# What does the Lorentz factor actually mean?

by arindamsinha
Tags: intuitive, lorentz factor, physical meaning
P: 181
 Quote by ghwellsjr No, it is not the same. You have stated that the Lorentz factor is the ratio of time passage rates between two observers. That is wrong. Instead, it is the ratio of time passage rates between the coordinate time of an inertial reference frame and an observer (or a clock) that is moving in that frame. Even if you want to consider a second observer at rest in the frame, he cannot observe the Lorentz factor ratio between his own clock and that of the moving observer. Your definition should obviously be incorrect to you because it is symmetrical, which only means that the ratio could not ever be anything other than 1. You have to at least make one of the observers different than the other one in order to have a ratio that is greater than 1. That difference is that one of the observers is at rest in an inertial reference frame in which times at distant locations have been synchronized to his clock. We imagine that there are many synchronized coordinate clocks throughout the reference frame at every possible location. Then, the moving observer is comparing the time passage rate of his clock to the time passage rate of whichever clock he is closest to as he is moving past these imaginary coordinate clocks. He finds that those clocks are ticking faster than his own but he's not comparing his one clock to just one other coordinate clock, he's comparing his one clock to many other coordinate clocks as they appear to be flying past him. Does that help?
Yes, I can see what you are saying. I was taking the underlying assumption that one of them is moving at a certain velocity w.r.t. the coordinate clock.

I suppose we can extend my definition a bit more and say:
• The Lorentz factor is 'the ratio of time passage rates (or time-speeds) between two observers, one of whom is stationary and the other is inertially moving with a certain velocity'

This includes the possibility that the velocity is 0, in which case, the Lorentz factor will be 1.

Does that sound more like it?

 Quote by ghwellsjr Obviously, this is not the only physical meaning of the Lorentz factor...
You are right, I am not saying this is the only definition (intuitive or otherwise) of the Lorentz factor, but one possible intuitive description.
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P: 4,162
 Quote by arindamsinha Yes, I can see what you are saying. I was taking the underlying assumption that one of them is moving at a certain velocity w.r.t. the coordinate clock. I suppose we can extend my definition a bit more and say: The Lorentz factor is 'the ratio of time passage rates (or time-speeds) between two observers, one of whom is stationary and the other is inertially moving with a certain velocity' This includes the possibility that the velocity is 0, in which case, the Lorentz factor will be 1. Does that sound more like it?
No, that is still not right.

It's between one observer and one reference frame.

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P: 15,569
 Quote by ghwellsjr Nevertheless, I don't see anything wrong with his question: ... Obviously, this is not the only physical meaning of the Lorentz factor
and hence the problem. Why pick that one application of the factor and crown it and say this one is the real meaning? It isn't. It is simply one application of the Lorentz factor, whose meaning is given by the definition.

If I were going to talk about the meaning of the Lorentz factor as something different from its definition then I would talk about its derivation, not its applications.
P: 181
 Quote by ghwellsjr No, that is still not right. It's between one observer and one reference frame. Please go back and read my posts.
I have read your posts in complete detail, even before my earlier responses. Still, I don't understand the difference. We are talking about one observer who is stationary w.r.t. the coordinate frame and one who is traveling. I believe we are saying the same thing.

Let me know where specifically you are disagreeing on this.
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P: 4,162
 Quote by DaleSpam and hence the problem. Why pick that one application of the factor and crown it and say this one is the real meaning? It isn't. It is simply one application of the Lorentz factor, whose meaning is given by the definition. If I were going to talk about the meaning of the Lorentz factor as something different from its definition then I would talk about its derivation, not its applications.
In arindamsinha's first post, he quoted a definition of Lorentz factor from wikipedia:
 Quote by arindamsinha The Lorentz factor is defined as: γ = 1/√(1-v2/c2) = 1/√(1-β2) = dt/dτ where: v is the relative velocity between inertial reference frames, β is the ratio of v to the speed of light c. τ is the proper time for an observer (measuring time intervals in the observer's own frame), c is the speed of light.
This definition had an extension that ended in dt/dτ. I'm just trying to explain what that extension means in the context of a simple, understandable, intuitive, physical way, which is what he asked for. Einstein categorized this explanation as a physical meaning of the equations along with length contraction, as well as numerous other applications throughout the rest of the paper. If arindamsinha had asked about length contraction, then we would be talking about that and not about time dilation. We're just focusing on time dilation because that is what the equation he quoted is focused on.
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P: 4,162
 Quote by arindamsinha I have read your posts in complete detail, even before my earlier responses. Still, I don't understand the difference. We are talking about one observer who is stationary w.r.t. the coordinate frame and one who is traveling. I believe we are saying the same thing. Let me know where specifically you are disagreeing on this.
You're talking about two observers and one coordinate frame. We calculate the time dilation of both observers in exactly the same way. We take their speed and plug it into the equation you posted on your first post and from that we calculate gamma. Then we can use the formula I posted on post #4, dτ/dt = 1/γ, to calculate the Proper Time of each observer with respect to the coordinate time. The Proper Time for your first stationary observer will pass at the same rate as the coordinate time. The Proper Time for your second traveling observer will pass at a slower rate than the coordinate time.

