The speed of light. What's the story, really?

[jtbell]

If enough energy was pumped into a space vessel I think it might well become singularitised eventually.

We can produce a high relative velocity between you and the space vessel not only by accelerating the space vessel, but also by accelerating you. How can accelerating you turn the space vessel (which as far as its passengers know is just sitting there minding its own business) into a black hole?

 

[pawprint]

We can produce a high relative velocity between you and the space vessel not only by accelerating the space vessel, but also by accelerating you. How can accelerating you turn the space vessel (which as far as its passengers know is just sitting there minding its own business) into a black hole?

I took that position when I wanted to semantically enlighten ghwellsjr. I was criticised until I pointed out that that answer was about semantics, not physics.

Why do you keep saying subjective?

Extending your argument logically we find the Universe rotating at better than 0.9999997 c around Geneva. Not?

 

[ghwellsjr]

I took that position when I wanted to semantically enlighten ghwellsjr. I was criticised until I pointed out that that answer was about semantics, not physics.

I have not yet been enlightened. I pointed out to you that the time that the pilot experiences and sees on his clock is not merely in his mind (which is what subjective means), it is objective and it's called Proper Time. All reference frames agree on the time that the pilot reads on his clock. I have no idea why you refer to the pilot's experience as subjective so maybe you should try to enlighten me some more.

 

[pawprint]

I have not yet been enlightened. I pointed out to you that the time that the pilot experiences and sees on his clock is not merely in his mind (which is what subjective means), it is objective and it's called Proper Time. All reference frames agree on the time that the pilot reads on his clock. I have no idea why you refer to the pilot's experience as subjective so maybe you should try to enlighten me some more.

If you know this I don't understand your perplexity. I agree that any observer anywhere can, with the right equipment, agree on both Proper Time and, provided the observer also has data on another observer's relativity, both the second observer's Proper and Subjective Time, which are related but not identical.

The pilot is not experiencing Proper Time, nor are we. We can only infer it. I call that subjective.

 

[salvestrom]

We can produce a high relative velocity between you and the space vessel not only by accelerating the space vessel, but also by accelerating you. How can accelerating you turn the space vessel (which as far as its passengers know is just sitting there minding its own business) into a black hole?

In any situation with relative velocity there exists a reality that at some point, recent or distant, one of the two objects in question was accelerated to its velocity. That will always be the one that is actually moving, regardless of the symmetry and usefulness of treating that object as stationary in its own rest frame, or the ability of that observer to tell they are moving.

 

[pawprint]

In any situation with relative velocity there exists a reality that at some point, recent or distant, one of the two objects in question was accelerated to its velocity...

Yes. I'll be more careful in the future. I wouldn't want my mass (relativistic or otherwise) to exceed a safe limit :{)

 

[jreelawg]

In any situation with relative velocity there exists a reality that at some point, recent or distant, one of the two objects in question was accelerated to its velocity. That will always be the one that is actually moving, regardless of the symmetry and usefulness of treating that object as stationary in its own rest frame, or the ability of that observer to tell they are moving.

Are you certain this is how relativity works? I thought that velocity was meaningless without a frame of reference.

Lets say you accelerate, in the opposite direction of your "unknown absolute velocity" or whatever you are referring to. You should now have reduced your velocity, and your clock should be effected.

So you should be able to tell which direction and how fast your moving in reference to some absolute frame in space time.

I thought that relativity says that there is no fixed space time that you can move in reference to? Am I wrong?

 

[pawprint]

...I thought that relativity says that there is no fixed space time that you can move in reference to? Am I wrong?

You aren't wrong. Relativity implies that. However we now have a great deal of high precision astronomical data from which we can infer certain things.

At least 25 years ago the 'Great Attractor' was hypothesised. It was thought to be a massive gravity sink in the direction of the constelation Leo. It was deduced from doppler observations, and suggested that the Milky Way and other galaxies were being attracted in that direction. A rough figure of 350 - 400 km/s was given. (Article published in Scientific American 1985 from memory.)

