Approaching Speed: Is it Different for Photons?

[moatasim23]

Are approching speed and relative speed different quantities in all circumstances or only for photon?

 

[HallsofIvy]

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").

 

[Doc Al]

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.

 

[HallsofIvy]

Ah, thanks, Doc. Yes, that would be different.

 

[harrylin]

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.

 

[moatasim23]

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?

 

[yuiop]

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.

 

[ghwellsjr]

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.

 

[yuiop]

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.

 

[m4r35n357]

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?

 

[harrylin]

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".

 

[moatasim23]

But the answers of the two are almost identical at low speeds.Arent they?

 

[ghwellsjr]

Yes.