Do you see that the Proper Time for each observer can be easily calculated from the formula no matter what the speed is? And do you see that a coordinate frame does not require any observer to be stationary nor does it require any particular number of observers, not even one? Finally, do you see that if you use a definition for the Proper Time (or for time dilation) that does not include a specified coordinate frame, but rather is just between two observers, then it won't work because whatever you say about the passage of time for one of them with respect to the other one can also be said about the two observers if you interchange them and that would create a dichotomy. You can't say that the ratio of the times between A and B is the same as the ratio of the times between B and A unless both ratios are one.
P: 181
 Quote by ghwellsjr You're talking about two observers and one coordinate frame. We calculate the time dilation of both observers in exactly the same way. We take their speed and plug it into the equation you posted on your first post and from that we calculate gamma. Then we can use the formula I posted on post #4, dτ/dt = 1/γ, to calculate the Proper Time of each observer with respect to the coordinate time. The Proper Time for your first stationary observer will pass at the same rate as the coordinate time. The Proper Time for your second traveling observer will pass at a slower rate than the coordinate time. Do you see that the Proper Time for each observer can be easily calculated from the formula no matter what the speed is? And do you see that a coordinate frame does not require any observer to be stationary nor does it require any particular number of observers, not even one? Finally, do you see that if you use a definition for the Proper Time (or for time dilation) that does not include a specified coordinate frame, but rather is just between two observers, then it won't work because whatever you say about the passage of time for one of them with respect to the other one can also be said about the two observers if you interchange them and that would create a dichotomy. You can't say that the ratio of the times between A and B is the same as the ratio of the times between B and A unless both ratios are one.
Yes, but one observer is at rest w.r.t. the coordinate reference frame. So what's the issue? I do not mean observers as in 'human beings who happen to be at that location', but just the point of view.
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P: 4,162
 Quote by arindamsinha Yes, but one observer is at rest w.r.t. the coordinate reference frame. So what's the issue? I do not mean observers as in 'human beings who happen to be at that location', but just the point of view.
Suppose you take your two observers, one at rest and one traveling, and you transform to a new reference frame using the Lorentz transformation process such that now both observers are traveling in the opposite direction at the same speed. How will you interpret their respective Proper Times or their respective time dilations?
P: 181
 Quote by ghwellsjr Suppose you take your two observers, one at rest and one traveling, and you transform to a new reference frame using the Lorentz transformation process such that now both observers are traveling in the opposite direction at the same speed. How will you interpret their respective Proper Times or their respective time dilations?
OK, thanks. I got what you mean.
P: 372
 Quote by arindamsinha The Lorentz factor is used ubiquitously in relativity for transformation between frames and in describing various relationships. This is all great mathematically, and well understood in its applications in relativity. I am wondering if there is a simple and understandable explanation of what the Lorentz factor really is. I mean, is there any intuitive, physical way in which it can be explained? (for example, it is a conversion factor between such and such...) Any opinions on how we might be able to describe the 'real meaning' of the Lorentz factor in some intuitive and easily understandable way?
A very precise measuring system is available using reflected light. The light clock can serve as a standard unit of measure by reflecting light from a mirror to a detector.
In the U frame of reference, one unit of time is defined as the distance light moves from the origin to a mirror m located perpendicular to the x axis and return. If the light source is not moving, the distance is 2d for 1 time unit.
Observer A moves a distance (a) while a light wave composed of multiple photons, moves from the origin a distance d. Because light speed c is constant and independent of the source, the A-clock photon must have an x component equal to (a) to compensate for A's motion. The p component becomes the active part of the A-clock. Since p is less than d, the photon will move a greater distance to reach m, resulting in a tick rate less than that for U.
The clock photon for B will have a greater compensating component b, and thus a smaller active component q, resulting in a tick rate less than that for A.
The tick rate is a non-linear function of (clock speed)/(light speed), or v/c, with each observer clocking a different photon. The observer is not aware of his slow clock rate, because all processes involving photon interaction, including biological/(chemical) are also ocurring at the same rate, i.e. his perception is altered. Gamma(the Lorentz factor) equals d/(the vertical component) with v/c substituted for (the x component)/d. Gamma for A = d/p with v/c substituted for a/d.
The 't' factor has been omitted, because it is common to each vaiable and d, and to emphasize that the clock is in fact counting distances.

