Time travel question

Morepower (Craig) · #1 20-07-2010, 12:29 PM

I was watching a doco the other night where they had a hypothetical train that circled the Earth at the speed of C. They said the passangers on the train travelled non stop for a week, but the people on the Earth experienced the train travelling for 100 years ? My question is if you were fuelling the train would you put a weeks worth of fuel in it or 100 years worth ?

Thanks
craig

GeoffW1 (Geoff) · #2 20-07-2010, 01:07 PM

Hi,

I think a week.

Cheers

SkyViking (Rolf) · #3 20-07-2010, 01:33 PM

If, from the passengers point of view, the trip took one week then the train couldn't have travelled at c. If travelling at the speed of light the trip would seem instantaneous for the passengers, so if a week passed then the speed must have been a bit lower than c, but that's probably just a journalistic error.
Regarding the fuel then it would be tempting to say that since time itself pass more slowly on the train then the fuel is also consumed at a slower rate so only one weeks worth should be necessary.
But in the extreme case of the train actually travelling at c, and the trip therefore being instantaneous for the passengers, the fuel consumption would thus have to be zero. That would be a bit of a paradox, so I'd say the fuel consumption is always the same in both frames of reference. Therefore I vote for loading the train with 100 years worth of fuel.
Interesting question by the way.

Moon (James) · #4 20-07-2010, 02:14 PM

For the train to accelerate from 0 to C and back down again would take an infinite amount of fuel, right? As it speeds up, the mass approaches infinity.

SkyViking (Rolf) · #5 20-07-2010, 02:22 PM

But it didn't travel at c because the journey took one week in the train's frame of reference.

CraigS · #6 20-07-2010, 04:41 PM

I thought I was beginning to understand this stuff until I read:

http://www.physorg.com/news163738003.html

What (I think) they're saying in this article, is that there is a situation where the train could actually have been been going for a 100 years whilst the earthbound folk, (albeit, now in orbit - so not earthbound), experience the train's trip as only one week ! (ie: the opposite of the 'classical twins' paradox).

So with a few changes to the original conditions, I think the train needs a 100 years worth of fuel (?).

Cheers

sjastro · #7 20-07-2010, 06:41 PM

Fuel consumption is analogous to the biological aging of the passengers on the train. In other words the passengers will age at a "normal" rate in the train's frame of reference irrespective of it's speed.
So you would be adding fuel according to the time of travel of the train not on the time that has elapsed on Earth.

Of course as has already been mentioned it's not actually possible for any object with mass to attain the speed of light. The Lorentz equations break down.

Regards

Steven

SkyViking (Rolf) · #8 20-07-2010, 08:06 PM

Quote:

Originally Posted by sjastro

Fuel consumption is analogous to the biological aging of the passengers on the train. In other words the passengers will age at a "normal" rate in the train's frame of reference irrespective of it's speed.
So you would be adding fuel according to the time of travel of the train not on the time that has elapsed on Earth.

I thought so too at first, but that would mean less fuel is consumed the faster the train goes... surely not?

bojan · #9 20-07-2010, 08:32 PM

Quote:

Originally Posted by SkyViking

I thought so too at first, but that would mean less fuel is consumed the faster the train goes... surely not?

Well, you still need to accelerate to reach the final speed.
So, faster travel, more fuel.

SkyViking (Rolf) · #10 20-07-2010, 08:58 PM

I think the final answer must be that the elapsed time in either frame of reference doesn't matter, because only the amount of fuel needed to reach the final travel speed is necessary, and that amount is universal.

Colin_Fraser · #11 20-07-2010, 11:07 PM

Obviously not the same train that travelled to Ballarat from Melbourne on Monday. It ran out of fuel at Bacchus Marsh. True.
The people on the train travelled for 1 week. Relative to the trains time or Earths time?

sjastro · #12 20-07-2010, 11:17 PM

Quote:

Originally Posted by SkyViking

I thought so too at first, but that would mean less fuel is consumed the faster the train goes... surely not?

In the train's frame of reference you are using proper time, not Earth's frame of reference which is time dilated. Since proper time runs faster then the time in the Earth's frame of reference this accounts for the length contraction in the train's frame of reference (as distance= velocity X proper time).

