Understanding Relativity: Chat with a Physics Professor Online for Clarity

[physwizard]

c) the speed of light is relative to the observer but constant relative to the space it's occupying.

One of the principles of relativity is to ditch the concept of 'space' and rather focus on how distances are measured in this 'space'. And how would you be able to measure its speed relative to the 'space it's occupying' ? In physics, you only talk about things which you can measure and for which a concrete procedure can be carried out to measure it. Otherwise, its not physics, its not even science.
To understand relativity you need to critically think about the concepts of space and time. This is something you can understand only by pondering over it with assistance from some book, and not by discussing in the forum. Two books come to my mind here - one is the small book written by robert resnick(i don't know if it is still in publication) and the other one is the one by Einstein written for the lay reader. These are good starts but no book can really hand it over to you you need to put the pieces together yourself.
So I don't really think anything written on this forum can really help you you got to meditate upon it yourself with assistance from written material. Good luck!

 

[Dale]

well, its somewhat the cause of confusion.
here's how i feel the conversation went.
can you add velocities in an intertail frame of reference? yes. if an inertial frame sees two objects going two velocities, you can add and subtract them.

This is the separation velocity. All of the velocities are calculated in a single frame.

If B sees A going 0.8, would A see B going 0.8?

Here you are asking how velocities in one frame transform into another. This is a fundamentally different question, so it shouldn't be surprising that it requires a different formula.

if two objects each going 1.6 away from each other from one inertial frame, can they do so from their own inertial frame?

Again, this is asking how velocities transform between frames.

 

[phillip1882]

I'm not trying to be difficult, I'm just trying to state what logically makes sense to me.

a) Do you mean that if the speed of B relative to A is v, the speed of A relative to B is not -v? SR states very clearly that the speed of A relative to B is -v.

yes. either velocity should change, allowing the property of addition to apply, or the property of addition should not apply to any observer.
[

b) Are you talking about how the rate of change of the coordinate distance between A and B is 1.6c, in the coordinate system in which A has velocity -0.8c and B has velocity 0.8c? This couldn't be any other way, because it's part of what we mean by things like "position", "time", "distance", "rate of change", etc. It doesn't have anything to do with what theory of physics is the most accurate. There isn't even a change of coordinates involved, since we're doing everything in one coordinate system.

I'm not quite sure how to respond to this, as i feel I've addressed this several times already.

c) Here I can't even guess what you mean.

okay. we all agree that, no matter what direction you fire a light beam in air, it will have the same velocity.
so i would argue the speed of light relative to air is constant. however, if i ran toward toward a light beam, i would measure a different velocity of light.
if a star emits a light beam, the light beam relative to the background radiation, (the light and electromagnetic radiation of other stars) will always be the same value. therefore i would argue that the speed of light relative to the star is changing. can you prove other wise? please no math, just logic.

Not just because there's a "law" that says no. I showed you how to use SR to calculate the velocity of B in the inertial coordinate system that's comoving with A. You didn't reply to that.

okay, you're really hung up on this, so let me do the following.
let me make the following assumptions.
1) there is an instantaneous method of determining distance and time.
2) there is no fundamental speed limit
3) the detector of distance and time can only measure in one direction.

<-----OO------>    O----->
 50          60             50
what's actually occurring, according to an impartial observer.

<-----------O|O             <-O
     110                          10
what's occurring according to the 60 m/h ball.

