1821

Physics, Astronomy & Cosmology / Why do people insist that NASA made up the Moon landings?

« on: 06/08/2009 23:08:41 »

Ok, but the fact there was a transmission coming from there doesn't mean that astronauts landed there.

They might have claimed that we sent machines which landed on the moon and transmitted those radio signals but that’s stretching it a bit too far. Not to mention that the conspiracy theorists never admit this possibility. It’s clear that they never thought of something this obvious – or if they did they weren’t prepared to explain it. I’d like to see them claim that we had the ability to send machines and not people. When the astronauts went to the moon they broadcasted radio waves continuously (to monitor life signs etc). Russian scientists must have monitored all those transmissions just waiting for the opportunity to prove it was a hoax or find something wrong or wait for an accident/screw up. Those scientists can tell the origin of those radio waves by triangulating the radio signals. Thus it was quite easy to tell that those signals originated from the surface of the Moon. If someone wants to say that they were actually machines sending out those signals then let them do so and we’ll address that when it comes.

Then again I can’t prove to you that I’m not a machine myself, can I?

1822

Physics, Astronomy & Cosmology / Why do people insist that NASA made up the Moon landings?

« on: 06/08/2009 13:11:00 »

It would have been trivial for the Russians to prove that the US was not on the moon at that time. All that had to do is claim that radio transmissions weren’t coming from the moon. But since they were and that would be impossible to fake with even modern technology the conspiracy idiots ignore this all too simple fact.

1823

Physics, Astronomy & Cosmology / If I give an object some potential energy, does its mass increase?

« on: 06/08/2009 12:37:17 »

That's not my definition, Pete. That's Einstein's.

Huh? Where did you get that impression from?

You didn’t address the concerns in my response, i.e. that merely defining a term does not justify its name. You'd have to demonstrate how it has the properties associated with inertia. I.e. if you choose to “define” mass as another name for rest energy then you have to justify it. I can call myself a pot, but that doesn’t justify you pushing me into an oven! I explained that in relativity mass is always defined in terms of momentum. All you did so far is to tell me how, in slow speeds, objets which have rest mass is “related” to momentum, i.e. presumably by p = mv. There is an important difference since if mass is defined in terms of momentum then anything which has momentum has mass. This does not hold with the definition you chose. And you didn’t specify under what conditions that pertains to. If you assert that it holds in all possible applications then you are mistaken since (1) only holds in special spacetime coordinates (i.e. those corresponding to in inertial frames only frames) (2) even then only when the object is not subject to  stress.

In any case this is all still off topic since none of it addresses the original question. If the OP (HankRearden) has no further questions then it seems to me that the conversation is over, at least for me

1824

Physics, Astronomy & Cosmology / can a body reach velocity of light ?

« on: 05/08/2009 14:34:12 »

Hi ,

Is it possible to reach an object to the speed of light or exceed it ?

No. To be precise, no object, which can be at rest in one inertial frame, can move at the speed of light or faster as observed in another inertial frame. The intersection of two waves/lines is not an object so it doesn’t pertain to your question.

1825

Physics, Astronomy & Cosmology / If I give an object some potential energy, does its mass increase?

« on: 05/08/2009 11:39:49 »

A few last comments:

Farsight; when you defined rest mass = rest energy/c^2 all you did was give rest energy a new name with new units. Merely defining a term does not justify its name. You’d have to demonstrate how it has the properties associated with inertia. Even then you’d only be using your own pet notion of mass as it pertains to acceleration. In relativity mass is always defined as to how I relates to momentum. You also claimed that a domain wall is an abstraction but you didn’t justify that claim other than claiming, “we see no such thing.” Vacuum domain walls were left over from phase transitions in the early universe. Just because you don’t see then it doesn’t mean they don’t exist or never have existed. Heck, it wasn’t until recently that black holes were actually detected but we spoke of them very meaningfully before that. Same with cosmic strings. Heck, I can’t see the other side of the universe but I’m pretty sure its there. You might not approve of the use of the term “mass” in cosmology and relativity texts/journals, but it’s there.

1826

Physics, Astronomy & Cosmology / If I give an object some potential energy, does its mass increase?

« on: 04/08/2009 06:43:55 »

I do use it that way. IMHO rest mass is rest energy, only it's not actually at rest. It's just moving in some circular or back-and-forth type fashion so it isn't moving in aggregate with respect to me. Now, show me some negative mass!

Let me get this straight. When you use the term “mass” you mean nothing more and nothing less than “rest mass” = “rest energy/c^2” and you use the symbol “m” to represent it, correct? If so then I know of no situation where there is negative rest energy so there is no example I know of where your “mass” is negative. I assume you use it in the expression for momentum, i.e. p = gamma*m*v, right?

