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Author Topic: If you travel faster than light, could you look back and see past events?  (Read 16509 times)

Offline quamic

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Say you are in a space ship moving away from the planet, for all purposes in a straight vector, at or faster than light speed, how about 1ly/h. For example, if you travel for one hour and you turn around, will you be able to see (through a telescope/viewer) events on earth that occurred a year before you left?
« Last Edit: 14/04/2013 11:05:26 by chris »


 

Offline JP

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You can't travel faster than the speed of light, so the question doesn't have an answer.
 

Offline quamic

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You can't travel faster than the speed of light, so the question doesn't have an answer.

Based off of empirical evidence the question can have one of three answers: yes, no, or maybe. I'm wondering if you could supply a source for your information, or why you bothered to answer the topic if you had no real information to provide.
 

Offline lightarrow

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Based off of empirical evidence the question can have one of three answers: yes, no, or maybe. I'm wondering if you could supply a source for your information, or why you bothered to answer the topic if you had no real information to provide.
LOL
It's as if one asked you: "what happened if you suddenly understood that you have to study relativity?" and you answered: "it's impossible" and the counter-reply were: "can you supply a source of information of your claim?"   :)
Did you grasp the latent message here?  ;)
 

Offline Bill S

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Hi Quamic, Welcome!

Whilst I am obliged to agree with JP, I would say: don't be put off by getting an (appropriately) abrupt answer to your first question.

May I suggest that you read (if you have not done so already) J Richard Gott's "Time Travel in Einstein's Universe".  It may not answer your question, but, if it doesn't, it should help you to re-phrase it so as not to meet a dead end.   
 

Offline evan_au

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Hypothetically, if someone did manage to develop a faster-than light spaceship (like Star Trek), you would be able to look back through a powerful telescope and see yourself launching from Earth.

Einstein showed that you can't accelerate up to the speed of light, let alone beyond it. But other researchers are looking for ways that you might be able to go faster than light without actually reaching the speed of light first(!) As you might expect, this is not easy, and at this time no-one has announced a method that looks even remotely practical.
 

Offline JP

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Are serious researchers actually trying to figure out how to go faster than light?  The only ideas I'm aware of are those that use general relativity (e.g. wormholes) to go between two points quickly without actually traveling FTL.  If you managed to take a wormhole, then yes, you would see things happening on earth before you left, but you haven't at any point gone faster than the speed of light.  There's a subtle, but important distinction there.

But it's not at all clear if even these general relativity ideas even work in reality, since they tend to require exotic matter with properties that we've never seen.  Such matter may be possible or may not be.
 

Offline flr

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In my opinion the current ideas on how to travel FTL 'globally' (but still bellow c locally) with wormholes and exotic matter and so on, only show one thing: The impossibility of going FTL from any practical point of view.

Maybe in 1000s years from now, if still around and if our brain evolve toward more intelligent, maybe humans will find a way to go in space or to go FTL, but that I think will require completely new physics.

PS: But why in such a hurry?
 

Offline flr

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Say you are in a space ship moving away from the planet, for all purposes in a straight vector, at or faster than light speed, how about 1ly/h. For example, if you travel for one hour and you turn around, will you be able to see (through a telescope/viewer) events on earth that occurred a year before you left?

Now back to your question: Since it is more SF than science, let's fantasize (for argument only) that Star Trek Enterprise ship will give you 1 hour free FTL ride taking you 1 ly away from Earth.

Also, let's assume that the ship' sensors observing the  Earth are based on capturing photons that Earth emits. Let's fantasize that somehow those sensors can capture photons even when traveling FTL.
Your question was: how and at what time will you see the Earth?

My guess is that there are 3 situations:
i) while traveling FTL away from Earth:
    - probably you will see Earth going backward in time.   
ii) the ship stop (i.e. it is no longer FTL) at a distance of 1 ly from Earth.
    - while the ship is stopped, you will see yourself on Earth  1 year before leaving
iii) the ship returns at FTL
    - you should see fast forwarding during 1 hour of FTL travel back to Terra, the 1 year of history starting from the time you left minus 1 year to the time you arrived  + 2 hour.

However, the most likely scenario is that there is no Star Trek Enterprise ship to give free FTL rides :)

 

Offline lightarrow

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Now back to your question: Since it is more SF than science, let's fantasize (for argument only) that Star Trek Enterprise ship will give you 1 hour free FTL ride taking you 1 ly away from Earth.

