weird space travel fact

[DrNeb]

James4 just blew my mind.

now i have to pick up the pieces...

[markophoto]

and me DrNeb!

So, how do we reconcile the fact that a photon travels from anywhere to anywhere instantly, and the fact that light travels 186K(ish) miles per second and therefore there will be a limit to how far light travels in (say) a year.

Am I trying to combine two ideas that I shouldn't

In other words are they both just facts:

1 - Light travels at 186K miles per second.

2 - To a photon time stands still so gets anywhere instantly.

Am I making sense? I am to me, but might not be to you!

Marko

[themos]

"A year" only makes sense if you specify the observer that carries the clock.

Two events can appear "simultaneous" to one observer and "not simultaneous" to another.

1 and 2 are facts.

Add 3: there is no non-zero lower limit to travel time (as experienced by the traveller), no matter what the distance between departure and arrival.

[spaceboy]

Is time not a concept of measurement only? To consider something of going forward or back in time would be to consider it possible to change our own destiny. IE: We are in some way able to prevent death by traveling back wards in time to our youth or to travel forward in time with out aging?

[DrNeb]

Ok - i'm useless with all of this.

Dumb question incoming.

time stands still for the photon, am i right in thinking that time doesnt actually stand still, it is just moving so fast that it appears to the photon that time is moving much slower?

so - if you were to replace the photon with a biological subject, in this case a person. say that it was possible for the person to travel just like a photon. the person would arrive at the destination X amount of light years old and basically a pile of dust and not in mint condition despite, to the subjects knowledge he/she arrived instantly.

Oh no... i've gone cross eye...

[themos]

There's no known way of going back in time. We can only change the speed of going forwards. As far as the individual is concerned, he/she will still live three-score and ten, as perceived in the ordinary sense. It's just that others (including whole planets, galaxies etc) may have travelled further forwards in time (and died or gone supernova).

[themos]

No, DrNeb, it doesn't work like that. We might be able to encode ourselves as a bunch of photons, though, and then the photons on arrival (anywhere) would be as young as when they left (the rest of the universe will have gone forward in time, though). Things with mass cannot do this, but subjective travel time can be reduced without limit (ok, you run into intractable radiation problems as you start going really fast because the vacuum of space will start behaving like a brick wall).

[Zakalwe]

I was recently trying to figure out by what factor does time slow down and at what speed? As Rob says 'time is the only constant'. As a result of this the answer is, if you travel at half the speed of light, time slows down to half normal time for you the traveller. If you could travel at the speed of light time would stop completely for you. However, because of problems with infinite mass, etc, etc, (I use etc, etc, to pretend I know lots more reasons, that really I don't - it always worked in exams!) you can't ever travel that fast - But! Photons do all the time. In fact its the only speed they can travel at. We often say to the public, 'just imagine, the light from Andromeda has taken 2.5 million years to travel to your eye'. And that's true, but it was only 2.5 million years on our watches. For the photon it took no time at all. It arrived here instantly - it's clock is still at 12.00 noon, the same instant that it left. The photon experiences no time passing and it travels anywhere instantly. What fun!

"Time is the only constant". There is no such thing as a constant for time. In a universe governed by Newtonian mechanics there was such a thing as universal time. But that was incorrect. in the universe of Special Relativity and General Relativity, time is constant only within a frame of reference. So time runs slower or faster to an observer outside a frame of reference.

For the photons leaving the Andromeda Galaxy, time for them seems to pass at "normal" speed. If they had watches on their little massless arms, then the second hands would tick as normal. 2.5 million years would pass on their long journey to us. So it is wrong to say that from their frame of reference that they get here in no time. However, if you synchronised their watches with a watch on Andromeda, then their watches would run at a different rate. If it was possible to see their watches from the outside then it would appear that the second hand on their watches was not moving. But to the photon, the second hand moves as normal. Of course, we wouldn't be able to verify this, because the information about the time displayed on the watchs back in Andromeda cannot get to us faster than the speed of light (as information cannot travel faster than c)

A similar thing happens if you fall into a black hole (because, under General Relativity, there is no way to differentiate from moving at speed, to being in a gravity well). If I was approaching the event horizon (and I ignore the tidal affects ripping me apart), then time passes normally for me. The second hand on my watch moves as normal. If you were observing me from a safe distance, then you would see the second hand move slower and slower, until I finally reached the event horizon and the second hand would appear to stop. But for me, time passes as normal. If I looked at you, the second hand on your watch would appear to be moving faster and faster. Once I reached the event horizon, your second hand would be infinitely fast, but for me, time would pass as normal. Time is only constant when measured within the same frame of reference.

Photons can travel at the speed of light because they have no mass. Only entities with mass cannot be accelerated to c (the Lorenz-Fitzgerald equations will give the increase in mass and energy required as a body accelerates).

[themos]

But to the photon, the second hand moves as normal.

I don't think it does. The elapsed time for an observer is given by the "proper time", integrated along their spacetime trajectory. For photons, this "proper time" is always zero. Integrating zero gives zero.

[Zakalwe]

No, DrNeb, it doesn't work like that. We might be able to encode ourselves as a bunch of photons, though, and then the photons on arrival (anywhere) would be as young as when they left (the rest of the universe will have gone forward in time, though). Things with mass cannot do this, but subjective travel time can be reduced without limit (ok, you run into intractable radiation problems as you start going really fast because the vacuum of space will start behaving like a brick wall).

