# I am here but i am also over there, can this be?

Andy
Heres a nice question for everyone probably going to be worded really badly.

Do you think it would be against a fundamental law of the Universe to have a paricle in 2 different places at the same point in time?

Basically i am asking if you think that time travel is possible.

Dave
Maybe, not in a back to the future way tho...

Greetings Andy !
Originally posted by Andy
Do you think it would be against a fundamental law of the Universe to have a paricle in 2 different places at the same point in time?

Basically i am asking if you think that time travel is possible.
An important thing for you to understand is that Special
Relativity has removed absolute simultainity from the
physical vocabulary. Simultainity is relative, just like time
itself.

What this means is that you can observe the "same" particle
in two different places at the "same" time if the light or
other type of influence that you use to observe the particle
traveled towards you for different time periods relative to you
(at c - the speed of light).

Moving on to Quantum Mechanics two more things can be said:
First of all, in QM we have wave-particles. This means that
a particle is not a point in space. The WF that discribes
the particles can be spread over huge distances and into
separate parts. Of course, when you observe = interact with the
particle you only see it in one particuilar place.

Second, according to QM it is impossible to select an individual
particle for observation the same way it is impossible to
separate individual waves from the sea. So, you can't
really know for certain that you're observing the "same"
particle.

Live long and prosper.

Staff Emeritus
Gold Member

Originally posted by drag
Greetings Andy !

An important thing for you to understand is that Special
Relativity has removed absolute simultainity from the
physical vocabulary. Simultainity is relative, just like time
itself.

What this means is that you can observe the "same" particle
in two different places at the "same" time if the light or
other type of influence that you use to observe the particle
traveled towards you for different time periods relative to you
(at c - the speed of light).

Actually, I don't think this is quite true. Suppose there were two points in the trajectory of a particle with the retarded times t1 and t2 - the times when the object was at points r1 and r2 wrt the observer. Then r1 = c(t-t1) and r2 = c(t-t2). t is the time at which we observe the object in two different places.
Then r1 - r2 = c(t1-t1). So the average velocity of the object would have to be [r1-r2]/[t1-t2] = c Since no object is traveling with an average velocity of c, we can never view an object in two places at once.

Moving on to Quantum Mechanics two more things can be said:
First of all, in QM we have wave-particles. This means that
a particle is not a point in space. The WF that discribes
the particles can be spread over huge distances and into
separate parts. Of course, when you observe = interact with the
particle you only see it in one particuilar place.

Second, according to QM it is impossible to select an individual
particle for observation the same way it is impossible to
separate individual waves from the sea. So, you can't
really know for certain that you're observing the "same"
particle.

Live long and prosper.

However, we are attempting [as I vaguely understand it] to collapse the wave function for a "single particle" in two quantum states. By this it is supposed that the same "particle" will exist in two places at once. I had heard that the folks at the University of Colorado had done this. I have looked for this and was unable to find the report. I recently heard Kaku comment on this experiment. He said that we have not been able to do this yet.

Originally posted by Andy
Heres a nice question for everyone probably going to be worded really badly.

Do you think it would be against a fundamental law of the Universe to have a paricle in 2 different places at the same point in time?

Basically i am asking if you think that time travel is possible.

No - time travel is impossible. Always and forever.

Is it now? Why and how?

Time travel to, say the future, happens all the time. Time travel to the past is not expressly forbidden either (it raises a lot of logical paradoxes sure). But when you state that it is impossible, back it up!

And to address the question: Its been answered. Quantum physics prohibits it.

Proving that time travel is impossible is simple. It doesn't need any physics.

Time has no quality of measurement. To say one is traveling describes physical movement - in physical dimensions. Time cannot be traveled, the word doesn't even compute into a non-spatial dimension.

Staff Emeritus
Gold Member
And to address the question: Its been answered. Quantum physics prohibits it.

If this is fundamentally true, then what are the folks at the University of Colorado doing?

Originally posted by Ivan Seeking
If this is fundamentally true, then what are the folks at the University of Colorado doing?

What they're doing isn't important - what is important is that they are not doing this.

I'm sure they are doing a lot of things. You'll have to be more specific before we can clarify anything.

Originally posted by Ivan Seeking
Actually, I don't think this is quite true. Suppose there were two points in the trajectory of a particle with the retarded times t1 and t2 - the times when the object was at points r1 and r2 wrt the observer. Then r1 = c(t-t1) and r2 = c(t-t2). t is the time at which we observe the object in two different places.
Then r1 - r2 = c(t1-t1). So the average velocity of the object would have to be [r1-r2]/[t1-t2] = c Since no object is traveling with an average velocity of c, we can never view an object in two places at once.
What ?! It appears to me that you fail to take GR into
account. It is possible that the light coming from one event
has been "caught up in traffic", the same way you observe
gravitational lensing of objects seeing them at different time
periods all together.
Originally posted by Ivan Seeking
However, we are attempting [as I vaguely understand it] to collapse the wave function for a "single particle" in two quantum states. By this it is supposed that the same "particle" will exist in two places at once. I had heard that the folks at the University of Colorado had done this. I have looked for this and was unable to find the report. I recently heard Kaku comment on this experiment. He said that we have not been able to do this yet.
What ?! I was providing Andy with some general background from QM
to adress the issue of the "same" particle and what
QM says particles really are. I did not talk about collapsing
the WF in different places or something. Also, I've never
heard of such attempts at all, if you can provide a relevant
link on this subject I would appreciate it (that's really weird).