Okay. Imagine I have two balls each traveling apart at 50 miles an hour. after 1 hour, they are 100 miles apart. to determine the distance, the 50 mile an hour ball fires a 60 mile an hour ball toward the other 50 m/h ball. each ball has a diameter of 1 mile, so each ball can know how far away they are based on their own size, and each carries an accurate method for determining time.
the 60 m/h ball can't detect his own motion of course, but he sees the distant 50 m/h ball approaching him in a very odd manner. despite the fact he can detect the distance between himself and the distant ball as roughly 100 miles, it seems that the distant ball covers a distance of 600 miles in a time span of 10 hours, despite the fact the ball is traveling toward him. he concludes some sort of time space dilation must be occurring.
he doesn't know how much time dilation is occurring yet, but he can safely assume that space is being dilated by a factor of 6, and guesses that the ball must be traveling toward him at a rate of 10 miles/hour. now that the 50m/h ball has reached him, the 60 mile/hour ball bounces of the 50 m/h ball and is now traveling 60 m/h in the opposite direction. wow. the other ball is now a distance of 1100 miles away from him, despite the fact that only 10 hours have passed. he knows space is being dilated by a factor of 6, so 1100/6 = 183 miles must be the actual distance between him and the distant ball. dividing the two, he concludes that the ball must be traveling at a rate of 18.3 miles an hour. he doesn't know if time is being dilated yet, so he checks. it takes a very strange 18.3 hours for the ball to reach him, instead of the expected 10! "since I've already factored in space dilation, the only explanation must be time dilation." he reasons. dividing the two, he concludes that time must be dilated by a factor of 1.83. factoring in both,velocity must be being dilated by a factor of 3.27. therefore the original ball must have been going a speed of 600/10 = 60, 60/3.27 = 18.3 miles per hour, and the other must be going the same in the opposite direction. his job finished, he reports back that the velocity between the two 50 mile an hour balls is roughly 36.6 miles/hour.

 

[Nugatory]

so i would argue the speed of light relative to air is constant. however, if i ran toward toward a light beam, i would measure a different velocity of light.

You can argue that all you want, but it's just not true. That's why we do experiments, to check our intuitive arguments against how the world really works. The experiments say that if you run towards a light beam you won't measure a different velocity of light, although you will measure a blue shift.

please no math, just logic.

Hard to do, because logic is a branch of mathematics.

 

[Fredrik]

yes. either velocity should change, allowing the property of addition to apply, or the property of addition should not apply to any observer.

SR and pre-relativistic classical mechanics are both defined to be consistent with some basic symmetry assumptions about space, in particular rotation invariance. This ensures that both of these theories will agree that if the velocity of A relative to B is v, the velocity of B relative to A is -v.

I'm not saying that a theory in which the velocity of B relative to A is different from -v is logically inconsistent, but I am saying that such a theory has nothing to do with relativity.

I'm not quite sure how to respond to this, as i feel I've addressed this several times already.

I haven't seen you do it. If A's position at time t is given by $x_A(t)=-vt$, and B's position at time t is given by $x_B(t)=vt$, then the distance between them at time t is, by definition of distance, $x_B(t)-x_A(t)=vt-(-vt)=2vt$. If we denote this by d(t), then the rate of increase of the distance is defined to be d'(t), and this is equal to 2v. Note that everything follows from the definitions. There's no physics in this problem.

okay. we all agree that, no matter what direction you fire a light beam in air, it will have the same velocity.

Right, every theory that's consistent with rotational invariance would say this, so we don't even have to use SR. Pre-relativistic classical mechanics says the same thing.

however, if i ran toward toward a light beam, i would measure a different velocity of light.

According to pre-relativistic classical mechanics, yes. According to SR, no. And experiments say that SR is right about this (or at least a lot less wrong than pre-relativistic classical mechanics).

You're just telling me what your intuition is telling you, and your intuition is telling you all the things that pre-relativistic classical mechanics says, because that theory was made to agree with our intuition. SR on the other hand was made to agree with the idea that you won't measure a different velocity of light when you're running toward the light source. (In 1905, this could be motivated both by Maxwell's equations and the Michelson-Morley experiment).

if a star emits a light beam, the light beam relative to the background radiation, (the light and electromagnetic radiation of other stars) will always be the same value. therefore i would argue that the speed of light relative to the star is changing. can you prove other wise? please no math, just logic.

This is like asking me to run without using my legs. Proofs are mathematical, and they're done within the framework of a theory. It doesn't make sense to ask for a theory-independent proof that doesn't rely on math, because there's no such thing.

I also don't understand what you said before that, so I don't even know what you're asking me to prove.

let me make the following assumptions.
1) there is an instantaneous method of determining distance and time.
2) there is no fundamental speed limit

Both of these assumptions contradict SR, but are consistent with pre-relativistic classical mechanics. So I hope you won't be mentioning anything relativistic later, because then you'll be contradicting yourself as well.

