IM Egdall

I think it has already been said that the argument with the mass is not entirely convincing. But it is funny that all what you said is completely correct except for the word "mass". Had you said "electrical charge" or "half-integer spin" then your statement would have been a great argument.

Ya, good point. It occurred to me as I read this that two colliding photons can (and do) produce matter particles if their energy is high enough. So, as you point out, the charge and half-integer spin are much better arguments.

Assume a single electron (or any particle) passes through a double-slit. Per quantum field theory, the electron's wave function travels through both slits. The results is the electron is detected in one location at the detector screen. But over time with a number of electrons passing through the apparatus one at as time, you get an interference pattern at the detector screen.

So here is my question: Per general relativity, the electron (or any other particle) is a source of spacetime curvature (gravity). So could a sensitive-enough gravity detector placed near one slit tell in principle which slit the electron went through?

Or maybe a better question is: Is a particle's wave function a source of spacetime curvature?

And here is another thing that comes to mind. Photons have no mass. Matter particles (electrons, neutrinos, quarks) do have mass. So how can particles which have no mass make up particles that do have mass? So it appears from this that photons cannot build up to make matter particles.

I thought that it was more along the lines that the time was so short that energy had a considerable amount of variation under HUP

[math]\Delta E\Delta t \gtrsim h[/math]

 

in very layman's terms the time of existence is so short that the universe doesn't realise the energy sum is incorrect - ie the energy used to create them is paid back in annihilation before anyone can notice it is missing

You are talking about virtual particles, which appear out of "empty" space and collide. They are a particle and antiparticle pair. One has positive energy and the other has negative energy. So when they collide, no energy is released. The amount of time a pair exists is determined by the HUP, as you note.

I was talking about real particles. Here they are a particle and antiparticle pair. But they both have positive energy. So when they collide, they release energy.

As I understand it, the big bang was the expansion of space. And the universe at time zero may have been finite or infinite.

Inflation theory says a tiny moment after the big bang, the universe expanded exponentially. So couldn't we say this was an explosion of space.

Here's how I think it works. A photon is not a particle with "very little mass". It is a particle with zero mass. The conservation law is conservation of mass/energy.

Mass is converted to energy by E = mc^2. So say an electron and anitelectron(positron) collide, annihilate each , and produce photons. The total mass/ energy of the electron and positron before the collision equals the total energy of the photons after the collision. Mass/energy is conserved.

And of two photons have enough energy, when they collide, they can produce an electron and a positron. Again the energy of the photons before collision has to be enough to equal the mass/energy of the electron and positron after.

This mass/energy consrevation works for all particle interactions.

Basically, if the event horizon is larger than the object's surface, then it is considered a black hole. I believe that in the case of supermassive black holes at galactic cores, the event horizons are substantially larger than the singularities underneath them. This is what sets it apart from a neutron star, which doesn't have as much gravity to have that effect.

 

If black holes (stellar) were infinitely dense, then the singularities would contract to a diameter infitely smaller than 12 miles, which defies logic, if you ask me. This is kind of a subject of debate right now, since most older theory considers a singularity to be an infinitely dense point, such as all of those neutrons and electrons compacted into a point much smaller than the sum of the diameters of all the particles, whereas Hawking has kind of broken from the pack and reclassified the black hole singularity as being finitely dense, essentially the sum that I just described. I believe this makes much more sense than the standard larger-than-life mythology of black holes of old.

Good stuff here. Thanks. But again what does the 12 miles refer to and how is it calculated?

Think about how an electron's orbital diameter is about 1.06*10^-10m, or an angstrom, across and the size of a proton is about 1*10^-15m across. Now collapse the two into each other to where there's no space in between any of the particles and this thing which was the size of a star is now about 12 miles across. That's a black hole. It's not an infinitely dense point.

Are you saying the event horizon is about 12 miles in diameter?

And if there is no space between the particles at the black hole center, then isn't the center infinitely dense?

It was a parable.

Oh. I guess I take things too literally.

