This whole fast than light time travelling thing

[Driller]

I've been reading around this for a while along with relativity and I get it mostly with one sticking point.

The clever folk seem to suggest that what you see as a result of the speed of light dictates the movement of the object through time.

One example was that if you move away from a clock at the speed of light then the clock hands would appear to not move and thus, for the moving person, time at the clock would be slower.

But the thing is, even though the clock hands look like they're not moving, surely they actually still are moving and hence the whole thing would be just an optical illusion?

If I look at someone through blue coloured glass that person may look blue but they're not really.

Now obviously I'm not as intelligent as those pesky astrophysicists, so can someone explain this to me please?

[Toltec]

What you see is not the clock hands, but the light reflected from those hands.

Imagine that the clock is a steady stream of rounds fired from a gun towards you, each one has a number on it which you can read as it passes you. If you are moving away from the gun at the same speed as the bullets you just stay alongside one round so that is all you ever see and the count does not change.

Now that is not at all correct in terms of relativity and light, however it illustrates that what you see is the information about what the time/count was when that marked round left the gun, not what the time is at the gun when it reaches you.

[budgie smuggler]

It's not an optical illusion, time dilation really does occur due to a difference in velocity (and/or gravity) between two observers.

[Toltec]

To make it even stranger for you, if you were holding a clock that a person next to the clock you are moving away from were looking at it too would appear to be running slowly to them.

[Beati Dogu]

Worth a read:

"The Hafele–Keating experiment was a test of the theory of relativity. In October 1971, Joseph C. Hafele, a physicist, and Richard E. Keating, an astronomer, took four cesium-beam atomic clocks aboard commercial airliners. They flew twice around the world, first eastward, then westward, and compared the clocks against others that remained at the United States Naval Observatory. When reunited, the three sets of clocks were found to disagree with one another, and their differences were consistent with the predictions of special and general relativity."


https://en.wikipedia.org/wiki/Hafele%E2%80%93Keati...

[Squiggs]

Yep ..... I've come across what I think is an 'anomaly' when moving faster than the speed of light ........
If I were in a room, in a machine that could 'move' me faster than the speed of light, and if I were in that time machine but was able to stick my arm out, and flick a switch that turned a light bulb on at the exact same time as I started my machine ....... then I'd never see the light from that bulb come on,
If the light from that bulb can't reach me and if I could amazingly quickly 'steer' my machine back to that room the light still wouldn't have been switched on.

So travelling faster than the speed of light to arrive back at your original place of departure .... can only take you back in time?????????

The whole thing frazzles the (limited) capabilities of my brain!

[Simpo Two]

Squiggs said:

Yep ..... I've come across what I think is an 'anomaly' when moving faster than the speed of light ........
If I were in a room, in a machine that could 'move' me faster than the speed of light, and if I were in that time machine but was able to stick my arm out, and flick a switch that turned a light bulb on at the exact same time as I started my machine ....... then I'd never see the light from that bulb come on,
If the light from that bulb can't reach me and if I could amazingly quickly 'steer' my machine back to that room the light still wouldn't have been switched on.

So travelling faster than the speed of light to arrive back at your original place of departure .... can only take you back in time?????????

The light was switched on but you didn't see it because you were moving away from it too fast.

When you circle round to go back to the switch you will meet the light coming towards you...!

[Toltec]

Firstly I'd suggest you use a remote switch or at least a pole otherwise you will lose your arm.

Travelling faster than light might let you see the room before you turn the light on, you could then return to the room and see your earlier self looking back at the room.

You could use a pair of wormholes, but then you would destroy the universe.

[Squiggs]

Toltec said:

Firstly I'd suggest you use a remote switch or at least a pole otherwise you will lose your arm.

Toltec said:

Travelling faster than light might let you see the room before you turn the light on, you could then return to the room and see your earlier self looking back at the room.

Yep ...... that's what does my brain in!? I'm travelling faster than the speed of light - which with my limited mental capacity suggests to me that I should be travelling into the future to see future events - but instead it takes me back in time to see historical events rather than forwards.

[Orchid1]

I've always wondered about stuff like this as well.

If you were in a spaceship orbiting earth and set off faster than the speed of light once around the planet would you not arrive back at your starting point before you've even left and thus have traveled back in time briefly or am I talking nonsense??? :s

[budgie smuggler]

Orchid1 said:

I've always wondered about stuff like this as well.