You are adding fuel based on proper time not on Earth's dilated time.

Regards

Steven

xelasnave · #13 21-07-2010, 02:58 PM

er...How much fuel is needed to accelerate a grain of sand to the speed of light ?...
alex

bojan · #14 21-07-2010, 03:07 PM

Quote:

Originally Posted by xelasnave

er...How much fuel is needed to accelerate a grain of sand to the speed of light ?...
alex

Infinite amount

xelasnave · #15 21-07-2010, 03:15 PM

Never ask a question unless you know the answer

.

I find speculation about things that can never happen interesting as long as one keeps their feet on the ground

.

And it would be impossible to achieve an orbital speed at C I expect

...
alex

AdrianF (Adrian) · #16 21-07-2010, 07:01 PM

Quote:

Originally Posted by Morepower

I was watching a doco the other night where they had a hypothetical train that circled the Earth at the speed of C. They said the passangers on the train travelled non stop for a week, but the people on the Earth experienced the train travelling for 100 years ? My question is if you were fuelling the train would you put a weeks worth of fuel in it or 100 years worth ?

Thanks
craig

Was that "Into the Universe" with Stephen Hawkings?
Was a good documentary I have it on my iPhone and have watched it a few times.....still trying to come to grips with his theory of time travel

.

Adrian

Morepower (Craig) · #17 21-07-2010, 07:55 PM

Quote:

Originally Posted by AdrianF

Was that "Into the Universe" with Stephen Hawkings?

Yeah it was, it has been good so far.

Some people say time travel is not possible and personally i'm not really sure. But if it is not then why do we have to adjust the clocks to run faster on GPS satellites ?

I was leaning toward a weeks worth of fuel but wasn't sure when I started thinking about what the guy who fuelled it must be thinking as the train is still going ~100yrs later.

mbo (Pat and Kirk) · #18 23-07-2010, 03:18 PM

Hi all,
I didn't see the doco (yet), but I'm guessing they weren't implying that the train actually reached 100% light speed (impossible according to relativity, and 99% of scientists). To get a time dilation of 1 week = 100 years (a factor of 1:5200) requires a speed of about 0.99999998 the speed of light. (from http://www.1728.com/reltivty.htm)
I'm very rusty on relativity, so this may all be rubbish, but ...
I believe a key point with the fuel consumption is that the mass of the train, (and the fuel) appear to be different depending on your frame of reference, also the distances traveled appear different depending on the location of the observer.
The observer on the train would see the mass of the train increased by a factor of 5200, also the distance (and the earths circumference) would appear to _contract_ for those on the train by the same factor. In the end all of numbers should balance out, so that 1 weeks travel at 1/5200th the distance does the same number of laps of the earth as seen by the earth observer over 100 years.
Those on the train find that fuel consumption has gone up to billy-o because their mass has increased. so they use up their 100 years worth of juice in one week! (I'm still missing something here? the fuel weight went up by the same factor ??)
So in short, my guess is 100 years worth of fuel needs to be put on the train.

sjastro · #19 23-07-2010, 05:58 PM

Here is a real life example similiar to the train problem.

The LHC can accelerate protons up to 99.99% c.
The energy required to accelerate the proton is based on it's rest or inertial mass not on it's relativistic mass at 99.99% c.
In fact if you would not be able achieve 99.99% c based on the relatvistic mass.

The whole point is that in the proton's frame of reference at 99.99% c, it doesn't experience mass increase or time dilation.

The same applies to the train's frame of reference. An observer on the train will not experience any relativistic effects bar distance contraction due to proper time.

So therefore the train is supplied with one weeks fuel. Fuel consumption is a property specific to the train's frame of reference.

Regards

Steven

renormalised (Carl) · #20 23-07-2010, 06:45 PM

One question....who's paying for the repairs done on the rails, or tube system, this train is running through and/or on, because at 0.99999998% of lightspeed, the amount of heat generated via friction between the rails and the wheels of the bogies on the train, or it's motion through the transit tube if it's a maglev train, will be enormous!!!!!

Plus the shielding on the train would have to be metres thick and as dense as lead to keep the passengers from being fried by gamma radiation....and extremely high energy gamma rays at that!!!!.