(btw, purhaps that is possible but with gravity waves used
as detectors, I have a thread on this in Theo. Physics and
I'd wish someone would finally seriously answer it or at
least provides a good physicly back-uped guess.)

Live long and prosper.

Antiproton
Originally posted by Izzle
Proving that time travel is impossible is simple. It doesn't need any physics.

Time has no quality of measurement. To say one is traveling describes physical movement - in physical dimensions. Time cannot be traveled, the word doesn't even compute into a non-spatial dimension.

I'd say hours, minutes and seconds are particularly measurable. As was stated earlier, we are traveling through time constantly. The question is can we go in a different 'time direction'. As far as I've seen, there is no literature that shows it as an impossibility. Now, what happenes once you get back there...thats a whole slew of fun discussions.

Staff Emeritus
Gold Member

Originally posted by drag
What ?! It appears to me that you fail to take GR into
account. It is possible that the light coming from one event
has been "caught up in traffic", the same way you observe
gravitational lensing of objects seeing them at different time
periods all together.

aaaaahhhh. You are correct oh great one. In fact, I just now thought of some double images of stars that have been seen lensing around in deep space. Sorry, I was in SR mode.

What ?! I was providing Andy with some general background from QM
to adress the issue of the "same" particle and what
QM says particles really are. I did not talk about collapsing
the WF in different places or something. Also, I've never
heard of such attempts at all, if you can provide a relevant
link on this subject I would appreciate it (that's really weird).

I know that I have the basic story correct. Exactly how this is supposed to work is out of my league [beyond arm waving anyway]. As I said, I have been unable to find information about the initial reports that I heard, perhaps a few years ago now, but since Kaku just mentioned this about a week ago, it is apparently still an active area of research. I will try to find something.

As for electrons, they all look alike to me.

Of course, "the electron is not as simple as it looks".
---(William) Lawrence Bragg, British Physicist(1890-1971)

Staff Emeritus
Gold Member
Originally posted by Izzle
What they're doing isn't important - what is important is that they are not doing this.

However it would seem that you state opinions as if they were facts. Clearly some very intelligent people don't think this issue is resolved. How many physicists are on your research team?

Staff Emeritus
Gold Member
Originally posted by Izzle
Time has no quality of measurement.

Then what detemines the frequency of a Cesium clock? Why does it change frequencies with it's state of motion? Why is it's frequency affected by gravity. What is the mechanism for these changes?

Andy
OK thanks for all of the replies but i guess I am going to have to a bit more blunt with this, would it be possible for me to go backwards in time to meet myself but more importantly to interact with myself?

Originally posted by Antiproton
I'd say hours, minutes and seconds are particularly measurable. As was stated earlier, we are traveling through time constantly. The question is can we go in a different 'time direction'. As far as I've seen, there is no literature that shows it as an impossibility. Now, what happenes once you get back there...thats a whole slew of fun discussions.

Nope. hours and minutes are not a measurement of the time dimensional.

Also - the deal with people even thinking at all that time travel is possible is just because they're taking something they understand (space) and implying that something they don't understand (time) has similar qualities.

Time does not travel, there is no measurement of time, it has no directions, no level, no quality - these are all just incorrect implications...

Antiproton
I don't see how you can assert that our 'telling' time isn't a measurement of 'Time'. Hours, seconds and minutes are just as arbitrary as meters and yards, yet you have no problem with that.

Further more, I think you're making too big a distinction between Time and Space. The more we learn about it, the more the differences between the two are slurred.

I cannot accept your flat denials of a concept yet unknown to us without some kind of scientific basis. My understanding of General Relativity and Quantum Mechanics dictates to me that time travel is not explicitly forbidden. Therefore it is till possible to theorize to that effect.

I mean, let's look at the most basic principle of Time. Now, we are in the present. 10 minutes from now, we will be in the future, which with then be the present (that was great fun to type). If that's not traveling in time, what is it? That's no different than saying "First we were at 2m, and now we are at 4m. We have traveled".

Can you argue scientifically that they are fundamentally different? Other than it just seems harder to reverse the direction of movement in Time.

Originally posted by Antiproton
I don't see how you can assert that our 'telling' time isn't a measurement of 'Time'. Hours, seconds and minutes are just as arbitrary as meters and yards, yet you have no problem with that.

Further more, I think you're making too big a distinction between Time and Space. The more we learn about it, the more the differences between the two are slurred.

I cannot accept your flat denials of a concept yet unknown to us without some kind of scientific basis. My understanding of General Relativity and Quantum Mechanics dictates to me that time travel is not explicitly forbidden. Therefore it is till possible to theorize to that effect.