The faster you want to go, the more energy is required to accelerate you to that speed. To accelerate you to the speed of light, you need an infinite amount of energy (the energy required can be calculated by the Lorentz-Fitzgerald equations)

[markophoto]

I'm going to have to read this through again......

and then ask more questions...

[spaceboy]

Is time not a concept of measurement only? To consider something of going forward or back in time would be to consider it possible to change our own destiny. IE: We are in some way able to prevent death by traveling back wards in time to our youth or to travel forward in time with out aging?

So is this is wrong then.

Is it not the person doing light speed is aging at the same speed as if not doing LS. And the observers are aging at the same speed they just aren't aging the same in relation to each other ???? Although the two would I take it be in different dimensions, time would still move at the same pace for both in relation to the dimension they are in. As the clock ticks the same for both and not actually going faster or slower it would be correct? to say that in fact neither are going forwards or backwards in time at all. When the 2 dimensions over lap then the difference would be apparent between the traveler and the observer but in actual fact neither have actually changed the flow of time.

Man the past few days have been hard on my half dead brain cells.

[themos]

I think you've got it, spaceboy, except instead of "dimension" use the more impressive sounding "worldline". We are all like spaghetti, we each live on our own strand. Most often, the strands all go pretty much together (from the viewpoint of a huge speed like the speed of light) so we don't notice the weird relativity effects. But when one strand extends to another room, instead of being on the plate with the rest, then we notice the weird relativity effects.

[Zakalwe]

So is this is wrong then.

Is it not the person doing light speed is aging at the same speed as if not doing LS. And the observers are aging at the same speed they just aren't aging the same in relation to each other ???? Although the two would I take it be in different dimensions, time would still move at the same pace for both in relation to the dimension they are in. As the clock ticks the same for both and not actually going faster or slower it would be correct? to say that in fact neither are going forwards or backwards in time at all. When the 2 dimensions over lap then the difference would be apparent between the traveler and the observer but in actual fact neither have actually changed the flow of time.

Man the past few days have been hard on my half dead brain cells.

Correct.

Forget about dimensions though, Replace that with speed (in the case of Special Relativity), or gravitational field (in the case of GR). For each entity, according to their unique frame of reference, time appears to travel normally. All cocks within their frame of reference are in agreement. The entities only realise that their clocks differ when the clocks are compared to one that was contained in a different frame of reference*

An example of this is atomic clocks that are carried in satellites (especially those in geosynchronous orbits due to the distance involved). The atomic clocks of those satellites run at a different speed to Earthbound clocks, because the Earthbound clocks are in a stronger gravitational field. However, we on the ground perceive a second as a second. An observer on the satellite also perceives a second as a second. It is only when both are compared is it apparent that time has moved art a different rate. To the satellite-bound observer the Earth-bound clock has run more slowly. To the Earth-bound observer, the clock on the satellite has run quicker.

Either one that was moving at a different relativistic speed, or one that was contained in a different gravity well.

[spaceboy]

So you say speed and gravity. Do you mean like traveling at a faster speed causes higher G's and so effects time ???

[GazOC]

The effects on time of gravitational fields was the reason that Mercurys's orbit was always slightly out when calcuated with Newtonian physics.

[spaceboy]

So G-force can still be experienced in a vacume

[GazOC]

If you are accelerating.

[Zakalwe]

So you say speed and gravity. Do you mean like traveling at a faster speed causes higher G's and so effects time ???

No.

You would only experience G-force either in the vicinity of a massive body or when under acceleration (in fact, to the observer there is no difference in the two).

In comparison, you could be travelling very very fast and experience zero-G.....which is exactly what trainee astronauts experience* when they go on the "vomit-comet". Inside the plane there is nothing to give away the fact that the are travelling very fast towards the ground.

*The same as a body in orbit. It is, in fact, falling very fast towards Earth. However as it is falling towards a sphere and not a flat plain, the ground has "retreated" away from it. Hence the body is constantly falling, or is in free-fall.

[Zakalwe]

So G-force can still be experienced in a vacume

Of course. How else does the Moon stay orbiting the Earth*?

*or the Earth stay orbiting the Sun?

[spaceboy]

This has thrown me ............

The atomic clocks of those satellites run at a different speed to Earthbound clocks, because the Earthbound clocks are in a stronger gravitational field.

Either one that was moving at a different relativistic speed, or one that was contained in a different gravity well.

LS is not effected by time due to speed causing G. But as it has no mass how can it create G ???????

I think I'm confusing myself now.. I don't think I'm reading some thing right.

Please resolve the speed , gravity thing again.

[Zakalwe]

This has thrown me ............

LS is not effected by time due to speed causing G. But as it has no mass how can it cause G to effect time ???????

I think I'm confusing myself now.. I don't think I'm reading some thing right.

Please resolve the speed , gravity thing again.

A strong gravitational field warps the space-time continuum. Time within a strong gravitational field moves at a different rate to the time experienced by an observer not in a strong gravitational field. Time also moves at a different rate for an observer moving at relativistic speed when compared to an observer who is not moving at relativistic speeds

[spaceboy]

Scrub the last one!! Everything is a blurr I need a Valium.

[spaceboy]

Is it and if so how is gravity created by LS?

[Zakalwe]

Who said it was?