Relativity

[fast]

I know. Apparently the math works out such objects cannot obtain c, but I was trying to overlook that lil fact to illustrate the point that c will appear slower despite being the same. Kind of. That's one reason I diverted to scenarios with 1/4c and 1/2c, to eliminate the pain of the lil fact I was trying to ignore.

Consider this:

If an object is moving at 1/4c and emits a light, the light is not traveling at 1.25c
If an object is moving at 1/2c and emits a light, the light is not traveling at 1.5c

In both cases, light is traveling at c. Speaking of it as if c is relative seems misleading when it's a constant. It being constant is crucial. It's a central tenet of relativity. The postulate doesn't assume the very thing the theory shows. It supports the theory. I'm not trying to undermine the theory. I just want an accurate perspective on the postulate.

You seem to have reverted to talking about the speed of light without mentioning what that speed is in reference to. Relativity cites what the reference frame is... it is any reference frame in which the measurement is made. In your train thought experiment, that reference frame is the train. For the guy standing by the tracks watching the train speed by, it is his measurement instrument. Both will measure the speed of light (whatever the source) to be the same. This isn't intuitive but is what is actually measured in experiments and what the theory explains.

When we speak of objects in motion traveling at a certain speed, we can be explicit and say not just the speed but what it's relative to. For instance, if I say the cow is moving at 3MPH, the truth is that it's not 3MPH relative to just anything. The cow isn't (for instance) traveling 3MPH relative to the speed of the surface of Venus. The theory of relativity demands that we speak of movement in terms of being relative to something else. Yet, you catch me reverting when speaking of the speed of light. That's because it's immune to the theory. We can speak of light speed being relative, but we need not do so. It's a genuine constant. If it weren't, it wouldn't be used as a postulate in support of the theory.

When one is on the caboose traveling at C - 1MPH and shines the light towards the engine train, it will move at the speed of light (train speed + 1MPH). The speed of light, period. It will appear unbelievably slow to the person cutting the light on (since he's traveling at c-1MPH) and will actually see the photons in vivid motion taking off slowly relative to him. Still, the light is traveling at c, a universal truth inherient to the cosmos. Sure, the person on a nearby planet will see the light as passing furiously fast, but if c is c and nothing slower or faster, then how things appear to observers (slow or fast) alters the actual MPH of c not one bit.

 

[Kharakov]

As I understand it C is not dependent on an inertial frame.

Throw a baseball forward in a jet cabin and the velocity relative to the ground is the speed of the jet plus the speed of the ball.



Emit a laser pulse from a jet, on the ground the velocity is C.

Two spaceships at rest synch clocks. One accelerates away. On each ship a second is still a second. Nothing appears tp change in the two frames to observers in the frames.

Relativity is consistent and predictive, but to me we are missing something fundamental not that I have a clue what.

Light is the ruler. It also encodes relative velocity in its wavelength/frequency, if you know what it was emitted by (light might include this information, but doesn't reveal it to us).

 

[bilby]

I know. Apparently the math works out such objects cannot obtain c, but I was trying to overlook that lil fact to illustrate the point that c will appear slower despite being the same. Kind of. That's one reason I diverted to scenarios with 1/4c and 1/2c, to eliminate the pain of the lil fact I was trying to ignore.

Consider this:

If an object is moving at 1/4c and emits a light, the light is not traveling at 1.25c
If an object is moving at 1/2c and emits a light, the light is not traveling at 1.5c

In both cases, light is traveling at c. Speaking of it as if c is relative seems misleading when it's a constant. It being constant is crucial. It's a central tenet of relativity. The postulate doesn't assume the very thing the theory shows. It supports the theory. I'm not trying to undermine the theory. I just want an accurate perspective on the postulate.

You seem to have reverted to talking about the speed of light without mentioning what that speed is in reference to. Relativity cites what the reference frame is... it is any reference frame in which the measurement is made. In your train thought experiment, that reference frame is the train. For the guy standing by the tracks watching the train speed by, it is his measurement instrument. Both will measure the speed of light (whatever the source) to be the same. This isn't intuitive but is what is actually measured in experiments and what the theory explains.