Also, let's assume that the ship' sensors observing the  Earth are based on capturing photons that Earth emits. Let's fantasize that somehow those sensors can capture photons even when traveling FTL.
Your question was: how and at what time will you see the Earth?

My guess is that there are 3 situations:
i) while traveling FTL away from Earth:
    - probably you will see Earth going backward in time.   
ii) the ship stop (i.e. it is no longer FTL) at a distance of 1 ly from Earth.
    - while the ship is stopped, you will see yourself on Earth  1 year before leaving
iii) the ship returns at FTL
    - you should see fast forwarding during 1 hour of FTL travel back to Terra, the 1 year of history starting from the time you left minus 1 year to the time you arrived  + 2 hour.

However, the most likely scenario is that there is no Star Trek Enterprise ship to give free FTL rides :)
I know that you have warned that this is more SF than science, but, even speculating, I can't understand on which basis you are speculating: aproaching light speed you would see nothing from where you are coming and infinite light from where you are going.
 

Offline Pmb

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Quote from: quamic
Based off of empirical evidence the question can have one of three answers: yes, no, or maybe. I'm wondering if you could supply a source for your information, or why you bothered to answer the topic if you had no real information to provide.
He did give you "real information." What you basically asked was "What does physics predict/say if you did this thing which is impossible according to physics?" That's not a meaningful question.

The source of his information is the special theory of relativity. The emperical evidence comes from many experiments and is constantly being demonstrated as true in particle accelerators all over the world.

If you want to understand this then you need to learn special relativity and that means learning a ton of math. I've created a website to teach special relativity at
http://home.comcast.net/~peter.m.brown/sr

In the following web page
http://home.comcast.net/~peter.m.brown/sr/uniform_accel.htm

A charged particle subjected to a uniform electric field experiences uniform acceleration in its own frame of reference. I show that such a particle comes arbitrarily close to moving at the speed of light but never equals it. See
http://home.comcast.net/~peter.m.brown/sr/uniform_accel.htm

In this web page
http://home.comcast.net/~peter.m.brown/sr/inertial_mass.htm

I show that the inertial mass of a particle becomes infinite at the speed of light and imaginary when faster than the speed of light, meaning that it's impossible for a particle which starts out moving at less than the speed of light will never move at or faster than the sspeed of light.

In this web page
http://home.comcast.net/~peter.m.brown/sr/work_energy.htm

I show that the kinetic energy of a particle moving at the speed of light is infinite.

Emperical evidence can be found in the physics literature, namely the American Journal of Physics. You can find it online. Do search using the term "Resource" and search for articles giving experimental results.
It can be shown using special relativity that if a body is subjected to a finite force then the energy required to accelerate it to the exact speed of light is infinite and thus impossible. As JP explained, using general relativity it might be possible to cheat physics and skip over the space between two points using a spatial shortcut known as a wormhole. However this isn't really traveling at the speed of light since the speed of the body as measured by spatial dispacements never exceeds the speed of light. The reason it works is because the wormhole is a shortcut in space between two positions in space.

So the answer to your question as you stated it is neither yes, nor o maybe since you asked what would the laws of physics tell you if they're wrong. That has no answer.
« Last Edit: 13/04/2013 13:57:50 by Pmb »
 

Offline flr

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I know that you have warned that this is more SF than science, but, even speculating, I can't understand on which basis you are speculating: aproaching light speed you would see nothing from where you are coming and infinite light from where you are going.

I had in mind something like this [ignoring the effect of such impossible accelerations]:
 - Travel FTL few sec. Stop 'almost suddenly' for 1 ns and make a photo of Earth and label the photo as (1).
 - travel FTL another few sec. . Stop again 'almost suddenly' for 1ns to make a picture of Earth, and label the photo as (2).
 - .... and so on... for step number (i) which produce the photo labeled as (i)
 
Let's consider the case of moving away from Earth.
Putting together all these photos in the order of increasing the label (i), the traveler should see a movie of the Earth backward in time.