Since then we've had at least two high resolution looks at the globe of CMB radiation. Doppler observations and calculations agree with the 'Great Attractor' hypothesis. The figure is now generally quoted as 480 km/s.

Proper Time, as mentioned frequently in this thread, can be inferred from such observations. The Earth, being in orbit, is an accelerating body. (It is forever falling towards the Sun). However the acceleration is constant and velocity changes over a year are insignificant compared to the apparent relative velocity of our galaxy and others.

So although there is no 'Ether' as was believed until the 1880s, we can now answer the 'absolute velocity' question that Michelson and Morely's famous experiment was designed for.

The Wikipedia articles give lots of detail.

 

[ghwellsjr]

If you know this I don't understand your perplexity. I agree that any observer anywhere can, with the right equipment, agree on both Proper Time and, provided the observer also has data on another observer's relativity, both the second observer's Proper and Subjective Time, which are related but not identical.

How can you agree with me regarding any other observers when I said nothing about any other observers? And I still have no idea what you mean by Subjective Time. Is this your own personal definition or can I look up the subject somewhere?

The pilot is not experiencing Proper Time, nor are we. We can only infer it. I call that subjective.

Are you saying that the pilot's clock does keep Proper Time but he has a different experience of time? Are you saying that the pilot's clock might say that it only took him two years to make a long trip but when he got back he really aged a lot longer, in fact the same amount as those he left behind?

 

[salvestrom]

Are you certain this is how relativity works? I thought that velocity was meaningless without a frame of reference.

Lets say you accelerate, in the opposite direction of your "unknown absolute velocity" or whatever you are referring to. You should now have reduced your velocity, and your clock should be effected.

So you should be able to tell which direction and how fast your moving in reference to some absolute frame in space time.

I thought that relativity says that there is no fixed space time that you can move in reference to? Am I wrong?

Um. Let me be clear. My post is an opinion. However, in SR and GR accelerating frames are not considred symmetric.

I never mentioned an unknown absolute velocity...

You are correct that there is no "preferred" reference frame.

Nonetheless, the reality that nothing in the universe is stationary (everything has some amount of velocity and is going somewhere with it), shouldn't, in the opinion of this weekend physicist, be confused with actually suggesting that a spaceship traveling at 200000km/s past a local planet is stationary and it is the planet that is moving (at 200000km/s). No force has, in the past, acted upon this planet to accelerate it to such a speed. The crew of the spaceship, however, hopefully aren't incredulous enough to dismiss their recollection of blasting off from their homeworld and firing their engines to get their vessel upto 2/3c.

The maths is largely indifferent to anyone's point of view. Lorentz Transformations go both ways, each observer says the other is moving more slowly through time, and so on. The so-called Twin Paradox restores some sense of reality back to the whole ordeal by revealling that only one of them has actually experienced a slower rate of time (but been blissfully ignorant of it).

This - all of it - doesn't interfere with the maths and therefore the theory's usefulness. Why people have such a problem with it is beyond me. It's really just a question of philosophy. Or sanity.

 

[Drakkith]

How do you know that Drakkith? If enough energy was pumped into a space vessel I think it might well become singularitised eventually. I think you've suggested a viable way to make lightweight black holes!

An object must have the mass in its rest frame to become a black hole. Accelerating an object to any speed will not cause this to happen, as it's invariant mass does not change.

[Edit] I remember now- CERN is hoping it will happen. There was uninformed public outcry when they confirmed the rumour. Informed word was that any such BH formed at CERN would be so small and evaporate into Hawking Radiation so quickly as to pose no threat even to the detectors.

This is incorrect. The black holes would have been formed from the collision between protons, not the acceleration and resulting velocity.