When we speak of objects in motion traveling at a certain speed, we can be explicit and say not just the speed but what it's relative to. For instance, if I say the cow is moving at 3MPH, the truth is that it's not 3MPH relative to just anything. The cow isn't (for instance) traveling 3MPH relative to the speed of the surface of Venus. The theory of relativity demands that we speak of movement in terms of being relative to something else. Yet, you catch me reverting when speaking of the speed of light. That's because it's immune to the theory. We can speak of light speed being relative, but we need not do so. It's a genuine constant. If it weren't, it wouldn't be used as a postulate in support of the theory.

When one is on the caboose traveling at C - 1MPH and shines the light towards the engine train, it will move at the speed of light (train speed + 1MPH).

as measured by an observer next to the tracks, yes.

The speed of light, period. It will appear unbelievably slow to the person cutting the light on (since he's traveling at c-1MPH) and will actually see the photons in vivid motion taking off slowly relative to him.

NO! It will appear to him to flee from his torch at the speed of light. That's the whole point! EVERY observer, no matter how they are moving, measures any light as traveling at c.

Still, the light is traveling at c, a universal truth inherient to the cosmos. Sure, the person on a nearby planet will see the light as passing furiously fast,

Again, NO. Like EVERY observer, he will measure it as traveling at c

but if c is c and nothing slower or faster, then how things appear to observers (slow or fast) alters the actual MPH of c not one bit.

There is no 'actual' mph. mph are ALWAYS relative to a reference frame.

 

[skepticalbip]

I know. Apparently the math works out such objects cannot obtain c, but I was trying to overlook that lil fact to illustrate the point that c will appear slower despite being the same. Kind of. That's one reason I diverted to scenarios with 1/4c and 1/2c, to eliminate the pain of the lil fact I was trying to ignore.

Consider this:

If an object is moving at 1/4c and emits a light, the light is not traveling at 1.25c
If an object is moving at 1/2c and emits a light, the light is not traveling at 1.5c

In both cases, light is traveling at c. Speaking of it as if c is relative seems misleading when it's a constant. It being constant is crucial. It's a central tenet of relativity. The postulate doesn't assume the very thing the theory shows. It supports the theory. I'm not trying to undermine the theory. I just want an accurate perspective on the postulate.

You seem to have reverted to talking about the speed of light without mentioning what that speed is in reference to. Relativity cites what the reference frame is... it is any reference frame in which the measurement is made. In your train thought experiment, that reference frame is the train. For the guy standing by the tracks watching the train speed by, it is his measurement instrument. Both will measure the speed of light (whatever the source) to be the same. This isn't intuitive but is what is actually measured in experiments and what the theory explains.

When we speak of objects in motion traveling at a certain speed, we can be explicit and say not just the speed but what it's relative to. For instance, if I say the cow is moving at 3MPH, the truth is that it's not 3MPH relative to just anything. The cow isn't (for instance) traveling 3MPH relative to the speed of the surface of Venus. The theory of relativity demands that we speak of movement in terms of being relative to something else. Yet, you catch me reverting when speaking of the speed of light. That's because it's immune to the theory. We can speak of light speed being relative, but we need not do so. It's a genuine constant. If it weren't, it wouldn't be used as a postulate in support of the theory.

When one is on the caboose traveling at C - 1MPH and shines the light towards the engine train, it will move at the speed of light (train speed + 1MPH). The speed of light, period. It will appear unbelievably slow to the person cutting the light on (since he's traveling at c-1MPH) and will actually see the photons in vivid motion taking off slowly relative to him. Still, the light is traveling at c, a universal truth inherient to the cosmos. Sure, the person on a nearby planet will see the light as passing furiously fast, but if c is c and nothing slower or faster, then how things appear to observers (slow or fast) alters the actual MPH of c not one bit.

