If two particles are moving with the speed 0.8c towards each other, what is the relative velocity of first with respect to second and vice-versa.

0.8c 0.8c
---------> <-----------

If two particles are moving with the speed 0.8c at opposite direction of each other, what is the relative velocity of first with respect to second and vice-versa.

0.8c 0.8c
<----------- ------------>


If two particles are moving in same direction, what is the relative velocity of first with respect to second and vice-versa.

0.8c
------------->
0.8c
------------->

or

0.8c
------------->
0.8c
------------->

Please help.

Very interesting question.

Well at those speeds you will need to use einstein's special/general theories.
As you know nothing can travel faster than c.

Read your textbook. It should be explained there.

WRONG! Dark travels faster than the speed of light, surely this is obvious! It doesn't matter where you want to go in the universe at the speed of light, dark will always get there first!

I think this is one of Terry Pratchett's general quantum theories.

However... I'm not sure what the Speed of Dark is though, like c, it may also be measured in parsecs. Any ideas?

Hmmm... So would that make the first question +1.6 and +1.6 parsecs
The second -1.6 and -1.6 parsecs with both in the same direction having zero relative velocity?

OK smarty pants, tell me where I'm wrong!

(N.B I'm not taking into consideration the expansion rate of the universe nor that the answer to Life, the Universe and Everything is 42.)

Play Bonny!

I've never read Terry Pratchett, but this was stated in The Neverending Story 2.

1 c
This might sound unusual because 0.8 + 0.8 = 1.6
But remember, speed is distance/time
So when the speed is limited at 1.0 c, the distance must change.
Indeed, the distances as seen from the other particle become shorter.
If you are travelling a 0.8c you don't notice anything in your distances.
But when you look at the other particle it looks like its distances are shorter.

If you move as fast as the other you simple don't see movement

Nope! It's the time that changes.

This has already been proved by synchronising a couple of atomic clocks, one of which went on a moon mission and, on return, was found to show a time ahead of the one which had stayed on Earth.

There's also a thing called the twins paradox:
As I originally heard it, one twin stays on Earth while the other travels at the speed of light to Andromeda, the Astronaut twin ages 56 years during the round trip while the one on Earth gets bored waiting and goes home for his tea.

OK, so It's an interesting piece of Pub quiz trivia which I've forgotten the end to, but for a better explanation which hurts my head see here.

Play Bonny!

Space-time is absolute, but both space and time are not. In that respect you are right. Space and time change.

It is the other way around. The travelling person gets only a few years older (the time he needs to make his round trip). But for the person on earth it is many centuries.

This, BTW, rises question what the age of the universe is. 15 billion years in our time calculation, but how much in the time calculation of other moving galaxies?

jlinkels

Well, I told you I'd forgotten the proper quote, thanks for the correction.

I always liked the closing credits of Men in Black where the camera zooms out past the planets, the Milky Way Galaxy, through the Universe, and you find out that the universe is really only an atom in a larger creation. Nice concept! (Or did I get it wrong again!)
This metric relates to the time it takes for our planet to rotate round our sun. The wee green men probably don't have a conversion chart to change our "years" to whatever their units of time are even if they knew we existed.

Ohya!... Ma' heid's startin' tae nip!


Play Bonny!

As relatives get older, their velocity decreases.

But their volatility increase, so go figure. I'm closer to Social Security than I like to think, but I'm looking forward to Thanksgiving, when I should be able to make some older geezers explode. Such fun.

Actually this is not a problem at all for The Standard Model. We observe that all galaxies on the largest scale (some local scales in which some galaxies are gravitationally bound to others presently move toward each other) are all moving away from each other at amazing velocities and that velocity is increasing. This is occurring everywhere in The Observable Universe. We can only imply from such a large sample that the parts of the whole Universe we can never see (experience in any way) behave in a similar fashion.

This is why what is erroneously labeled "The Big Bang" is as strong a theory for over a century now that it is. Time and technology have only made it stronger especially back to around 480,000 years after the Big Bang ( see http://www.physicsoftheuniverse.com/..._timeline.html). The TLDR version is that the Universe started as a high energy (HOT!!) dense place that expanded and cooled over time and can be divided into Epochs of the dominant characteristics able to exist at such levels of energy.

Prior to 10^-6 (.000001) seconds after the Big Bang it appears that the Universe was a Quark-Gluon soup. Just recently the Large Hadron Collider has reproduced the calculated energy levels of this Epoch and Quark-Gluon soups appeared in situ, not once but many millions of times. Obviously this and other recent experiments have served to increase the validity of BBT.

There is a "wall" however beyond which it is theorized we cannot go and that is Planck Time which is 10^-23 seconds after the Big Bang (that's a "1" preceded by a decimal point and then 23 zeros till we reach the "1", seconds). This is why no credible scientist will ever comment on what occurred before that time since there apparently can be no information, no evidence whatsoever, pass to us from before that time. There are learned people who attempt to go past this barrier but this is only possible Mathematically and presently there is no way to test the premises of such equations, especially since we have yet to understand the fundamental nature of Gravity.

BTW the most accurate estimations we have show it is 13.7 billion years to the Big Bang, not 15.

the question has a issue
"If two particles'

define "particle"????

if it is a photon then c is the answer to all 3 questions

if something else ?
then that changes things

also is this in " perfect" flat "spacetime"
if the "particle" is very massive then the .8c acceleration given to it will warp it's local spacetime
and the spacetime of the other particle used as the "standing still reference"


so
without defining things
there is no real answer

yes this stuff can be like drinking a "PanGalacticGargleBlaster "

Tried to read something about Imaginary time and Imaginary mass once but thought it best I stopped before what little gray matter I have collapsed onto itself and then exploded.

http://hyperphysics.phy-astr.gsu.edu...einvel.html#c1