3 ms·

I was reading this article carefully. I noticed that he uses the word "stationary" to mean both "absolutely stationary" and "relatively at rest". The reasoning

by nyc111 2y ago

I was reading this article carefully. I noticed that he uses the word "stationary" to mean both "absolutely stationary" and "relatively at rest". The reasoning in this article is based on conflating and mixing the two meanings. So for instance when he writes "photons are always in motion" [1] he assumes that there are objects that are not always in motion, that is, they are in absolute rest. But he also writes "specifically its 'rest mass' m, which is the mass as measured by an observer who is stationary relative to the object." In this quote, he defines the word "stationary" as relative rest. This rhetorical trick is so common in physics, that's why I wanted to mention it. The trick is to define the same word twice with its opposite meanings and use the word with both meanings sometimes even in the same sentence. I wonder what is the name of this trick in logic. [1] This quote is from a different articla: https://profmattstrassler.com/waves-in-an-impossible-sea/waves-in-an-impossible-sea-commentary-and-discussion/chapter-5/ https://profmattstrassler.com/waves-in-an-impossible-sea/wav...

SetTheorist 2y ago
Generally, "equivocation" is the fallacy of using the same word with different meanings in an argument. https://en.wikipedia.org/wiki/Equivocation https://en.wikipedia.org/wiki/Equivocation
kccqzy 2y ago
I read the same article and I didn't find "absolutely stationary" in the article. My physics is rusty but even from my recollection of high school physics there is no such thing. It's clear that the word stationary means zero velocity, and first year high school physics taught that any measurement of velocity must be done in a reference frame. The idea that a photon is always in motion is true regardless of which reference frame you pick; it does not imply there are objects that are in absolute rest.
thfuran 2y ago
There is no absolute reference frame that would distinguish "absolutely stationary", as you call it, from "relatively at rest". There is only relative motion. Photons are "always in motion" in that there does not and cannot exist any massive object with respect to which a photon is at rest.
- jerf 2y ago
  I do not know where I first saw it, but I saw someone observe that the shocking thing about Einstein's theory of relativity is not that everything is relative; that was understood reasonably well for a long time before him. What is shocking is that the speed of light in a vacuum, or the speed of causality, is a constant. You can derive huge swathes of relativity from that fact alone. As a result, of all the speeds, when discussing the speed of light (which conventionally means "in a vacuum" unless otherwise mentioned), you can in fact ignore the question of reference frame, with the exception of you don't want to use a reference frame itself moving at the speed of light. But other than that, an article exclusively confining itself to the discussion of the speed of light in fact doesn't need to worry itself about the relativity of reference frames. For that speed alone, you can't level the complaint against it that it ignores the issue of different frames, because it uniquely doesn't matter. (Relatedly: It is frequently given as the reason you can't reach or exceed the speed of light is some stuff about masses rising. While mathematically true in its own way, I think there's a cleaner reason to explain why you can't reach or exceed it, which is that you can't even get closer to it. No matter what you do, the speed of light is c. You accelerate to a thousand miles a second in some direction, and how much closer are you to c? The answer is, none. Light continues fleeing from you, in all directions, at c. There is no "get really close to c somehow and then just push yourself over really hard" because there is no "get close to c" in the first place. No matter how hard you accelerate, in what direction, in what order, in what manner, you not only can't get "close" to c, you can't even get closer. For similar reasons, in this paragraph, I don't need to qualify in which reference frame you go a thousand miles per second different than before, because it doesn't matter for c. You can't even get slightly closer to it, let alone "exceed" it somehow. It is an absolute.)
  - thfuran 2y ago
    That motion is relative was understood. That even notions like simultaneity also depend on reference frame I think was shocking. And I'm not really sure what it would mean for light to not be at rest in any reference frame but for it to not have the same speed in all reference frames. I think that would be even more shocking than light having a constant speed.
  - 2y ago
elashri 2y ago
It is not a fallacy. In special relativity the important concept is the frame of reference. For photons there is not no inertial frame of reference where photon will be stationary (relative or absolute means nothing actually). The whole concept of measurement (which you will need to describe something as stationary) depends on an observer travelling at speed velocity less than speed of light. If you Try to assign a inertial frame of reference to a photon you will break laws of special relativity - photons always travel in speed of light from perspective of any inertial observer whether it is moving or at rest (verified by experiment) - relativity prohibited mass less particles (like photon) to be stationary. E = mc^2 doesn't apply in photon and apply only on stationary objects. The complete equation is E^2 = (p^2 c^2) + (m c^2)^2 Where m is the rest mass. In photon case the second terms vanishes and the energy is merely the first term. -Time and space behave differently for photons. In relativity, time dilation and length contraction become extreme at light speed. A photon does not experience the passage of time in the way objects with mass do. Therefore, trying to define a reference frame for a photon would lead to contradictions in our understanding of spacetime. So in general it doesn't mean much absolute vs relative on photon case.
- nyc111 2y ago
  "In special relativity the important concept is the frame of reference." I study motion not special relativity. Motion is a fundamental concept. We don't need to use Einsteinian lingo to study motion.
- nyc111 2y ago
  "For photons there is [] no inertial frame of reference where photon will [appear to] be stationary (relative or absolute means nothing actually)." I added my comments to your sentence. Obviously, if photon appears to be stationary, this means relative to something else. So relative and absolute are not meaningless.
deleted 2y ago
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by nyc111 2y ago

