Light speed spaceship time travel conundrum

[Jebus HChrist]

Greatly appreciate the contributions to this question of mine, I guess the issue for a designer of the spacecraft is what should the instruments display regarding speed and distance travelled, since the travellers are experiencing time at a different rate to their fellow earthlings. Maybe the first display would be the true speed of the craft say 0.9999 of a light year 2nd instrument apparent speed from the travellers perspective which would be say 50 times speed of light 3rd how far time had slowed down compared to Earth (true speed divided by apparent speed) in this case 50 times slower than earth. This is assuming the instruments could work, after all clocks will slow down the faster they travel. I would also add the older you get the faster time goes, but I'm that bloody old dave "OP" thanks very much.

 

[ZapperZ]

I must have experienced a time-dilation here, or it took you 2 years to respond to your post.

Zz.

 

[Jebus HChrist]

I must have experienced a time-dilation here, or it took you 2 years to respond to your post.

Zz.

Seems like only 2 weeks to me, something about time flys when you're enjoying yourself!

 

[harrylin]

Let’s say we have a spaceship capable of traveling at the speed of light, and a planet we can visit 50 light years away. From earth’s perspective if we can track the ship we would be able to see it was traveling at the speed of light and we would also see it has taken 50 years to complete the journey.

From the travellers perspective time will slow down, so let’s say to them the time only lasts 1Year. Doesn’t this mean that they would experience the journey as if they traveling 50 times the speed of light? If they have instruments on board showing speed what will these instrument show, speed of light or 50 times speed of light?

I’m not a student I’m old and love this stuff hence maybe some young whippersnapper can answer this question for me.

JHC

Hi JHC,

You gave a nice example of what Einstein meant with "the velocity of light in our theory plays the part, physically, of an infinitely great velocity".

Indeed, it is possible to travel that great distance with an experienced time that is arbitrarily short; calculated in Earth's distance measures, and reckoned with the travelers' clocks, their "speed" exceeds by far the speed of light.

I suppose that you understand that this is a special way of reckoning, thanks to using such a mixed bag of units.

However, what their instruments will show depends very much on how they are tuned. If they use a kind of radar-Doppler then the instruments are auto-tuning to the inertial system in which they are in rest. Then the distances* will simply look shorter and the speed will look the same as measured from earth.

Cheers,
Harald

*PS: in fact they have no time to do an active radar measurement of that planet after take-off. Thus they would need to utilise passive clues to estimate the distance.

 

[yuiop]

Doc Al you say “But to the spaceship the distance to the planet is much less than 50 light years”. So for arguments sake would you agree that the travellers could experience the journey as though it were 1 year, and that it would therefore appear from their perspective that they were traveling 50 times the speed of light. My original question still stands.

JHC

Lets say at the time the ship launched, a "spaceship launched" signal is sent from Earth to the distant planet. When the ship arrives at the distant planet, the proud astronaut announces he has traveled from Earth at 50x the speed of light. The greeter at the planet says "That is odd, because the launched signal arrived just before you and it was traveling at 1X the speed of light. Did you take the scenic route?".

 

[sicarius]

Lets say at the time the ship launched, a "spaceship launched" signal is sent from Earth to the distant planet. When the ship arrives at the distant planet, the proud astronaut announces he has traveled from Earth at 50x the speed of light. The greeter at the planet says "That is odd, because the launched signal arrived just before you and it was traveling at 1X the speed of light. Did you take the scenic route?".

Actually, yes and no. You are right about the arrival times, but the astronaut would not announce that he has traveled 50x the speed of light. He would announce that he has traveled 300 trillion miles in one year. There is a difference.

To the traveler, the passage of time slows down to 1/50th of what it is here on earth. What that means is during a given relative interval (1 minute travel time verses 1 minute Earth time) the traveler actually takes in 50x the light. That means that from the travelers perspective, a light year would be 50x the size. If you take the distance they travel as a numerical constant by saying they are 300 trillion miles apart (roughly 50 light years from Earth's perspective) then everything makes a little more sense. To the traveler's eyes they never violated the speed of light, the speed of light was just raised from their relative perspective (or more precisely a year is 50x as long).

