Q I speed of light

[kris h]

I was watching QI last night stephen fry was talking about time travel and the speed of light .

This is what he said if i was travelling at the speed of light to a planet 4 years would pass on earth i just dont get how this would happen.

So it takes light from the sun 8mins to get to earth so lets say ive

got my own STARBUG travelling to pluto at the S.O.L say it takes

40mins (Guessing) so there i am at pluto also great views of neptune .On earth the year is 2014 return back to earth 2018 but for me 80mins have just gone bye so if this is true time travel to the future is possible but its somthing i just get my head round it

I believe that when going at the speed of light time would slow down for me in the space ship but every where else would be as normal.

Just my little view on time travel. speed of light

"GREAT SCOTT" !

[ollypenrice]

Hi,

I'm not sure where you get your four years from (2014 to 2018.)

1) To get your head round it, the first thing to understand is that being in a gravitational field is the same as being in an accellerated environment - ie in something which is either speeding up or slowing down or indeed turning. All produce 'G' as in the popular phrase 'G force' though that is not a helpful phrase scientifically. Just standing on the surface of the earth you are in a field of 1G due to the earth's gravity. You can get the same effect by getting in a jet and accellerating; at the right rate of accelleration you'll feel 1G. You'll feel the same 1G at a certain rate of cornering in a racing car and again the same 1G when you put the brakes on with the right pressure. Okay?

2) Oddly, even astonishingly, Einstein showed that in accellerated environments time slows down. It really does though, and this has been proven hundreds or thousands of times in experiments. Take 2 atomic clocks which agree what time it is to incredible precision. Take one for a blast in a fast jet, causing it to spend some time in an accellerated environment, bring it back and now it is running slow by comparison with its partner clock. You could say that it is also 'younger' than its partner clock.

3) There is no 'correct' time anywhere in the universe. The rate at which time passes for anyone, anywhere, depends on how accellerated or unaccellerated their environment is. So when your twin stays at home and you blast off somewhere ideally a bit more distant than Pluto, and then return, you will heve spent a lot of time being accellerated. During this accelleration time has slowed down for you, so when you return you will be younger than your twin.

This is sometimes wrongly called the Twins Paradox, but that is really something slightly different.

Olly

[tinvek]

if i recall the program correctly, he was refering to if you travelled to a planet orbiting our nearest star (excluding the sun) proxima centauri which is 4.2 light years, hence the 4 year difference in time. as olly said time is not a constant and is altered by gravity and velocity, the effect is sufficient that GPS has to allow for the clocks on the satellites going out of sync with earth and atomic clocks have even detected the miniscule difference in time between the top and bottom floors of very tall buildings.

[Marky]

Both velocity and gravity affect the rate that time passes.

If you are either in a strong gavitational field (like near a black hole) or travelling very quickly (near the speed of light) your time will appear to run slower to a stationary observer (or someone in weak gravitational field) watching you. From your persepective your clock will seem to run at a normal speed but the rest of the universe will appear to speed up.

So as you travel near the speed of light clocks back on earth would appear to run faster so more time will have passed for people back on earth than for you. On arriving back, you may have only been away for a few days but on earth many years may have passed. It's called time dilation, not really time travel.

Some bloke called Einstein worked it all out a while back.

In simplified form Dt' = Dt/sqr(1-v2/c2)

Dt' is the time interval between two events (click ticks) for the moving person as viewed by an external observer and v is the velocity of the moving person (c=speed of light). Dt is the time interval for the external observer. Dt' gets bigger the closer v is to c, i.e. time runs slower for the moving person.

Hope that helps...

[Shibby]

Don't forget that we constantly travel forwards through time! It's only sending information backwards that causes theoretical issues.

[Nillchill]

it is not possible to travel backward in time, time does not exist, time is purely a measurement of our position in the universe relative to a previous position, to be able to travel backwards in time would require the prediction, calculation of position of every atom and molecule in the universe as well as the means by which you get them there, and as far as I can say, there is no way of achieving this. and i;m guessing there never will be, because it hasn't happened yet. therefore it will never happen

[Asur84]

There was a program on channel 4 by Stephen Hawking on this and I actualy find this subject really fascinating as the laws of physics dictate that nothing can actualy travel faster than the speed of light however as Olly says the gravity does appear to slow time down and this is due to the fact that, as Nillchill says, time does not exist. In theory time is actualy slightly slower around the pyramid of Giza than elswhere due to the sheer mass of the pyramid.

Jonthon.

[Quatermass]

Not only does time slow down near dense matter but it also goes indescribably slowly when waiting for the number 388 bus when its raining or maybe this is due to a large black hole just behind the Bungay shopper that i haven't noticed yet again only a theory at this stage but I have my suspicions

[Asur84]

We have a similar phenomina in Manchester but for all buses.

