# Lorentz Question II: Doppler Shifted Freq. & Time Dilation

• Chrisc
In summary, Frames A and B are at rest relative to each other, and each measures the frequency of the light signals from the other. The frequency of the light signals is affected by time dilation for each frame. Frame A calculates the Lorentz factor for frame B according to their relative speed. If the motion of the third frame is in the same direction as B, frame A's measurements of the third frame will be different by +0.511 meters per meter and -0.339 seconds per second.
Chrisc
In free space, two frames A and B are at rest with respect to each other.
They each flash light signals toward the other at an equal and constant rate.
(i.e. on for one second, off for one second)
Each then begins to move toward the other at a significant constant speed.

••••• Assumption 1•••••
They cannot determine the equality of their motion in the equality of the (doppler shifted) frequency of the light signals measured by both.
Although it is reasonable to assume their procedure to acquire equal motion is verified in their measurements,
they can only agree their tests measure "relative" motion between them.

••••• Assumption 2•••••
Each now measures the (doppler shifted) frequency of the others light signals increased proportional their relative speed.

••••• Question 1•••••
Does the time dilation of either frame affect the frequency of light signals measured by the other?

Chrisc said:
••••• Assumption 1•••••
They cannot determine the equality of their motion in the equality of the (doppler shifted) frequency of the light signals measured by both.
Although it is reasonable to assume their procedure to acquire equal motion is verified in their measurements,
they can only agree their tests measure "relative" motion between them.
OK. Once they are done accelerating, the only thing that matters (or that they can determine) is their relative speed.
••••• Assumption 2•••••
Each now measures the (doppler shifted) frequency of the others light signals increased proportional their relative speed.
OK. The observed frequency increases with their relative speed, but is not simply proportional to it.
••••• Question 1•••••
Does the time dilation of either frame affect the frequency of light signals measured by the other?
The observed frequency of light is definitely affected by time dilation. According to frame A, frame B's clocks and light sources--since they are moving with respect to A--are subject to the usual time dilation. The relativistic Doppler shift takes all that into account. See: http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/reldop2.html#c1"

Last edited by a moderator:
OK, so far so good.
••••• Assumption 3 •••••
I will take the position of frame A, I measure the relative speed of the distant frame B is 0.75c.
Calculating the time dilation and length contraction of frame B, I find for every second I measure, they measure 1.511 and for every meter I measure, they measure 0.661 meters.
Had I taken the position of frame B, the above would be correct with respect to frame A.

••••• Question 2 •••••
With this knowledge stated above, will any measurement B makes of the motion of a third frame be with respect to my frame, different by +0.511 meters per meter and -0.339 seconds per second, if the motion of the third frame is in the same direction as B?

Chrisc said:
OK, so far so good.
••••• Assumption 3 •••••
I will take the position of frame A, I measure the relative speed of the distant frame B is 0.75c.
Calculating the time dilation and length contraction of frame B, I find for every second I measure, they measure 1.511 and for every meter I measure, they measure 0.661 meters.
Had I taken the position of frame B, the above would be correct with respect to frame A.
OK. Sounds good. The "Lorentz factor" (usually called gamma) is 1.51.

••••• Question 2 •••••
With this knowledge stated above, will any measurement B makes of the motion of a third frame be with respect to my frame, different by +0.511 meters per meter and -0.339 seconds per second, if the motion of the third frame is in the same direction as B?
I think what you're asking is this. If B measures a moving clock and meterstick (moving in that third frame) to be dilated and contracted by some factor X, will frame A say measure them to be dilated and contracted by a factor of 1.51*X? The answer is no; it's more complicated than that.

To take a specific example, let's say that third frame was moving at the same speed 0.75c with respect to frame B. To find the Lorentz factor as seen by frame A, we first need the relative speed of the third frame with respect to A. That would be:
$$V' = \frac{V_1 + V_2}{1 + V_1 V_2/c^2}$$

When you plug in the numbers, you get V' = 0.96c. Which gives you a Lorentz factor of 3.57.

Thank you Doc Al.
You've gone one step further than I am ready to.
I want to make sure I understand the physical ramifications of this first
before I go continue, as my question is specifically the physical ramifications of the next step.
I will review the formulas again before posting.
I do not use LaTex so it might get messy.
If need be I will post the formulas in an image format.

I've studied the equations again and I have no issue with what you've explained, and I can now see you have already answered my original question re: priming the original measures of A or B with gamma.