You seem to be stuck on a Newtonian view of reality. Newton was wrong although since we as, humans, are so unfamiliar with anything moving at any appreciable velocity with respect to ourselves that we don't notice the error, although at such low relative velocities that error is insignificant.

In the real universe, things are very different. For instance, one of the problems Einstein faced in trying to formulate a better explanation for reality was that the measurement of the speed of light from stars made no sense using Newtonian mechanics. The Earth orbits the sun at about 30 Km/sec. This means that the Earth will be moving toward a star on the ecliptic at 30 Km/sec at one point in its orbit and 30 Km/sec away from that star six months later. This makes a 60 Km/sec difference in our relative velocity with that star. This speed difference did not show up in the speed of light from that star when it was measured. Both points in the orbit gave the same answer for the speed of the light from that star. Even more mysteriously at the time, the speed of the light from all stars measured the same even though it was pretty much known that stars had significant and various velocities with respect to our sun.

The theory of relativity resolved the dilemma and made sense of the measurements even though many reject it because it is contrary to intuition.

 

[Kharakov]

Don't forget that gravity also propagates at the speed of light.

 

[fast]

The Earth orbits the sun at about 30 Km/sec. This means that the Earth will be moving toward a star on the ecliptic at 30 Km/sec at one point in its orbit and 30 Km/sec away from that star six months later. This makes a 60 Km/sec difference in our relative velocity with that star. This speed difference did not show up in the speed of light from that star when it was measured.

I had actually wondered about that

 

[skepticalbip]

Don't forget that gravity also propagates at the speed of light.

Hey, I never fell quite that fast when tripping over a stool.

 

[Kharakov]

Don't forget that gravity also propagates at the speed of light.

Hey, I never fell quite that fast when tripping over a stool.

It, like light, is in specific amounts. So while light travels at the same velocity, but imparts different momentum, gravity travels at its velocity, but imparts different acceleration depending on the source.

Now, the idea is, can you find something that refracts gravity?

 

[fast]

NO!

The implications of "no" is that the speed of light is relative to the speed I'm traveling, which means the true speed relative to another object puts the speed of c in excess of c.

 

[beero1000]

NO!

The implications of "no" is that the speed of light is relative to the speed I'm traveling, which means the true speed relative to another object puts the speed of c in excess of c.

No. That is the whole point - my measurement of distance and time will be different than yours in an exact relationship that leads us both to conclude that the light moves at c.

 

[bilby]

NO!

The implications of "no" is that the speed of light is relative to the speed I'm traveling, which means the true speed relative to another object puts the speed of c in excess of c.

Only because you are using the (incorrect) classical formula for addition of velocities.

You are using the approximation expected by Galileo and Newton:

\(v = v_1+v_2\)

But the actual formula is:

\(v = \frac{v_1+v_2}{1+(\frac{v_1.v_2}{c^2})}\)

which approximates the same results when \(v_1 . v_2\) is a lot smaller than \(c^2\), which it is at the kinds of speeds humans are typically familiar with.

When the product of the two velocities approaches c², the difference between the classical approximation and the relativistic result becomes significant.

It may be counterintuitive that if you watch two spacecraft fly past each other at 90% of light speed, that the crew of one spacecraft will see the other approaching at less than .9c+.9c = 1.8c; but it is true nonetheless, and can be shown experimentally.

The actual result of adding .9c+.9c is approximately .9945c, and that is the apparent closing speed of one spacecraft as viewed from the other.

 

[Underseer]

Some of you may be curious why the topic of Relativity is coming up more often these days.

William Lane Craig is doing very badly in his quest to shore up his version of the Kalam Cosmological argument. He tried to use modern physics to add credibility to the cosmological argument, and so now he has been backed into a corner and the only way to salvage his version of the Kalam argument is to claim that relativity is false, because that is the only way he can still argue that the universe has a cause.

So he's been reduced to trying to "disprove" relativity with a syllogism.

Craig is widely considered one of the best apologists (by average Christians, not by actual Christian academics), so many of them repeat his arguments without actually understanding said arguments.