I mean, let's look at the most basic principle of Time. Now, we are in the present. 10 minutes from now, we will be in the future, which with then be the present (that was great fun to type). If that's not traveling in time, what is it? That's no different than saying "First we were at 2m, and now we are at 4m. We have traveled".

Can you argue scientifically that they are fundamentally different? Other than it just seems harder to reverse the direction of movement in Time.

Being at time zero and then time 10 is moving THROUGH TIME (so you say).

and then you say "first we are at 2m, and now ere at 4m".

That's not moving through space - it's moving through time and space.

You failed to isolate movement through space.

And you can't possibly isolate it.

Andy

as i said before,

Origionally posted by Me!
would it be possible for me to go backwards in time to meet myself but more importantly to interact with myself?

Originally posted by Andy
OK thanks for all of the replies but i guess I am going to have to a bit more blunt with this, would it be possible for me to go backwards in time to meet myself but more importantly to interact with myself?
No.
But who knows what theory will some physict discover tomorrow...
Reality is more extraordinary than any fantasy, or so they say...

Live long and prosper.

Andy
But who knows what theory will some physict discover tomorrow...

So i take it that you don't think much of these theories about time travel then.

Originally posted by Andy
So i take it that you don't think much of these theories about time travel then.
I am not aware of such theories. Basicly GR tells us that
if we were to travel faster than c - the speed of light
then we would go back in time. But, since there is no known
or even seriously theorized way to exceed c, it just remains
a weird result of messing with GR's equations.
(Science sucks, right ? It ruins all the fun stuff
of most sci-fi. But, it can sometimes present even more
exciting possibilities and these may actually be real ! )

Live long and prosper.

Andy
I am not aware of such theories. Basicly GR tells us that
if we were to travel faster than c - the speed of light
then we would go back in time. But, since there is no known
or even seriously theorized way to exceed c, it just remains
a weird result of messing with GR's equations.
(Science sucks, right ? It ruins all the fun stuff
of most sci-fi. But, it can sometimes present even more
exciting possibilities and these may actually be real ! )

Yea i agree, what i meant when i said theories about time travel was the theoretical possibilities that time travel could possibly happen if you could travel faster than the speed of light, i personnally think that by traveling faster than the speed of light you are only ever going to see backwards in time, obviously have no evidence for this but this is just as much evidence as anyone else has got saying that it could happen. i think.

Staff Emeritus
Gold Member
Originally posted by drag
I am not aware of such theories. Basicly GR tells us that
if we were to travel faster than c - the speed of light
then we would go back in time. But, since there is no known
or even seriously theorized way to exceed c, it just remains
a weird result of messing with GR's equations.
(Science sucks, right ? It ruins all the fun stuff
of most sci-fi. But, it can sometimes present even more
exciting possibilities and these may actually be real ! )

Live long and prosper.

We have at least 6 or 8 designs for time machines [See Hawkins’s The Future of Spacetime]. Some argue that they would actually work; that is if they could ever be built and if we had a few thousand suns worth of energy. Some approaches don't necessarily require so much energy but rather exotic technologies e.g. perfect conductors or exotic matter.

Antiproton
It could be something as trivial (Ha!) as interaction with an exotic manifestation, like the theoretically predicted but as of yet undetected superluminal 'tachyon'. Time reversed calculations indicate there would be no impossibilities associated with time travel with objects already moving faster than light.

Though it does bring up interesting thoughts about what it would be like if you could do it. Our QM class came to the conclusion that if you could travel through time, you could be nothing but an observer...changing the past would not be possible. Our logic was convoluded, but it came to some interesting thoughts...

Of course particles are confined otherwise you and I would extend to infinity. Of course particles have waves and the wave effect extends to infinity in theory; in reality it grows weaker with distance and is neutralised by the action of stronger waves.
Particles can be split and this is sometimes taken as the same particle in two different places.
A particle in a high velocity collision with a thin film is shattered but its wave structure is not. As the wave leaves the other side of the film it takes with it sufficient force carrier (in the form of corpuscular quanta) to reform the particle; this force carrier is replaced in the film by the force carrier of the shattered particle, this is called tunneling.
Unfortunately scientist like to 'jazz up' up their findings partly to be heard above the crowd and partly to gain fame; the result is a mass of confussing and totally unneccessary jingoism.

Andy
So its just aload of rubbish that people like to think makes sense even though it doesnt, just theories based upon other theories that we don't understand either.

ObsessiveMathsFreak
Most modern physics is just a load of rubbish.

This has a lot to do with the difficulty of modern equations, in paticular the equations of general relativity.

We can't ask 'what is time'
That would be like asking 'what is matter'

these questions are in the realm of metaphysics.

On time travel, I think time travel would be beyond our scope. I mean what would happen if someone went back in time and killed themselves.
What Would happen?!

Time...that one indeed we cannot answer.

The equations of GR are not too difficult, just not easy.

Staff Emeritus