••••• Assumption 4•••••
The equations of mechanics express no quantitative value without a bench mark such as rest. Absolute rest was assumed in order that the equations held throughout the universe. With the abolishment of absolute rest from the laws one of two dynamics was needed to uphold the laws.
1) a Lorentz transformation
2) a new absolute

•••••• Assumption 5•••••
Every measurement in inertial frames including the speed of light holds to the equations of mechanics because the Lorentz factor divides all measurements of the dimensions length and time "into" the speed of light: 1/sqrt 1-v^2/c^2. (ignoring mass for now)

••••• Question 3 •••••
It seems to me this means, in a physical sense, the Lorentz transformation is the formula by which we discover (through physically real measurements) the speed of light is a physically real absolute by which the equations of mechanics are upheld in all frames?

Chrisc said:
••••• Assumption 4•••••
The equations of mechanics express no quantitative value without a bench mark such as rest. Absolute rest was assumed in order that the equations held throughout the universe. With the abolishment of absolute rest from the laws one of two dynamics was needed to uphold the laws.
1) a Lorentz transformation
2) a new absolute
The notion of "absolute rest" in mechanics was abandoned long before Einstein. Before the Lorentz transformations there were the (still extremely useful) Galilean transformations. (At low speeds, the Lorentz transformations are well approximated by the Galilean transformations.)

•••••• Assumption 5•••••
Every measurement in inertial frames including the speed of light holds to the equations of mechanics because the Lorentz factor divides all measurements of the dimensions length and time "into" the speed of light: 1/sqrt 1-v^2/c^2. (ignoring mass for now)
Not sure what you're saying here. Realize that in the standard treatment, the Lorentz transformations are derived based upon the assumption that the speed of light is invariant.

••••• Question 3 •••••
It seems to me this means, in a physical sense, the Lorentz transformation is the formula by which we discover (through physically real measurements) the speed of light is a physically real absolute by which the equations of mechanics are upheld in all frames?
I don't understand what you're asking here. Again, the Lorentz transformations have the invariant speed of light built into them. They answer the question: How must position and time measurements transform in order to assure that the speed of light is the same for all observers?

Doc Al said:
Again, the Lorentz transformations have the invariant speed of light built into them. They answer the question: How must position and time measurements transform in order to assure that the speed of light is the same for all observers?

That is the essence of my question. They state how, but in stating how do they not at least hint why?
In that position and time "do" change by the amounts defined by the Lorentz transformations, are the Lorentz transformations defining a physical property and/or physical constant of space and time?

Hello Chrisc.

Doc Al is of course correct but perhaps it is better for you to realize that the Lorentz Transforms were "designed by us" to to agree with the physics of the situation, the postulate that c is the same for all inertial observers. It is not the transforms that make the physics, but they reflect the "reality" of the situation.

Matheinste.

matheinste said:
...It is not the transforms that make the physics, but they reflect the "reality" of the situation.

Thanks matheinste, that is the way I understand it.
Are you aware of any reasons given for the "reality" of the situation?

Hello Chrisc.

---Thanks matheinste, that is the way I understand it.
Are you aware of any reasons given for the "reality" of the situation?----

No.

Matheinste.

Chrisc said:
Thanks matheinste, that is the way I understand it.
Are you aware of any reasons given for the "reality" of the situation?

It's geometry.

## What is the Lorentz transformation equation for Doppler shifted frequency?

The Lorentz transformation equation for Doppler shifted frequency is given by f' = f0 / γ(1 ± v/c), where f' is the shifted frequency, f0 is the original frequency, γ is the Lorentz factor, v is the relative velocity between the source and observer, and c is the speed of light.

## How does the Doppler effect impact the perceived frequency of light from a moving source?

The Doppler effect causes the perceived frequency of light from a moving source to be either higher or lower than the actual frequency depending on the relative motion between the source and observer. If the source is moving towards the observer, the frequency will appear higher, while if the source is moving away, the frequency will appear lower.

## What is the relationship between time dilation and the Doppler effect?

The Doppler effect is a result of time dilation, which is a consequence of the theory of relativity. Time dilation causes time to appear to slow down for an observer in a frame of reference moving at a high velocity relative to another frame of reference. This results in a difference in the perceived frequency of light from a moving source.

## How does the Lorentz transformation equation account for time dilation?

The Lorentz transformation equation includes the Lorentz factor, which is a function of the velocity of the source and the speed of light. This factor accounts for the time dilation effect and adjusts the perceived frequency of light accordingly.

## Can the Doppler effect be observed in everyday life?

Yes, the Doppler effect can be observed in everyday life. Some common examples include the change in pitch of a siren as an ambulance or police car passes by, the change in pitch of a train horn as it approaches and passes, and the shift in color of light from a star that is moving towards or away from Earth.

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