# Speed of light isn't constant relative to all observers.

#### Zeno

Registered Senior Member
http://calgary.rasc.ca/algol_minima.htm

The eclipses come farther apart in time when the Earth is moving away from Algol. Therefore, the light is passing the Earth more slowly. Imagine you are standing still and that you have a line of people running past you at a constant speed and they are equally spaced apart from each other. If you are standing still and then start walking in the same direction that they're running, there will be more time in between each person reaching you and they are passing you more slowly. If you believe that the speed of light is constant relative to all observers then you will need to explain why the above reasoning doesn't apply to light.

http://calgary.rasc.ca/algol_minima.htm

The eclipses come farther apart in time when the Earth is moving away from Algol. Therefore, the light is passing the Earth more slowly. Imagine you are standing still and that you have a line of people running past you at a constant speed and they are equally spaced apart from each other. If you are standing still and then start walking in the same direction that they're running, there will be more time in between each person reaching you and they are passing you more slowly. If you believe that the speed of light is constant relative to all observers then you will need to explain why the above reasoning doesn't apply to light.

The speed of light in a vacuum is a universal constant - that is it moves at a set rate unless otherwise acted upon. It's apparent speed relative to someone else in motion is, largely, irrelevant.

http://calgary.rasc.ca/algol_minima.htm

The eclipses come farther apart in time when the Earth is moving away from Algol. Therefore, the light is passing the Earth more slowly. Imagine you are standing still and that you have a line of people running past you at a constant speed and they are equally spaced apart from each other. If you are standing still and then start walking in the same direction that they're running, there will be more time in between each person reaching you and they are passing you more slowly. If you believe that the speed of light is constant relative to all observers then you will need to explain why the above reasoning doesn't apply to light.
From the Earth's perspective, it is Algol that is moving away from the Earth, thus the distance between Earth has increased between each Eclipse and the extra time it takes the light to travel that distance at c between eclipses means we see the eclipses spaced further apart in time. Doppler shift is only dependent on the relative speed between the source and observer and not on which one we consider as moving.

It takes a longer time between eclipses because the light, relative to us, is no longer moving at c. The speed of the Earth around the Sun is 66,629 mi/hr. We are not moving directly towards and away from Algol, so our speed in that direction is actually 66,629 mi/hr * cos(17.516339) = 63,539.49055 mi/hr. So the speed of light relative to us when we are moving away is 186,264.3501 mi/sec. So the time taken increases by (186,282/186,264.3503) = 1.000094757. So 2.867321 days * 1.000094757 = 2.867592699 days. That's very close to 2.8675875347 days when the Earth is moving away from Algol at maximum speed.

Last edited:
That's exactly my point. It takes a longer time between eclipses because the light, relative to us, is no longer moving at c. The speed of the Earth around the Sun is 66,629 mi/hr. We are not moving directly towards and away from Algol, so our speed in that direction is actually 66,629 mi/hr * cos(17.516339) = 63,539.49055 mi/hr. So the speed of light relative to us when we are moving away is 186,264.3501 mi/sec. So the time taken increases by (186,282/186,264.3503) = 1.000094757. So 2.867321 days * 1.000094757 = 2.867592699 days. That's very close to 2.8675875347 days when the Earth is moving away from Algol at maximum speed.
No, the light is still moving at c relative to the Earth., it just has to travel a longer distance.

This demonstrates how Doppler shift work for light:
If you are the source, the light expands outward from you at c equally in all directions. The two observers (the red and blue dots) here are stationary to the source and receive the waves at the same frequency as the source emits. like this:

If the dots were moving from right to left at the relative speed with respect to source, then tthe red dot would be running away form the expanding waves and the blue dot rushing to meet them,

However, if you are the red or blue dots, and there is a relative motion between you and dots, the following is happening: The first part of a wave is emitted and expands outward at c from the point of emission equally in all directions. The source is moving, so is in a different spot when the next part of the wave is emitted, and this new spot is the center of that part of the wave that expands outward at c. The next part is emitted from another point and is the center of the light emitted at that moment and so on. Like this:

No part of the waves are ever moving at anything but c relative to the red or blue dots, but the red dot sees longer waves at a lower frequency and the blue dot sees shorter waves at higher frequency.

The point is that having the dots moving from right to left or having the source moving from left to right are just two different views of the same thing, but if you are the source the light is seen as traveling at c with respect to yourself, and if you are the dot you measure it as traveling at c with respect to you.

Last edited:
Nice animations, but viewing the second one, Zeno could say that surely the moving (yellow) dot should be able to determine that the light leaving its rear end travels a greater distance per unit time than the light leaving its front end. The whole explanation is quite a bit more complicated to explain, and involves relativity of simultaneity (ROS), which most newbies to relativity find difficult to understand. Einstein's train car thought experiment (chapter 9 from the 1920 book Relativity) demonstrates ROS well, but even that should be taken in the context of the previous chapters. It took me a really long time (years) to understand SR, but I'm glad I stayed at it until it finally made sense.

Last edited:
I just thought I should clarify that the "newbie to relativity" I was referring to was Zeno, not Janus. I know Janus understands, and is virtually an expert on SR.

