IM Egdall

"mom-energy" =

[

"space-time 4-displacement" between events A,B

]

divided by

[

"proper time"

]

Intuitively, that seems a better definition, on the 'grounds' that the invariant interval dS is "directly accessible" to every reference frame, without any naive notions, of the perceived-to-be-moving particle somehow "reporting its proper time". To my mind, distinguishing "invariant interval" (same for all observers) from "proper time" (specific to a special reference frame) helps clarify this concept.

I agree.

IME - that's not the definitions I would have used - take a look at this page on Gravitational Time Dilation

 

I gave the definition of coordinate time in special relativity (flat spacetime -- zero gravity). You gave the definition in general relativity (curved spacetime).

I understand, that the "mom-energy" 4-vector is calculated, from quantities, derived from two different reference frames. First, the observer measures a 4-displacement, [math]\Delta x^{\mu}[/math], between two events, on the trajectory, of some particle, propagating through their laboratory rest frame. Then, that 4-displacement is divided by the proper time, [math]\Delta \tau[/math], elapsing between those two events, in the particle's own rest-frame. I.e. "the observer measures the displacement, but the particle reports the time".

 

In special relativity, momenergy is calculated in a single reference frame. The 4-displacement is measured in that frame, as you indicate. But the proper time is invariant -- it is the same for all (uniformly moving) frames of reference.

Thanks, Dr Rocket for your enlightening comments.

You have misconstrued relativity. Time runs at 1 second per second. Always. Everywhere. To talk about the "rate of tiime" is meaningless. Since rate of something = change in something/change in time. When applied to time this yields rate of time = change in time/change in time = 1.

 

What is true is that clocks measure proper time and proper time is dependent on the world line of the clock. That world line is reflects acceleration, hence gravitational effects for anything not in free fall. But to compare clocks meaningfully one must compare them at points of intersection of their respective world llines.

 

The effects of which you speak are coordinate effects and only approximate a true comparison of the proper times measured by clocks.

 

This not "traveling into the past" or "traveling into the future" in any meaningful sense. It does illustrate that time is a local concept and that there is no such thing as a true global notion of time.

But if I were to travel in a rocket into space and back ar relativistic speed, the time interval for my trip is less than the time interval for the trip as observed on Earth. In other words, my clock on the spaceship and my calendar measure say a trip of 10 years. But the clocks and calendars on Earth measure a time interval for the round-trip of say 30 years. Say I leave in the year 2012. So when I return, my calendar would say it is the year 2022. But Earth calendars say it is the year 2042. I have only aged 10 years, but 30 years have gone by on Earth. In that sense, I have traveled into the future.

In principle, at a fast enough speed, I could travel in the rocket for say one year, and thousands of years would have passed by when I return to Earth. Ain't this a kind of time travel?

Do not the rocket clock and Earth clock intersect at two world lines, at the beginning of the trip and at the end of the trip? Comparing them we see they do not show the same elapsed time.

Please enlighten me as to how this is incorrect.

The last equation says "mom-energy = mass x displacement / proper-time". So, some hypothetical CR, impacting earth's atmosphere at high energy, is perceived by "outsiders", e.g. earth observers, to have high mom-energy; whilst perceiving "internally" a small amount of proper-time elapsing ? So, there is a connection, between what the particle perceives (proper-time); and what other observers perceive (mom-energy)? If a high-energy particle is perceiving little proper-time elapsing, then is that why such particles are difficult to accelerate, i.e. you apply what you think is a large force, but the particle perceives the force applied for only a brief moment ??

I do not think you are correct here.

Proper Time

Proper time is the spacetime interval in time units. It is absolute -- the same value for all uniformly moving frames of reference. Proper time is not the "perceived" time, as you call it.

Co-ordinate Time

The perceived time is relative or different in different reference frames. It is called co-ordinate time -- the time interval in the reference frame chosen.

Momenergy

In the momenergy vector, the momentum and energy are relative, but the mass is absolute. (Similar to the relative space and time intervals and the absolute spacetime interval.) They are related by:

mass ^2 = energy^2 - momentum^2

The momentum and energy change with relative motion, but the mass is invariant. Say in my reference frame, a particle is at rest. So it's momentum is zero. Thus its mass equals its energy. (This is E=mc^2 with c in units like light-years per year so c = 1).

Now say the same particle is moving at velocity v in your reference frame (you are moving at v with respect to the particle). Here the momentum is not zero. The particle's energy, however, as you measure it, is greater than the energy I measure. Why? Because your measurement includes the particle's rest energy plus its energy of motion. So you measure greater momentum and greater energy for the particle than I do. But when you take the difference of the squares of the energy you measure and the momentum you measure, you get the same value I get for the mass.