Jonathon.

[Quatermass]

Well time just sped up today I got my new telescope! check out my blog link for a mad monatge of the frantic unwrapping almost at warp speed, well pleased...

[Asur84]

Well time just sped up today I got my new telescope! check out my blog link for a mad monatge of the frantic unwrapping almost at warp speed, well pleased...

Nice piece of gear ther Quartemass, I am looking forward to your posts on obs reports from that bad boy.

I had a quick look up tonight but no joy theres a bit of clouud cover so not getting to see much.

Jonathon.

[Bried]

I think what Stephen Fry was referring to was Einsteins's Special Theory of relativity (1905) rather than his General theory of relativity (1916 i think).

Einstein was troubled by James Clerk Maxwell's equations that predicted a specific speed of light. The equations did not refer what this speed was relative to ie. waves on the sea travel relative to the ocean. Michaelson and Morley conducted an experiment in the late 1800's to try to detect an ether which light could to travel relative to. They were unsuccessful in detecting an ether because it doesn't exist and concluded that light did in fact travel at the same velocity for all observers no matter how fast they were moving (in a seperate inertial frames of reference)

The fact that light travels at the same speed for all observers has some amazing consequences. I will try to explain briefly what these consequences are.

If you are traveling in an inertial frame of reference in uniform motion (in a train and not accelerating and not turning) and you have a device called a light clock then you can time how long a journey takes.

A light clock is a device that in reality doesn't exist (i don't think) but helps to explain the implications. A pulse of light is sent up the light clocks tube from the bottom to the top and then down again. This takes a certain amount of time which can be calculated depending upon the length of the light clock. If this clock is travelling with you on the train and the train is travelling close to the speed of light then to the person on the train the pulse of light travels up and then down and the pulses path remains vertical. However, an observer standing at the side of the track would observe the light pulse to travel along an angle rather then straight up. This means that the path the light has taken to the observer at the side of the track is longer than the path that the light takes to the observer on the train.

If light is the same speed for all observers then time on the train must be passing by slower than time at the track because the light relative to the track has a longer distance to cover.

This is a very very brief description of special relativity and there is loads more than this but simply too much to go into. There are some great "paradoxs" which turn out not to be paradoxs after all, such as the twins paradox, the train in tunnel paradox. There is also General Relativity which has already been mentioned on this thread and relates acceleration to gravity. This is responsible for gravitational lensing which was proved right by Arthur Eddington in 1919 (I think) when he observed stars to be in different locations next to the sun in a solar ecplise. It is also our current best theory of gravity.

There are loads of examples on the net. Some key search words to look for are the Twins Paradox, Train in tunnel paradox, The lorrentz Transformation, Michaelson Morley Experiment, Light clock, length contraction.

There are some pretty good books out there too. The Brian Cox one called Why E=mc^2 is pretty good and Richard Feynman's books are amazing.

I hope I haven't bored anyone with this huge post but relativity is one of my favorite subjects.

Brian

[Asur84]

Cheers for the insight Brian I think I'm going to look up that book.

Jonathon.

[Moonshane]

Well time just sped up today I got my new telescope! check out my blog link for a mad monatge of the frantic unwrapping almost at warp speed, well pleased...

well time for US would speed up but not you. you have more mass as you are now hugging and inseparable from your telescope.

[Moonshane]

Stephen Hawking's latest book gives an excellent description of relativity in relation to speed and distances etc. it goes something a bit like this : imagine someone in a plane flying at say 500 miles an hour. they cover therefore around 700 feet per second as you watch them fly through the air. someone bouncing a ball on the plane will see the ball bounce up and down on the same spot and cover no or very little distance in that second; someone on the ground (if they could see through the plane and only see the ball) would see the ball travel 700 feet in that one second. that's one effect of speed on time and distance which are effectively the same thing in some ways and assuming I understand it!

the other one was that where there's a strong magnetic field which 'stretches' space-time and like eg a heavy ball on a thin rubber sheet, two things traveling the distance from one side to the other will take different times to cover the same distance across 'space-time' as one has to travel along the distortion created by the gravitational field.

please correct me if I'm wrong as I think I understand this bit!

[Quatermass]

well time for US would speed up but not you. you have more mass as you are now hugging and inseparable from your telescope.

LOL your not wrong there moonshane my wife says I cant take it to bed with me tonight.

this photo show clearly how time indeed sped up at 2.30 this after noon:headbang:

[Moonshane]

whatever you do, don't stand there telling it that it's beautiful and you've never been so impressed with anything in your life etc (not within your wife's earshot anyway).

I feel some decorating duty coming your way soon!

[Quatermass]

whatever you do, don't stand there telling it that it's beautiful and you've never been so impressed with anything in your life etc (not within your wife's earshot anyway).