"In May and November, the Earth is moving at "right angles" to the line to Algol. During this time we see minima happening regularly at their 2.867321 day intervals. However, during August, the Earth is rapidly moving towards Algol at about 107,229 km/hr as explained on my How Fast Are We Moving? page. (The Earth moves approximately 202 times its own size in one day.) So in 2.867321 days the Earth moves about 7,379,039 km closer to Algol. _But the varying light from Algol doesn't know this - its light waves left Algol 93 years ago and are travelling at a constant speed._ The result - we "catch a bunch of minima early" during August as shown on Chart 2. Exactly the opposite happens during February - the Earth is moving away from Algol that fast and it takes longer for the group of minima to reach us so we see them taking longer between events. How long? 7,379,039 km divided by the speed of light 299,792.458 km/sec is 24.61382 seconds. So in May and November when we are not moving towards or away from Algol - the period seems constant. It is our rapid movement towards or away from the events in August and February that causes the timing differences."

I assume that light is passing the earth at c when the earth isn't moving towards or away from Algol.

In February the earth is moving away from Algol and the time between the eclipses is 2.8675875347 days and the light is passing the earth at 186,265 mi/sec.

In May and November the earth is not moving towards or away from Algol and the time between eclipses is 2.867321 days and the light is passing the earth at 186,282 mi/sec.

In August the earth is moving towards Algol and the time between eclipses is 2.8670608912 days and the light is passing the earth at 186,299 mi/sec.

What you are describing is Relativistic Doppler Shift - a well-known phenomenon.
When two objects are moving away from each other, any regular pulses between them will be attenuated.
https://en.wikipedia.org/wiki/Relativistic_Doppler_effect

This has nothing to do with the speed of light, which is always measured to be c - a fact which has been confirmed experimentally.

I don't think Zeno was implying that the constant speed of light changed absolutely, only that it seems that way to the observer on planet earth. While I appreciate you all taking the time to answer a question from a "newbie," I believe an apology is in order for the arrogant and disgusting display of intellectual bullying that you all thought was appropriate to someone that may have actually wanted to learn something or become a valuable contributor to the community.

Last edited:
Sock puppets are not allowed on sciforums. Your account has not been permanently banned this time, but don't do this again.
Let's see....
When the earth is not moving away from or towards Algol the time between eclipses is 2.867321 days so (186,282 mi/sec)*(86,400 sec/day)*(2.867321 days) = (4.61488571 * 10^10 miles between eclipses). (4.61488571 * 10^10 miles between eclipses) / (2.8675875347 days between eclipses when the earth is moving away from Algol) = (1.609326884 * 10^10 miles/day). (1.609326884 * 10^10 miles/day) / (86400 sec/day) = 186,265 miles/sec.
The frequency of the light isn't being measured to it seems that the Doppler effect has nothing to do with the change in time between eclipses.

I don't think Zeno was implying that the constant speed of light changed absolutely, only that it seems that way to the observer on planet earth.
OK, but it doesn't seem that way to an observer on Earth. So he's wrong, and he doesn't seem to have an interest in getting it right.

I believe an apology is in order for the arrogant
There is a certain amount of arrogance in stating things about a subject as if one knows what one is talking about - without bothering to read up on it first. The second sentence in the OP is factually incorrect, and everything else that follows is moot after that error.

A more appropriate thing for the OP to do would be to ask questions to find out where one has gaps in one's knowledge.

If the OP had led with "Do the eclipses of Algol as seen from Earth over the course of the year indicate that light is passing Earth more slowly?"
And then we could have said "No. Here's why." and we would have fallen over each other trying to help the OP learn.

Last edited:
*
The frequency of the light isn't being measured to it seems that the Doppler effect has nothing to do with the change in time between eclipses.
These eclipses - like any periodic signal - are effectively a ticking clock.

As we move away, each "tick" takes a little longer to reach us - the frequency of eclipses lowers. This is the eclipse equivalent of a red shift.
As we move toward, each "tick" takes less time to reach us- the frequency of eclipses lowers. This is the eclipse equivalent of a blue shift.

But the speed of light that delivers these signals remains the same.

... someone that may have actually wanted to learn something...
If the OP wants to learn something, it would require asking questions rather than making statements in ignorance.
If the OP wants to learn something, it would require reading the responses and acknowledging the corrections in his misunderstanding.

Let's see....
As you can see, the OP is just doubling down on his own incorrect ideas and has, as yet, shown no interest in learning.

This is not our first time seeing this behavior, and it certainly won't be the last.

By the way, sockpuppets are forbidden, and will be perma-banned.

Moderator note: Theseus was a sock-puppet created by Zeno to give the false impression that somebody else supported his position in this thread.

Theseus has been permanently banned.

Under ordinary circumstances, the Zeno account would also have been permanently banned. However, Zeno is a longstanding (more than 10 years) member of this forum, so I am cutting him a little slack, just this once. Maybe he just forgot the rules. Any repeat of this behaviour will incur a permanent ban, however.

Zeno will be absent from sciforums for 2 weeks, to give him time to reflect on this poor choice he made.