Time Machines: Can Airplanes Reach the Speed of Light?

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Discussion Overview

The discussion revolves around the concept of time machines in relation to airplanes and their speeds, particularly the idea of whether airplanes could achieve time travel if the air they move through were to approach the speed of light. The conversation touches on theoretical implications of relativity, time dilation, and the nature of speed in relation to time travel.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • Some participants propose that if air could move at near the speed of light, it might allow airplanes to achieve speeds that could theoretically lead to time travel.
  • Others argue that there is no clear evidence supporting the idea that moving near the speed of light would facilitate building a time machine, emphasizing that the speed of the airplane and the air do not simply add up in a way that could exceed the speed of light.
  • A participant explains the relativistic velocity addition formula, indicating that no combination of speeds less than the speed of light can result in a speed greater than light.
  • Time dilation is mentioned as a well-established phenomenon, but some participants clarify that it only applies in one direction (into the future) and does not equate to the concept of a time machine as commonly understood.
  • There is a discussion about the Hafele-Keating experiment, which demonstrates time dilation effects in moving clocks, but participants note that this does not equate to building a time machine.
  • Some participants assert that any object moving faster than another can be considered a "time machine" in terms of relative motion, but they also acknowledge that the practical implications of this are limited.
  • There is a debate about the interpretation of time dilation and whether participants have a correct understanding of its implications.

Areas of Agreement / Disagreement

Participants generally disagree on the implications of time dilation and the feasibility of airplanes functioning as time machines. While some accept the concept of time dilation, there is no consensus on its application to the idea of time travel or the conditions under which it might occur.

Contextual Notes

Participants express uncertainty regarding the definitions of terms like "time machine" and the conditions necessary for achieving speeds that might lead to time travel. There are also unresolved questions about the interpretations of relativity and time dilation.

cdux
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If airplanes' highest speed depends directly on the air it is moving through and not the ground, does it mean we can build a time machine if only the air moves near the speed of light?
 
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Sorry but I can't make heads or tails of this! First there is no clear evidence that "moving near the speed of light" will help you build a time machine! There are some ways of interepreting relativity that lead to the idea that going faster than the speed of light has an effect on time but it is not clear whether that interpretation is true and, in any case, there is no way of going faster than the speed of light. And, finally, why talk about "air moving near the speed of light" when there is no better way of getting air to move "near the speed of light" (relative to what, by the way?) than the airplane itself.
 


cdux said:
If airplanes' highest speed depends directly on the air it is moving through and not the ground, does it mean we can build a time machine if only the air moves near the speed of light?

I'm going to assume that you mean this: If the air were moving at near the speed of light and the plane was moving at a speed with respect to the air such that it's speed plus the speed of the air was greater than the speed of light, does it mean that we can get the plane moving faster than light?

The answer is no. This is because the speed of the air and the plane do not add up that way (no velocities do).

The formula you would have to use is

[tex]\frac{u+v}{1+\frac{uv}{c^2}}[/tex]

Here u is the speed of the air and u the speed of the plane with respect to it. c is the speed of light (in a vacuum).

If you play around with this, you will quickly see that no matter how large u an v are (as long as they are less than c) the answer comes out to being less than c.

However, if u and v are very small compared to c, like they are in everyday experience, the answer comes out to being very nearly the same as u+v.

This is why we would say that the speed of the plane with respect to the ground is equal to the velocity of the air plus the airspeed of the plane. At normal speeds, using the simpler equation gives you an answer so close to the real answer that the difference is much much smaller than the uncertainty of the speeds that we start with. In addition, the simple equation is more intuitive to work with.
 


HallsofIvy said:
Sorry but I can't make heads or tails of this! First there is no clear evidence that "moving near the speed of light" will help you build a time machine!
I had the impression time dilation is consensus accepted.
 


cdux said:
I had the impression time dilation is consensus accepted.
Time dilation is well established, but it only goes in one direction.
 


cdux said:
I had the impression time dilation is consensus accepted.

Both consensus-accepted and well and thoroughly experimentally verified. But that has absolutely nothing to do with building a time machine - at least what most people are thinking of when they say "time machine".

You might want to google the Hafele-Keating experiment to see what can be done by moving clocks around in airplanes.
 


Nugatory said:
Both consensus-accepted and well and thoroughly experimentally verified. But that has absolutely nothing to do with building a time machine - at least what most people are thinking of when they say "time machine".
I thought it was accepted that any object moving faster than another is effectively in a time machine going into the future in relation to the slower object. It's also seen experimentally and on GPS satellites.
 


cdux, yes you're right it would indeed be a "time machine". In fact, every time there is relative motion you have a "time machine". Even if I wave my arms around they end up traveling in time relative to my body. As you mentioned, this only works going into the future.

Everybody else seems to have assumed you were talking about a classical time machine which allows for going into the past...and your post did seem to lead towards that direction.

So yes, it would be a time machine. HOWEVER, contrary to the implication of your post, adding the stipulation "if only the air moves near the speed of light?" doesn't make it any simpler or easier to accomplish any appreciable time travel.
 


cdux said:
I thought it was accepted that any object moving faster than another is effectively in a time machine going into the future in relation to the slower object. It's also seen experimentally and on GPS satellites.

By your argument, everything is a time machine because SR always applies to moving objects. Problem is that the time shift is very small, usually and it's just a one way trip. Not a lot of practical use, I think.
 
  • #10


sophiecentaur said:
By your argument, everything is a time machine because SR always applies to moving objects.
Yes.
 
  • #11


cdux said:
Yes.

So what is your point?
 
  • #12


cdux said:
I thought it was accepted that any object moving faster than another is effectively in a time machine going into the future in relation to the slower object. It's also seen experimentally and on GPS satellites.

That's why I put in that disclaimer about what people usually mean when they talk abut a "time machine" - and you'll notice that the H-K experiment I pointed you at is literally using an airplane to study this effect.

You may have the sense of the dilation backwards as well?
 
  • #13


Nugatory said:
You may have the sense of the dilation backwards as well?
Why do you say that?
 
  • #14


cdux said:
Why do you say that?

It wasn't clear whether you were thinking the moving clock would be the faster one or the slower one.
 

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