So the point would be that rotating a bucket through space is not technically equivalent to moving the rest of the universe through space around the bucket, BUT because moving that much of the universe would bring space along with it, either way the only thing that is moving through space is the bucket.

boy, this is getting tough to sort out. I think rotating the bucket is technicallly equivalent to rotating the rest of the universe. This is Einstein's Principle of General Covariance -- there is no "preferred" frame of reference. Any reference frame is as valid as any other one. So the bucket spins in the universe frame, and the universe spins in the bucket frame. And both ways of looking at it produce identical physical results.

I also think your talk of moving through space for the bucket but not moving through space for the universe is not quite right. There is no absolute space in relativity -- so space doesn't move along with the universe. Both the bucket and universe are moving though space and through time or spacetime.

Because the length of the life of atom or particle has nothing to do with the passage of time.

Sure it does. This is demonstrated in particle accelerators all the time. The faster the particles move, the longer they live on average. This is because of time dilation -- time for the particles rus slower than time for the laboratory. So from out point-of-view in the laboratory reference frame, the particles live longer than the same particles at rest.

Fro example, see link: http://www.physlink....perts/ae611.cfm

 

 

In a seperate vein, the Big Bang is said to have exploded from an infinite singularity. It it was infinite, the explosion would be infinite as well, and that would eliminate the possibility that the universe will collapse upon itself. Or am I wrong in thinking I read that the singularity was infinite?

Do you have a link for this infinite singularity? I think whether the universe is finite or infinite is still an open question in cosmology.

Okay wait... you're saying that a rotating frame of reference (say where a bucket is at rest and the universe is spinning around it) is equally valid as a frame of reference where only the bucket is spinning?

 

If I go outside and twist my head around, and consider my head's frame of reference, then the sun is moving around me at many times the speed of light. How can that be valid?

 

(Though its distance to me doesn't change. Is it only the rate of change of distance between something moving and something at rest that cannot exceed c? In polar coordinates with my head at the origin, the sun's movement when I twist my head seems minor.)

Yes in your reference frame, that dizzy feeling you get when you spin around is caused by the frame dragging produced by the rotation of the rest of the universe.

And yes, when you spin around, from your point-of-view, stars that are far enough away are moving faster than the speed of light. But you can not use this effect to send a signal faster than the speed of light -- so relativity rules are not violated.

No this is not semantics.

 

The distance between your eyes and your nose will always be the same no matter your state of motion. The carrot at the end of the stick will always be at the same distance of the donkey, but there is nothing absolute in this. The distance between you and the horizon will always be 11 kilometers, no matter you speed at the surface of the Earth. Again there is nothing absolute. It is relative through geometry (between your height and the radius of the Earth).

When someone correctly states that "the light beam is always traveling at the same speed relative to you.", it means what it says: Relative. Point.

The distance between your eyes and your nose are always the same to you, because you are moving at the same velocity as your eyes and nose. BUT another observer moving relative to you will measure the distance as shortened (along the direction of motion.) And the greater the speed, the more the shortening. That is why we say space is relative.

But the speed of light is the same for all observers, no matter what their uniform motion. So it is absolute.

Now I'm feelin too dum for this thread.

Welcome to the club.

Bolded mine.

Right. I suppose everyone must agree on your statements. So do I.

 

What does it mean?

 

1. the light beam is always traveling at the same speed relative to you.

That means Speed of Light is relative. (you wrote it, not me, I simply agree with you).Not absolute as you may find several times in litterature and on the Web.

 

2. As a matter of consequence, the observed fact that SOL is constant means that it has something to do with the relation between the observer and the observed phenomena. That is another way to say that SOL is relative.

 

My interpretation of the above is that SOL is a kind of horizon.

I think we are getting into semantics here.

No matter what your uniform motion with respect to a beam of light, you will always measure that light's speed (in a vacuum) as the same value, c. This is what is meant by the speed of light is absolute. (If the speed of light were relative, then observers moving at different speeds would measure different values for the speed of light. )

Frame dragging is accepted science -- a recognized prediction of general relativity. Testing it, however, has been a challenge because iti s a very small effect for the rotating Earth.