I feel some decorating duty coming your way soon!

No way I am so bad at decorating and diy..

But back on the subject of time and space Mr Hawkins Universe programme was really great I still cant take it all in how vast the universe is. This hobby is so cool and Im learning so much from you guys on this forum although I confess to feeling a right old newbie when reading some of the stuff from the astro boffins. I have booked a course with the open university though so watch out Quatermass might end up a professor one day

[Asur84]

No way I am so bad at decorating and diy..

But back on the subject of time and space Mr Hawkins Universe programme was really great I still cant take it all in how vast the universe is. This hobby is so cool and Im learning so much from you guys on this forum although I confess to feeling a right old newbie when reading some of the stuff from the astro boffins. I have booked a course with the open university though so watch out Quatermass might end up a professor one day

I felt the same, I had to watch them twice just to catch some of the info to understand.

I'm thinking of going back to uni myself though.

Jonathon.

[Quatermass]

I felt the same, I had to watch them twice just to catch some of the info to understand.

I'm thinking of going back to uni myself though.

Jonathon.

Well the open university courses are an easy option. I did a degree in theology 10 years ago and enjoy learning new things and now I am getting older have more time to study subject that interest me. The course I am going for only a simple introduction to astronomy but if I get on well with it can take more advanced courses relating to my interest in the universe time and space. Learning on line suits me better these days.

[Asur84]

I'm going to have a look into that as I can then fit it around work.

Jonathon.

[Bried]

Stephen Hawking's latest book gives an excellent description of relativity in relation to speed and distances etc. it goes something a bit like this : imagine someone in a plane flying at say 500 miles an hour. they cover therefore around 700 feet per second as you watch them fly through the air. someone bouncing a ball on the plane will see the ball bounce up and down on the same spot and cover no or very little distance in that second; someone on the ground (if they could see through the plane and only see the ball) would see the ball travel 700 feet in that one second. that's one effect of speed on time and distance which are effectively the same thing in some ways and assuming I understand it!

the other one was that where there's a strong magnetic field which 'stretches' space-time and like eg a heavy ball on a thin rubber sheet, two things traveling the distance from one side to the other will take different times to cover the same distance across 'space-time' as one has to travel along the distortion created by the gravitational field.

please correct me if I'm wrong as I think I understand this bit!

The bouncing ball is a similar explanation to the light clock but instead of a bouncing ball there is a pulse of light.

The bit you mentioned about the heavy ball on the rubber sheet is an analogy used to explain how gravity warps spacetime in general relativity. I've never heard of a magnetic field warping spacetime though. Most of my studies have been with special relativity rather than general relativity though so it could be something I haven't encountered yet. (I'm trying to understand the mathematics of special relativity before I move on to general.)

What I do understand from my knowledge of general relativity with regard to warping of spacetime is that bodies such as planets want to move in a straight paths. Because, for example the sun is such a massive body it's mass warps spacetime analogous to the rubber sheet. Planets want to move in a straight line but can't because of the warping and therefore take the straightest path they can which happens to be bent into an orbit. The bending due to warping of spacetime also happens with light as I mentioned earlier with Arthur Eddington's confirmation of general relativty during the 1919 solar eclipse. You have to remember though that the rubber sheet analogy is wrong in the sense that it relies on gravity to weigh the central region down whilst marbles are rolled around.

Einstein derived the general theory from postulating that acceleration has the same effect as gravity. If you are in a lift in space (no gravity) and then accelerate at 9.8m/s the effect you would feel would be the same as gravity on earth.

You wouldn't be able to tell if you were on Earth or in an elevator accelerating in space.

Einstein then imagined acceleration at much higher velocities and imagined the effects that this would have on a beam of light held horizontal in the lift. Because of the high acceleration the beam of light would appear to bend downward. This is analogous of holding a hose pipe with water squirting out horizontally. If you then accelerate it upward the flow of water bends downward.

Taking into consideration that Einstein had postulated that acceleration and gravity have the same effects he then came to the conclusion that if acceleration bends light then so should gravity. I think this is how he came up with the idea that gravity is the warping of spacetime due to an objects mass. My knowledge of General Relativity is not so hot at the moment so apologies if I have made any errors.

Hope this helps and sorry for rambling again.

Brian

[Bried]

I forgot to mention above with regard to the bouncing ball that it only really explains the motion of the light pulse. A ball can be bounced at many different velocities whereas every observer no matter what frame of inertial reference they are in will always agree on the same speed for light. Which I think is 299,792,458 metres per second.

[Moonshane]

sorry Brian - it was obviously too late for my pea brain. I did of course mean gravitational rather than magnetic field.

In my own (small) mind if mass = potential energy then something with mass and also moving must have more energy and thus more mass and thus more gravity? So more gravity and/or speed will warp space-time by a larger degree?