If you were in a spaceship orbiting earth and set off faster than the speed of light once around the planet would you not arrive back at your starting point before you've even left and thus have traveled back in time briefly or am I talking nonsense??? :s

That's about it, and also the plot to Superman IIRC Moot point though, given there are no time travelers that we know of therefore we can assume FTL is not possible.

[Gary C]

One thought experiment is

Two people are traveling towards each other in rockets and will meet at point A, now the person at point A sees both rockets coming towards him at 3/4 the speed of light, so relative to him, they are traveling towards at each other at more than the speed of light.

However, relative to each person in the rocket, as time is traveling slower, the other rocket is moving towards them at less than the speed of light.

In theory as I understand it, at the speed of light, time stops

Experimentally proved by accelerating briefly existing isotopes to high speeds and measuring if they exist at points down the beam, they travel much further than simple time/distance would expect.

[tbourner]

YouTube will explain it!
Search for "time dilation" or "twins paradox" or something.

[Moonhawk]

If you got into a spaceship that could travel at 99.9% the speed of light and set off for Alpha Centauri and then turned around and came back again - somebody here on earth would observe that it took you just under 9 years to complete the trip.

For you on the spaceship however - the journey would appear to take only 5 months.

The closer you get to the speed of light - the faster time appears to run in the universe around you.

Ignoring the practicalities of doing so - if you could travel exactly at the speed of light, time would appear to run infinitely fast in the universe around you and as such you would see the entire history of the universe play out in an instant.

Conversely - we observe photos of light from distant galaxies that has taken millions, if not billions of years to reach us (as measured from earth) - however from the perspective of the photons of light - they arrived on earth at the exact same moment they left the host galaxy.

Edited by Moonhawk on Wednesday 9th May 13:51

[Derek Smith]

I read a science fiction book, one that had an uncanny similarity to the storyline of Stargate, where a woman was lost towards a black hole through some devious manoeuvering of her husband. The interpretation of relativity in the book was that the woman would live to the end of the universe because she would be traveling at the speed of light at the time she reached the event horizon.

Spaghettification has something to say about a fundamental error of the scientific basis of the book, but I remember someone suggesting that if a black hole was big enough, then it would not be a problem. Big in this case means really big.

[Timmy40]

Derek Smith said:

I remember someone suggesting that if a black hole was big enough, then it would not be a problem. Big in this case means really big.

Are we talking like, Dianne Abbot big?

[Toltec]

Timmy40 said:

Derek Smith said:

I remember someone suggesting that if a black hole was big enough, then it would not be a problem. Big in this case means really big.

Are we talking like, Dianne Abbot big?

There are some arguments as to whether information is lost or preserved when entering one of those.

[Moonhawk]

Derek Smith said:

Spaghettification has something to say about a fundamental error of the scientific basis of the book, but I remember someone suggesting that if a black hole was big enough, then it would not be a problem. Big in this case means really big.

Yep - once a black hole becomes big enough - it's theoretically possible cross the event horizon without being spaghettified (although you would be at some point once inside - since all possible paths once inside the event horizon lead towards the signularity)

https://en.wikipedia.org/wiki/Spaghettification#In...

Edited by Moonhawk on Wednesday 9th May 16:49

[Derek Smith]

Moonhawk said:

Derek Smith said:

Spaghettification has something to say about a fundamental error of the scientific basis of the book, but I remember someone suggesting that if a black hole was big enough, then it would not be a problem. Big in this case means really big.

Yep - once a black hole becomes big enough - it's theoretically possible cross the event horizon without being spaghettified (although you would be at some point once inside - since all possible paths once inside the event horizon lead towards the signularity)

https://en.wikipedia.org/wiki/Spaghettification#In...

Edited by Moonhawk on Wednesday 9th May 16:49

The way it was explained was that an object would accelerate towards the black hole and then, at the event horizon, be traveling at the speed of light. (The book focused on the time dilation earlier though. The bloke had tried to kill his Mrs by sending her towards the black hole. However, typically selfish female, she was not only still alive, but would outlive him.)

My understanding is that if one goes at the speed of light then subjective time stops. So the person will instantly be transported to a/ the end of the universe, or, 2/ the end of that particular black hole, whichever comes sooner.

So the guy will either blink into existence at the same time they blink out, or be in the middle of a pretty big bang.

I can't remember the name of the book. It's almost there but not quite, rather like the woman in the book.

[Timmy40]

Here's a interesting article about what would happen if you fell into one....

http://www.bbc.co.uk/earth/story/20150525-a-black-...