There is a problem with this definition of mass. It can only be used in special cases, i.e. it can’t be used in general circumstances. Especially when it comes to the mass density of, say, a magnetic field or when you have a body under stress or if your speaking of the mass of a fluid which has pressure.

I dispute this.  IMHO in the early days of the universe the energy density was high, so processes within it would have run slowly just as they do in a region of high gravitational potential, hence the initial expansion would appear to be rapid. Nowadays the galaxies aren't pushing each other apart, the space between them is expanding because it's still under pressure.

I don’t understand what you mean by this? Please explain what pressure you are referring to. And what I said does not merely refer to the expansion of space but that it’s expanding at an accelerating rate.

The dimensionality of energy is pressure x volume, and there's nothing negative that I can see. Nor is there anything negative in a region of uniform space. Introduce a sun, and you've introduced positive energy to one portion of that region. You've also introduced a gradient in the energy density into the region we call a gravitational field. The active gravitational mass is telling you how much energy is there. I can't find any negative energy or negative mass anywhere.

Do you know what a relativist means when the speak of “negative active gravitational mass density”? I’ve provided a web page to describe this term

http://www.geocities.com/physics_world/gr/active_grav_mass.htm

First off nobody knows why this is happening so its no wonder you can’t find anything. The term “negative active gravitational mass density” means nothing more and nothing less than there is gravitational repulsion and that the source of gravity is less than zero. E.g. for a relativistic fluid with proper energy density u and pressure p, in the weak field limit Einstein’s field equation becomes  del^2 phi = 4pi*G/c^2(u + 3p). The term (u+3p) is called the “active gravitational mass density”. When it’s less than zero (i.e. “negative mass) there is gravitational repulsion. This happens when there is negative pressure (i.e. tension). This happens in a vacuum domain wall. The tension is so large as to overwhelm the energy density and this gives rise to a negative active gravitational mass density

I just can't see it Pete. Sorry.

All you have to do is look at the situation of a body under tension. I’ve worked out an example at
http://www.geocities.com/physics_world/sr/inertial_energy_vs_mass.htm

Express the stress-energy-momentum tensor in coordinates where a body is moving. Let the speed be small so that p = mv. This “m” is what you call mass, right? The situation I’ve described m is a function of the tension (i.e. negative pressure) in the body and if it’s large then M will be less than zero. But it is nor proportional to the bodies rest energy. That’s why I don’t use the concept of mass as rest energy/c^2.

1827

Physics, Astronomy & Cosmology / If I give an object some potential energy, does its mass increase?

« on: 03/08/2009 03:04:29 »

For more on inertial forces see my web page http://www.geocities.com/physics_world/gr/inertial_force.htm

Nice site. Thank you Pmb.

That's my personal website so thank you kindly for the compliment.
This forum is a bit weird. When I type there is a very long time delay before the letters appear. It's too hard to type directly into the window so I use an editor and do a cut and paste. There's a lot going on here that I wish I could keep track of and respond to.

A region of empty space is not empty of energy, and OK, in this respect one can ascribe mass to it (he said grudgingly). A planet is surrounded by a region of space which exhibits a gravitational field, and there is energy "density" in that region of space. But the gravitational field itself is only there because there's a gradient in the energy density. No gradient means no gravitational field. You can see this very easily if you consider uniform space. There's no gravity at all.

MTW give the stress-energy-momentum tensor for the gravitational field as that derived by Landau and Lifshitz. On 465 they give an expression for it for a weak gravitational field. The energy density is given in terms of phi,j*phi,j. For a uniform gravitational field phi = gz/c^2 an so the energy density is uniform. The field is still there though, i.e. you'd still "fall down".

Try this: if you're in a windowless box in free-fall you can't feel any force and you can't feel any acceleration. As far as you're concerned there is no gravitational field, you're just floating in space in an inertial reference frame, and nothing is falling down at all. But if I snap my fingers and give you a window, you change your mind pronto. You switch your reference frame from the box to the ground, and you are utterly convinced of the existence of that gravitational field. But you still can't feel any force acting upon you.  I snap my fingers again to give you a soft landing, whereupon you do feel a force which intensifies then levels off, but persists. You feel the force when you're standing on the ground, not when you're falling down.   

