see light going by at all. The light is constantly there unless something blocks it's path as far as what they see.

And this is wrong. They would still see the light going past them at 100% of c. This is what it *means* when we say that the speed of light is the same in all reference frames.