SetTheorist 2y ago

Generally, "equivocation" is the fallacy of using the same word with different meanings in an argument. https://en.wikipedia.org/wiki/Equivocation https://en.wikipedia.org/wiki/Equivocation

kccqzy 2y ago

I read the same article and I didn't find "absolutely stationary" in the article. My physics is rusty but even from my recollection of high school physics there is no such thing. It's clear that the word stationary means zero velocity, and first year high school physics taught that any measurement of velocity must be done in a reference frame. The idea that a photon is always in motion is true regardless of which reference frame you pick; it does not imply there are objects that are in absolute rest.

thfuran 2y ago

There is no absolute reference frame that would distinguish "absolutely stationary", as you call it, from "relatively at rest". There is only relative motion. Photons are "always in motion" in that there does not and cannot exist any massive object with respect to which a photon is at rest.

jerf 2y ago
I do not know where I first saw it, but I saw someone observe that the shocking thing about Einstein's theory of relativity is not that everything is relative; that was understood reasonably well for a long time before him. What is shocking is that the speed of light in a vacuum, or the speed of causality, is a constant. You can derive huge swathes of relativity from that fact alone. As a result, of all the speeds, when discussing the speed of light (which conventionally means "in a vacuum" unless otherwise mentioned), you can in fact ignore the question of reference frame, with the exception of you don't want to use a reference frame itself moving at the speed of light. But other than that, an article exclusively confining itself to the discussion of the speed of light in fact doesn't need to worry itself about the relativity of reference frames. For that speed alone, you can't level the complaint against it that it ignores the issue of different frames, because it uniquely doesn't matter. (Relatedly: It is frequently given as the reason you can't reach or exceed the speed of light is some stuff about masses rising. While mathematically true in its own way, I think there's a cleaner reason to explain why you can't reach or exceed it, which is that you can't even get closer to it. No matter what you do, the speed of light is c. You accelerate to a thousand miles a second in some direction, and how much closer are you to c? The answer is, none. Light continues fleeing from you, in all directions, at c. There is no "get really close to c somehow and then just push yourself over really hard" because there is no "get close to c" in the first place. No matter how hard you accelerate, in what direction, in what order, in what manner, you not only can't get "close" to c, you can't even get closer. For similar reasons, in this paragraph, I don't need to qualify in which reference frame you go a thousand miles per second different than before, because it doesn't matter for c. You can't even get slightly closer to it, let alone "exceed" it somehow. It is an absolute.)
- thfuran 2y ago
  That motion is relative was understood. That even notions like simultaneity also depend on reference frame I think was shocking. And I'm not really sure what it would mean for light to not be at rest in any reference frame but for it to not have the same speed in all reference frames. I think that would be even more shocking than light having a constant speed.
- 2y ago

elashri 2y ago

It is not a fallacy. In special relativity the important concept is the frame of reference. For photons there is not no inertial frame of reference where photon will be stationary (relative or absolute means nothing actually). The whole concept of measurement (which you will need to describe something as stationary) depends on an observer travelling at speed velocity less than speed of light. If you Try to assign a inertial frame of reference to a photon you will break laws of special relativity - photons always travel in speed of light from perspective of any inertial observer whether it is moving or at rest (verified by experiment) - relativity prohibited mass less particles (like photon) to be stationary. E = mc^2 doesn't apply in photon and apply only on stationary objects. The complete equation is E^2 = (p^2 c^2) + (m c^2)^2 Where m is the rest mass. In photon case the second terms vanishes and the energy is merely the first term. -Time and space behave differently for photons. In relativity, time dilation and length contraction become extreme at light speed. A photon does not experience the passage of time in the way objects with mass do. Therefore, trying to define a reference frame for a photon would lead to contradictions in our understanding of spacetime. So in general it doesn't mean much absolute vs relative on photon case.

nyc111 2y ago
"In special relativity the important concept is the frame of reference." I study motion not special relativity. Motion is a fundamental concept. We don't need to use Einsteinian lingo to study motion.
nyc111 2y ago
"For photons there is [] no inertial frame of reference where photon will [appear to] be stationary (relative or absolute means nothing actually)." I added my comments to your sentence. Obviously, if photon appears to be stationary, this means relative to something else. So relative and absolute are not meaningless.

deleted 2y ago

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