Any object only "feels" a force on it when there is stress imposed in the body by the force field. If you're falling in a uniform gravitational field you'd feel no force. But the same would happen for a charged body in a uniform electric field too. If you're in a non-uniform gravitational field then you'd feel tidal forces since they'd impose stresses in your body. It's odd that people don't make this obvious comparison with the electric field!

there's no such thing as negative mass

This is wrong. I think you said this because you use the term "mass" as another name for rest energy. That's a problem. In any case dark energy is due to the presence of negative active gravitational mass density. Any time you have repulsive gravity, such as in inflation and dark energy, then you're talking about negative active gravitational mass density. It's also conceivable that there can be negative inertial mass if a body could exist that could support large enough values of tension. You probably never thought of this because you associate of mass with rest energy. Negative inertial mass means that the momentum of a body is opposite to its velocity and that's quite possible in special relativity given large enough values of negative pressure (i.e. tension) compared to the bodies energy.


If you want to learn more about this analyze the stress-energy-momentum tensor for continuous media. E.g. see either Tolman or Rindler.

Pete

1828

Physics, Astronomy & Cosmology / How is the distance between two light pulses calculated?

« on: 02/08/2009 16:25:21 »

I now see my mistake. The solution is as follows. From

d’ = cT’ = gamma*c[d/(c - v)](1 - v^2/c^2)

Factor out (1 - v^2/c^2) to (1+v/c)(1-v/c)

d’ = gamma*[d/(1 – v/c)] (1+v/c)(1-v/c) = d*gamma(1+v/c)

This reduces to

d’ = d*sqrt[(c+v)/(c-v)]

Which is the right answer.

1829

Physics, Astronomy & Cosmology / If I give an object some potential energy, does its mass increase?

« on: 01/08/2009 22:04:44 »

The important point is that gravity is a pseudoforce like Einstein said, because no energy is added to the system.

Einstein never said that gravity was a pseudoforce. Einstein argued that the gravitational force is of the same nature as inertial forces. Some Newtonians argue that since there is no source of such a force that it’s not “real” and hence terms like pseudoforce, apparent force, and fictitious force were coined. But others disagreed with that notion and Einstein was one of them. Einstein argued that inertial forces should be thought of as being real. It’s also wrong to assume that there is no potential energy in the gravitational field or that there is no mass equivalence to that potential energy. Einstein’s field equations are non-linear because gravity itself is a source of gravity.

For more on inertial forces see my web page http://www.geocities.com/physics_world/gr/inertial_force.htm

Pete

1830

Physics, Astronomy & Cosmology / Re: How is the distance between two light pulses calculated?

« on: 01/08/2009 12:56:39 »

The nice thing about forums like this is that sometimes merely asking the question to someone else stimulates the mind and the solution comes to you. After I asked the question last night I started thinking about the way I asked it. That led me to better understand how to solve the problem, although the solution still evades me.

Let two light pulses, separated by a distance d, be moving parallel to the x-axis. At t = 0 let the leading pulse be at x = 0 and the trailing pulse be at x = -d. After a time delta t = T the trailing light pulse reaches S’s origin clock, which has moved a distance vT in the mean time.

Define the distance between the pulses as measured in S’ to be d’ = cT’ where T’ is the time it takes for the second pulse to reach the clock from its initial position. Determine T’ by the Lorentz transformation. Define the following two events

Event 1: First light pulse arrives at clock. Coordinates of event (x,t) = (0,0)
Event 2: Second light pulse arrives at clock. Coordinates of event (x,t) = (vT, T)

The temporal displacement between these two events is delta t = T. The spatial displacement between these two events is delta x = vT. We wish to find d’ = cT’. The Lorentz transformation for time is

t’ = gamma(t – vx/c^2)

The temporal difference between the two events is then

delta t’ = gamma(delta t – v delta x/c^2)

Substitute delta t’ = T’, delta t = T and delta x = vT

T’ = gamma(T – v(vT)/c^2) = gamma(T – v^2T/c^2) = gamma*T*(1-v^2/c^2)

d’ = cT’ = gamma*cT*(1-v^2/c^2)

cT = vT + d   ----->    T = d/(c – v)

d’ = cT’ = gamma*c[d/(c – v)](1 – v^2/c^2) = d*gamma*(1 – v^2/c^2)

d’ = d*sqrt[(1-v/c)/(1+v/c)]

which is the wrong answer. [:-'(]

1831

Physics, Astronomy & Cosmology / How is the distance between two light pulses calculated?

« on: 01/08/2009 02:05:00 »

In order to keep myself from getting rusty right in relativity I'm working through the book Introduction to Tensor Calculus, Relativity and Cosmology, by D. F Lawson. One of the problems has me a bit stumped. It reads

Two light pulses are moving in the positive direction along the x-axis of the frame S, the distance between them being d. Show that, as measured in S', the distance between the pulses is
 
d*sqrt[(c+v)/(c-v)]

The only part covered up to this point is the Lorentz transformation (LT) I assume that the author wants the reader to use the LT to arrive at this expression. Can someone you give me a hint how to start?

