# Time Machines: Can Airplanes Reach the Speed of Light?

• cdux
In summary, airplanes' highest speed depends directly on the air it is moving through and not the ground, which means we can build a time machine if only the air moves near the speed of light.
cdux
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?

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

$$\frac{u+v}{1+\frac{uv}{c^2}}$$

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.

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

cdux said:
Yes.

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?

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

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.

## 1. Can airplanes actually reach the speed of light?

No, airplanes cannot reach the speed of light. According to the theory of relativity, the speed of light is the maximum speed that can be reached in the universe. The fastest airplanes can only reach speeds of about Mach 3, which is much slower than the speed of light.

## 2. How close can airplanes get to the speed of light?

Airplanes can get close to the speed of light, but they will never be able to reach it. As mentioned before, the speed of light is the absolute speed limit in the universe. The fastest airplanes, such as the Lockheed SR-71 Blackbird, can reach speeds of about Mach 3.3, which is only a fraction of the speed of light.

## 3. Why can't airplanes reach the speed of light?

There are several reasons why airplanes cannot reach the speed of light. One reason is that as an object approaches the speed of light, its mass increases exponentially, making it more and more difficult to accelerate. Additionally, the amount of energy required to reach the speed of light is practically impossible for current technology to generate.

## 4. Can technology ever advance to the point where airplanes can reach the speed of light?

It is highly unlikely that technology will ever advance to the point where airplanes can reach the speed of light. As mentioned before, the amount of energy required is immense and currently impossible for us to generate. Even if we were able to overcome this hurdle, the effects of time dilation would make it impractical for humans to travel at such speeds.

## 5. Is it possible to travel through time with airplanes?

No, airplanes are not capable of time travel. Time travel is a concept that is still purely theoretical and has not been proven to be possible. While airplanes can travel at high speeds and experience time dilation, they cannot travel through time in the way that is often depicted in science fiction.

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