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-   -   Time travel in aircraft (https://www.pprune.org/tech-log/10530-time-travel-aircraft.html)

Brakes...beer 27th Jan 2001 23:50

Time travel in aircraft
Ages ago I remember reading an article about Einstein's Theory of Relativity (I think), which illustrated a point about space and time with an example of a train driver on the London-Glasgow route. Over the course of his working life, back and forth, five days a week he would age something like eight minutes less than his wife, who stayed 'motionless' at home.

Can any physicists enlighten me on this in simple terms. If it's true, I'm going to go for long-haul!

compressor stall 28th Jan 2001 06:47

Yes interesting, although i would have thought that eight minutes is a huge difference for only travelling 150 km/h, even over several decades. But I do not know the maths behind this.

The other factor to consider is the fact that as you move further out from earth's gravitaional field, time relative to earth's time will speed up...hence twins one on earth and one in space, when reunited the one from space will have aged further. This has been demonstrated by an atomic clock in space and one on the ground.

There have been other expereiments involving atomic clocks on aircraft. One of which involved flying it around the world and comparing it to the base clock, and the one which has me puzzled: flying 2 around the world, one clockwise and one anti clockwise, supposedly they came back reading different times. Why is this?

Back to your wanting to go long haul...The aforementioned effect on a pilot would ameliorate the effect of the [train driver's] aging, but to what extent I do not know (back to the text book for some quantitive figures). Stay tuned.

Those who restrain desire do so because theirs is weak enough to be restrained.
William Blake

NIMBUS 28th Jan 2001 09:20

Must be a pretty quick train!
Time dilation equations only become a factor at speeds approaching the speed of light.

I've got the theory somewhere, and I'll try to find it and post it here!

compressor stall 28th Jan 2001 16:51


I thought that they were a factor at any speed...just more pronounced as you get faster. Is it an exponential increase?


Those who restrain desire do so because theirs is weak enough to be restrained.
William Blake

[Steve] 28th Jan 2001 17:48

Time dialation as predicted by Einstein is


Given that c is approx 3x10^8 m/s (or about 1.08x10^9 km/hr), then time dilation is approx


So at normal speeds, the dilation of time is rather hard to measure.

At 200Km/hr, time dilation is 1 / (1 - 3.429x10^-14) (ish).

So if you spent 20 years working 5 days a week, travelling for 8 hours per day, without holidays, sick, or long service leave, you would age about 5 microseconds less than your stationary spouse.

At 800Km/hr, time dilation is much larger! If you spent the same amount of time in an aircraft cruising at this velocity, you would age almost 1 millisecond less than your train driving brother.

For these reasons, I cannot recommend long duration flights as a method to achieve immortality.

mik 28th Jan 2001 19:52

On 22/11/1975, the U.S. navy performed an experiment when they flew a P3C carrying atomic clocks for 15 hours at a true speed fo 270 knots. Special relativity prdicted that elapsed time would be reduced by 5.6 nanoseconds.

However, General Relativity says that because the gravitation field was reduced, elapsed time would be greater by 52.8 nanoseconds. So overall, theory predicted that the crew aged by 47.2 nanoseconds more than they would have if they'd stayed on the ground.

The result of the experiment was that the atomic clocks said that 47.1 extra nanoseconds had elapsed. Yet again Einstein was right!


[This message has been edited by mik (edited 28 January 2001).]

Per Ardua Ad Asda 29th Jan 2001 20:54


I thought that you were supposed to age at SLOWER rate when travelling at high speed?

Through Difficulties to the Supermarket....

twistedenginestarter 29th Jan 2001 21:17

I thought that you were supposed to age at SLOWER rate when travelling at high speed?

So did I but its worse for me because I did a degree in Physics.

Anyway, how about Per Ardour ad Asda?
;) ;) ;)

mik 30th Jan 2001 00:41

You do age slower at a slower rate at high speed, but you age faster if gravity is less, which was the case in the P3C as it was at altitude.

If it had flown for 15 hours at 270 knots at say 200', then the crew would have "gained" 5.6 nanoseconds.


fly4fud 30th Jan 2001 01:47

always thought the effect was related to the speed of lite, e.g. the closer one gets to it, the slower time "passes". If one could reach and maintain the speed of lite, no ageing, going faster and here it is: the time machine....whooa ;)

... cut my wings and I'll die ...

Horsepower 30th Jan 2001 15:54

Travelled back in time to the Aran Islands recently. Elapsed time was 1:20, but when I arrived it was the 1950s :)

CaptainSquelch 30th Jan 2001 17:59

if you apply theoretical stuff like relativity to everyday examples it is easy to oversee factors that really count, and I personally think you should to make the calculation possible.

However in this case the simplification has gone too far.

I real life you age much faster when you fly a lot. This is due to the combines efects of sleep loss, radiation, low humidity, jet lag, rapid climatic changes, bad food, warm beer, chaep wiskey etc. etc. etc. . . . . .