This mass is the magnitude of the momenergy vector. And we both measure the same mass, the same momenergy magnitude for the particle -- regardless of how we are (uniformly) moving with respect to that particle.

i think that you can't travel to the past...but you can to the future... because if you go fast enough... time will slow down for you... so on earth it could be 100 years...but 20 years for you! <--- just a theory

You are correct. But this is actually an every day effect. Say you leave your home and drive to work or school in the morning and return home that evening. Per special relativity, because of your motion in your car, time has run a tiny bit slower for you than time in your home. So you return home a tiny bit into the future. We don't notice this effect because it is so miniscule, but it is real.

Oh, and you can and do travel into the past as well as the future simply by changing altitude. Per general relativity, time runs slower as it gets closer to the Earth (gravitational time dilation). So say you are at home and you go upstairs to bed. All night long, time upstairs runs a tiny bit faster for you than time downstairs. So when you come down to breakfast in the morning, you are travelling into the past. Again, this is a miniscule amount so you don't notice it. But it is real.

We are all time travellers.

Well, if you're far off, then I'm off the grid with this: If time is weaved into the fabric of the universe, how come Einstein found no need to insert time into E=MC2 ? Doesn't it take energy to bend space-time? Doesn't Mass bend space-time? Doesn't it take energy for me to make any sense here?

 

Yes it takes energy to bend or curve spacetime, And yes mass also bends or curves spacetime. Per E=mc2, mass and energy produce equivalent physical effects. So mass is a source of gravity (spacetime curvature) and space is a source of gravity (spacetime curvature). (In fact, it is difficult to come up with an exact definition of what mass is.)

More formally, it is momenergy which is the source of gravity or spacetime curvature, as represented in the stress-energy tensor. And so-called momenergy is a four dimensional spacetime vector with energy as the time component, momentum as the space component, and mass as the magnitude of the vector. See link:

http://physics.bu.ed...8_Momenergy.pdf

Scientific American has published a simplified explanation, with 10 minute video, of QE.

Clever video! Or I should say "clever John Bell".I'll give the link to my students in my quantum emtangement class. Thanks.

So-called empty space contains the non-zero Higgs field, and other fields like gravity and electromagnetism. There are also virtual particles popping in and out of existance. And so-called dark energy, whatever that is. Etc. So space is hardly a void.

My pet peeve on popular science TV programs (and in some popular science books) is that they talk about new unsubstantiated physics without emphasing there is as yet no supporting evidence for these theories, e.g. string theory.

 

Well if it is a crisis, no one thinks it is an important one.

The vast majority of climate scientists do! It is just that people in general here in the US anyway are not nearly as alarmed. If people are worked up enough and communicate this, then politicians respond. But people here generally aren't so worked up yet. THAT"S THE PROBLEM.

They either believe the mis-information put out by WSJ and other sources (a big oil priority?). OR they figure it'll happen some time in the distant future, so why worry now.

But the longer we produce more and more greenhouse gases, the more difficult it will be to mitigate and the more dire the consequences. And there may be some point of no return.

.o

They would use stealth, out of grave necessity. They would have "all the time in the world" to go back and remove evidence of their time travel. If ETs can travel great distances, they may also be able to time travel.

Ya, good point. I was thinking about this too. Maybe if future scientists have the technology to make "time machines", they also may have the ability to cover up their visits to our time in some way. But, then again, it may not be so easy to do this. So perhaps Hawking's idea might be a good one. Fun to think about, but no way to know for sure (currently).

Well, they could be on an alternate timeline.

If such a thing as alternate timelines really exist.

Interesting letter to the editor in WSJ refuting their climate change editorial.

http://online.wsj.co...0727472662.html

I find it so frustrating that the experts in the field of climate science have to fight so hard to get the public to accept what the evidence is clearly telling them: global warming is real and human activity is a primary cause. Some of the compelling data is give in this thread. For a summary, read my scenceforums blog - It's Relative:

http://blogs.scienceforums.net/IME/

This is not just a discussion about which theory may be correct. It bears on global climate changes (which have already begun), for which the impact on human life (and other life forms) is disasterous. Can we risk the deaths - yes the deaths - of thousand, perhaps millions of humans by doing nothing now? I have tried to keep an open mind about this issue, but the more I read, the greater I feel the likelihood of global warming is way too high a risk to just ignore.

To me the question is -- why does the Wall Street Journal print editorial after editorial denying climate change? A big-oil connection? A Rupert Murdock insistance? Do the writers there really, truly believe what they are saying? To me, it degrades the reputation of the newspaper and makes me suspect the soundness of their other opinion pieces.

Scientists will make a distinction. The observable universe is called exactly that. "Universe" generally means everything, even beyond the observable limit.

I have found scientists to be sloppy on this point. They often write or say "universe" when they are referring only to the "observable universe". So you have to be careful in trying to understand which one they are referring to.

