Time Trip - questions and answers from BBC News

servantx

servantx

Quantum Scribe
Source: http://www.bbc.co.uk/science/horizon/2003/timetripqa.shtml

How widely accepted is the theory that we can travel in time?

The Future
According to Professor Paul Davies "Scientists have no doubt whatever that it is possible to build a time machine to visit the future". Since the publication of Einstein’s Special Theory of Relativity in 1905, few, if any, scientists would dispute that time travel to the future is perfectly possible.

According to this theory, time runs slower for a moving person than for someone who is stationary. This has been proven by experiments using very accurate atomic clocks. In theory, a traveller on a super high-speed rocket ship could fly far out into the Universe and then come back to Earth at a time hundreds or thousands of years in its future.

Another consequence of special relativity is that gravity slows time down. So, another way of time travelling to the future would be to go and sit next to a black hole or a neutron star, both of which are very massive and have huge gravitational fields. When you went back to Earth, it would have aged more than you.
 
ScientificAmerican.com
First Teleportation Between Light and Matter
October 4, 2006

Source: http://www.scientificamerican.com/article.cfm?id=first-teleportation-betwe

At long last researchers have teleported the information stored in a beam of light into a cloud of atoms, which is about as close to getting beamed up by Scotty as we're likely to come in the foreseeable future. More practically, the demonstration is key to eventually harnessing quantum effects for hyperpowerful computing or ultrasecure encryption systems.
 
According to this theory, time runs slower for a moving person than for someone who is stationary. This has been proven by experiments using very accurate atomic clocks. In theory, a traveller on a super high-speed rocket ship could fly far out into the Universe and then come back to Earth at a time hundreds or thousands of years in its future.
Click to expand...


I suppose technically one could call it 'time travel'....though it's not quite what most people have in mind. It doesn't actually require a huge long journey in space..if one can get the spaceship close to the speed of light then time will slow down more and more as one approaches that speed.

The trouble is.....to avoid being squashed by G forces, it would take at least 6 months to accellerate to even 98% of light speed. Then comes the biggest problem of all. The relativistic effects become exponential with each further tiny increase in speed....BUT....so does the increase in the mass of your rocket. Which in turn means you need a larger amount of fuel to propell that mass.....and so on !

And at that speed, you have only to hit a single atom of intergalactic hydrogen on the way......and you are history.
 
Which in turn means you need a larger amount of fuel to propell that mass
Click to expand...
If you carry your fuel with you, as you surely will, then the mass of the fuel will increase at the same rate as that of the vehicle. Thus the thrust from the perspective of the traveler need not change. It seems too often that people think that the fuel is somehow not affected by relativity.
 
If you carry your fuel with you, as you surely will, then the mass of the fuel will increase at the same rate as that of the vehicle. Thus the thrust from the perspective of the traveler need not change. It seems too often that people think that the fuel is somehow not affected by relativity.
Click to expand...

Precisely. Though you would expend fuel, thus reduce rest mass, during the acceleration portion of the trip you end up with three huge problems that are very apparent:

1. F = ma. If you rearrange the equation by dividing each side by "m" you have,

a = F/m.

If you add the Lorentz Transformation to the mass then the issue is obvious. As you approach the speed of light the denominator on the right side tends to infinity. No matter how much fuel you carry along its mass "m" tends to infinity and the differential increase in acceleration "a" on the left side per unit of force "F" input on the right side tends to zero no matter how large the force input. The force simply can't overcome the inertia of the mass in the denominator as it tends to infinity.

2. You have expended your fuel in the acceleration phase. We assume that at some point you might want to stop somewhere and enjoy the view. But where are you going to get the fuel for the braking phase?

3. Lorentz contraction. The actual effects of this aren't apparent in Special Relativity. In General Relativity, where differential geometry is used in place of differential calculus, the spacetime surrounding your craft doesn't contract linearly. It folds up into a cone along your axis of translation (movement) that will, at the speed of light, be a singularity of infinite density. As you closely approach the speed of light the entire spacetime of the universe is tending to be directly in front of you along your path of movement and it is also contracting toward you. All the mass of the universe is compacting into a blue sheet and you are on a path to run into all of it simultaneously. That will probably ruin your entire afternoon.
 
Another consequence of special relativity is that gravity slows time down.
Click to expand...

Servantx,

Just a technical detail, but gravity isn't present in Special Relativity, which is the reason for the term "special" in the name. It is the special case of relativity where there is no gravitation involved. General Relativity incorporates the case where gravity is present. Other than that your statement was correct. Gravity slows your clock relative to an observer in a lesser gravitational field.
 
Thanks for the details. My post was copy and pasted from the BBS News article. Not my own words.
Click to expand...

Thanks for the clarification. It points to the fact that when reading science related articles in the popular media the writer(s) is rarely familiar with the subject matter.
 
If you carry your fuel with you, as you surely will, then the mass of the fuel will increase at the same rate as that of the vehicle. Thus the thrust from the perspective of the traveler need not change. It seems too often that people think that the fuel is somehow not affected by relativity.
Click to expand...


Which would surely mean that for any chemical fuel, it would become less effective. If a pound of fuel now weighs a ton, the chemical bond strengths don't change for relativity.......but that chemical energy now has to push apart 2000 times as much mass, using the same energy.

Although the amount of chemical energy released would be the same, it is the speed of the propellant that determines the speed of the rocket. More massive fuel would have greater inertia and thus less able to travel at higher speed. A ton of fuel blasted out at 10mph is not going to accelerate you much !
 
...may be a solar sail, light drive propulsion (laser beams), or even Alcubierre drive would be more apporiate for space travel in coming future.....
 
Back
Top