Nice work theo`s, talking about nanoseconds while overseeing the years. Find a formula that includes the above facors. Good luck


Pom Pax 31st Jan 2001 22:45

compressor stallrelative to a fixed point in space one aircraft is moving at the speed of rotation of the earth plus its ground speed and other the other minus its ground speed.
Neil Armstrong used to joke that the U.S. owed him overtime because the flight time to the moon and back differed from the elapsed time recorded on earth.

Brakes...beer 2nd Feb 2001 16:34

Fascinating stuff, many thanks. Until the long-haul hypersonic market on Jupiter picks up, I think I'll stick to Channel-hopping for the sheer fun of it.

compressor stall 2nd Feb 2001 19:19

Pompax, but did they not end up at the same place, in roughly the same time?

Hence they both have covered the same distance in the same time, and their origin/desination has moved on in space, but seeing they both ended up there/depart from there it is basically irrelevant in determining their difference.

Those who restrain desire do so because theirs is weak enough to be restrained.
William Blake

Dr. Red 3rd Feb 2001 08:20

Compressor Stall,

<font face="Verdana, Arial, Helvetica" size="2">...did they not end up at the same place, in roughly the same time?

Hence they both have covered the same distance in the same time, and their origin/desination has moved on in space, but seeing they both ended up there/depart from there it is basically irrelevant in determining their difference.</font>
That comment stands in classical physics, but I believe it breaks down when relativity is applied; a very-high-velocity man "gains more time" than his stationary twin, and retains that temporal advantage even when the two are reunited on earth. I never could get my head around that bloody relativity!


Smoketoomuch 3rd Feb 2001 23:41

It all depends on who underwent the acceleration - so although they both ended up in the same place/time they aged differently. Errr, I think.

Prof2MDA 4th Feb 2001 00:01

Just a couple of points here:

1. The time is relative, so in reference to the comment that the two aircraft going opposite ways around the Earth would differ because one is moving "with" the Earth's rotation, that would be fine if you were comparing it to a clock at some "observation point" outside of the system.

2. The point that there is less gravity at altitude having more affect is quite valid, although in the one example we were comparing two aircraft so ostensibly that fact is washed out.

3. The differences from a relativistic viewpoint are virtually impossible to measure, but if we were able to compare the two there might be a difference as the westbound aircraft is going to be travelling a lot slower across the ground due to the the winds, so compared to a ground based clock, one has moved faster than the other.

Of course, problem with that is that both aircraft are moving with the wind, so relative to _each other_ the effect of wind should wash out. I'll leave that to you to ponder, as I need to shower now and have no time to write anymore!

[Steve] 4th Feb 2001 07:20

Here's some basic relativity for you.

Imagine two aircraft travelling away from each other at a significant velocity.

One aircraft sends a message at the speed of light (a radio transmission?) to the other.

This message takes a finite time to travel between one aircraft and the other.

From the point of view of BOTH aircraft the messages either approach or recede from them at the speed of light (c)

From the point of view of a observer on the aircraft sending the message, the message leaves them at c relative to them, and must travel slightly further than the distance between the aircraft (since the other is receeding).

From the point of view of an observer on the second aircraft, the message is sent, with a relative speed of c and needs to travel the distance the aircraft are apart, while the other aircraft receeds from the point at which the message was sent.

Thus, the time taken for the message to travel beween the two aircraft differs depending on which aircraft you're on.

A rather more interesting (if potentially useless) question is what would an imaginary observer riding a photon traveling between the aircraft observe?

Of course this is special relativity, and does not take into acount the effect of acceleration. Gravity is the same as acceleration (as far as relativity is concerned).
A similar effect means that if you look very closely at the receeding aircraft, it is shorter by just enough that a person in the tail shining a torch to the cockpit will measure c to be exactly the same, even though the beam is travelling at c with respect to an external (or any other) observer.

Of course they will report the same about your aircraft.

compressor stall 4th Feb 2001 14:59


Understand that point, but thinking it through, I still disagree.

Ve= Earths speed around its orbit
Va= Aircraft speed around earth

The 2 aircraft are launched from Greenwich and fly around the world at the same latitude in the same time. Greenwich at the time of launch is facing the sun. It is also facing the sun at time of landing.

Rough relative speeds (avoiding all the calculus) to a person at a point fixed in space.

Aircraft A at greenwich: Ve + Va
Aircraft A over date line: Ve - Va

Aircraft B at Greenwich: Ve - Va
Aircraft B over date line: Ve + Va

So the aircraft relative to a fixed point in space are actually covering the same distance at the same speeds (albeit in different order).

Maybe the error came up because Greenwhich was not in the same position relative to the fixed point in space when the aircraft finished their journeys?

Those who restrain desire do so because theirs is weak enough to be restrained.
William Blake

[This message has been edited by compressor stall (edited 04 February 2001).]

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