Imagine I have two balls each traveling apart at 50 miles an hour. after 1 hour, they are 100 miles apart. to determine the distance, the 50 mile an hour ball fires a 60 mile an hour ball toward the other 50 m/h ball. each ball has a diameter of 1 mile, so each ball can know how far away they are based on their own size, and each carries an accurate method for determining time.
the 60 m/h ball can't detect his own motion of course, but he sees the distant 50 m/h ball approaching him in a very odd manner. despite the fact he can detect the distance between himself and the distant ball as roughly 100 miles, it seems that the distant ball covers a distance of 600 miles in a time span of 10 hours, despite the fact the ball is traveling toward him. he concludes some sort of time space dilation must be occurring.

There's nothing like that going on here. Consider an external observer who sees ball A go at -50 mph, ball B at 50 mph and ball C at 60 mph. From ball C's point of view, ball C is stationary and ball B is 100 miles away and moving towards ball C at 10 mph. This will take 10 hours. From the external observer's point of view, the position of ball C at time t (in hours, after it left ball B) is -50+60t, and the position of ball B at time t is 50+50t. Set the two equal and solve for t (to find the value of t at which both balls have the same position). The result is 10. The external observer and ball C agree that it takes 10 hours. There's no time dilation or anything like that at all.

he doesn't know how much time dilation is occurring yet

Your assumptions imply that there's no time dilation. Your conclusions appear to be a result of combining coordinate assignments made by different coordinate systems in a bad way.

 

[Dale]

yes. either velocity should change, allowing the property of addition to apply, or the property of addition should not apply to any observer.

I don't know what is so hard to understand here.

In any frame you can take velocities measured in that frame and use $v-u$ to get the separation velocity between V and U that frame. In any frame you can take velocities measured in that frame and use $(v-u)/(1-vu/c^2)$ to get the velocity of V in U's frame.

What is in any way unclear or inconsistent about the above?

 

[phillip1882]

in any frame you can take velocities measured in that frame and use v−u to get the separation velocity between V and U that frame. In any frame you can take velocities measured in that frame and use (v−u)/(1−vu/c2) to get the velocity of V in U's frame.

What is in any way unclear or inconsistent about the above?

re read this again please.
if i can add and subtract v and u to get the velocity between v and u in a frame of reference, and time dilation and space dilation doesn't change velocity, why would v see a different velocity with respect to u?

This is like asking me to run without using my legs. Proofs are mathematical, and they're done within the framework of a theory. It doesn't make sense to ask for a theory-independent proof that doesn't rely on math, because there's no such thing.

well, no.
for example i can prove that the force of gravity must change with mass.
if if we accept Newtons first hypothesis, Force = Mass* acceleration, and we know all falling objects accelerate at the same rate, then when mass changes force must change with it.

now. we have two hypothesis of Einstein. 1) the laws of physics are constant with respect to all frames of reference. okay i agree with this, there's no argument from me here. the speed of light is constant with respect to all frames of reference. okay i disagree with this part, because in my opinion it leads it impossible to detect a particle that could potentially be moving faster than the speed of light.

the Michel Morley experiment doesn't prove light is constant in all frames of reference. what the Michel Morley experiment proves is that you cannot push a light beam that's traveling through medium any faster.

here's three experiments i would recommend.
#1) measure the speed of light in a strong wind versus a calm day. i strongly suspect that if the medium changes velocity so will the speed of light.
#2) measure the speed of light on an airplane that's going faster than the speed of sound. I strongly suspect the speed of light will measure the same value, despite the fact the plane is in motion, which again doesn't prove light is constant to all observers, just constant to the medium it's traveling in.
#3) have the light beam that's fired inside the plane exit the plane and hit a target on the ground. the light beam will change velocity, for the same reason as it would in experiment #1.
if you disagree with this assessment, please try these experiments for yourself.

we know for a fact that the speed of light changes depending on the medium it's in. so how can the speed of light possibly be constant for all observers?

and again i would like to state that sound has this same property.

 

[Mentz114]

...
...
here's three experiments i would recommend.
#1) measure the speed of light in a strong wind versus a calm day. i strongly suspect that if the medium changes velocity so will the speed of light.
#2) measure the speed of light on an airplane that's going faster than the speed of sound. I strongly suspect the speed of light will measure the same value, despite the fact the plane is in motion, which again doesn't prove light is constant to all observers, just constant to the medium it's traveling in.
#3) have the light beam that's fired inside the plane exit the plane and hit a target on the ground. the light beam will change velocity, for the same reason as it would in experiment #1.
if you disagree with this assessment, please try these experiments for yourself.

we know for a fact that the speed of light changes depending on the medium it's in. so how can the speed of light possibly be constant for all observers?

and again i would like to state that sound has this same property.