Here is what I'm thinking; define the distance betwen the two pulses by how long to takes both pulses to pass by the origin. Call that time T. Then d = cT. dt = T = d/c. Do the samething in frame S'. This gives

dt' = gamma(dt - vdx/c^2)

dx = 0

dt' = gamma(dt) = gamma (d/c)

d' = cdt' = d*gamma

This gives the wrong result. Hence my question.

1832

Physics, Astronomy & Cosmology / If I give an object some potential energy, does its mass increase?

« on: 30/07/2009 02:24:04 »

Hi Farsight!! How are you?

Conversations about mass can get confusing because its almost guaranteed that different people will be using different definitions of mass. What I spoke about above is often referred to as relativistic mass. What Farsight is talking about is what is often referred to as rest mass. Farsight’s comments can only be applied to special circumstances. Namely for bodies not subjected to stress.

1833

Physics, Astronomy & Cosmology / what are the backgrounds needed to be a physicist

« on: 29/07/2009 20:48:36 »

I'd been computer-programming with loops and variables and whatever for about 2 years before they started teaching us algebra at school.
I don't think my teacher realised why I wanted to call my variables with meaningful names rather than just a,b,c, x,y,z etc. She just thought I was being awkward. []

That's a useful thing to do when programming a computer. It’s an awkward thing to do with math. Each for good reasons.

1834

Physics, Astronomy & Cosmology / what are the backgrounds needed to be a physicist

« on: 29/07/2009 03:39:41 »

ok pmb, thanks, now does anyone have any likns that can help wit all that math so that i may can teach myself some of it.

There’s no substitute for a textbook but if you’d like you can search using Google. I assume that so far you haven’t even learned algebra yet, correct? I did a search for you and here’s what I found as an example of where to start

http://www.wtamu.edu/academic/anns/mps/math/mathlab/

1835

Physics, Astronomy & Cosmology / what are the backgrounds needed to be a physicist

« on: 28/07/2009 18:46:45 »

If you’re just entering high school then I recommend taking the obvious courses, i.e. chemistry and physics. Then, as emphasized above, study math. To be precise, take geometry, algebra, trigonometry and calculus. If they offer statistics then take that too. Computer courses are also important. To become a physicist you have to take so much math as an undergraduate that if you take math courses for your electives then you can get a second major in math. That’s what I did.

1836

Physics, Astronomy & Cosmology / If I give an object some potential energy, does its mass increase?

« on: 27/07/2009 12:52:52 »

Thank you. It's nice to be here.

1837

Physics, Astronomy & Cosmology / If I give an object some potential energy, does its mass increase?

« on: 26/07/2009 00:38:25 »

So matter can basically be thought of as just potential energy because matter can be turned into energy and theoretically vice-versa, does that mean when I lift up a plate and increase it's potential energy, i've technically increased its mass?

You're thinking of mass-energy. The term "potential energy" refers to a different kind of energy. There are two ways the potential energy of a body can change. One can change the internal potential energy of a body. That kind of change will cause the rest mass of the body to change. The same thing will happen if you were to change the internal kinetic energy of the body increases (e.g. if the particles which make up the body vibrate faster).

Another way to change the potential energy of a body is to change its position in a field. Depending on what one means by "mass" its also possible that the mass by changing body’s potential energy. If the field is a gravitational field and one changes the position of the body in the gravitational field then there will be change in the mass of the body. This mass is related to the body’s rest mass m_0 by (let T = proper time)

m = (dt/dT) m_0

For they typical kind of gravitational field (i.e. corresponding to a time-orthogonal spacetime)

dt/dT = 1/sqrt[ 1 + 2*Phi – v^2/c^2]

where Phi = gravitational potential

The mass is then

m = m_0/sqrt[ 1 + 2*Phi – v^2/c^2]

Don’t confuse this with proper mass (aka “rest mass”)

Wanhafizi wrote – “I saw a documentary of LHC experiment, they were saying, mass of the protons will increase as the energy increase.”

They were speaking of the inertial mass of the proton, not its proper mass. When particle physicists speak of mass they are often referring to proper mass. However when relativists use the term “mass” they can mean one of several things. For example, in the lecture notes from Alan Guth’s “The Early Universe” course at MIT he explains that light has mass according to its energy. In this context he’s referring to what other relativists refer to as “relativistic mass.” This kind of mass is often spoken of in general relativity. It also goes under various other names such as simply “mass”  but also inertial mass, relativistic mass, active gravitational mass and passive gravitational mass.