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Your diagram and explanation are very difficult to follow.
 Quote by phyti A very precise measuring system is available using reflected light. The light clock can serve as a standard unit of measure by reflecting light from a mirror to a detector. In the U frame of reference, one unit of time is defined as the distance light moves from the origin to a mirror m located perpendicular to the x axis and return. If the light source is not moving, the distance is 2d for 1 time unit.
You have established that for a stationary light source and observer, the light starts at 0, goes directly up to the mirror at m for a distance of d and then reflects back down for another distance of d for a round-trip distance of 2d. This part I understand.
 Quote by phyti Observer A moves a distance (a) while a light wave composed of multiple photons, moves from the origin a distance d. Because light speed c is constant and independent of the source, the A-clock photon must have an x component equal to (a) to compensate for A's motion. The p component becomes the active part of the A-clock. Since p is less than d, the photon will move a greater distance to reach m, resulting in a tick rate less than that for U.
This part I don't understand. You imply that the light only travels a distance d along the first diagonal but it doesn't reach m and you don't show the return path. You seem to be implying that length contraction is in play here which it is not so I'm very confused about your diagram and your explanation.
 Quote by phyti The clock photon for B will have a greater compensating component b, and thus a smaller active component q, resulting in a tick rate less than that for A. The tick rate is a non-linear function of (clock speed)/(light speed), or v/c, with each observer clocking a different photon. The observer is not aware of his slow clock rate, because all processes involving photon interaction, including biological/(chemical) are also ocurring at the same rate, i.e. his perception is altered. Gamma(the Lorentz factor) equals d/(the vertical component) with v/c substituted for (the x component)/d. Gamma for A = d/p with v/c substituted for a/d. The 't' factor has been omitted, because it is common to each vaiable and d, and to emphasize that the clock is in fact counting distances.
I've never heard of "compensating component" and "active component". Are these your terms?

I think what you have discovered is that if we plot the reciprocal of gamma as a function of normalized speed, we get a quarter of a circle. I made a similar plot some time ago to show the normalized age of the traveling twin (compared to the stationary twin) as a function of normalized speed (beta):

This was discussed in this thread, as well as others.

Although your plot might also describe the relationship of 1/gamma to speed, it is not labeled as such and has no discernable connection to a light clock, at least as far as I can understand. Maybe you could explain it some more and show how it relates to your diagram.
P: 846
 Quote by arindamsinha The Lorentz factor is used ubiquitously in relativity for transformation between frames and in describing various relationships. Wikipedia describes this as: ---------- The Lorentz factor is defined as: γ = 1/√(1-v2/c2) = 1/√(1-β2) = dt/dτ where: v is the relative velocity between inertial reference frames, β is the ratio of v to the speed of light c. τ is the proper time for an observer (measuring time intervals in the observer's own frame), c is the speed of light. ---------- This is all great mathematically, and well understood in its applications in relativity. I am wondering if there is a simple and understandable explanation of what the Lorentz factor really is. I mean, is there any intuitive, physical way in which it can be explained? (for example, it is a conversion factor between such and such...) Any opinions on how we might be able to describe the 'real meaning' of the Lorentz factor in some intuitive and easily understandable way?
Great question, arindamsinha. There is a very simple physical explanation. It is a mathematical representation of the way nature has given different observers different instantaneous 3-D cross-section views of the 4-dimensional universe.
 Mentor P: 15,569 What, no picture?
P: 846
 Quote by DaleSpam What, no picture?
I figured the sketches were getting annoying.
 P: 372 This post is intended to explain gamma/(the Lorentz factor) in terms of physical processes and minimal math instead of theoretical statements. The light clock consists of an integrated light emitter/detector, and a mirror, separated by a rod of length r. The clock counts a unit of time (t=1 tick) when a photon moves the length of the rod to the mirror, and returns to the detector. There are two observers, U who is not moving, and A who is moving at .6c relative to U on the Ux axis. Each has a copy of the clock with the rod oriented perpendicular to the x axis. Since the outbound path equals the inbound path, we only need to consider the first path. With U and A at the origin, each clock emits multiple photons (shown as a blue quarter circle because object motion is restricted to the +x axis.) For U the photon moves a distance r (.5 tick). For A the intersection of the circular arc and rod determine which photon becomes part of the clock. [1]The photon path ct can be resolved into the vt component which compensates for the motion of A and the st component which is the active part of the clock. When a photon arrives at the U mirror, the A clock photon has not reached the mirror because the photon speed relative to the rod is s. If r' equals the path length to the mirror for the A photon, then t'/t = r'/r = c/s = gamma. [1] A vector can be expressed in components suitable for the situation.
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P: 4,162
 Quote by phyti This post is intended to explain gamma/(the Lorentz factor) in terms of physical processes and minimal math instead of theoretical statements. The light clock consists of an integrated light emitter/detector, and a mirror, separated by a rod of length r. The clock counts a unit of time (t=1 tick) when a photon moves the length of the rod to the mirror, and returns to the detector. There are two observers, U who is not moving, and A who is moving at .6c relative to U on the Ux axis. Each has a copy of the clock with the rod oriented perpendicular to the x axis. Since the outbound path equals the inbound path, we only need to consider the first path. With U and A at the origin, each clock emits multiple photons (shown as a blue quarter circle because object motion is restricted to the +x axis.) For U the photon moves a distance r (.5 tick). For A the intersection of the circular arc and rod determine which photon becomes part of the clock. [1]The photon path ct can be resolved into the vt component which compensates for the motion of A and the st component which is the active part of the clock. When a photon arrives at the U mirror, the A clock photon has not reached the mirror because the photon speed relative to the rod is s. If r' equals the path length to the mirror for the A photon, then t'/t = r'/r = c/s = gamma. [1] A vector can be expressed in components suitable for the situation.
Like I said in my previous post #29, you have discovered a graphical relationship that relates speed to the reciprocal of gamma but it has nothing to do with your explanation of a light clock.