I like Hawking's point that there is empirical evidence time machines do not exist. He asks "where are all the tourists from the future visiting the present?"

Pacman3456: The problem with your idea is it does not agree with what we observe. The expansion of the universe shows itself between clusters of galaxies. The distance between them is expanding. But within galaxies themselves, we do not see this expansion. For example, the distances between stars with our Milky Way galaxy are NOT expanding. Why? Gravity within galaxies overcomes the expansion of space.

To use the balloon analogy, physicists tell us to imagine coins pasted to the surface of a balloon. As you blow up the balloon, the surface expands, the distance between the coins expands, but the coins remain the same size. (The coins here represent galaxy clusters.)

But please keep on thinking!

I did a google search and found a link which says Ivar Giaever is involved with the Heartland Institute and Cato Institute. These organizations are reportedly funded by Exxon Mobil, Phillip Morris, and the American Petroleum Insititute (amongst others). To me, this makes his statements denying human-induced global warning suspect.

And, the link notes, Giaver has not submitted a single scientific paper on the subject for a peer-reviewed journal. Link:

http://littlegreenfootballs.com/page/257775_Ivar_Giaevers_links_to_Exxon_M

Got it. Thanks swansont and DrRocket.

Old explanations was that the act of measuring one parameter disturbs the measurement of the other conjugate -- like determining a particle's position disturbs its momentum. But this is a simplification. New test with neutrons verifies the uncertainty is there independent of the disturbance! Because a so-called particle also behaves like a wave, and this wave-like nature makes determining both its position and momentum to arbitrary accuracy impossible. Cool!

http://www.scienceda...20116095529.htm

Sorry DH once more, I can not describe my question more detailed. Is is really so dificult to grasp the problem, I am asking?

 

Whether measurements of the speed of light in a glass rod (flint 170 000 km / s), will show the difference of speed, if the first measurement of the speed will be executed when the rod is in a position parallel to the speed of the Earth, and the second measurement, when the rod is in a position transverse to the motion of the Earth .

Does the speed of light in a glass rod (or other medium) depends on the motion of the Earth?

OK, I'll try to answer your question: The answer is no. The speed of light in a glass rod will show no difference, whether the rod is parallel to the Earth or perpendicular to the Earth. This result does not violate special relativity. Why? Because it is being performed in a single frame of reference. The source of the light, the glass rod, and the device which measures the speed of that light are all on the Earth at rest with respect to each other -- i.e. in a single frame of reference.

The motion of the Earth has no effect because the entire experiment and the Earth are moving together. This, in essence, is what Galileo said in his Dictum on uniform motion.

(There is a caviat here. We are assuming here the Earth is moving in uniform motion -- no change in speed and/or direction. The spinning of the Earth around its axis and the motion of the Earth in its curved orbit around the Sun are not uniform motion. However, there effects are so small in this experiment as to be considered negligible.)

Could be. If you consider electrons anti-matter. Then they balance protons. They already balance as charge.

This is wrong. For one thing, matter and antimatter have the same mass but opposite charge. But protons are much more massive than electrons -- so they cannot be matter-antimatter pairs.

Positrons have the same mass as electrons and have opposite charge. A positron and electron make up a matter-antimatter pair.

Analogies are pretty useless with string theory. It is a VERY abstract and mathematical theory.

Maybe this is the key problem with string theory. It is not based on a physical principle, like quantum mechanics (uncertainty principle) or special relativity (absolute speed of light) or general relativity (equivalence principle).

Bart, in your link the author is saying the Michelson Morley (MM) experiments did not show that the speed of light is absolute. The paper is correct.

Einstein's light postulate of special relativity says:

Light travels at the same speed with respect to all uniformly moving reference frames.

OR light travels at the same speed no matter what the speed of the source of that light.

But the MM experiments were done in a single reference frame. The light source, the test apparatus, and the detector (film I think) were all at rest with respect to each other (and the Earth.) So MM says nothing one way or another about whether the speed of light is absolute.

However, a number of other tests have shown the speed of light is absolute. DeSitter's famous binary star analysis in 1913 was the first one. In 1977, MIT physicist Ken Brecher used a similar technique to verify Einstein's light postulate to one part in a billion.

sorry, i dont have a link. I think i heard it in a documentary about the neutrino research at SNOLAB.

See http://neutrinoscience.blogspot.com/2011/09/arriving-fashionable-late-for-party.html

Thanks for the replies dudes,or even dudettes. Why does the inflation, whatever is responsible for it, only work on Super cluster scale?

I think you mean the expansion of space. Gravity within galaxy clusters overcomes the expansion. As I understand it, things inside galaxy clusters are expanded by the expansion of space, but this expansion has been stopped by the strength of gravity (spacetime curvature). But out there in so-called empty space, there are no galaxies, thus so little gravity that the expansion dominates.