----------------

I also believe that the travel of non-zero rest mass objects arbitrary close to 'c' is not practically possible. If 1 kg object accelerated at 0.999999999999999999999999999999999999 from the speed of light will hit a planet, it will vaporize instantly that planet or even a Sun emitting a soup of crazy highly energetic particles. Having so fast ships can spell danger for a civilization: a slight miscalculation and their planet will instantly vaporize from the collision with a fast ship.
Our telescopes haven't notice such events (objects dissapearing suddenly in a plasma glow emitting crazy particles). So, large objects must have limited speeds relative to us.
------------------------------------------

 

Offline JP

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The problem is that there is no known way of accelerating a camera to FTL speeds then stopping it suddenly to do this.  I'm all for though experiments, but when you start breaking laws of physics to do them, the results don't tell you anything about physical reality.
 

Offline Pmb

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The problem is that there is no known way of accelerating a camera to FTL speeds then stopping it suddenly to do this.  I'm all for though experiments, but when you start breaking laws of physics to do them, the results don't tell you anything about physical reality.
One can always place a camera in space and capture images of earth. When doing so they are all looking back in time. We just can't look back before today.
 

Offline yor_on

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Made me think a little that one. Assuming we look at a astronomical picture, each photon on it might represent a different age, thinking that way. Normally we define a time and a 'now' from the moment the picture was taken, but using 'c' each photo should consist of 'photons/waves' of a different time mix (sources).
 

Offline Pmb

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Made me think a little that one. Assuming we look at a astronomical picture, each photon on it might represent a different age, thinking that way. Normally we define a time and a 'now' from the moment the picture was taken, but using 'c' each photo should consist of 'photons/waves' of a different time mix (sources).
Yes! Precisely!
 

Offline evan_au

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Quote
will you be able to see (through a telescope/viewer) events on earth that occurred a year before you left?
You could achieve the same effect (but within our currently forseeable technology) by launching a rocket containing a large, foldable mirror, and positioning it half a light year away. You could then see events happening on Earth a year ago.

It will take hundreds or thousands of years for the mirror to reach the right position for deployment, even with acceleration powered by nuclear rockets. And it would require a telescope far beyond our current capabilities to see anything in this mirror.
If you really want to see the past, why not take a "selfie" on your smartphone?
 

Offline dlorde

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If you really want to see the past, why not take a "selfie" on your smartphone?

Absolutely! There is more amazing physics used by the technology in an average smartphone than most people can imagine. Who needs fantasy FTL when you've got a smartphone?
 

Offline bizerl

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If you really want to see the past, why not take a "selfie" on your smartphone?

It certainly brings into question the concept of "present" when everything we see, hear, touch etc. is actually a past event. Even if we see something "live", time has passed between the event occuring, and the light reaching our retinas, and then further time for the event to be recognised in our brain as an event.

In regards to the original question, if somehow Jiminy Cricket rose from the dead as a zombie, had enough fairy dust and somehow managed the 1 light year per hour speeds, wouldn't "conventional" theoretical physics then kick in and possible send our insect friend back in time? Then he could probably send the pictures to Earth and they would still be of the past!

Of course, if he kept going, he might make it all the way back to the big bang. Lucky for him, zombies are already dead.  [xx(]
 

Offline eddysciencefan

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unfortunately no you wouldn't be able to see anything behind you as the light would not be traveling fast enough to reach you retinas as you would accelerate away from it. if you were traveling at the speed of light exactly then the image would remain the same, as the light and you eyes are moving along together, much like looking at another person on a train moving the same speed relative to you looks like he is standing still.

fortunately however if you could travel that speed then you could see earth in the past by traveling far enough away (1 light year per year) and looking through a telescope, the image you see would be from the past. you could even tailor the distance to look back to the required time.

theoretically if you can travel faster than light you could set of and catch up the light that left earth from whenever you wanted to see it and observe through a telescope and see the romans/greeks/egypt or however else tickled your fancy
 

Offline dlorde

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... fortunately however if you could travel that speed then you could see earth in the past by traveling far enough away (1 light year per year) and looking through a telescope, the image you see would be from the past. you could even tailor the distance to look back to the required time.

theoretically if you can travel faster than light you could set of and catch up the light that left earth from whenever you wanted to see it and observe through a telescope and see the romans/greeks/egypt or however else tickled your fancy
Of course, we already look back in time when we look out at other star systems; one can imagine someone on a planet out in Andromeda looking back at us and seeing the Earth as it was 2.5 million years ago (they'd need a good telescope).
 

Offline JustWondering51

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Sorry for reviving an old thread, but I found this discussion very interesting (so much so I made an account to ask a follow up question).

A lot of the arguments above talk about the impossibility of speed of light travel, although we could argue the existence of wormholes that could near instantaneously transport us very far.