If you are asserting that light requires a medium to propagate, this would be contrary to observed fact, contrary to relativity and provocative in the extreme considering how many people you have ignored in this thread.

The speed of light in a vacuum is the universal constant. Intoducing media is irrelevant to the issues you've raised.

 

[D H]

re read this again please.
if i can add and subtract v and u to get the velocity between v and u in a frame of reference, and time dilation and space dilation doesn't change velocity, why would v see a different velocity with respect to u?

Simple. You can't add and subtract velocities as vectors.

Strictly speaking, the only time it is valid to add or subtract velocities is if one of them is zero. That one apparently can add or subtract velocities is a consequence of the errors becoming immeasurably small in the world of everyday events where velocities are less than one millionth of the speed of light (670 miles per hour).

Just because it appears that something is valid in our ordinary, everyday world does not mean that it is universally valid. Newtonian mechanics falls apart in the realms of the very small, very large, and very fast. Extrapolating your everyday experiences to domains orders of magnitude removed from those experiences is, simply put, wrong.

 

[Fredrik]

if i can add and subtract v and u to get the velocity between v and u in a frame of reference, and time dilation and space dilation doesn't change velocity, why would v see a different velocity with respect to u?

That first thing follows immediately from the definitions of position and derivative, and is the same in all classical theories of motion (including SR). That other thing follows from the definition of special relativity, which states that inertial coordinate systems are related by Poincaré transformations. (Lorentz transformations if they have the same origin).

You can't just jump to conclusions based on what your intuition is telling you, and ignore what the theory is saying.

well, no.
for example i can prove that the force of gravity must change with mass.
if if we accept Newtons first hypothesis, Force = Mass* acceleration, and we know all falling objects accelerate at the same rate, then when mass changes force must change with it.

You just used a theory and mathematics. These are the two things you told me not to use.

the speed of light is constant with respect to all frames of reference. okay i disagree with this part,

You disagree with the postulate that gave us the most successful scientific theories in human history?

because in my opinion it leads it impossible to detect a particle that could potentially be moving faster than the speed of light.

This conclusion is incorrect. And in SR, a particle either stays slower than c through its entire existence, or stays faster than c through its entire existence. Particles of the second type are believed not to exist.

I'm not even going to address the rest of your misconceptions. You need to think about whether you want to learn relativity or not. If you do, then you need to start asking questions about relativity, and stop making arguments based on your intuition and your ignorance of what the theory says.

 

[Fredrik]

Simple. You can't add and subtract velocities as vectors.

He was asking why you can't do that. I posted a complete answer to that in #24. Unfortunately, he's not interested in an answer that uses mathematics or is based on special relativity.

 

[Dale]

if i can add and subtract v and u to get the velocity between v and u in a frame of reference, and time dilation and space dilation doesn't change velocity, why would v see a different velocity with respect to u?

Because velocities in one frame don't directly tell you anything about velocities in another frame. You have to transform into the other frame to get velocity in the other frame.

This has nothing to do with special relativity. This has to do with coordinate transformations. If a value is coordinate-dependent (like velocity) then the value in one coordinate system doesn't tell you anything about the value in another coordinate system, you must transform to the new coordinate system.

 

[Dale]

the speed of light is constant with respect to all frames of reference. okay i disagree with this part, because in my opinion it leads it impossible to detect a particle that could potentially be moving faster than the speed of light.

This is simply wrong. Here is a thread where we discussed this at length:
https://www.physicsforums.com/showthread.php?t=683408&page=2

See in particular posts 23 and 28.

It is not impossible to detect v>c, the fact that we have not detected any particle with v>c is due to the behavior of all known particles, not due to a limitation of our measuring devices.

 

[Samshorn]

If i can add and subtract v and u to get the velocity between v and u in a frame of reference, and time dilation and space dilation doesn't change velocity, why would v see a different velocity with respect to u?

It wouldn't, but there is more to Lorentz covariance than just time dilation and length contration. You're overlooking the relativity of simultaneity, and the physical basis for that.