Many other Christians and Muslims have already abandoned Kalam and have switched to the contingency argument simply because time is irrelevant with the wording of the contingency argument.

 

[steve_bank]

Don't forget that gravity also propagates at the speed of light.

Has that been measured or detected? I remember something based on an underground laser ring gyro here in Washington.

 

[steve_bank]

As v -> C a singularity occurs, you can't get to C at least in the math. When v is small to C you get Newtonian mechanics.

Quantum..small
Newtonian..big and slow
Relativistic ... fast

 

[Kharakov]

Don't forget that gravity also propagates at the speed of light.

Has that been measured or detected? I remember something based on an underground laser ring gyro here in Washington.

I annoyed some grad students up that way a while back, because I had a multiplication error in my equations and I thought their results were inconclusive, but it wasn't gravity wave experiments exactly- I was bugging them about particle# (instead of mass) dependent GR. They were doing some extension of the Cavendish experiment, similar to one that is being conducted in satellites (can't recall its name).

They used a torsion balance to determine that it was indeed mass, and not # of particles, that determined gravitational acceleration. I still think it could be a cover up.... not really, the reasons to do so are quite alarming.


Ohh, yeah. I think it was confirmed because gravity waves...

 

[fast]

NO!

The implications of "no" is that the speed of light is relative to the speed I'm traveling, which means the true speed relative to another object puts the speed of c in excess of c.

Only because you are using the (incorrect) classical formula for addition of velocities.

You are using the approximation expected by Galileo and Newton:

\(v = v_1+v_2\)

But the actual formula is:

\(v = \frac{v_1+v_2}{1+(\frac{v_1.v_2}{c^2})}\)

which approximates the same results when \(v_1 . v_2\) is a lot smaller than \(c^2\), which it is at the kinds of speeds humans are typically familiar with.

When the product of the two velocities approaches c², the difference between the classical approximation and the relativistic result becomes significant.

It may be counterintuitive that if you watch two spacecraft fly past each other at 90% of light speed, that the crew of one spacecraft will see the other approaching at less than .9c+.9c = 1.8c; but it is true nonetheless, and can be shown experimentally.

The actual result of adding .9c+.9c is approximately .9945c, and that is the apparent closing speed of one spacecraft as viewed from the other.

I have no argument.

Back to the drawing board, as they say.

 

[beero1000]

Don't forget that gravity also propagates at the speed of light.

Has that been measured or detected? I remember something based on an underground laser ring gyro here in Washington.

Yes. IIRC, that's a prediction of general relativity that has been confirmed up to a factor of around ±10^-15. Einstein, amirite?

 

[Jon Osterman]

The "speed of gravity" being c is built into General Relativity, which has also been comprehensively tested. Indeed, if gravity propagated at speeds <c then gravity would not be an infinite range force and all of astrophysics would fall apart.

 

[Loren Pechtel]

Only because you are using the (incorrect) classical formula for addition of velocities.

You are using the approximation expected by Galileo and Newton:

\(v = v_1+v_2\)

But the actual formula is:

\(v = \frac{v_1+v_2}{1+(\frac{v_1.v_2}{c^2})}\)

which approximates the same results when \(v_1 . v_2\) is a lot smaller than \(c^2\), which it is at the kinds of speeds humans are typically familiar with.

When the product of the two velocities approaches c², the difference between the classical approximation and the relativistic result becomes significant.

It may be counterintuitive that if you watch two spacecraft fly past each other at 90% of light speed, that the crew of one spacecraft will see the other approaching at less than .9c+.9c = 1.8c; but it is true nonetheless, and can be shown experimentally.

The actual result of adding .9c+.9c is approximately .9945c, and that is the apparent closing speed of one spacecraft as viewed from the other.

I have no argument.

Back to the drawing board, as they say.

It occurs to me: You call yourself "fast" but you don't know what fast really means!

 

[fast]

Is vsub2 c?
Is vsub1 c-1MPH?