Approaching Speed: Is it Different for Photons?

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Discussion Overview

The discussion revolves around the concepts of "approaching speed," "closing speed," and "relative speed," particularly in the context of special relativity and their application to photons and other entities. Participants explore the definitions and distinctions between these terms, as well as their implications in various scenarios, including hypothetical situations involving moving cars.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that "approaching speed" refers to the relative speed of an object measured against another object it is nearing.
  • Others argue that "closing speed" is a specific term that describes the rate at which two objects approach each other, which can differ from relative speed in general.
  • A participant mentions that Einstein's views on relative speed have evolved in popular usage, suggesting that "relative speed" is often equated with the speed of an entity relative to a reference system.
  • In a hypothetical scenario involving two cars, participants discuss the relative speed and closing rate, providing a formula for calculating relative speed that incorporates relativistic effects.
  • Some participants assert that simply adding speeds to find closing speed is incorrect without considering relativistic adjustments, emphasizing that it does not represent the speed of any tangible object.
  • There is a mention of the practical application of closing speed in calculating time to collision, despite its limitations in representing actual speeds.
  • A participant questions whether special relativity applies only when the observer is one of the two objects in relative motion, leading to a clarification about the concept of "rest frame."
  • Another participant notes that the results of relative speed and closing speed are nearly identical at low speeds.

Areas of Agreement / Disagreement

Participants express differing views on the definitions and implications of approaching speed, closing speed, and relative speed. While some points of clarification are made, no consensus is reached on the fundamental distinctions or applications of these concepts.

Contextual Notes

Participants highlight the importance of definitions and the context in which these terms are used, indicating that assumptions about speed may vary based on the reference frame and the entities involved.

moatasim23
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Are approching speed and relative speed different quantities in all circumstances or only for photon?
 
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What, exactly, do you mean by "approaching speed"? My interpretation would be that it is the relative speed of an object, measured relative to some other object it is nearing ("approaching").
 
moatasim23 said:
Are approching speed
Do you mean closing speed? (The rate at which two things approach each other as measured in some third frame.)

If that's what you mean then, yes, closing speed and relative speed are different in general, not just for photons.
 
Ah, thanks, Doc. Yes, that would be different.
 
moatasim23 said:
Are approching speed and relative speed different quantities in all circumstances or only for photon?
As you know, Einstein disagreed with that. But nowadays it has become popular to mean with "relative speed" the same as what commonly is meant with "speed" - the speed of an entity relative to a reference system. Those who use such a definition for "relative speed" then must use such expressions as "approaching" or "closing" speed for the speed of one entity relative to another one, as measured with a reference system in which both are moving. That is generic, for all entities and not only for photons.
 
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Doc Al said:
Do you mean closing speed? (The rate at which two things approach each other as measured in some third frame.)

If that's what you mean then, yes, closing speed and relative speed are different in general, not just for photons.

If we consider a car A moving with speed v towards another car B traveling with speed 2v towards A.What is the relative speed and what is the closing rate?
 
moatasim23 said:
If we consider a car A moving with speed v towards another car B traveling with speed 2v towards A.What is the relative speed and what is the closing rate?
The relative speed of A according to B (or vice versa) is (v + 2v)/(1+v*2v/c^2) where v<c and the closing speed is 3v.
 
yuiop said:
moatasim23 said:
If we consider a car A moving with speed v towards another car B traveling with speed 2v towards A.What is the relative speed and what is the closing rate?
The relative speed of A according to B (or vice versa) is (v + 2v)/(1+v*2v/c^2) where v<c and the closing speed is 3v.
Yes, the so-called "closing speed" is the incorrect answer when you simply add the two speeds together without doing the division part of the correct formula. It doesn't represent anything real. It's not the speed of anything, just two numbers added together and given a name so that we can talk about how wrong it is.
 
ghwellsjr said:
Yes, the so-called "closing speed" is the incorrect answer when you simply add the two speeds together without doing the division part of the correct formula. It doesn't represent anything real. It's not the speed of anything, just two numbers added together ...
It does have one practical application. If the distance between the two objects is d, then the time to collision is d/(closing speed) but as you correctly point out it is not the speed of any single tangible object and is not a violation of SR or an example of superluminal speed.
 
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  • #10
If I'm paraphrasing correctly, effectively special relativity only "applies" when the observer is actually one out of the two. Is there a technical term for that?
 
  • #11
m4r35n357 said:
If I'm paraphrasing correctly, effectively special relativity only "applies" when the observer is actually one out of the two. Is there a technical term for that?
Special relativity effectively applies for all these cases (see my earlier reply). The technical term for measurements "when the observer is actually one out of the two" objects that are in relative motion, is that object's "rest frame".
 
  • #12
But the answers of the two are almost identical at low speeds.Arent they?
 
  • #13
Yes.
 

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