I know your thinking "Hold on, the speed of light is a constant!" and you are right. The term "year" is the variable here. If you define a year as the length of time it takes for the Earth to make one revolution around the sun, then the traveler would actually be in space for the full 50 years. From their perspective they see the Earth make 50 complete revolutions around the sun, and thus know that they have traveled for 50 years. If on the other hand you are defining a year as the amount of time that passes from our perspective as the Earth revolves around the sun (ie how much we age in a "year") then the traveler would only experience 1 year in space, and since a year passes at 1/50th the rate for them the term "light year" would have a different meaning and contain 50x the distance.

Since we are measuring a speed, which is distance over time, and time is relative, then the actual speed has to be relative in the same way.

 

[DaveC426913]

Actually, yes and no. You are right about the arrival times, but the astronaut would not announce that he has traveled 50x the speed of light. He would announce that he has traveled 300 trillion miles in one year. There is a difference.

To the traveler, the passage of time slows down to 1/50th of what it is here on earth. What that means is during a given relative interval (1 minute travel time verses 1 minute Earth time) the traveler actually takes in 50x the light. That means that from the travelers perspective, a light year would be 50x the size. If you take the distance they travel as a numerical constant by saying they are 300 trillion miles apart (roughly 50 light years from Earth's perspective) then everything makes a little more sense. To the traveler's eyes they never violated the speed of light, the speed of light was just raised from their relative perspective (or more precisely a year is 50x as long).

I know your thinking "Hold on, the speed of light is a constant!" and you are right. The term "year" is the variable here. If you define a year as the length of time it takes for the Earth to make one revolution around the sun, then the traveler would actually be in space for the full 50 years. From their perspective they see the Earth make 50 complete revolutions around the sun, and thus know that they have traveled for 50 years. If on the other hand you are defining a year as the amount of time that passes from our perspective as the Earth revolves around the sun (ie how much we age in a "year") then the traveler would only experience 1 year in space, and since a year passes at 1/50th the rate for them the term "light year" would have a different meaning and contain 50x the distance.

Since we are measuring a speed, which is distance over time, and time is relative, then the actual speed has to be relative in the same way.

Don't go filling people's heads with crazy ideas.

The traveler does not see a light year as 50x normal size. A light year will be the same distance light travels in one year. And he will measure the speed of light as 300,000km/s.

The traveller in the spaceship experiences no strange changes at all. Time dilation is never experienced in one's own FoR; it is only observed in others' FoR.

However, if he looks out the window, he will see several things of particular note:
1] Everything along his path of travel will appear compressed. Planets will be disks, distances to stars will be much shorter, the entire universe will be squashed along his direction of travel.

This is how he can reconcile getting to this distant planet so fast. He measures the star as only 1/50th of the distance it was when measured on Earth.

2] The light and other radiation will be blue-shifted in front of them and red-shifted behind them.
3] Everything will appear to be moving slowly - including the Earth's passage around the sun.

But in his spaceship, he will experience time passage as normal.

 

[yuiop]

Since we are measuring a speed, which is distance over time, and time is relative, then the actual speed has to be relative in the same way.

You are forgetting that distance is relative too. While his clocks slow down relative to clocks on Earth, his perception of distance changes too, so if he measures time using his own clocks and distance using his own rulers, then he gets the correct speed.

 

[physicscrazy]

if this could really happen then we all have unlocked the laws of nature and energy because if we travel at a light speed then after reaching a limit if we look at the Earth then we would be in future and have traveled through time

 

[physicscrazy]

another thing can happen and the person or anything inside the spacecraft will slow down and when he returns on the Earth after a long time his age will be slowed down and he will grow slower than all of us and when we reach at the age of 70 he will still be at the age of 20 or 30