What if we were to put aside the whole notion of us trying to get ahead of the light and instead think of the light coming to us. Consider the following scenario.

Suppose 10 light years from here there is a planet or some sort of celestial object that acts as a near perfect mirror. Maybe it's an extremely smooth planet composed mainly of shiny metal, or maybe it's just a planet with sentient life and they have conveniently placed a very large near perfect mirror on the surface of their planet.  Or maybe some savy scientists create a giant mirror and send it out on a ship going almost the speed of light for 10 years.   

Now supposed that instead of putting a bunch of research into an FTL ship, we develop an EXTREMELY powerful telescope such that we can observe this mirror (maybe it's a telescope that observes light through micro worm holes to retain resolution, or maybe it's just VERY good optics).

In theory our planets reflection would take 10 years to get to this reflective object, bounce of the mirror and then take ten years to get back, so if we pointed our telescope at it we'd be staring at ourselves 20 years in the past. 

To me that almost sounds doable, even if it's at a smaller scale (say we place a mirror the equivalent of 1 light day away) we'd be able to see 2 days into the past.

What do people more educated than me think? Does this sound like a more feasible way of looking into the past rather than trying to "beat the light" to a location?
« Last Edit: 04/09/2013 00:46:19 by JustWondering51 »
 

Offline dlorde

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To me that almost sounds doable, even if it's at a smaller scale (say we place a mirror the equivalent of 1 light day away) we'd be able to see 2 days into the past.

What do people more educated than me think? Does this sound like a more feasible way of looking into the past rather than trying to "beat the light" to a location?
Well it's more feasible to the extent that that the physical arrangement is within the laws of physics, so it could be attempted; but a light day is 26 billion kilometres (by comparison, the voyager spacecraft is at the edge of the solar system, 15 billion kilometres distant).

Have you considered how large the mirror would have to be to even be detectable visually (or how  powerful the telescope)? And what would you expect to actually see? I suspect you'd be lucky to see the sun as a single pixel with a mirror as big as the orbit of Mercury (I'd be amused if some enterprising physicist could do some calculations).
 

Offline Pmb

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Say you are in a space ship moving away from the planet, for all purposes in a straight vector, at or faster than light speed, how about 1ly/h. For example, if you travel for one hour and you turn around, will you be able to see (through a telescope/viewer) events on earth that occurred a year before you left?
If you were truly moving faster than light then how do you propose to capture the light in a camera or to view it otherwise? In any case the answer is yes. There have been theories about how faster than light travel could happen using things like warp bubbles and wormholes. This isn't really faster than light travel but it does allow one to effectively travel faster than light. It has never been proven that those things cannot exist, at least not yet. However its rather easy to do this much more simply. Just arrange a series of cameras along the path you would have traveled by a ship moving FTL.

By the way, observing images that were created in the past can very easily be observered in the present. We do it every time we look in the mirror or look at a photograph.
 

Offline yor_on

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I was thinking of how a distance shrink in a relativistic motion, as locally measured from the ship. Although that isn't traveling faster than light, it do have the effect of allowing that ship to 'propagate' :) ftl, if defined from its origin, if ignoring the 'time dilation' its origin would find.

My point is that the closer to light you move quamic, the shorter the distance you will find yourself to travel. You shrink your universe, locally measured, through motion.

The way to explain it 'universally' is using the 'time dilation' observed from 'at home' (earth) studying your local clock relative earths. In Earths frame of reference the ships clock slows down, lights speed in a vacuum remaining the same, explaining why you can travel 'so far'. In the ships frame of reference it is the room that shrink in the ships direction of motion, while lights speed in a vacuum stays the same.

Locally defined the ships definition is as true as Earths. And so you, depending on frame chosen, mixing it a little :) actually do ftl, although never faster than light.
=

Thinking again, I'm wrong, if we use Earths frame of reference looking at the ship. That ship won't ever be observed doing ftl in any way, presuming we had a way to follow it as the light reflected/coming from it will become more and more redshifted the faster it moves away, relative Earth. But we might be able to argue that the shrinking we measure locally, in the ships frame, is a result of ftl :) as defined from some 'commonly same' universal point of view. Locally it's not possible though, to define it that way. Because lights speed in a vacuum is a limit for mass, and bosons too. The last just means that we have no way of measuring tachyons, if they now exist, that I know of?
« Last Edit: 08/09/2013 15:51:28 by yor_on »
 

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