Your problem is that you claim that there is something significant when a photon hits the moving rod at the 80% mark. In fact, there are photons hitting both rods all along their trips to their respective mirrors. So what? There is no significance to the fact that a photon hits a rod at any particular time. What matters is when a photon hits the moving mirror, which you don't show. If you would continue the diagonal line up to the location of where the mirror would be when it hits it and then measure the time it takes for the photon to hit the mirror, you would see that it take 1.25 times as long as it takes for the photon to hit the stationary mirror which gives the correct illustration "in terms of physical processes".
P: 372
 Quote by ghwellsjr Like I said in my previous post #29, you have discovered a graphical relationship that relates speed to the reciprocal of gamma but it has nothing to do with your explanation of a light clock.
The hyperbola is the inverse function of the quarter circle, a fact of geometry.

 Your problem is that you claim that there is something significant when a photon hits the moving rod at the 80% mark. In fact, there are photons hitting both rods all along their trips to their respective mirrors. So what? There is no significance to the fact that a photon hits a rod at any particular time. What matters is when a photon hits the moving mirror, which you don't show. If you would continue the diagonal line up to the location of where the mirror would be when it hits it and then measure the time it takes for the photon to hit the mirror, you would see that it take 1.25 times as long as it takes for the photon to hit the stationary mirror which gives the correct illustration "in terms of physical processes".
The rod is only a visual aid to indicate a rigid device moving with A, and represents the photon path as imagined by A. It helps with those new to SR. The photon doesn't hit the rod. The successful photon has to have the correct angle to intercept the A mirror. How would it 'know' this? It doesn't. As stated, the successful photon has a horizontal vt component that equals the speed of the clock. The vertical st component moves toward the mirror but at less than c. The intersection of the arc of photons and the path/(rod) to the mirror is determined by the speed of A. The angle of the ct photon relative to vertical is a function of v/c. A simple examination shows, a lesser speed, a smaller angle and a greater s,
a greater speed, a greater angle and a lesser s.

The drawing is intended to show why 1 tick for the A clock is longer than 1 tick for the U clock for the general case, which it does. The reason is the constant and independent speed of light. The degree of dilation is a function of v/c. If the path is extended, it will not show anything new. Gamma is still the ratio c/s or (A time)/(U time).
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P: 4,162
 Quote by phyti If the path is extended, it will not show anything new. Gamma is still the ratio c/s or (A time)/(U time).
Exactly and that's because the ratio between ct and st is the same for all t. You just have a different size triangle involving ct, st and vt. I just don't know why you think there is anything significant for the specific value of t corresponding to when the photons hit U's mirror. If you had used the value of t corresponding to when the photons hit A's mirror, then st will be equal to r, the distance to U's mirror from O, and ct would be the distance to A's mirror from O, and ct/st = c/s = gamma. If you would draw that diagram, which is the diagram that everyone else draws, then it seems like it would be more helpful to those new to SR.

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