If time travel is possible, dont you think we would of met people from the future?

Peace.

I dont believe in tooth fairies.

Peace.

The concept of time has two seperate veins. One is the time we experience as "now". The other is time as it relates to physics. When experiments have been done with atomic clocks that demonstrate time variants between the earth and space shuttles, one thing seems clear to me. Although there is a definite difference in time relating to physics, both observers on earth and in space experience the same "now" regardless of that difference. There is no "Tooth Fairy" story here. There is a question in my mind that if a more extreme time difference is possible (say a full year or a decade) would not the observers still experience the same "now", just from different time perspectives? What do you think?

Hi "Strange F8".
This space/time/speed thing has always tied my head in knots, i just can"t understand it & it"s really frustrating !
My Ma-In-Law, jets all-over the world due to her occupation.
Just this year she"s been to Australia, the USA a few times (mainly New York & Chicago), Paris, Milan & Barcelona.
She"s doing all this jetting about, while i trundle from job to job in my LDV van (top speed 75mph).
Her NOW, is still the same as my NOW.
If i travel very fast, its still the same time, i"m just moving quickly!
Aren"t i ?

It's called uneducated

Time Travel Possible? Maybe but is it effective? no

because the only thing we could do with the power of Time Travel is observe nothing more

If time travel is possible, dont you think we would of met people from the future?

Peace.

What's even more interesting is that, when you observe the outside world from your ship, it also appears to slow down.
I think most people assume that if your ship time slows down and you look outside the window, that it would appear that the outside
time would speed up, but it doesn't, it also appears to slow down. It's a bit counterintuitive but when you start drawing graphs and using Lorentz transformations to plan the trip, it all adds up.
The secret is in the fact that there is no universal simultaneity, which is something a human brain can have a hard time
imagining.

Well it's just common sense really, if you're traveling at almost the speed of light, of course things will appear to slow down outside the rocket. Being as light is reaching you at a lesser frequency.

You only see objects because of the light refracted from the object entering your eye. So while traveling at a speed close to the speed of light, less refracted light from an object enters your eye.

Of course, traveling at the speed of light, everything will appear to stop. Going faster than the speed of light, you'll see hardly anything.

So, time is not slowing down on the outside as light is at the normal frequency. If that makes sense...

You are talking about Doppler effect, but that's not really the core of it, because it doesn't matter in which direction you are traveling.
If it were simply because of Doppler effect, then time would speed up for events that are in front of you.
So its still very far from common sense.

The thing is light travels at c, and you are traveling at say 0.99c, and you hit a beam of light, but the speed of collision
is not 1.99c , but still c.

I wonder... If traveling over the speed of light, say 0.5 over the speed of light, the light inside your own rocket would - in effect - dissapear leaving you in darkness. Unless you looked at the light-source.

No matter the speed you travel up to C light will still travel at C to an observer.

Ah I see, Interesting.

if you travel 1 hr into the future , in one hours time ,you did nothing

No no, I was referencing light to signify time.

As the frequency of light slows down - which in effect would contribute to the appearence of time slowing down - Everything around you is as normal as it was when you left.

I wonder... If traveling over the speed of light, say 0.5 over the speed of light, the light inside your own rocket would - in effect - dissapear leaving you in darkness. Unless you looked at the light-source.

I know it sounds absurd but such is the case. It all comes down to the inertial frame of reference. When it comes to light the inertial frame always seems like a standstill. A good example is say you're on a spaceship traveling .5 the speed of light and you point a flashlight along your trajectory. The light emited would still be traveling at C not 1.5 the speed of light. Once you start to talk about massive bodies this starts to fall apart though. Not the inertial frame per se but addition of velocity. Say you're in a train going 50 mph and you toss a ball towards the front of the train at 20 mph. If you measured from inside the train the ball would be going 20 mph. If the refernce were outside the train it would appear to be going 70 mph. And if you tossed it out the window forward (in a vacuum) it would hit a stationary object going 70 mph. Particles without mass don't work that way though.

I know it sounds absurd but such is the case. It all comes down to the inertial frame of reference. When it comes to light the inertial frame always seems like a standstill. A good example is say you're on a spaceship traveling .5 the speed of light and you point a flashlight along your trajectory. The light emited would still be traveling at C not 1.5 the speed of light. Once you start to talk about massive bodies this starts to fall apart though. Not the inertial frame per se but addition of velocity. Say you're in a train going 50 mph and you toss a ball towards the front of the train at 20 mph. If you measured from inside the train the ball would be going 20 mph. If the refernce were outside the train it would appear to be going 70 mph. And if you tossed it out the window forward (in a vacuum) it would hit a stationary object going 70 mph. Particles without mass don't work that way though.