 

[jreelawg]

Nonetheless, the reality that nothing in the universe is stationary (everything has some amount of velocity and is going somewhere with it), shouldn't, in the opinion of this weekend physicist, be confused with actually suggesting that a spaceship traveling at 200000km/s past a local planet is stationary and it is the planet that is moving (at 200000km/s). No force has, in the past, acted upon this planet to accelerate it to such a speed. The crew of the spaceship, however, hopefully aren't incredulous enough to dismiss their recollection of blasting off from their homeworld and firing their engines to get their vessel upto 2/3c.

I see what your saying, but your still only saying that the spaceship is the one moving relative to the home worlds reference frame.

The home world has been set in motion by the sun it's revolving around, and the solar system has be set into motion in the context of the galaxy it's in and so forth.

So for example if the spaceship goes in a direction opposite to the Earth's orbit, so that it stands still relative the sun, but is moving relative to the earth, and had to accelerate to get to that "velocity", who is moving?

If you were to be in a spaceship, and departed from earth, accelerating in a speed and direction, which put you into a position where you were for a moment stationary in relation to the center of the galaxy, how fast do you think the earth, would be moving relative to you?

It depends entirely on the frame of reference. Any kind of acceleration prior is meaningless unless you can somehow retrace an exact point of origin.

 

[jreelawg]

You aren't wrong. Relativity implies that. However we now have a great deal of high precision astronomical data from which we can infer certain things.

At least 25 years ago the 'Great Attractor' was hypothesised. It was thought to be a massive gravity sink in the direction of the constelation Leo. It was deduced from doppler observations, and suggested that the Milky Way and other galaxies were being attracted in that direction. A rough figure of 350 - 400 km/s was given. (Article published in Scientific American 1985 from memory.)

Since then we've had at least two high resolution looks at the globe of CMB radiation. Doppler observations and calculations agree with the 'Great Attractor' hypothesis. The figure is now generally quoted as 480 km/s.

Proper Time, as mentioned frequently in this thread, can be inferred from such observations. The Earth, being in orbit, is an accelerating body. (It is forever falling towards the Sun). However the acceleration is constant and velocity changes over a year are insignificant compared to the apparent relative velocity of our galaxy and others.

So although there is no 'Ether' as was believed until the 1880s, we can now answer the 'absolute velocity' question that Michelson and Morely's famous experiment was designed for.

The Wikipedia articles give lots of detail.

Is it the case that an objects clock as effected by relative velocity, is only ultimately effected as referenced by this great attractors frame of reference?

As in, when two objects meet in a single reference frames. The differences in there clocks attributed to their experiences of relative velocity, come from their velocity relative to this great attractor?

 

[pawprint]

An object must have the mass in its rest frame to become a black hole. Accelerating an object to any speed will not cause this to happen, as it's invariant mass does not change.

This is incorrect. The black holes would have been formed from the collision between protons, not the acceleration and resulting velocity.

The black holes, which have not yet been observed, are expected as CERN further ramps up its energy. To say that the black hole is formed by the collision of protons is true. But it ignores the fact that the protons must carry a huge relativistic mass before a black hole can be formed "from the collision of protons". Formation of black holes by collision of protons without relativistic mass is unlikely in the extreme.

 

[pawprint]

Is it the case that an objects clock as effected by relative velocity, is only ultimately effected as referenced by this great attractors frame of reference?

As in, when two objects meet in a single reference frames. The differences in there clocks attributed to their experiences of relative velocity, come from their velocity relative to this great attractor?

I'm not certain what you're asking jreelawg. It is the case that clocks in the Milky Way (and elsewhere) are running relatively slower (though not by much) than they would if the Great Attractor did not exist. It is also the case that clocks close to the Great Attractor, and unmoving in relation to it, run slower still.

A clock on an Earthly table runs a liitle faster than one on the floor. Some say it's because of gravity. But gravity is indistinguishable from acceleration as Einstein pointed out in his Special Theory of Relativity over 100 years ago.

Does this help?