NASA launched Gravity Probe B in 2004 to measure frame dragging. After lots of problems, the team says they have confirmed Einstein's prediction to 15%. (NASA Science Report - Dec. 2008).

Will continue to be valid the relativity theory?

 

There are already many objections to the relativity theory: the paradoxes, experiments with speeds superluminicas, experiments with light stopped, the speed of light greater than C (João Magueijo and the data from the quasar), the experiment of Martin Grusenick, etc. , etc. I think we need a radical examination of the theory. By the way i see on youtube a proposal for an experiment very interesting:

 

Wht paradoxes contradict relativity?

What superluninal experiments (outside of recent CERN neutrino issue)?

Experiments with light stopped, as I understand it, do not contradict relativity - do you have a reference that says it does?

I am not familiar with Magueijo and Grusenick -- are their claims recognized by mainstream science?

Outside of the CERN neutrino measurements, I think relativity is on very solid ground. and that could be a measurement issue.

Please provide links to the above questions.

I'm confused.

 

Reason 1: Suppose you're rotating a tremendous amount of mass around the bucket, which causes gravitational forces that have the exact same effect as the classical pseudo-forces present when rotating just the bucket. Now suppose that you remove some of this tremendous mass (half, or nearly all, or whatever). The gravitational force effect should decrease. With a negligible enough mass rotating around the bucket, the force on the bucket will be negligible. Then, if the forces involved in rotating the bucket are equal to forces when rotating the universe around the bucket, then it must be the case that rotating a bucket of water in a mostly empty universe will not cause the water's surface to curve. -- This is an intriguing idea, because it means that the mass of the water in the bucket depends on the amount of mass in the universe. But I've never heard of that before. It would imply that the gravitational pull on the mass in the bucket by all the rest of the mass in the universe is what gives the water its inertia.

 

Reason 2: Suppose you're rotating a tremendous amount of mass around the bucket, which causes gravitational forces that have the exact same effect as the classical pseudo-forces present when rotating just the bucket. Now suppose that you also rotate the bucket to match the rotation of the tremendous mass. What happens in this coordinate system should be the same as in another coordinate system, such as a frame that is also rotating with the universe and bucket. In the latter frame, nothing is rotating, and so there should be no curvature of the water's surface. This means that in the former frame (in which everything is rotating) there should be no surface curvature. Does this mean that the fictitious forces evident when rotating a bucket cancel out the forces that would be caused by rotating the universe around the bucket?

Good arguments. Let me try to answer them:

As to reason 1 -- I think you are rediscovering the equivalence of gravity and inertia. See link: http://philsci-archive.pitt.edu/1287/1/A1085285

As to reason 2 -- I believe if the bucket rotates to match the rotation of the universe, then there is no relative rotation between the two. This is equivalent to a reference frame where the bucket and universe are at rest. I think you have to consider only relative rotation. There is no absolute rotation in Einstein's construct.

 

Here is a Gif i made for my experimental media class as a tribute to Carl Sagan.

Hope you enjoy.

Nice!

I agree with Iggy. Well said.

Why do you say this?

I got the idea that a spinning universe with no other mass/energy besides the water bucket would still cause the water to become concave from Brian Greene, The Fabric of the Cosmos, p .417:

" whereas standard Machian reasoning would claim that the water would stay flat in the bucket spun in an infinite, empty universe, general relativity diagrees. What Pfister and Braun results show is that a sufficiently massive rotating sphere is able to completely block the usual influence of the space that lies beyond the sphere itself."

Again, I do not get this last part. A sufficiently massive rotating sphere implies a universe WITH mass/energy, doesn't it?

This doesn't sound right to me. Do you have a reference?

The water becoming concave was explainable before GR and certainly doesn't need frame dragging (a very small effect that requires sensitive instruments to detect) to explain it.

 

The idea of this thought-experiment is to show one effect of Einstein's Principle of General Covariance. Per general relativity, any reference frame will do. Newton says the only legitimate reference frame is absolute space itself. The bucket is spinning with respect to absolute space.