Doppler effect ads to the slowing down, but even if you don't use light flashes as reference time still slows down.
There is a difference between regular Doppler effect and relativistic Doppler effect, in that for relativistic calculations time dilation is considered.
Time dilation in itself really has nothing to do with Doppler effect.

The metaphor usually used for measuring events without using light as a medium (for observing) is having an "army" of observers
in one reference frame, meaning for every point in space there is an observer measuring events, which means
measurement is not affected by travel of light. And even then time slows down.
Regarding traveling beyond speed of light, first of all, since no matter how fast you go, light always travels at speed c away from you (you can never catch light, you can only accelerate into infinity, but light will always travel 299792458 m/s away from you)
But in theory, if you did travel faster than light , it would be in no way different from traveling below speed of light, because
time would go backwards, and it would be the same, only like in a mirror compared to traveling below speed of light.

But you can't cross c, because c is a constant no matter how fast you go.
If you go at 299 792 457 m/s, light won't go away from you at 1m/s, but will still go away from you at 299792458 m/s,
so reaching light speed is like having a cat chase its own tail, because the faster the cat is spinning, then faster the tail is getting away,
same way, no matter how fast you go, light will always be faster than you by C which is a constant.

It goes in the other way too.
If you are going TOWARDS a light source at near 0.99999999c , the collision with the photons will not happen at
1.999999999c speed, but will happen at c speed.
In other words C+C=C , or you can't add speeds as vectors between different inertial frames

Ah, same as what capeo said, but with examples - makes it easier on my eyes - I'm going to have to really read more into this, it's mind boggling.

So basically, everything inside the bubble is constant while everything outside the bubble is also constant but with a different consistency (so to speak)?

So basically, everything inside the bubble is constant while everything outside the bubble is also constant but with a different consistency (so to speak)?

It seems to me, in my short time on these boards there is much confusion about what time really is. So Ill go ahead and make a post about it using some simple mathematical examples to improve understanding. As a disclaimer.. I will treat time and its behavior as in the case of special relativity, Those of you more endowed in quantum physics maybe be inclined to argue, but this is a simplistic sakes kind of thread. Maybe should have started a new topic but well see. Ill try and keep it simple and to the point, as most people are not as enthused as me about math and physics.

First lets start by defining what we mean by time. Time is a human idea, it is a scalar quantity we use to measure things or keep track of things. Much like labeling drawers on your tool chest. What time is not is a place. The past or the future do not exist in any real sense, only awareness of them because we perceive time as universal and linear. So looking at stars is not actually looking in the past, what you see when look at a star is light that may have taken millions of years to get, but I assure you the light is in the present, as is the star in question. Because you can not see the star over the vast distance does not mean it is in the past. Say I get a letter from China that took 6 days to reach me, the letter like the light is in the present, it just took "time" to get to me. Furthermore China is not existing in the past because I can not see it.

Now we keep track of time with seconds, the time standard in science. Surprising that most don't know what a second is. Keeping with the whole simple definition thing a second is...
It is the amount of time it takes a cesium-133 atom to transition between two ground states. Which corresponds to 9,192,631,770 periods of radiation for transition between states to occur.

Now on space-time and space travel.

Space is big, Sitting here in our homes and modern lifestyles we often forget that, if even realize it all. For instance the milky way is approximately 90,000 light years in diameter, our local group is is around 10 million light years diameter. Im sure many of you have heard of a light year, but lets go ahead and define it for everyone. A light year is...
The amount of time it takes light to travel in 1 year or 31 556 926 seconds. Now since we know how fast light travels in a vacuum (299 792 458 meters per second, well call it c from now on) and how many seconds are in the average year, we can calculate how far light goes in a year c*(seconds in a year) and we see light goes 9.46 × 10^15 meters (quick break for notation the ^ symbol means raised too) or roughly 5.88 × 10^12 miles.

So that means, that traveling at c it would take approximately 90 000 years to cross the milky way or roughly 10 million years to cross our local group. The fastest man made object was the satellite Helios which traveled at approximately 150 000 miles per hour or 67 056 meters/second or .02% the speed of c. Electromagnetic radiation such as radio waves also travel c in a vacuum. I made an earlier post with a mathematical example which I am going to copy here, baring no one has a problem with that .