 

[salvestrom]

I see what your saying, but your still only saying that the spaceship is the one moving relative to the home worlds reference frame.

The home world has been set in motion by the sun it's revolving around, and the solar system has be set into motion in the context of the galaxy it's in and so forth.

So for example if the spaceship goes in a direction opposite to the Earth's orbit, so that it stands still relative the sun, but is moving relative to the earth, and had to accelerate to get to that "velocity", who is moving?

If you were to be in a spaceship, and departed from earth, accelerating in a speed and direction, which put you into a position where you were for a moment stationary in relation to the center of the galaxy, how fast do you think the earth, would be moving relative to you?

It depends entirely on the frame of reference. Any kind of acceleration prior is meaningless unless you can somehow retrace an exact point of origin.

In the situations you describe each component has a shared velocity. By this I mean the velocity of the galaxy toward the Virgo cluster, or the solar system's velocity about the galaxy. Since they share this velocity its effects will be equal and can be put aside. We are then only interested in the additional velocities, the additional time dilation. A ship that is stationary relative to the galaxy wouldn't need to have any additional velocity. It could hover over the "north pole" of the galaxy and just be carried along by the gravity.

Such a ship would record time on Earth varying as it orbits about the sun and galaxy, as the various velocities combine and cancel. Indeed, a third observer not caught up in the galaxy's gravitational influence will witness the time on earth, for some parts of the orbit, running slightly faster than the polar ships clock. This makes me giggle. Don't ask why.

 

[jreelawg]

I'm not certain what you're asking jreelawg. It is the case that clocks in the Milky Way (and elsewhere) are running relatively slower (though not by much) than they would if the Great Attractor did not exist. It is also the case that clocks close to the Great Attractor, and unmoving in relation to it, run slower still.

A clock on an Earthly table runs a liitle faster than one on the floor. Some say it's because of gravity. But gravity is indistinguishable from acceleration as Einstein pointed out in his Special Theory of Relativity over 100 years ago.

Does this help?

Yeah, but I'm not talking about gravity or acceleration, I'm talking about velocity.

 

[pawprint]

Yeah, but I'm not talking about gravity or acceleration, I'm talking about velocity.

I've looked back through the thread. Is it post #62 that you want elucidated? And sorry for the delay, I had a verification problem.

 

[alexg]

The black holes, which have not yet been observed, are expected as CERN further ramps up its energy. To say that the black hole is formed by the collision of protons is true. But it ignores the fact that the protons must carry a huge relativistic mass before a black hole can be formed "from the collision of protons". Formation of black holes by collision of protons without relativistic mass is unlikely in the extreme.

You keep using the term 'relativistic mass'. Use of it is pretty well outmoded as it is quite misleading. What we are actually talking about is momentum and energy, not mass. Cern hopes to create micro BH because the energy of two protons colliding is so high.

 

[pawprint]

You keep using the term 'relativistic mass'. Use of it is pretty well outmoded as it is quite misleading. What we are actually talking about is momentum and energy, not mass. Cern hopes to create micro BH because the energy of two protons colliding is so high.

Once again the difference seems to be about semantics, not physics. I call it relativistic mass, you call it momentum. The energy is the same no matter how we describe it.

 

[alexg]

Once again the difference seems to be about semantics, not physics. I call it relativistic mass, you call it momentum. The energy is the same no matter how we describe it.

Yes, but the term 'relativistic mass' has lead many to think that an objects gravity increases as the 'relativistic mass' increases.

 

[pawprint]

Yes, but the term 'relativistic mass' has lead many to think that an objects gravity increases as the 'relativistic mass' increases.

Your point is well taken. I am happy to be corrected and will endeavour to keep this in mind.

 

[pawprint]

In the same vein I now understand what hgwellsjnr was getting at, although it took a different thread to find it. When I used the term 'subjective' I should more accurately have referred to 'frames of reference'. Thank you hgwellsjnr.