Einstein says there is no absolute space. The bucket itself is a legitimate reference frame, per the Principle of General Covariance. But in this frame, the bucket is at rest. So what makes the water go concave" From this point-of-view, the rest of the universe is rotating in the opposite direction. And through frame-dragging, this causes the water to go concave!

I learned this from Brian Greene's The Fabric of the Cosmos, beginning of Chapter 14. He also gives references in the endnotes: D. Brill and J. Cohen, Phys. Rev. vol 143, no. 4, 1011 (1966) and H. Pfister and K. Braun, Class. Quantum Grav. 2, 909 (1985).

Here's what I think I undertstand:

Newton's bucket: a bucket of water is made to spin. The water rises up at the edge of the bucket. Looking at it per general relativity (GR) from two frames of reference:

(1) Ground reference frame - Bucket spins and makes water form concave surfrace.

(2) Bucket reference frame - From bucket's point-of-view, it is stationary and rest of universe is spinning. But if bucket at rest, what makes the water surface concave?

General relativity says: the rotation of the rest of the universe causes the water to become concave in case 2. This is due to frame dragging.

Imagine a massive rotating object, a huge hollow sphere (a shell) with the water bucket at its center. Since the particles which make up the sphere are moving, and moving particles have momentum, which are a source of gravity per GR -- spacetime inside (and outside) the hollow sphere is dragged by this rotational motion. So the space inside the sphere starts spinning in the same direction. This in turn causes the water surface in the "stationary" bucket to form a concave shape.

And calculations show that for a shell mass/energy on par with that contained in the universe, the frame dragging produces the same concave water surface as when the universe is stationary and the bucket is rotating.

Here's what I don't understand:

If there were no other mass/energy in the universe, per GR the water would still become concave. But In the bucket reference frame, it is at rest. With no mass/energy in the rest of the universe, there would be no frame-dragging effect. Right? So what makes the water become concave? What am I missing?

Does anyone else agree with Janus?

 

I don't totally understand this (empiracally determined constant), but Wikipedia differentiates between law and theory of science.

I disagree with Wikipedia here. The language of scientists is too imprecise to lend itself to strict definitions.

The use of the terms "Law" and"Theory" are arbitrary. A scientific theory can be one with no compelling evidence to support it (like string theory) -- or one with volumes of evidence supporting it (like quantum theory or the theory of general relativity). Unfortunately, physicists use the same word "theory" for both cases.

So what makes a "theory" a "law"? Again it is arbitrary. There is no meeting of physicists or committee of experts which decides to promote a theory to a law. It is just a matter of convention. Saying Newton's Law of Gravity is really no different than saying Newton's theory of gravity.

And per Einstein's theory of gravity (general relativity), Newton's gravity theory is only an approximation for when 1) motions are small compared to the speed of light, and 2) gravity is "weak". (A stellar object whose escape velocity is small compared to the speed of light is considered a source of weak gravity.) So why do we call it Newton's Law of gravity? Just habit from our history.

If time was purely a subjective human phenomenum then surely all humans in one time zone would not be able to agree on the measurement of time etc. But largely we can and do agree on its measurement.

 

I agree that time is not a subjective human phenomenon. But it is relative. How an observer measures how fast time passes on a clock depends on the observer's motion relative to that clock.

The reason we agree on time's measurement in our everyday lives is because our everyday speeds relative to each other are such a small percentage of the speed of light. Light travels at about 670 million miles an hour in a vacuum. So even commercial airplane speeds (about 550 miles an hour relative to the Earth) are less than a millonth the speed of light.

Einstein's formula on the rate of time's passage and relative motion is the so-called Lorentz factor or the square root of (1 - v^^2). Here v is relative velocity as a percentage of the speed of light. A v of a millionth the speed of light results in an extremely small factor for a change in the rate of time's passage. So we don't notice it. But it is real -- and measurable with ultra-high accuracy clocks.

If in some future age, people regularly travel in spaceships at hundreds of millions of miles an hour, they would know from first-hand experience that time is relative.