Now it has also been stated in other threads that time, is relative to the observer, there is no universal time, but what does that mean? It turns out that speed effects time. Lets use a simple math example to show this.

We need a formula for this, we will use . This is how time is treated in special relativity and we get this from Lorentz Transformations, which I will spare you all the math of how we arrive at this equation, just know its correct.

Lets say we have two friends bob and tom, tom wants to travel to a planet 20 light years away. In his spaceship we tom's clock, while bob remains behind on earth with his clock.

For simplicities lets define our variables.
we'll call dt and it is the time as seen by bob.
we'll call dt' and it is the time relative to the traveler in our reference frame.(Tom)
v is the velocity relative to Tom, the traveler.
c is the speed of light
and the square root ill write as (equation)^.5
I know, dont get discouraged yet it will make more sense in a minute.

We know [the planet] is 1.89 x 10^17 meters away (get that by light year*20 light years). Now lets say we invent a new type of engine that allows us to travel at 80% the speed of light, or .8c. We then set off for our new planet and wonder how long it will take us to get there.

Well I am sure most of us have seen this equation before from a high school physics or math class
velocity equals distance divided by time. Since we know our velocity, .8c (units in meters per second) and we know our distance 1.89 x10^17 meters we can calculate roughly the time it takes us to get to this planet. So plug in the knows, use some simple algebra and we get 7.9 x10^8 seconds or roughly 25 years. This what here on earth we would guess and indeed it is what bob measures. But what about tom's clock?

Time to use that scary looking equation we saw earlier.
We are looking for dt', the time as seen by tom's clock. so we have dt=7.9 x10^8 seconds, v=.8c, we plug these numbers in and rearrange our equation.

dt'=(dt)(1-(v^2/c^2))^.5 and we get tom's clock shows 15 years have gone by.

So if at the start of our endeavor, Bob and Tom are twins and 30 years old. When tom arrives at this new star, he will seem to be 45 while bob will seem to be 55.

Now to you quantum physics buffs you may want say, we need to add to this! but again its just a simplified example to show that time is relative to the observer, not a universal constant.

edit: forgot I found this nifty little applet to help visualize it.
Time dilation applet
Well it appears theres no embed functionality so will have to just clicky the link.

I had made a pretty in depth post on time dilation before, with a simple math example. For the people saying this is mumbo-jumbo, its really not complicated math, so please take the time to read through the simple example as its pretty easy to understand.

I am going to just quote my original post, since it applies here.
Now it has also been stated in other threads that time, is relative to the observer, there is no universal time, but what does that mean? It turns out that speed effects time. Lets use a simple math example to show this.

We need a formula for this, we will use . This is how time is treated in special relativity and we get this from Lorentz Transformations, which I will spare you all the math of how we arrive at this equation, just know its correct.

Lets say we have two friends bob and tom, tom wants to travel to a planet 20 light years away. In his spaceship we tom's clock, while bob remains behind on earth with his clock.

For simplicities lets define our variables.
we'll call dt and it is the time as seen by bob.
we'll call dt' and it is the time relative to the traveler in our reference frame.(Tom)
v is the velocity relative to Tom, the traveler.
c is the speed of light
and the square root ill write as (equation)^.5
I know, dont get discouraged yet it will make more sense in a minute.

We know [the planet] is 1.89 x 10^17 meters away (get that by light year*20 light years). Now lets say we invent a new type of engine that allows us to travel at 80% the speed of light, or .8c. We then set off for our new planet and wonder how long it will take us to get there.

Well I am sure most of us have seen this equation before from a high school physics or math class
velocity equals distance divided by time. Since we know our velocity, .8c (units in meters per second) and we know our distance 1.89 x10^17 meters we can calculate roughly the time it takes us to get to this planet. So plug in the knows, use some simple algebra and we get 7.9 x10^8 seconds or roughly 25 years. This what here on earth we would guess and indeed it is what bob measures. But what about tom's clock?

Time to use that scary looking equation we saw earlier.
We are looking for dt', the time as seen by tom's clock. so we have dt=7.9 x10^8 seconds, v=.8c, we plug these numbers in and rearrange our equation.

dt'=(dt)(1-(v^2/c^2))^.5 and we get tom's clock shows 15 years have gone by.

