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Richyiee
Topic Author
Posts: 57
Joined: September 2nd, 2008, 5:20 pm

Time dilation

A clock on a rocket travelling near the speed of light away from earth will run slower than a clock on earth right ? But to the clock on the rocket earth is travelling near the speed of light away from the rocket so the clock on earth should run slower than the clock on the rocket ? am i missing something

hayes
Posts: 1181
Joined: July 18th, 2008, 11:24 am

Time dilation

Of course you're missing something, but I have no idea what! I love getting mixed up with this stuff, thinking outside the rules of physics we experience everyday. I remember as a kid reading about what happens if 2 rockets go in opposite directions from the planet at the speed of light! My guess, (I will try and google it/ read up on it a bit more at a later stage) is this:The earth and the rocket started in the same place, (in relation to each other of course - The fact they may already be whizzing around the cosmos is irrelevant). It is the rocket that started moving away (again, relative to the eart), exerting new forces on itself and the universe while the earth exerted no new force.It is for this reason that you could say the earth is staying still, while the rocket is moving away close to the speed of light, which then sets you up for the whole time issue...Another possibility could perhaps be to do with the difference in mass between the 2 objects, but again, just a guess.Nice question.

ppauper
Posts: 70239
Joined: November 15th, 2001, 1:29 pm

Time dilation

ah, special relativity !

frenchX
Posts: 5911
Joined: March 29th, 2010, 6:54 pm

Time dilation

It's a question of frame.There are two frames: the earth one and the rocket one.The clock in the rocket is slower in the EARTH frame. For the guy inside the rocket, he doesn't see a time change of his clock. But he could see a change on the Earth clock . The contrary nowthe earth moves towards the rocket with a high speed. In the earth frame we see no change but in the ROCKET one, the guy sees that the earth clock is slower.The effect is the same but in two different frames Keep in mind that in YOUR frame you are IMMOBILE

hayes
Posts: 1181
Joined: July 18th, 2008, 11:24 am

Time dilation

Ah good stuff. I was hoping you'd post an answer! (frenchX, not ppauper...).If you couldn't answer it then there'd be little hope for the rest of us!Thanks for that, much appreciated.

frenchX
Posts: 5911
Joined: March 29th, 2010, 6:54 pm

Time dilation

Thanks Hayes Physics is nice but special relativity is very counter intuitive.A lot of paradoxes

Richyiee
Topic Author
Posts: 57
Joined: September 2nd, 2008, 5:20 pm

Time dilation

so when the rocket returns to earth, will rocket man be younger or earthman ?

frenchX
Posts: 5911
Joined: March 29th, 2010, 6:54 pm

Time dilation

Since we live on earth (the reference frame is the earth one), the rocket man will be younger than us.In the earth frame,T_rocket=T_earth/sqrt(1-(v_rocket/c)^2) (it's the time dilatation effect)You can prove it with Lorentz transformation or with the Doppler effects formulas in special relativity.It's the famous Langevin twin paradox.

Richyiee
Topic Author
Posts: 57
Joined: September 2nd, 2008, 5:20 pm

Time dilation

but to the rocket man, his reference frame being the rocket, we would be younger than him ? how can he be younger than us and we are younger than him

Collector
Posts: 4682
Joined: August 21st, 2001, 12:37 pm

Time dilation

QuoteOriginally posted by: Richyieebut to the rocket man, his reference frame being the rocket, we would be younger than him ? how can he be younger than us and we are younger than himAssume earth is travling at a constant speed in one direction (and in rest for the observer on earth) then anything leaving earth and returning to earth must have traveled as far as the earth plus additional distance, it must have traveled further. So yes in this case it is easy to say who was traveling faster. In other cases it is much more difficult to find out what frame is the fastest (depending on various assumptions that many of them fit experiments, but give different conclusions). But yes when an objest is traveling at constant speed (at rest with itself) and another object is leaving and returning to the other frame it is easy to know what frame traveled faster relative to the other one. And what frame left the other frame and returned, this can be felt by acceleration, but acceleration is not direct cause of the time dilation, it is just pointing towards change in velocity for the affected observer and thereby clearly what observer left and returned. Of course this can be made much more difficult with both frames accelerating etc ... but main principle is the same.
Last edited by Collector on January 31st, 2011, 11:00 pm, edited 1 time in total.

Richyiee
Topic Author
Posts: 57
Joined: September 2nd, 2008, 5:20 pm

Time dilation

i dont agree to this, to the rocket man earth has accelerated left and returned whilst it is traveling at constant speed.given only 2 frames of reference how can you say one is traveling faster than the other? there is only one measure - the distance between them which is either getting larger or smaller
Last edited by Richyiee on January 31st, 2011, 11:00 pm, edited 1 time in total.

Collector
Posts: 4682
Joined: August 21st, 2001, 12:37 pm

Time dilation

QuoteOriginally posted by: Richyieei dont agree to this, to the rocket man earth has accelerated left and returned whilst it is traveling at constant speedyou dont agree, but experiments agree with this.The beautiful woman sitting in her chair on earth never felt any acceleration, but her twin sister leaving earth in a space ship felt acceleration, this they can communicate and agree on both of them, so yes the situation is assymetrical and this can easily be messured!The situation is Assymetrical and not only in theory, but fully measurable. You are back to the big discussion of Philosphy Professor Dingle in the 1950....well experiments showed he lost the discussion, but yes there are many interesting asspect that not have been fully explored around special relativity theory, but this is not one of them (in my opinion)
Last edited by Collector on January 31st, 2011, 11:00 pm, edited 1 time in total.

Richyiee
Topic Author
Posts: 57
Joined: September 2nd, 2008, 5:20 pm

Time dilation

acceleration is felt because the earth has a far greater gravitational field than the tiny rocket. Now what if the twin sister left in a big blob the same mass as the earth ? I think its interesting becuase if you cant answer this question then time dilation is a stupid concept, unless somehow gravity is involved in the equation
Last edited by Richyiee on January 31st, 2011, 11:00 pm, edited 1 time in total.

Collector
Posts: 4682
Joined: August 21st, 2001, 12:37 pm

Time dilation

QuoteOriginally posted by: Richyieeacceleration is felt because the earth has a far greater gravitational field than the tiny rocket. Now what if the twin sister left in a big blob the same mass as the earth ? I think its interesting becuase if you cant answer this question then time dilation is a stupid concept, unless somehow gravity is involved in the equationWhat about time dialtion in ariplains, do not a airplain compared to a person sitting in a chair at same location on earth have approximately the same gravitation. Do you not feel acceleration when taking off in a airplain... And yes time dilation according to SR is measured also here with very accurate clocks. And naturally time was slowed down for the airplain that we know went faster based on several factors. There are lots of papers published around this, actually more than 3400 papers on relativity theory published already before 1922... a good place to start...
Last edited by Collector on January 31st, 2011, 11:00 pm, edited 1 time in total.

rmax
Posts: 6080
Joined: December 8th, 2005, 9:31 am

Time dilation

Collector - good replies.To summarise: The twin paradox has nothing to do with one twin being older and the other being younger - that is a physical fact not a paradox. The key to the twin paradox is how (under Lorentz transforms) can you possibly tell which one is the younger: as Richiyee so correctly pointed out.The key (as per collector) is that the twin that has experienced the acceleration will be the younger. One twin must have experienced an acceleration to get to the speed of light, and hence the two frames are not identical under SR. The acceleration is not the gravity felt under the earth. For acceleration frames refer to General Relativity.Now however you look at, one twin will always experience an acceleration as to compare the twins you will have to have one return back to the place of origin otherwise you will have to send a signal faster than light to compare the twins ages.

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