Remember, speeds have meaning only in terms of a given system of space and time coordinates. To test your ingenuity, see if you can imagine two relatively moving systems of space and time coordinates such that a given pulse of light has the same speed in terms of both of them. Feel free to adjust the scaling AND tilt the space and time axes as necessary to achieve this. (Hint: It's possible.)

 

[phillip1882]

It is not impossible to detect v>c, the fact that we have not detected any particle with v>c is due to the behavior of all known particles, not due to a limitation of our measuring devices.

okay then use this measuring device relative two the two particle a and -a!
don't use light itself.
if i fire two light beams at each other, use this device to determine the speed between the two light beams.

in a vacuum, light has a medium. not matter, background radiation. you can never fully eliminate background radiation because it exists everywhere, and any sort of attempt to block it will generate background radiation itself. show me a way to completely eliminate background radiation, and fire a light beam through it. i would dearly love to know what happens.

 

[Dale]

okay then use this measuring device relative two the two particle a and -a!

That gives you the separation velocity in the device's frame, not the velocity of -a in a's frame nor the velocity of a in -a's frame.

in a vacuum, light has a medium. not matter, background radiation.

This is completely illogical. The background radiation IS light. It is the light given off by a black body at ~3 K, just like the light from a typical lightbulb is given off by a blackbody at ~3000 K. So what you are saying here is that light has a medium and that medium is light.

 

[phillip1882]

That gives you the separation velocity in the device's frame, not the velocity of -a in a's frame nor the velocity of a in -a's frame.

derive the equation for velocity addition (a1 +a2)/(1 +a1*a2/c^2)
with the assumption that two particles can detect motion faster than the speed of light; because apparently its possible.

This is completely illogical. The background radiation IS light. It is the light given off by a black body at ~3 K, just like the light from a typical lightbulb is given off by a blackbody at ~3000 K. So what you are saying here is that light has a medium and that medium is light.

in a round about sort of way yes. if you can believe the illogical assumption that light is not relative to an observer no matter the speed, than i can believe the illogical assumption that light requires a medium in a vacuum, even if that medium is light itself.

i think if you completely eliminated light from a medium, and then tried to put light through a medium, the light would not just go straight through. it would immediately scatter.
light is made up of particles. but those particles require other nearby particles in order to travel. you can't have motion without a frame of reference, therefore light creates it own frame of reference.

 

[Fredrik]

As everyone can see, we have closed this thread. The discussion was clearly not going anywhere.

 

[Dale]

Sorry Fredrik, I wanted to leave phillip1882 with a response to his last post, in case he checks in. I know it is probably futile, but who knows?

derive the equation for velocity addition (a1 +a2)/(1 +a1*a2/c^2)
with the assumption that two particles can detect motion faster than the speed of light; because apparently its possible.

Sure, here is a brief outline of the proof. Assume the following three things:
1) The principle of relativity
2) The invariance of c
3) Superluminal motion can be detected

From 1 and 2 derive the Lorentz transform. From the Lorentz transform derive the velocity composition formula.

If you notice, the third assumption is irrelevant to the proof. There is no need to assume it, but there is no harm in assuming it either. In fact, the third assumption need not be assumed at all, it can be proven as I did in the posts I linked to earlier on the other thread.

in a round about sort of way yes. if you can believe the illogical assumption that light is not relative to an observer no matter the speed, than i can believe the illogical assumption that light requires a medium in a vacuum, even if that medium is light itself.

The difference is that the first "illogical" really means "unintuitive to me" while the second "illogical" really means "self contradictory". If light needs a medium and the medium for light A is light B then light B still needs a medium, unless light doesn't need a medium, in which case light B is not the medium for light A. Your premise contradicts itself.

i think if you completely eliminated light from a medium, and then tried to put light through a medium, the light would not just go straight through. it would immediately scatter.
light is made up of particles. but those particles require other nearby particles in order to travel. you can't have motion without a frame of reference, therefore light creates it own frame of reference.

Pure speculation with no relationship to mainstream scientific knowledge whatsoever.

I am sorry that you couldn't wrap your head around relativity this time. Hopefully after some time some of this will sink in. Good luck!

Just FYI, the most important thing about relativity is how well it matches experiment. Any other theory is going to have to be equivalent to relativity in all regimes tested to date. So you need to get used to time dilation and velocity addition, they are simply part of how the universe actually works. When reality doesn't fit with your mental picture, then you need to change your mental picture: http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html