So if at the start of our endeavor, Bob and Tom are twins and 30 years old. When tom arrives at this new star, he will seem to be 45 while bob will seem to be 55.

Now to you quantum physics buffs you may want say, we need to add to this! but again its just a simplified example to show that time is relative to the observer, not a universal constant.

edit: forgot I found this nifty little applet to help visualize it.
Time dilation applet
Well it appears theres no embed functionality so will have to just clicky the link.
Now as for time travel. In our example, Tom does not really "time travel" per say. To tom, everything is normal, there was no jump in time he thought occurred. Tom only appears to have traveled into Bob's future. Not future in a universal since, considering such a place does not exist.

Also, Northwest, hit the nail on the head with "no absolute simultaneity in universe". Time is relative to each observers frame of reference. There is no universal clock ticking by, no static moments in time.

Great stuff! The idea of no absolute simultaneity in the universe is somewhat troubling though. If Bob and Tom could actually observe one another thru this whole process, would not Bob see Tom as moving very slow (relative to Bob's perspective), but Tom would see Bob as moving very fast (relative to Tom's perspective)? Or is this idea seperate from absolute simultaneity?

No, both of them are actually running slower than the other guy, it's kind of hard to imagine, but when you draw a space-time diagram , it is very obvious.

The secret is in CHANGING frames. That's when one becomes slower than the other, but if they don't change frames, both are slower than the other.

So , for example, you have Bob traveling at C, and Tom still, or Tom traveling in c (in opposite direction) or Bob being still, it makes no difference...
well anyway, both of them have a slower time than the other guy.
So who who will reap the benefits of aging less? The one that first jumps into the frame of the other guy.
In other words, the one that decelerates to the other frame will have less proper time (proper time is the key concept here).

So, while they are each in their frames, both have slower time than the other guy,
if Bob decelerates/accelerates to Toms frame, then Bob will prove to Tom his time really was slower, and if
Tom decelerates/accelerates to Bobs time frame, then Tom will prove to Bob his time was the slower one.

So how does this fit together? They each think they are measuring time by comparing it to equal events, but
they are not really, each of them has a different definition of what time is.

Great stuff! The idea of no absolute simultaneity in the universe is somewhat troubling though. If Bob and Tom could actually observe one another thru this whole process, would not Bob see Tom as moving very slow (relative to Bob's perspective), but Tom would see Bob as moving very fast (relative to Tom's perspective)? Or is this idea seperate from absolute simultaneity?

Tick Tock Tick Tock
Time doesn't exist...therefore time travel doesn't exist.

...so sorry to beat that old dead horse again...but Truth is...even when it becomes cliche...

Tick Tock Tick Tock
Time doesn't exist...therefore time travel doesn't exist.

...so sorry to beat that old dead horse again...but Truth is...even when it becomes cliche...

As I said, I dumbed it down a bit to make the example simple. Northwest hit on it well. If Bob was able to observe Tom's clock while in flight he would observe it ticking slower than his. Also, If Tom was able to see Bob's clock, he too would observe it slower.

It is also important to note that the 15 years we calculated is not specifically for Tom's clock, rather the difference between the clocks. When the rocket stops, as Northwest pointed out, we assume that Tom enters bob's reference frame again. By doing so, he gains the benefits of the time difference between the clocks.

Sorry, math has never been my forte, so I have to work harder even to get the "dumbed down" math you presented. lol Bear with me please for more questions. What if Tom dies during his flight and the ship doesn't stop, but continues on into the universe? Would he, from Bob's perspective, just seem to live forever? Whether or not time travel is possible either practically or theoretically, this topic is so intriguing its kinda like a mental roller coaster - what a ride!

"It's not an issue of technology, it's one of economics."

The cost of operating a time travelling machine, in relation to the cost of electricity, would be ten trillion dollars, Dr Savage estimates.

10 trillion dollars is nothing if the richest people in the world pooled their money together.
Then, they could each deposit all the money they had left over in the bank and time travel into the future and then cash in their back accounts and realize a huge profit in only (to them) a few hours.

What if they arrived to some future totalitarian state where it had
become a criminal offense to have wealth?

It's easy for me to believe supernatural but...time travel?
I don't believed it.

It's funny how one can believe in supernatural which is hard to explain, and not believe in time travel which is easy to explain, and demonstrated too.
Now, if there was some kind of theory behind this belief, I'd listen, but this 10 minute couch thinking behind it I assume

But then, there are still flat-earth societies, so I'm not surprised