lmoh said:I'm sorry if this makes no sense, but do massless particles have energy then? If so, would there be any difference in the speed of some massless particle that has more energy than another? I have a minimal understanding of SR but I just wanted to ask.
lmoh said:I'm sorry if this makes no sense, but do massless particles have energy then? If so, would there be any difference in the speed of some massless particle that has more energy than another? I have a minimal understanding of SR but I just wanted to ask.
pawprint said:Although photons are massless they carry momentum acording to their SR energy. It is arguable whether a photon with zero momentum truly exists.
lmoh said:Ok, but is it possible to have photons (or other massless objects in general) with varying amounts of energy and would that difference in energy have any effect on its speed, especially in relation to other photons (or other massless objects) or will it always move at c?
All massless particles are defined to travel at c.lmoh said:I'm sorry if this makes no sense, but do massless particles have energy then? If so, would there be any difference in the speed of some massless particle that has more energy than another? I have a minimal understanding of SR but I just wanted to ask.
ghwellsjr said:Why do you keep saying subjective?
The pilot cannot truly say whether it it he accelerating forward, or the universe that is accelerating in the opposite direction
Every Frame of Reference will agree on what you are calling the subjective time on the pilot's clock. It's called Proper Time. It might be subjective according to Lorentz Ether Theory but it's objective in Special Relativity.pawprint said:Because everything is relative. The pilot cannot truly say whether it it he accelerating forward, or the universe that is accelerating in the opposite direction. He subjectively believes the first viewpoint rather than the second. Another observer with a different frame of reference might subjectively believe the effect to be somewhere between the two viewpoints.
Mark M said:Photons always move at c. Even if you are traveling at 500 million miles per hour, you will still measure them moving at the speed of light. All observers always agree on the speed of photons at c.
ghwellsjr said:Every Frame of Reference will agree on what you are calling the subjective time on the pilot's clock. It's called Proper Time. It might be subjective according to Lorentz Ether Theory but it's objective in Special Relativity.
pawprint said:Because everything is relative. The pilot cannot truly say whether it it he accelerating forward, or the universe that is accelerating in the opposite direction. He subjectively believes the first viewpoint rather than the second. Another observer with a different frame of reference might subjectively believe the effect to be somewhere between the two viewpoints.
Drakkith said:I'm not sure this is correct. Acceleration is easily measurable. If a pilot in a ship is accelerating at 1g there's no way he could say the rest of the universe is accelerating instead of him. Once he is no longer accelerating then it becomes relative as to who is moving at what velocity.
pawprint said:There's no argument here. The post referred to was not intended to be 'the truth', but only to explain the use of the term 'subjectively' as requested.
ghwellsjr said:Why do you keep saying subjective?
Drakkith said:As I said in my above post, this is not true. The mass of an object does not change. If it did, then at a certain speed anything would be transformed into a black hole, which does not happen.
pawprint said:If enough energy was pumped into a space vessel I think it might well become singularitised eventually.
jtbell said: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?
ghwellsjr said:Why do you keep saying subjective?
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 said: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.
ghwellsjr said: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.
jtbell said: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?
salvestrom said: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...
salvestrom said: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.
jreelawg said:...I thought that relativity says that there is no fixed space time that you can move in reference to? Am I wrong?
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?pawprint said: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.
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?pawprint said:The pilot is not experiencing Proper Time, nor are we. We can only infer it. I call that subjective.
jreelawg said: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 said: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!
[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.
salvestrom said: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.
pawprint said: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.
Drakkith said: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.
jreelawg said: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?
jreelawg said: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.
pawprint said: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?
jreelawg said:Yeah, but I'm not talking about gravity or acceleration, I'm talking about velocity.
pawprint said: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.
alexg said: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.